Solubilization-emulsification mechanisms of detergency

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 74, 169-215 (1993) 

Review 

Solubilization-emulsification mechanisms of detergency 

Clarence A. Miller a,* Kirk H. Raney b 

a Department of Chemical Engineering, Rice University, P.O. Box 1892, Houston, TX 77251-1892, USA 

b Shell Development Co., Westhollow Research Center, P.O. Box 1380, Houston, TX 77251-1380, USA 

(Received 14 November 1992; accepted 23 January 1993) 

Abstract 

The removal of oily soils from fabrics having high contents of polyester or other synthetic materials 

occurs largely by a solubilization-emulsification mechanism. A systematic investigation of this mechanism 

has been conducted during the past several years and is reviewed here. The research has utilized a variety of 

oily soils containing hydrocarbons, triglycerides, and long-chain alcohols and fatty acids and has included the 

determination of equilibrium phase behavior, the observation of dynamic behavior which occurs when 

surfactant-water mixtures contact oily soils, and measurement of soil removal from polyester-cotton fabrics. 

In most cases, pure surfactants and oils have been used for simplicity, but data showing the applicability of 

major conclusions to systems containing commercial surfactants are presented. Because typical anionic 

surfactants are too hydrophilic to achieve the desired phase behavior, the work has employed non-ionic 

surfactants and mixtures of non-ionics and anionics, One major conclusion is that maximum soil removal 

usually does not occur when the soil is solubilized into an ordinary micellar solution, but instead when it is 

incorporated into an intermediate phase such as a microemulsion or liquid crystal that develops during the 

washing process at the interface between the soil and washing bath. Indeed, for hydrocarbon and triglyceride 

soils, the washing bath is itself a dispersion of a surfactant-rich liquid or liquid crystalline phase in water for 

conditions of optimum detergency, i.e. the temperature of the surfactant solution is above - sometimes far 

above - its cloud point temperature. 

Key words: Detergency; Emulsification; Solubilization 

1. General remarks on detergency 

Fabric detergency is a surprisingly complex 

process involving interactions among aqueous 

detergent solutions, soils, and fabric surfaces. 

This Process may occur in an industrial setting 

in which large volumes of similarly soiled 

fabrics are washed, or in a household setting in 

which small amounts of fabrics containing 

differing amounts of a wide variety of soils are 

washed. Because of their unique ability to 

adsorb at both fabric-water and soil-water 

interfaces, surfactants play an essential role in 

soil removal processes. To achieve the desired 

levels of surfactants in the washing solution, the 

* Corresponding author. 

concentration of surfactants (typically nonionic, 

anionic, or both) in most liquid and powder 

detergent formulations is in the range 10-40% 

by weight [1]. 

Several factors influence the effectiveness of 

surfactants in laundry detergents. The 

composition of a fabric is important in 

determining the mechanism by which soils are 

lifted from it. Cotton fabric contains rough and 

irregularly shaped hydrophilic fibers [1,2]. In 

contrast, synthetic polyester fabric contains 

uniform cylindrical fibers which, being of a 

more hydrophobic nature than cotton, are more 

tenaciously covered by oil. The differences in 

surface properties of the two materials are 

demonstrated by the much smaller contact angle 

in air formed by water on a cellulose film (32º)

170 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 

than that by water on a polyester film (79º) [3]. 

In addition to fiber surface composition and 

morphology, the weave of the fabric can 

influence detergency, more loosely woven 

fabric typically being easier to clean. 

Not surprisingly, the amounts and types of 

soils present on fabric are key factors in 

determining the effectiveness of a detergent 

solution. Oily soils include such common soils 

as skin sebum, dirty motor oil, and vegetable oil. 

Clay is classified as a particulate soil. Scanning 

electron microscopy, both conventional and 

environmental, has proven quite useful for 

viewing the distribution of both oily and 

particulate soils within woven fabric samples 

[2,4]. While surfactants play the key role in 

removing oily and particulate soil, protease 

enzymes are commonly present in both powder 

and liquid laundry detergents to chemically 

break down polymeric protein soil stains such as 

blood, egg, and cocoa. Lipase enzymes are also 

now being used in detergents to hydrolyze 

triglyceride soils and thereby aid the surfactant 

in soil removal [5]. Bleaches, both peroxygen 

and chlorine-based, decolorize stains such as 

those from tea and wine by destroying the 

chromophores in the organic molecules 

adsorbed to the fiber surfaces [6]. 

Water hardness and temperature can 

profoundly influence detergent effectiveness. In 

hard water, e.g. 300 ppm hardness, calcium and 

magnesium ions may precipitate certain 

surfactants prior to their being able to act on the 

soil. The divalent ions also form complexes 

between soils and fabric which increase their 

attraction, making the soil more difficult to 

remove [1]. Builders such as zeolite, sodium 

tripolyphosphate (STPP) and sodium carbonate 

are used in powders to negate the effect of water 

hardness by either precipitating the divalent 

ions, as in the case of sodium carbonate, or 

sequestering the ions from the water, as in the 

cases of zeolite and STPP. The latter is 

particularly effective in this regard. Surfactant 

precipitation may occur in cold water for 

surfactants with high Krafft points [I]. Also, 

wash water temperature, in addition to changing 

the performance characteristics of the dissolved 

surfactants, determines the physical properties 

of oily soils left on the fabric. As the washing 

temperature is reduced, the viscosity of the soils 

increases or the soils may even solidify, making 

them more difficult to remove with the same 

level of agitation. 

Recent trends in washing habits around the 

world have made the proper choice of a 

surfactant system for a detergent formulation 

more critical than ever. Greater use of 

temperature-sensitive synthetic fabrics such as 

polyester or polyester-cotton blends as well as 

energy conservation have led to a world-wide 

trend to lower washing temperatures [7]. 

Phosphate limits or bans have resulted in the use 

of less effective builders or of no builders, so 

that surfactants are less protected from the 

negative impact of water hardness. Also, as a 

result of the effort to reduce the total amount of 

chemicals released to the environment as well as 

the volume of packaging materials, detergent 

manufacturers are formulating products with 

lower dosage requirements. As a result of these 

trends, the cleaning efficiency required from 

surfactant systems is steadily increasing. 

2. Mechanisms for removal of oily soils 

The trends described above can have a 

particularly negative impact on the removal of 

Oily soils from synthetic fabrics. Commonly 

encountered examples of this troublesome 

soil-fabric combination are dirty motor oil, 

cooking oil, or sebum on 100% polyester or 

polyester-cotton blends. Many studies have been 

performed to visualize the mechanisms by 

which such oils are removed from synthetic 

fabrics [8-12]. Although in practical situations 

the oily soils are trapped in the interstices 

between fabric fibers and as thin films along the 

fiber surfaces, most work has focused on the 

removal of oil drops from flat surfaces or 

individual fibers with the assumption that 

similar removal mechanisms would be relevant 

to removal from fabric. In fact, good correlation 

between Oil removal from flat films and from 

chemically similar fabric has been reported [8]. 

Of relevance to this type of study is Young 5 

equation, which relates the interfacial tension

C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 171 

between the surfactant solution and oil (γ ow), oil 

and solid substrate (γ os) and surfactant solution 

and solid substrate (gws) to the equilibrium 

contact angle q measured through the soil 

cos θ = γ ws − γ os 

γ ow 

(1) 

For quite hydrophilic surfaces like cotton, gws 

is smaller than g0s, and a contact angle greater 

than go is commonly achieved. Anionic 

surfactants which adsorb on the fabric with their 

negatively charged head groups oriented toward 

the detergent solution are particularly effective 

in reducing gws. in this case, the roll-up 

mechanism is operative: the water preferentially 

wets the fabric, causing the oily stains to be 

entirely lifted off the fibers into the washing 

solution. This behavior, shown schematicaily in 

Fig. 1(b) for soil removal from a flat surface. is 

enhanced on cotton fabric due to swelling of the 

cotton fibers with water which increases the 

hydrophilicity of the fabric surfaces [9,13]. 

For low surface energy, i.e. hydrophobic, 

materials such as polyester, a contact angle of 

less than 90º is usually observed, and small 

portions of the oily soil may be removed by 

hydraulic currents at the soil-water interface, as 

shown in Fig. 1(a). In Fact, if the fabric surface 

is initially completely covered by oily soil, no 

location is available for the surfactant solution 

Fig. 1. Mechanisms of liquid soil removal: (a) 

emulsification; (b) roll-up. 

to reach the fiber surface and undercut the soil. 

Observation of this "necking" or emulsification 

mechanism has been made by many 

investigators for mineral oils and mineral 

oil-polar soil mixtures on hydrophobic flat films 

and fibers [8-12]. Removal in this manner is 

enhanced by low interfacial tension at the 

oil-water interface which allows the oil film to 

be deformed easily to form small emulsion 

droplets. 

Several factors have been studied with regard 

to their effect on the emulsification mechanism 

for the removal of mixtures of mineral oil and 

polar organic alcohols or acids from polyester 

[8-10]. Such model systems, depending on the 

ratio of the non-polar and polar constituents, can 

be considered to be representative of sebum 

soils from the skin. The rate of emulsification of 

mineral oil-oleic acid mixtures from polyester 

(Mylar) films was found to change as the oleic 

acid content was varied [8], Other factors such 

as electrolyte concentration and temperature 

were also found to have large effects on the rate 

of soil removal by this mechanism [8,9]. In 

some situations, emulsification of 

non-polar-polar soil mixtures without external 

agitation, i.e. spontaneous emulsification, has 

been observed [ 13,14]. Emulsification, roll-up, 

and other adhesion and detachment phenomena 

involving oily soils and solid surfaces are 

reviewed in the accompanying paper [ 15]. 

Another mechanism of oily soil removal 

involves the formation of intermediate phases at 

the detergent solution-oil interface 

[11,13,14,16]. Apart from the recent studies 

described below, this mechanism has most often 

been reported for the removal of soils containing 

large quantities of polar constituents. The 

growth of liquid crystal occurs in these systems 

due to interaction at the interface between the 

polar soil constituents and the adsorbed 

surfactants. After growing to a sufficient extent, 

the intermediate phase is broken off by agitation 

and emulsified into the aqueous solution 

allowing fresh contact of the remaining soil with 

the detergent solution. 

Direct solubilization of oily soils into 

surfactant micelles can also occur to a 

significant extent if a large excess of surfactant


relative to oil is present and if the surfactant is 

above its CMC. The solubilization of very small 

oil drops from polymer fibers has been 

visualized for a variety of non-polar oils 

representative of liquid laundry soils [17]. 

However, soil solubilization rates are often 

enhanced when surfactant-rich phases, either 

isotropic or liquid crystalline in nature, are 

present in the washing solution. Such phases 

exist, for instance, when non-ionic surfactants 

are above their cloud points. These phases can 

either solubilize oily soils directly or interact 

with soil to form intermediate surfactant-rich 

phases such as microemulsions containing large 

amounts of oil. Under favorable conditions, the 

intermediate phases can be emulsified into the 

washing bath. A detailed discussion of this 

mechanism of soil removal is the subject of this 

review. 

Clear evidence exists that solubilization and 

emulsification are major factors in removal of 

oily soils from hydrophobic, synthetic fabrics 

[18,19]. Unlike roll-up, in which the interaction 

of the fabric with the oily soil and water is most 

critical, the solubilization-emulsification mechanism 

occurs primarily at the soil-detergent 

solution interface and is therefore directly 

infiuenced by the phase behavior of the 

corresponding oil-water-surfactant system. For 

example, the formation of intermediate liquid 

crystalline phases in fatty acid-surfactant-water 

systems has been explained by the equilibrium 

phase behavior of those systems [ 16]. Also, 

spontaneous emulsification phenomena in 

oil-water-surfactant systems have been shown to 

be predictable from equilibrium phase behavior 

[20]. Therefore, an understanding of the phase 

behavior in these systems is needed to predict 

the effectiveness of and/or to optimize detergent 

solutions for specific soil compositions and 

washing conditions. Available information on 

phase behavior is reviewed in the next section. 

In subsequent sections, systematic studies of 

dynamic contacting between aqueous surfactant 

solutions and oils as well as detergency studies 

using the same systems are reviewed in order to 

further explain the role of the solubilizationemulsification 

mechanism in practical detergency 

processes. 

3. Equilibrium phase behavior 

As indicated above, some knowledge of the 

equilibrium phase behavior of 

soil-water-surfactant systems is needed to 

understand solubilizationemulsification 

mechanisms of detergency. In this section, we 

review such phase behavior with emphasis 

given to model systems with well-defined 

components. Results are given for one- and 

twocomponent oils consisting of hydrocarbons, 

triglycerides, and/or long-chain alcohols or 

acids, representing soils such as lubricating oils, 

cooking oils and sebum. In many cases 

single-component specific alcohol ethoxylates 

are the surfactants. However, most features of 

the behavior described should be applicable to 

more complex systems as well, e.g. those 

containing multicomponent commercial 

surfactants. 

3. 1. Phase behavior of soil-free washing baths 

We begin with the fluids used for washing, 

i.e. rather dilute mixtures of surfactant and water 

with inorganic salts and/or various additives 

also present in some cases. As is well known, a 

typical hydrophilic surfactant above its Krafft 

temperature forms micelles in water at 

concentrations above its CMC. If the 

temperature or composition of a micellar 

solution is varied in such a way that the 

surfactant becomes less and less hydrophilic, the 

separation of another phase eventually results. 

If. for instance, the temperature of a micellar 

solution of an ethoxylated alcohol is increased, a 

second liquid phase begins to appear when the 

so-called cloud point temperature is reached. As 

Fig. 2 for the n-dodecyl pentaoxyethylene 

monoether (C12E5)-water system [21] shows, 

the cloud point is a function of surfactant 

concentration since clouding occurs when the 

coexistence curve forming the upper boundary 

of the aqueous surfactant solution L1 is crossed 

during heating. At temperatures well above the 

cloud point, the lamellar liquid crystalline phase 

La and yet another liquid phase L3, widely 

considered to consist of bilayers arranged in a 

sponge-like structure, are seen in this system for


Fig. 2. Phase diagram of C 12E 5-water system [21]. L 1, 

L 2, and L 3 denote isotropic liquids; Lα, H 1, and V 1 

denote lamellar, hexagonal and viscous isotropic 

liquid crystalline phases, respectively. Reprinted with 

permission of the Royal Society of Chemistry. 

relatively low surfactant concentrations. As the 

particles of a liquid crystal do not coalesce as 

readily as liquid drops, the dispersions of La are 

frequently less turbid than those of L 3, a 

property which can be used to locate phase 

transition temperatures at which the La and L 3 

phases form [22]. At the highest temperatures 

shown in Fig. 2 the surfactant-rich liquid phase 

L 2 coexists with water. 

For more hydrophilic surfactants such as 

n-dodecyl hexaoxyethylene monoether (C 12E 6), 

clouding occurs at higher temperatures. 

Moreover, the Lα and L 3 phases do not appear 

at low surfactant concentrations; the La phase 

transforms continuously into L 2, and the cloud 

point curve is the only feature of this part of the 

phase diagram (see Fig.3). Phase diagrams for 

various binary nonionic surfactant-water 

systems are given by Mitchell et al. [23]. 

Temperature effects are weaker for ionic 

surfactants and generally act in the opposite 

direction. Since the Debye length, a measure of 

the electric double layer thickness, is 

proportional to (kT) 1/2 , where kT is the 

characteristic free energy of random thermal 

Fig. 3. Phase diagram of C 12E 6-water system [23]. 

The symbols for the phases are as in Fig. 2 except 

that S is a solid phase and W is a water-rich liquid 

phase. Reprinted with permission of the Royal 

Society of Chemistry. 

motion, higher temperatures make ionic 

surfactant films more hydrophilic, with a greater 

tendency to curve toward an oil-in-water 

configuration. However, the addition of 

inorganic salts has the opposite effect, 

compressing electric double layers and causing 

ionic surfactant films to become less 

hydrophilic. In some cases, a- second liquid 

phase is ultimately formed as salinity increases, 

i.e. the behavior is similar to clouding of 

non-ionic surfactant solutions discussed above. 

This phenomenon was observed by McBain 

many years ago for aqueous soap solutions 

[23b]. Another example is shown along the 

upper boundary of Fig. 4 [24], with NaCl added 

to the sodium salt of a commercial ethoxylated 

sulfate based on a C 12-C 13 alcohol and 

containing an average of three ethylene oxide 

groups (Neodol 23-3S). As Fig. 4 indicates, 

multiphase regions involving the lamellar liquid 

crystal La are observed at even higher salinities. 

In other systems, for instance the Aerosol 

OT-NaCl-water system, the first phase formed 

upon increasing the salinity is the lamellar liquid 

crystalline phase [25,26]. Indeed, such behavior 

is typical for anionic surfactant-short-chain 

alcohol systems investigated for possible use in


Fig. 4. Phase behavior of mixtures of C 12E 3, Neodol 

23-3S, and NaCl brine, with the temperature and total 

surfactant concentration fixed at 30ºC and 5.4 wt.%, 

respectively [24]. B, NaCl brine; W(non), weight 

fraction of non-ionic surfactant in the surfactant 

mixture. Reprinted with permission from Dr. Dietrich 

Stemkopff Verlag. 

enhanced oil recovery, with the L 3 phase found 

at still higher salinities [27]. The addition of 

divalent cations (i.e. an increase in hardness) 

produces the same effects in these systems, 

although generally at lower electrolyte 

concentrations [27]. 

Whether a liquid phase or the liquid crystal 

forms upon the addition of salt apparently 

depends on the relative importance of reducing 

the effective electrical repulsion between nearby 

ions within a micelle and reducing it between 

micelles. If the former is the dominant effect, 

the micelle shape changes from spherical to 

cylindrical to planar as the effective surfactant 

head group area decreases, a sequence in 

accordance with well-understood surfactant 

packing considerations in micelles [28]. The 

large, bilayer sheets corresponding to the planar 

configuration with nearly equal head and tail 

areas arrange themselves into the lamellar 

phase. In contrast, if reducing the repulsion 

between relatively small micelles is the more 

important effect, the micelles eventually 

flocculate to form a "coacervate" or micelle-rich 

liquid phase. 

It appears from the available evidence that 

coacervation is the likely outcome of increasing 

salinity for anionic surfactants that are rather 

hydrophilic, while liquid crystal formation is 

probable for surfactants whose hydrophilic 

characteristics only slightly outweigh their 

lipophilic characteristics in the absence of salt. 

Figure 4 provides an example. The addition of 

less than 20% of the non-ionic surfactant 

n-dodecyl trioxyethylene monoether (C 12E 3) 

reduces the hydrophilic nature of the surfactant 

mixture sufficiently such that the liquid crystal 

phase forms instead of a coacervate when the 

NaCl concentration is increased. Such behavior 

can be explained as follows. Hydrophilic 

surfactants such as Neodol 23-3S require large 

concentrations of salt for the surfactant 

aggregates to become planar. Repulsion 

between small, nonplanar micelles is apparently 

reduced sufficiently so as to produce 

coacervation before the salinity increases 

enough for planar micelles to form. For less 

hydrophilic surfactants, less salt is needed to 

produce planar aggregates, and formation of the 

lamellar phase occurs before coacervation. 

The cloud point phenomenon discussed above 

for non-ionic surfactants may also be viewed as 

coacervation of a micellar solution induced by 

making the surfactant less hydrophilic. In this 

case. raising the the temperature reduces the 

interaction between the ethylene oxide chains 

and water and thereby reduces the repulsion 

between micelles or even reverses it to an 

attraction. Of course, the effective area of the 

ethylene oxide head group is also reduced, and a 

change from spherical to cylindrical micelles, 

which would facilitate coacervation by an 

entropic mechanism, can occur before the cloud 

point is reached. Unlike the situation for anionic 

surfactants, however, there do not seem to be 

any reports of the lamellar liquid crystalline 

phase forming directly from micellar solutions 

Or ethoxylated alcohols before clouding occurs 

as the temperature is raised in dilute binary 

systems. 

The cloud point of a non-ionic surfactant 

natulrally depends on its structure, increasing


for longer ethylene oxide and shorter 

hydrocarbon chains. Shifting the point of 

attachment of the ethylene oxide chain from the 

end to the central portion of the hydrocarbon 

chain depresses the cloud point. The addition of 

many common salts, e.g. sodium and potassium 

chlorides and sulfates, lowers the cloud point 

although the effects are much smaller than for 

ionic surfactants. However, some salts cause the 

cloud point to increase. The former effect is 

generally considered to stem from reduced 

hydration of the ethylene oxide chains resulting 

from competition with the added ions for the 

available water molecules. The latter effect 

occurs for ions such as I - , SCN - and most 

multivalent cations Which break the structure of 

water. For some salts the anion and cation have 

opposite effects, with the stronger determining 

the direction of the cloud point shift. These 

effects have been recently discussed by Mackay 

[29]. 

Other additives also influence the cloud point 

and other phase boundaries in non-ionic 

surfactant-water systems. Long-chain alcohols 

make the system less hydrophilic, as Fig. 5 

shows for the case of n-dodecanol added to 

mixtures of C 12E 5 and water [30]. 

Fig. 5. Phase behavior resulting from the addition of 

small amounts of n-dodecanol to I Wt'% C 12E 5 in 

water [30]; 30 denotes a three-phase region. 

Reprinted with permission from Academic Press. 

In this case, the cloud point is lowered by some 

23ºC when the alcohol content is only 10 wt.% 

relative to the surfactant. The temperatures for 

the other phase transitions are lowered as well. 

For the binary surfactant-water system, the 

phase rule constrains the three-phase regions, 

e.g. W + L 1 + Lα, to a single temperature, but 

the addition of the alcohol provides an 

additional degree of freedom and allows these 

regions to span a finite temperature range, as 

Fig. 5 indicates. 

The additive can, of course, be another 

surfactant. It is well known that the addition of 

an ionic surfactant greatly increases the cloud 

point of an ethoxylated alcohol by adding an 

electrical repulsion between micelles and 

thereby inhibiting coacervation [31]. The 

opposite occurs when a second but more 

lipophilic non-ionic surfactant is added, e.g. 

C 12E 3 to C 12E 6, as shown in Fig. 6 [32]. This 

system is particularly interesting because, as 

noted previously, the Lα and L 3 phases do not 

occur in dilute mixtures of water and pure C 12E 6. 

However, both these phases appear when only a 

few per cent of the more lipophilic surfactant 

has been added, due to some rather complex 

Fig. 6. Phase behavior of mixtures of C 12E 6 and 

C1 2E 3 in water. The total surfactant concentration is 

fixed at 1.0 wt.% [32].


phase behavior in the region indicated by the 

box. Although details are not given here, an 

interesting feature of this behavior is the 

existence of a four-phase region, which is 

constrained to a single temperature in this 

ternary system by the phase rule and which 

involves W, L 1, L 3 and La, phases [32]. Figure 6 

also shows a transition from W + L 3 to W + L 2 at 

58ºC for the C 12E 3-water system, which was 

clearly seen in this study using optical 

microscopy but does not appear in the existing 

phase diagram [23]. 

The addition of a non-ionic surfactant to an 

anionic surfactant makes the surfactant mixture 

more lipophilic and causes a shift from L1 to 

Lα, to L 3, as shown in Fig. 4. This sequence is, 

not surprisingly, consistent with that shown in 

Fig. 6 for increase in the C12E3 content of 

C 12E 6-C 12E 3 mixtures at constant temperature. 

The same sequence is found when a lipophilic 

alcohol is added to an anionic [27] or a 

zwitterionic [33] surfactant system. 

Although cationic surfactants are rarely used 

for cleaning purposes, they are useful for 

neutralizing charge build-up on fabric surfaces 

and for fabric softening. As an important trend 

in detergent formulation is combining all 

ingredients into a single mixture in order to 

eliminate the need for the separate addition of 

bleaches, fabric softeners, etc. during the 

washing process, it seems useful to include 

some information on phase behavior with 

cationic surfactants present. Moreover, some 

recent work suggests that mixtures of cationic 

and nonionic surfactants may be useful in 

removing oily soils, though not by solubilization 

mechanisms [34]. Because anionic and cationic 

surfactants attract one another, surfactant 

aggregation occurs readily with substantial 

neutralization of charge. When either the 

anionic or the cationic surfactant is present in 

substantial excess, the result is mixed micelles 

but with a much lower CMC than for the 

individual surfactants. When the two surfactants 

are present in almost equal amounts, the 

formation of a solid or liquid crystalline phase 

can be expected if the hydrocarbon chain 

lengths are sufficiently long. In some cases, 

vesicles have been found to form spontaneously 

upon mixing aqueous solutions with 

intermediate ratios of anionic and cationic 

surfactants [35]. 

3.2. Phase behavior of 

water-surfactant-hydrocarbon systems 

An important feature of the phase behavior of 

systems containing water, surfactants, and 

hydrocarbon soils is the existence of 

microemulsions, thermodynamically stable 

liquid phases containing substantial amounts of 

both water and oil. The formation of 

microemulsions requires that the 

surfactant-films which separate oil and water 

microdomains be rather flexible, and that the 

hydrophilic and lipophilic properties of the 

surfactant be roughly balanced. However, within 

conditions satisfying these overall constraints, 

the microstructure is quite sensitive to changes 

in the relative strength of hydrophilic and 

lipophilic interactions. In systems which contain 

comparable volumes of oil and water and which 

are dilute in surfactant but not so dilute as to 

preclude aggregation, packing considerations 

dictate that oil-in-water microemulsions coexist 

with excess oil for hydrophilic surfactants and 

water-in-oil microemulsions with excess water 

for lipophilic surfactants. Drop size' are of the 

order of 5-50 nm, increasing in size as the 

temperature, pressure, or system composition is 

changed to shift the surfactant closer to the 

condition of precise balance between 

hydrophilic and lipophilic properties. Very near 

this balance. the microemulsion becomes 

continuous in both phases and coexists with 

both excess water and excess oil. Interfacial 

tensions of this 'middlephase" microemulsion 

with both excess phases are frequently below 

0.0 1 mN m -1 - in some systems by an order of 

magnitude or more. 

The condition for which the hydrophilic and 

lipophilic properties are exactly balanced and 

the surfactant films have no spontaneous 

tendency 10 curve in either direction has been 

called the phase inversion temperature (PIT) or 

hydrophile-lipophile balance (HLB) temperature 

by Shinoda and Friberg [36] for the case of


non-ionic surfactants for which temperature is 

usually the variable of greatest interest. For 

ionic surfactants it is more common to speak of 

"optimal" conditions, e.g. optimal salinity [37]. 

Whatever one calls it, several criteria have been 

used to define the condition for balance in terms 

of readily measured experimental quantities. 

The most common criterion is equal volumetric 

solubilization in the microemulsion of the oil 

and water phases. The differences between the 

"optimal" conditions given by this and other 

criteria are small for practical purposes and will 

be ignored here. 

The effects of temperature and inorganic salts 

on making the surfactant more or less 

hydrophilic are basically the same as those 

described in the preceding section, and so are 

the effects of adding alcohols or additional 

surfactants, except that one additional factor 

must be considered - the relative solubilities of 

the surfactants and additives in the oil phase. It 

is the composition of the surfactant films 

separating oil and water domains that 

determines the microstructure of the 

microemulsion. In a mixture of two non-ionic 

surfactants the more lipophilic surfactant has a 

higher solubility in the oil phase and the 

surfactant films are thus more hydrophilic than 

the overall surfactant mixture. The magnitude of 

this effect for a given pair of surfactants 

depends on both the overall surfactant 

concentration and the water-to-oil ratio. 

Kunieda, Shinoda and co-workers have 

developed equations for predicting the 

dependence of the PIT on system composition 

for mixtures of two non-ionic surfactants [38] 

and for mixtures of an anionic and a non-ionic 

surfactant [39]. For instance, in the latter case 

the following relationship must be satisfied at 

the PIT 

W n = S sn + S onR ow [(1 - S sn)/(1 - S on)] (X -1) (2) 

where W n is the mass fraction of non-ionic 

surfactant in the overall mixture, S sn is the mass 

fraction of non-ionic surfactant in the surfactant 

films, S on is the mass fraction of non-ionic 

surfactant in the excess hydrocarbon phase, R ow 

is the mass fraction of oil in the oil-water 

mixture, and X is the total surfactant mass 

fraction in the system. The solubility of the 

anionic surfactant in the excess oil has been 

neglected. 

It is clear from this equation that a plot of Wn 

as a function of (X -1 - 1) at constant Row and 

temperature should yield a straight line from 

which values of Ssn and Son can be extracted. 

Figure 7 shows such plots for mixtures of C 12E 3 

and Neodol 23-3S at various temperatures along 

with the corresponding values of Ssn and Son. 

The oil phase is n-hexadecane and the aqueous 

phase is water containing 1 wt.% NaCl. As 

might be expected, nonionic surfactant 

solubility in the oil phase S on increases with 

increasing temperature. In contrast, the fraction 

S sn of nonionic surfactant in the films decreases. 

Since increasing temperature makes the nonionic 

surfactant less hydrophilic, it is reasonable 

that less of it would be required to achieve the 

Fig. 7. PIT results for the C12E3-Neodol 23-3S-1 

wt.% NaCl brine-n-hexadecane system [24]; X is the 

total surfactant mass fraction in the system. 

Reprinted with permission from Dr. Dietrich 

Steinkopff Verlag.


balance between hydrophilic and lipophilic 

properties at high temperatures. 

The PIT is also influenced by the composition 

of the oil phase, being higher for hydrocarbons 

with longer chains. The reason is that their 

penetration into the hydrocarbon chain region of 

the surfactant films tends to make the films 

curve toward a water-in-oil configuration. Such 

penetration is less for longer-chain 

hydrocarbons [40], probably due primarily to an 

entropic effect [41] except for short-chain 

hydrocarbons where energy effects have 

recently been shown to be important as well 

[42]. Reed and Puerto [43] have developed a 

scheme relating optimal conditions to the molar 

volume of the oil and the solubilization at 

optimal conditions. 

The other factor mentioned above as being 

necessary for microemulsion formation is the 

existence of flexible films. Films are most rigid 

for surfactants having long, straight 

hydrocarbon chains. Flexibility can be increased 

by promoting less ordered packing in the 

hydrocarbon chain region of the films, e.g. by 

using branched-chain surfactants or mixtures of 

surfactants with different chain lengths, or by 

adding short-chain alcohols. Increasing the 

temperature also promotes flexibility. Too much 

flexibility can be undesirable, however, because 

it reduces the solubilization capacity of a 

microemulsion. However, when the surfactant 

films become too rigid, the lamellar liquid 

crystalline phase forms. Barakat et al. 

determined the conditions in several systems for 

which the lamellar phase formed instead of a 

middle-phase microemulsion because of film 

rigidity [44]. Hackett and Miller investigated the 

detailed phase behavior near the transition [45]. 

The liquid crystal can also form when the 

amount of oil or water present becomes too low 

[46] or when the surfactant concentration of a 

middle-phase microemulsion becomes too high 

[47]. 

Kunieda and Shinoda [47] have presented 

ternary diagrams at several temperatures ranging 

from below to above the PIT for the 

C12E5-water-n-tetradecane system. We discuss 

below the use of such diagrams in interpreting 

the dynamic behavior which occurs when 

surfactantwater mixtures contact oil. 

3.3. Water-non-ionic surfactant- triglyceride 

systems 

Extensive studies have been made of 

microemulsions in hydrocarbon systems. Much 

less information is available for oils which are 

liquid triglycerides. An important difference is 

that triglycerides such as triolein which are of 

interest for detergency are of much higher 

molecular weight than simple straight-chain 

liquid hydrocarbons such as n-hexadecane. The 

higher molecular weight makes it much more 

difficult to incorporate such triglycerides into 

surfactant films, the result being a major 

reduction in solubilization in many systems. 

Figure 8 shows the general form of a 

water-nonionic surfactant-liquid triglyceride 

ternary diagram [48-50]. Deviations from this 

behavior occur at sufficiently high temperatures, 

a matter discussed further below. As indicated 

on the diagram, the phase designated D' is the 

same as that usually called L3 in binary 

surfactant-water systems, mentioned in section 

3.1. While, like the middle-phase 

microemulsions discussed above, the D' phase 

coexists with both excess water and excess oil 

for suitable overall system compositions, it 

differs from the microemulsions in that it can 

Fig. 8. Schematic phase diagram for non-ionic 

surfactant-water-triolein system at low temperatures 

[48]. O denotes a triolein-rich phase.


solubilize only small amounts of the oil phase. 

Note that solubilization of triglyceride in the 

lamellar liquid crystalline phase is also low. 

Behavior of the W + D' + O three-phase 

triangle has been studied as a function of 

temperature for pure non-ionic surfactants and 

triolein. Figure 9 shows the results for C12E3 

[48], a lipophilic surfactant that is already above 

its cloud point at 0ºC, well below the 

experimental temperature range. The surfactant 

content of the D' phase increases rapidly with 

temperature, the same as for the L3 phase in 

binary surfactant-water systems, but 

solubilization of triolein remains low. The 

solubility of surfactant in the triolein phase is 

substantial - more than 10% by volume even at 

the lowest temperature studied (30.5ºC) - and 

increases with temperature. For comparison, we 

note that the solubility of C12E3 in excess 

n-hexadecane in equilibrium with 

microemulsions at 30ºC is about 3% by volume 

(see Fig. 7). 

At about 40ºC the rate of increase with 

temperature of surfactant solubility in the 

triolein phase increases significantly. Simultaneously, 

water solubilization in this phase, 

previously rather low, rises dramatically. Such 

formation of water-in-oil microemulsions is one 

way the system can depart at high temperatures 

from the behavior shown in Fig. 8. 

Another way is illustrated in Fig. 10 by the 

Fig. 9. The W + D' + 0 region at several temperatures 

in the C12E3-water-triolein system [48]. 

Fig. 10. The W + D' + O and W + D + O regions at 

several temperatures in the C 12E 5-water-triolein 

system [48]. 

corresponding C 12E 5 diagram [48]. Just above 

64ºC a phase transformation occurs, and at 

higher temperatures the diagram shows a new W 

+ D + O three-phase region instead of the W + 

D' + O region. The D phase is able to solubilize 

considerable triolein and is thus more favorable 

for detergency than D' if it forms as an 

intermediate phase during washing. According 

to Fig. 10, the composition of the D phase shifts 

to become richer in oil with increasing 

temperature in a manner similar to that seen for 

microemulsion systems [47] although the 

surfactant concentration of about 40% is well 

above that observed in typical microemulsions. 

Details of the phase behavior in the temperature 

region where the D phase first appears, 

including the existence of two four-phase 

regions at two closely spaced temperatures, are 

given for another system by Kunieda and 

Haishima [50]. 

As indicated above, the inability of the 

hydrocarbon chain region of the surfactant films 

to incorporate the large triglyceride molecules is 

the chief reason for the poor solubilization. 

Similar poor solubilization and phase behavior 

have been seen in systems containing the 

anionic surfactant Aerosol OT, hydrocarbons 

with chain lengths of twelve and above, and 

NaCl brine [26]. Recently, Binks [51] has 

investigated further the phase behavior of some 

of these systems. 

If the surfactant films were made more flexi-


ble, i.e. if chain packing in this region were 

made more disordered, one might expect 

solubilization to increase. Clearly in some 

systems such as water-C12E5-triolein (Fig. 10), 

increasing temperature provides sufficient 

disorder for the D phase to form. In other 

systems such as water-C12E3-triolein (Fig. 9), the 

D phase does not form at any temperature. One 

might expect that adding amphiphilic 

compounds with chain lengths different from the 

surfactant or with branched chains would 

promote less ordered packing and formation of 

the D phase. As Fig. 11 shows, the addition of 

tert-amyl alcohol (TAA) does, in fact, favor 

formation of the D instead of the D' phase in the 

water-C12E4-triolein system [52]. In the absence 

of TAA the D phase is seen only over a range of 

about 0.2ºC near 55ºC, and not at all for the 

somewhat lower temperatures of Fig. 12 [48]. 

For the purposes of improving detergency, the 

use of TAA to promote solubilization of 

long-chain liquid triglycerides at relatively low 

temperatures is not attractive because its high 

solubility in water requires that it be used at 

Fig. 11. Partial phase diagram of the C 12E 4-tertiary 

amyl alcohol-water-triolein system: surfactant 

content, 16 wt.%; equal volumes of water and triolein 

[52]; LC denotes the lamellar or Lα phase. 

Fig. 12. Partial phase diagram of the C 12E 4-watertriolein-n-hexadecane 

system [48]. W. and 0. denote 

water-continuous and oil-continuous microemulsions. 

rather high concentrations. The same 

disadvantage applies to the use of hydrotopes, a 

possibility considered by Friberg and Rydhag 

[53]. Alander and Warnheim [54] managed to 

solubilize a mixture of medium-chain 

triglycerides (C 8-C 10) in aqueous solutions of a 1 

:2 mixture of sodium oleate and n-pentanol at 

25ºC, but they had less success with long-chain 

triglycerides. 

Recent results suggest that the use of 

doublechain surfactants with varying chain 

lengths, e.g. secondary alcohol ethoxylate 

surfactants, instead of straight-chain surfactants 

may prove effective for forming the D phase 

and thereby solubilizing reasonable amounts of 

triolein at temperatures suitable for warm water 

washing [55]. Here, too, the basic idea is that 

solubilization should be improved for surfactant 

films with disordered packing in the 

hydrocarbon chain region. 

3.4. Mixtures of hydrocarbons and triglycerides 

Microemulsion formation is easier in mixed 

soils of hydrocarbon and triglyceride than for


pure triglyceride soils. In the mixed soil 

systems, hydrocarbon molecules presumably 

penetrate the surfactant films, allowing oil 

droplets or oil microdomains of other shapes to 

form. Triglyceride and hydrocarbon are jointly 

solubilized within the microdomains. Figure 12 

shows phase behavior in the water-n-dodecyl 

tetraoxyethylene monoether (C12H4)-triolein-n hexadecane system for a particular overall 

surfactant concentration and equal volumes of 

oil and water phases [48]. The transition from 

the existence of a middle-phase microemulsion 

(D phase) in hydrocarbon-rich systems the D' 

phase in triolein-rich systems is clear. Note that 

the sequence of phases seen with increasing 

temperature over one temperature range for pure 

triolein is, omitting the excess oil phase, Lα, L2 + D'. D', W + D'. The same sequence occurs in 

,he oil-free system at modest surfactant 

concentrations. That is, the sequence of phases 

with triolein present is the same as that when it 

is absent, except that an excess oil phase is 

present and the transition temperatures are 

somewhat lower. This behavior is to be 

expected when solubilization is low. 

At intermediate oil compositions, D phase 

formation can be promoted by increasing either 

the temperature or the hydrocarbon content of 

the oil. By making the surfactant less 

hydrophilic, increasing the temperature 

promotes a surfactant film configuration where 

the hydrocarbon chains diverge, and thereby 

facilitates the solubilization of triolein. The 

transition between the W + D' + O and W + D + 

0 regions occurs by means of a narrow 

four-phase region W + D'+ D + O. Table I gives 

compositions of the four-phase region at one 

temperature. Clearly, solubilization of both 

hydrocarbon and triglyceride is greater in the D 

phase. Moreover, hydrocarbon is solubilized in 

preference to triolein in both phases. Similar 

phase behavior has been reported for another 

triglyceride [49]. 

Figure 13 shows interfacial tensions measured 

with the spinning drop apparatus at 30ºC for 1 

wt.% C12E4 with various mixtures of 

n-hexadecane and triolein [48]. For the pure 

hydrocarbon the tensions drops after 10 minutes 

Table 1 

Compositions in volume fractions of four coexisting 

phases of the C12E4-water-triolein-n-hexadecane 

system at 39.2ºC [48] 

to about 0.005 mN m -1 , which is a reasonable 

value for a system near its PIT. In contrast, the 

tension reached after some 3 h is about 0.2 mN 

m -1 for pure triolein. The higher tension is 

expected in view of the low solubilization of 

triolein. Mixed oils have intermediate values of 

interfacial tension. 

Fig. 13. Interfacial tensions (IFT) at 30ºC for 1 Wt'% 

C 12E 4 with various mixtures of triolein and 

n-hexadecane [48].


3.5. Water-surfactant-polar soil systems 

Hydrocarbons and triglycerides are frequently 

referred to as non-polar soils, while long-chain 

fatty acids and alcohols are termed polar soils. 

In this section we consider only the case of pure 

polar soils. 

Ekwall has studied the phase behavior of 

many anionic surfactant-water-polar soil 

systems [56]. Figure 14 is a partial ternary 

diagram at 50ºC for the water-sodium 

octylsulfonate-n-hexanol system, which has 

been investigated in recent years by Kunieda 

and Nakamura [57]. A matter of interest for later 

sections of this paper is that the region of 

coexistence of L1 and L2 phases near the 

water-hexanol axis is bounded by a three-phase 

triangle involving these phases and the lamellar 

liquid crystal La. In other cases, including the 

water-sodium octanoate-n-decanol system that 

was studied extensively by Ekwall's group, 

careful examination of the dilute region reveals 

that the D' (or L3) phase replaces Lα, in the 

three-phase triangle which terminates the L1-L2 region [58]. In this system a second three-phase 

triangle D'-Lα-L2 also exists, the arrangement 

Fig. 14. Phase behavior of the sodium 

octylsulfonate-water-nhexanol system in the dilute 

region at 50ºC [57]. Reprinted with permission of the 

American Chemical Society. 

being similar to that shown in Fig. 8 for 

triglyceride systems. In almost all diagrams of 

this type involving relatively long-chain 

compounds, the lamellar phase and its 

associated multiphase regions are prominent 

although other liquid crystalline phases may be 

present as well at high surfactant concentrations 

[56]. 

Kunieda and Nakamura [57] showed that the 

addition of NaCl to the system of Fig. 14 caused 

the D' phase to appear and the dilute portion of 

the phase diagram to resemble Fig. 8. The 

higher salinity is likely to make the 

surfactant-alcohol bilayers more flexible and 

enables them to assume the locally saddleshaped 

configuration necessary for formation of 

the sponge-like microstructure of the D' phase 

for slightly lipophilic conditions. Apparently 

only the lamellar structure is possible for more 

rigid bilayers. 

When the surfactant is non-ionic, the same 

behavior, i.e. the appearance of the D' phase and 

a shift from a diagram resembling Fig. 14 to one 

resembling Fig. 8, can be effected by increasing 

the temperature, as Kunieda and Miyajima [591 

showed for the water-n-dodecyl octaoxyethylene 

monoether (C12E8)-n-decanol system. 

Here. too. the ability to form the D' phase at 

temperatures above about 14ºC is probably the 

result of increased flexibility of the 

surfactant-alcohol bilayers. 

3.6. Mixtures of non-polar and polar soils 

As mentioned in section 3.2, the addition of 

rather lipophilic amphiphilic compounds reduce, 

the PIT of non-ionic surfactant-water-hydrocarbon 

systems. Long-chain alcohols and 

(undissociated) fatty acids ate of this type, as 

Fig. 15 shows for the addition of oleyl alcohol 

to systems containing water, n-hexadecane, and 

several non-ionic -surfactants [12]. Note that 5% 

oleyl alcohol in the system by oil phase reduces 

the PIT in the C12E6 about 35ºC. Similar results 

were found by the same authors for oleic acid. 

When enough polar soil is present that the 

system is above its PIT but the surfactant is


Fig. 15. PIT values for non-ionic surfactant-water-nhexadecane-oleyl 

alcohol systems [12]. Reprinted 

with permission of the American Oil Chemists' 

Society. 

below its cloud point, one might expect that 

phase behavior in the dilute region would be 

similar to that described in the preceding 

section, i.e. the L 1-L 2 region would terminate in 

a three-phase region involving either the D' or 

the La phase. Experiments at 30ºC with C 12E 7 

and oils having ratios of n-hexadecane to oleyl 

alcohol of 3/1 and 1/1, respectively, showed that 

such behavior did, in fact, occur, with the third 

phase being the lamellar liquid crystal Lα [60]. 

However, in contrast to the situations shown in 

Fig. 8 and 14 where the oil phase solubilizes 

modest amounts of water, L 2 phases in these 

systems extended to compositions containing up 

to 75-80% water which were in equilibrium with 

aqueous micellar solutions. Evidently, the 

presence of hydrocarbon and of the double bond 

in the alcohol chain makes the surfactant films 

sufficiently flexible so that the L 2 phase can 

invert continuously and become water 

continuous, ultimately reaching compositions 

comparable to those of the D' phase in systems 

such as that shown in Fig. 8. The relevance of 

this behavior to detergency is discussed later. 

When the long-chain alcohol is mixed with a 

liquid triglyceride instead of with a 

hydrocarbon, multiphase regions containing the 

D' phase are prominent, as is shown in Fig. 16 

[61] for the C 12E 6-water-triolein-oleyl alcohol 

system. The sequence of phases observed with 

increasing temperature in Fig. 16 for oils having 

oleyl alcohol contents exceeding about 20% is 

the same as was found for the water-non-ionic 

surfactant-triolein systems discussed in section 

3.3, as may be seen, for instance, along the 

right-hand boundary of Fig. 12 for water-C 12E 4- 

Fig. 16. Partial phase diagram of C12E6-water-triolein-oleyl alcohol system with 10 wt.% surfactant, 45 wt.% 

water, and 45 wt.% mixed oil [61]. The symbol IV denotes the four-phase region W + D'+ D + O.


triolein. The D phase is seen, however, for 

surfactant concentrations well above that of Fig. 

16. Note that the amounts of oleyl alcohol 

needed to depress the temperatures of the 

various phase boundaries are much greater than 

are shown in Fig. 15 for hydrocarbon systems. 

A likely explanation is that much of the oleyl 

alcohol is dissolved in the bulk triglyceride 

phase, leaving relatively little alcohol in the 

surfactant films which, to a large extent, 

determine the basic phase behavior by 

controlling the aggregate shape. 

4. Diffusion path analysis 

Being of rather short duration and typically 

involving small quantities of soils, detergency 

processes are strongly influenced by dynamic, 

diffusional phenomena which occur on a 

microscopic scale. Oily soil removal, in 

particular, depends on phase transitions which 

occur at the oil-washing solution interface. The 

preceding section described equilibrium phase 

behavior in both water-surfactant systems, 

representing the washing solution, and 

oil-water-surfactant systems, As demonstrated 

below, such equilibrium phase behavior can be 

combined with the theory of diffusion processes 

to interpret certain dynamic behavior such as 

intermediate phase formation and spontaneous 

emulsification that occurs during detergent 

processes. 

A mathematical technique called diffusion 

path analysis has been used with success in 

predicting certain dynamic phenomena in 

multicomponent solid and liquid systems when 

two phases not in equilibrium are brought into 

contact with one another [62-64]. Essentially, a 

time-invariant path of compositions can be 

plotted across an equilibrium phase diagram 

by-solving component transport equations with 

certain assumptions and boundary conditions. 

First, convection in the system from any source 

is assumed to be negligible. Second, the two 

phases are assumed to be semiinfinite in extent. 

This assumption simplifies the mathematical 

analysis and is probably reasonable at least for 

short times after contacting. Third, diffusion of 

each species is assumed to be dependent only on 

its own concentration gradient with a uniform 

diffusion coefficient in each phase. Also, local 

equilibrium is assumed at all interfaces which 

form; this means that the compositions at the 

interfaces are defined by equilibrium tie lines. 

Diffusion path analysis is most conveniently 

applied to three-component, i.e. ternary, 

systems. In this situation, the phase diagram can 

be represented in the form of a two-dimensional 

triangle as, for instance, in Fig. 8, with 

two-phase regions shown as regions of varying 

shape containing equilibrium tie lines and 

three-phase regions represented as triangles in 

which the compositions of the equilibrium 

phases are shown as the vertices. The analysis in 

this case consists of solving in each phase the 

following transport equations for two of the 

species 

(∂wi/∂t) = Di(∂ 2 wi/∂x 2 ) i = 1,2 (3) 

where x is the distance from the initial surface 

of contact, t is time and wi and D i are the mass 

fraction and diffusivity of species i, 

respectively. The value of W3 for the third 

species is found by invoking Σw i = 1. The 

semi-infinite phase assumption allows transformation 

of the above equations to ordinary 

differential equations in the similarity variable 

η i = [x/(4D it) 1/2 ]. Integration yields the following 

error function solutions for the diffusion path 

segment in each phase 

w i = A i + B i erf η I i = 1,2 (4) 

where A i and B i are constants which are 

evaluated from the boundary conditions. Since 

η i varies from - ∞ to + ∞ at each value of time, 

the set of compositions given by Eq. (4) is 

independent of time although the position x of a 

specific composition does vary with time. It is 

often convenient to plot the compositions or 

"diffusion path" directly on the equilibrium 

phase diagram. 

In evaluating A i and B i for phases in contact. 

iteration on a tie line is performed until the 

individual species mass balances at the interface 

are satisfied. In addition to obtaining the path of 

compositions that forms between the initial


phases, one may also calculate the relative 

velocities of all interfaces, and therefore the 

growth rates of intermediate phases. More 

detailed descriptions of the mathematical 

analysis and the specific error function solutions 

can be found elsewhere for a ternary system 

forming a single interface [63] or two or more 

interfaces [65]. 

The utility of diffusion path theory in ternary 

liquid systems was first shown for predicting the 

occurrence of spontaneous emulsification in 

alcohol-water-oil systems [63]. Specifically, 

when an alcohol-oil mixture denoted d in Fig. 

17 is brought into contact with water, 

spontaneous emulsification of oil drops in the 

water phase is observed. In this situation, the 

construction of a diffusion path between the 

initial compositions shows the formation of an 

interface with equilibrium compositions b and c 

connected by the tie line represented by the 

broken line. Spontaneous emulsification in the 

aqueous phase can be explained by the passage 

of that path segment from b to W through the 

corner of the two-phase envelope, thereby 

predicting the formation of small drops of 

oil-alcohol mixture below the interface. 

Experiments showed that, in the absence of 

interfacial turbulence, interfacial displacement 

Fig. 17. Schematic diffusion path in alcohol(A)water(W)-oil(O) 

system showing supersaturation 

leading to spontaneous emulsification. 

is proportional to the square root of time, as 

predicted by the theory [65,66]. This diffusion 

mechanism of spontaneous emulsification is 

distinct from other modes of spontaneous 

emulsification in which interfacial instability 

results in the mechanical dispersion of one 

phase in another [67]. 

Diffusion path analysis was later applied to 

oil-water-surfactant systems [20,64,68]. In these 

cases, the use of pseudoternary phase diagrams 

was required. For example, commercial 

surfactants are almost always complex mixtures 

containing numerous species of surfactants. 

Rather than solving the diffusion equations for 

each species, one can sometimes combine all 

surfactant components together and treat them 

as a pseudocomponent. Mixtures of 

hydrocarbons can also be considered as 

pseudocomponents. Although diffusion path 

studies are typically performed when 

single-phase systems are originally present, the 

ability to calculate diffusion paths in which one 

of the initial compositions is a stable dispersion 

of one phase in another, e.g. a liquid crystalline 

dispersion, has also been demonstrated [64]. 

5. Dynamic contacting studies 

Direct observation of the dynamic phenomena 

that occur when non-equilibrated liquid phases 

are brought into contact can be made in various 

ways. On a macroscopic scale, a liquid can be 

gently placed on top of another liquid in a tube, 

and rather large-scale phenomena can be 

observed. This simple technique was used in the 

early studies of spontaneous emulsification in 

oil-water-alcohol systems [63] and has been 

used with surfactant systems to monitor the 

formation of microemulsion and liquid 

crystalline phases between oil and surfactant 

solutions [68-71]. In these cases, the oil is 

gently layered on top of the aqueous phase, and 

dynamic phenomena are observed without 

magnification. Crossed polarizers aid in the 

identification of birefringent liquid crystalline 

phases. A shortcoming of this technique is the 

inability to observe events which occur 

immediately after the contacting of the two


phases. Also, the experimental scale increases 

the possibility of convection occurring due to 

the formation of adverse density gradients. 

Nevertheless, intriguing phenomena including 

oscillations in the interfacial position and abrupt 

changes in the interfacial velocity have been 

observed by Friberg and co-workers [70,71] in 

systems in which the lamellar liquid crystal is 

present, although generally at rather long times 

(a few days) after initial contact. For various 

reasons, diffusion path theory is not applicable 

for describing such phenomena, as these 

workers have pointed out. 

To facilitate the observation of dynamic 

phenomena at short times after contact, a 

microscopic technique was developed. A key 

aspect of the technique is the use of rectangular 

glass capillaries, having a path width 200 gm, 

tohold the sample [68]. Figure 18 shows a 

diagram of the sample cell that is 50 min in 

length. After the aqueous phase is imbibed 

about half way into the capillary, that end is 

Fig. 18. Rectangular glass capillary cell used in 

vertical-stage contacting experiments. 

sealed by a resin curable by ultraviolet light. The 

oil phase is then injected into the other end by 

use of a syringe. Initially, the resulting diffusion 

phenomena were observed using a conventional 

microscope with the cell in a horizontal 

configuration. However, due to the density 

difference between the two phases, overriding of 

the oil over the surfactant solution occurred, 

causing distortion of the interface and 

complicating the interpretation of the results 

[68]. 

A vertical-stage microscope was then 

designed that allowed the cell to be placed in a 

vertical configuration in a controlled 

temperature environment. Details of the 

microscope and contacting technique are given 

elsewhere [20]. In this configuration in a stable 

region of contact between the two initial phases 

can be maintained, allowing easy viewing. The 

vertical-configuration microscope was 

subsequently improved and equipped with a 

video imaging system [48,72]. The use of video 

taping and image analysis allows detailed 

review of phenomena which may be missed 

initially in real time, and improved 

determination of, for example, the velocities of 

interfaces and rates of formation of intermediate 

phases. 

The vertical-contacting technique was first 

used to study the dynamic contacting in 

water-anionic surfactant-oil systems 

representative of those used in enhanced oil 

recovery processes [20]. Intermediate phase 

formation and spontaneous emulsification 

experimentally observed in a brine-petroleum 

sulfonate-hydrocarbon system were found to be 

predictable from calculated diffusion paths 

based on relevant phase diagrams [64]. Widely 

varying but predictable phenomena were found 

as the salinity of the aqueous phase was varied. 

6. Diffusional phenomena in detergent systems 

6.1. Water - alcohol ethoxylate - hydrocarbon 

systems 

Studies of diffusional phenomena in systems 

having direct relevance to detergency processes


have recently been performed. Experiments 

were designed to investigate the effects of 

changes in temperature on the dynamic 

phenomena which occur when aqueous 

solutions of pure non-ionic surfactants contact 

hydrocarbons such as tetradecane and 

hexadecane [18,72]. These oils can be 

considered to be models of non-polar soils such 

as lubricating oils. The dynamic contacting 

phenomena, at least immediately after contact, 

are representative of those which occur when a 

detergent solution contacts an oily soil on a 

synthetic fabric surface. The following is a 

summary of the observed behavior interpreted 

through the use of schematic diffusion paths. 

Detailed phase behavior in such systems has 

been reported previously and was used in 

construction of the diffusion paths [47]. 

With C12E5 as the non-ionic surfactant at a 1 

wt.% level in water, quite different phenomena 

were observed below, above, and well above the 

cloud point when tetradecane or hexadecane 

was carefully layered on top of the aqueous 

solution. Below the cloud point temperature of 

31ºC, very slow solubilization of oil into the 

one-phase micellar solution was observed. An 

interesting phenomenon was observed at 20ºC 

involving a "volcanolike" instability which 

caused flow of the aqueous solution to the oil 

interface. This flow column, which is believed 

to have resulted from an adverse density 

gradient within the aqueous phase, is shown in 

Fig. 19 [72]. The upper tip of the column was 

observed to oscillate, probably due to gradients 

in interfacial tension along the oil-water 

interface (Marangoni flow). Of more importance 

to a detergency process, the schematic diffusion 

path shown in Fig. 20(a) explains why no 

intermediate phase formed between the water 

and oil. Also, due to the low solubility of oil in 

the dilute aqueous surfactant solution in this 

region of the ternary phase diagram, it predicts 

the quite slow solubilization of oil into the 

surfactant solution. At temperatures just below 

the cloud point temperature, an intermediate 

phase depleted in surfactant did form between 

the micellar solution and the oil. The schematic 

diffusion path in this case is shown in Fig. 

20(b). Once again, instabilities in the aqueous 

Fig. 19. "Volcano" instability in C 12E 5-water-ntetradecane 

system at 20ºC. The image is out of focus 

to allow observation of refractive index variations 

[72]. Reprinted with permission of Academic Press. 

Fig. 20. (a) Diffusion path well below the cloud point 

showing no intermediate phase formation; (b) 

diffusion path slightly below the cloud point showing 

the formation of intermediate phase W [72]. 

Reprinted with permission of Academic Press. 

ous phase occurred, in this case due to a density 

difference between the original micellar solution 

and the intermediate phase, causing flow of 

surfactant solution to the oil interface. 

Nevertheless, at all temperatures studied below 

the cloud point, only very slow solubilization of 

oil into the surfactant solution was observed. 

At 35ºC, which is just above the cloud point, a 

much different behavior was observed. The 

surfactant-rich L1 phase separated to the top of 

the aqueous phase prior to contacting by 

hexadecane.


Upon addition of the oil, the drops of the L1 phase rapidly solubilized the hydrocarbon to 

form an oil-in-water microemulsion containing 

an appreciable quantity of hydrocarbon. After 

depletion of the larger surfactant-containing 

drops, a front developed as smaller drops were 

incorporated into the microemulsion phase. This 

behavior is shown schematically in Fig. 21. 

Unlike the experiments carried out below the 

cloud point temperature, an appreciable 

solubilization of oil was observed in the time 

frame of the study, as indicated by upward 

movement of the oil-microemulsion interface. 

Similar phenomena were observed with both 

tetradecane and hexadecane as the oil phases. 

When the temperature of the system was 

raised to just below the phase inversion 

temperatures of the hydrocarbons with C12E5 (45ºC for tetradecane and 50ºC for hexadecane), 

two intermediate phases formed when the initial 

dispersion of L1 drops in the water contacted 

the oil. One was the lamellar liquid crystalline 

phase Lα (probably containing some dispersed 

water). Above it was a middle-phase 

microemulsion. In contrast to the studies below 

the cloud point temperature, appreciable 

solubilization of hydrocarbon into the two 

intermediate phases, shown 4 min after 

contacting in Fig. 22, was observed. A diagram 

of the phenomena observed is shown in Fig. 23. 

A similar progression of phases was found at 

35ºC using n-decane as the hydrocarbon. At this 

temperature, which is near the phase inversion 

Fig. 21. Schematic diagram showing conversion of 

the L 1 phase into an oil-in-water microemulsion at 

temperatures above the cloud point [72]. The symbol 

me denotes a microemulsion. Reprinted with 

permission of Academic Press. 

Fig. 22. Video frame showing intermediate phases 4 

min after contact in C 12E 5-water-n-tetradecane 

system at 45ºC near the PIT [72]. Reprinted with 

permission of Academic Press. 

Fig. 23. Schematic diagram showing the conversion 

of L 1 phase into a middle-phase microemulsion and a 

liquid crystal dispersion, for the experiment depicted 

in Fig. 22 [72]. Reprinted with permission of 

Academic Press. 

temperature of the water-C12E5-decane system, 

the existence of a two-phase dispersion of Lα 

and water below the middle-phase 

microemulsion was clearly evident. 

To compare the rate of tetradecane 

solubilization at temperatures below and near 

the phase inversion temperature, verticalcontacting 

experiments were performed between 

the L1 phase and oil at 40 and 48ºC. In both 

cases, the surfactant-rich phase was the 

equilibrium phase that separated from a 10% 

aqueous solution at that temperature. At 40ºC a 

liquid crystalline phase and an oil-in-water


micro emulsion formed between the L1 and oil 

phases. At 48 º C, a similar phase progression 

was observed, with a middle-phase microemulsion 

forming in place of the oil-in-water 

microemulsion. As shown in Fig. 24, plots of 

the position of the oil-micro emulsion interface 

versus the square root of time were straight lines 

in both cases indicating diffusion-controlled 

mass transfer. In Fig. 24, an arbitrary constant 

has been added to each position for ease of 

comparison, i.e. x = 0 is not the initial contact 

position. At any given time, the relative slopes 

of the two lines are indicative of the relative 

rates of oil solubilization. In this case, the rate of 

solubilization near the PIT is 2.2 times greater 

than at 40ºC. 

Well above the phase inversion temperatures 

for both hexadecane and tetradecane, 1 wt.% 

C12E5 exists as a dispersion of liquid crystal Lα 

in water. Rapid movement of the liquid crystal 

to the oil occurred upon contacting, causing 

extensive spontaneous emulsification of water in 

the oil phase. Eventually a layer depleted in 

liquid crystal formed near the oil interface. 

Figure 25 shows the spontaneous emulsification 

that occurred. Interpretation of the phenomena 

Fig. 24. Variation of oil- microemulsion interface 

with time at 40ºC and 48ºC following contact of the 

L 1 phase with oil for the C 12E 5-water-n-tetradecane 

system. 

Fig. 25. Video frame showing spontaneous 

emulsification observed 18 min after initial contact in 

the C 12E 5-water-n-hexadecane system at 60ºC [72]. 


by diffusion path analysis indicated that the oil 

was being converted into a waterin-oil 

microemulsion at this high temperature; this 

means that very little solubilization of oil into 

the aqueous phase was taking place. The 

spontaneous emulsification occurred due to the 

passage of the oil-phase diffusion path segment 

across the twophase water-micro-emulsion 

region, as shown in Fig. 26. 

Experiments similar to those described above 

were also performed using C12E4 as the 

surfactant [18]. This surfactant is more hydrophobic 

than C12E5 and therefore has a lower 

Fig. 26. Schematic diffusion path for the experiment 

depicted in Fig. 25. Point d represents the 

composition of the initial water-surfactant mixture; 

HC, S, and tie denote hydrocarbon, surfactant, and 

microemulsion. The last is oil-continuous in this 

case. Some multiphase regions are not shown [72]. 

Reprinted with permission of Academic Press.


cloud point (7ºC) and PIT with hexadecane 

(30ºC). Contacting experiments were performed 

below, at, and above the PIT. In contrast to 

C12E5, the structure of the 1 wt% C12E4 solution 

was a lamellar liquid crystalline dispersion in 

water at all the temperatures studied. With 

regard to the intermediate phases which formed 

at the different temperatures, the following 

results were obtained. Below the PIT, an 

oil-in-water microemulsion formed between the 

lamellar liquid crystalline dispersion and oil. At 

the PIT, the behavior was dependent upon the 

concentration of liquid crystalline material at the 

initial oil-surfactant solution interface. At the 

low concentration of liquid crystal present at a 

1% surfactant level, no intermediate phase 

formation was observed when the aqueous 

dispersion was contacted with hexadecane. To 

provide a higher level of liquid crystal at the 

initial point of contact, La drops were allowed to 

cream to the air-water interface prior to the 

contacting studies. These drops converted to 

concentrated La domains upon heating to 30ºC. 

With this initial aqueous phase structure, the 

formation of a middle-phase microemulsion 

layer was observed upon contacting with oil. 

Finally, if a pure lamellar liquid crystalline 

phase containing 25% surfactant was contacted 

with oil, swelling of the liquid crystalline phase 

was observed, as shown in Fig. 27. 

An interpretation of these results was made by 

comparing the qualitative diffusion paths, 

shown in Fig. 28, resulting from the different 

aqueous starting compositions [18]. The 

differences in behavior for the dilute (C), 

concentrated (B) and pure liquid crystal (A) 

cases were attributed to the relatively high 

solubility of C12E4 in the oil phase at this 

temperature, which influenced whether the 

diffusion path passed above or below the 

various three-phase regions that are present on 

the phase diagram. 

At temperatures of 40-50ºC, which are above 

the PIT, behavior similar to that for C12E5 was 

observed. Dissolution of the lamellar liquid 

crystalline phase into the oil resulted in the 

formation of a water-in-oil microemulsion and 

spontaneous emulsification of water in the oil 

phase. 

Fig. 27. Video frame showing swelling of the 

lamellar liquid crystalline phase 43 min after initial 

contact for the C 12E 4-water-n-hexadecane system at 

the PIT of 30ºC [18], Reprinted with permission of 

Academic Press. 

Dynamic contacting studies were also 

performed with hydrophobic additives 

combined with C 12E 5 [30]. C 12E 3 and 

n-dodecanol were added to C 12E 5 in proportions 

to yield cloud points near that of C 12E 4 

(approximately 7ºC). This study was conducted 

Fig. 28. Schematic diffusion paths representing 

different behavior observed at different surfactant 

concentrations for the C 12E 4-water-n-hexadecane 

system at the PIT of 30ºC [18] The symbol (mp µe) 

denotes a middle-phase microemulsion. Reprinted 

with permission of Academic Press.


ducted to determine whether formation of 

intermediate microemulsion phases containing a 

high proportion of oil could be obtained at 

temperatures lower than those for C 12E 5 alone. 

However, despite exhibiting phase behavior in 

the absence of oil similar to that of C 12E 4, the 

C 12E 5-C 12E 3 mixture behaved in a way 

intermediate to the behavior seen with C 12E 4 and 

C 12E 5 upon being contacted with hexadecane. 

Also, the microemulsion phases formed at 

various temperatures when the C 12E 5-dodecanol 

system contacted oil were essentially unchanged 

from those seen in the C 12E 5 system without any 

additive present. For example, rather than 

forming a middle-phase microemulsion with 

hexadecane at 30ºC, the two systems formed 

oil-in-water micro-emulsions. The contacting 

temperature had to be increased to 40ºC in the 

case of the C 12E 5-C 12E 3 System and 50ºC in the 

case of the C 12E 5-dodecanol system before 

middlephase microemulsion formation was 

observed. These differences between the two 

systems were attributed to differences in 

partitioning of the additive and the more 

water-soluble C 12E 5 between the oil and the 

microemulsion phases. The observed differences 

between the two additive systems would be less 

if smaller quantities of oil relative to the 

surfactant solution had been present. 

6.2. Water-alcohol ethoxylate-triglyceride 

(+ hydrocarbon) systems 

Triolein is a pure triglyceride suitable for use 

as a model for kitchen soils such as vegetable 

oils. Dynamic contacting studies similar to those 

described above for hydrocarbons were 

performed with triolein using aqueous solutions 

containing the three alcohol ethoxylates C 12E 3, 

C 12E 4 and C 12E 5 [48]. As described in the phase 

behavior section, ternary triolein - water-non- 

ionic surfactant systems exhibit different phase 

behavior than those containing straight-chain 

hydrocarbons. Specifically, the large size of the 

triolein molecules inhibits solubilization and 

formation of microemulsion phases. 

At low temperatures, schematic phase 

behavior like that shown in Fig. 8 is observed in 

which two three-phase regions are present in the 

ternary diagram. At these temperatures, the 

surfactantwater mixture is a dispersion of the 

liquid crystal La in water. When this dispersion 

is contacted with triolein, a water layer forms 

between the liquid crystal and the oil, and 

extensive spontaneous emulsification occurs in 

the oil phase [48]. This behavior can be 

explained in terms of a diffusion path which 

passes below the bottom three-phase region in 

Fig. 8. Spontaneous emulsification occurs in the 

oil phase due to passage of that diffusion path 

segment across the two-phase water-oil region. 

In general, insufficient surfactant is available at 

the interface to form an intermediate L 3 or D' 

phase due to the high solubility of non-ionic 

surfactants in triolein at these conditions. At still 

higher temperatures where C 12E 3 and C 12E 4 are 

in the form of aqueous dispersions of L 3 (greater 

than 30ºC for C 12E 3 and 55ºC for C 12E 4), similar 

behavior occurs except that even more vigorous 

spontaneous emulsification is observed. 

C 12E 5 exhibited behavior at approximately 

65ºC quite comparable to that observed with 

hydrocarbon systems near the PIT. In fact, as 

shown in Fig. 10, the D phase in that system 

contains almost equal volumes of triolein and 

water at 65ºC. When a concentrated lamellar 

liquid crystalline dispersion contacted triolein at 

that temperature, the D phase formed, first as 

lenses along the water-oil interface, then as a 

continuous layer. A schematic diffusion path 

corresponding to this behavior is shown in Fig. 

29. 

The dynamic contacting of C 12E 4. solutions 

with oil mixtures containing varying proportions 

of triolein and hexadecane was also studied 

[48]. For 3/1 hexadecane-triolein mixtures, 

behavior comparable to that found with the pure 

hydrocarbon system was obtained. Similarly, 

3/1 triolein-hexadecane systems behaved in the 

contacting experiments like the pure triglyceride 

system. However, contacting of the concentrated 

liquid crystalline dispersion at 38ºC with a 1/1 

mixture of triolein and hexadecane resulted in 

the formation of transient middle-phase 

microemulsion droplets. The failure to form a


Fig. 29. Diffusion path corresponding to observed 

behavior for the C 12E 5-water-triolein system at 

64.5ºC [48]. 

complete microemulsion layer during the course 

of the experiment probably resulted from the 

high solubility of the surfactant in the oil 

mixture. 

The latter experiment was repeated with 

tertiary amyl alcohol (TAA) added to the 

concentrated La dispersion at three different 

TAA/C12E4 ratios: 0.05, 0.10, and 0.20 by 

weight [52]. All three experiments showed the 

rapid formation of an intermediate phase, 

presumably the D or microemulsion phase. The 

higher the ratio of TAA to surfactant, the faster 

was the formation of the intermediate phase as a 

continuous layer. As indicated in the previous 

paragraph, the formation of a continuous 

middle-phase microemulsion layer did not occur 

in the absence of TAA. 

7. Oil drop-contacting experiments 

As discussed in the preceding section, 

contacting experiments using vertically oriented 

cells, and their interpretation using diffusion 

path theory, have provided a fundamental 

understanding of the conditions for the 

occurrence of intermediate phase formation and 

spontaneous emulsification for a variety of 

systems containing water, pure surfactants, and 

non-polar oils. However, when either surfactant 

mixtures, e.g. the C12E5-additive systems 

discussed above, or commercial surfactants that 

are themselves complex mixtures, are used, the 

conditions for which intermediate phases form 

during a vertical cell-contacting experiment are 

frequently rather different from those expected 

during washing experiments in the same system 

at the same temperature. The reason is 

differential partitioning of different surfactant 

species into the oil phase. As explained 

previously in section 3, the extent of differential 

partitioning depends on both the overall 

surfactant concentration in the aqueous phase 

and the water-to-oil ratio (see Eq. (2)). In 

particular, the surfactant remaining after some 

partitioning into the oil has occurred, the factor 

which largely controls whether an intermediate 

phase will form, is less hydrophilic for a 

washing experiment in which the water-to-oil 

ratio is very large than for a contacting 

experiment in which the volumes of oil and 

water are comparable. It should be noted that 

this problem does not arise for systems 

containing mixtures of anionic surfactants and 

hydrocarbon oils, because none of the individual 

surfactant species has appreciable solubility in 

the hydrocarbon phase. 

When the oil consists of one or more polar 

components or of both polar and non-polar 

components, there is another limitation of the 

vertical cell-contacting technique that is 

significant even when pure surfactants are used. 

Basically, both experiment and diffusion-path 

theory yield information on the behavior of the 

system during the early stages of contact when 

the oil phase remains at its initial composition 

except in a region near the surface of contact. 

However, when the volume of the oil phase is 

small, the diffusion of polar material out of 

and/or diffusion of surfactant into the oil can 

cause the composition of the entire oil phase to 

vary with time, i.e. no location exists where the 

oil has its initial composition. This effect is 

especially important when inverse micelles or 

other aggregates form in the oil that have mixed 

films of surfactant and polar compounds. As a 

result, situations occur in which an intermediate 

phase does not form on initial contact but de-


Fig. 30. Schematic illustration of contacting 

experiment in which a small oil drop is injected into 

an aqueous surfactant solution. 

velops later in the experiment when the oil 

composition becomes suitable. Examples of 

such behavior are discussed below. 

An oil drop-contacting technique was 

developed in which the water-to-oil ratio is 

large, as in practical washing situations. As 

shown in Fig. 30, a drop of oil, usually some 

10-100 mm in diameter, is injected into a 

horizontal rectangular glass cell by means of a 

very thin hypodermic needle. The cell, which is 

400 Rm thick, is inside a thermal stage modified 

to enable the drop to be observed by 

videomicroscopy from the moment of injection 

[24,60]. Since the drop must be viewed through 

the surfactant solution in which it is immersed, 

this technique works best when the surfactant 

solution is below its cloud point temperature. 

However, some experiments have been 

successfully carried out in which the initial 

surfactant solution was a dispersion of the 

lamellar liquid crystal in water, as discussed 

below. It is noteworthy that no similar limitation 

exists for the vertical cell technique, and indeed 

almost all of the experiments described above 

for non-polar oils were conducted above the 

cloud point temperature. 

7. 1. Experiments with surfactant mixtures and 

nonPolar oils 

Mixtures of anionic and non-ionic surfactants 

are now almost universally used in liquid 

detergents for laundry applications since they 

are more effective than anionics alone for 

washing synthetic fabrics at low temperatures. 

The oil dropcontacting technique was used to 

determine whether an intermediate 

microemulsion phase would form near the PIT 

for these mixed surfactant systems with 

hydrocarbon soils [24] in a manner similar to 

that described above for pure non-ionic 

surfactants with the vertical cell technique. 

The pure non-ionic surfactant C12E3 and the 

commercial anionic surfactant Neodol 23-3S 

were used in this study, i.e. the same 

combination as discussed above in the phase 

behavior section. The use of a commercial 

mixture rather than a pure anionic surfactant had 

minimal effect on the differential partitioning 

since all the individual anionic species in the 

mixture had very low solubilities in 

n-hexadecane, the hydrocarbon used. Because 

the volume of the oil drop injected was small, it 

dissolved little non-ionic surfactant, and the 

relevant PIT was that for which the surfactant 

composition Ssn in the films within the 

microemulsion phase was the same as the 

overall surfactant composition in the system. 

Data on Ssn for this system when the aqueous 

phase contains 1 wt.% NaCl are given in Fig. 7. 

As may be seen from Fig. 4, the initial washing 

bath, i.e. the oilfree mixture of the surfactant 

and a 1 wt.% NaCl solution, forms a dispersion 

of the lamellar liquid crystal in brine for 

surfactant compositions equal to the relevant 

values of Ssn. Contacting experiments were conducted for a 

surfactant mixture containing 78 wt.% of the 

nonionic surfactant [24]. According to Fig. 4, 

the relevant PIT is 30ºC. At 25ºC, no 

intermediate phase was observed and the drop 

diameter did not decrease appreciably with time. 

Thus, solubilization of hydrocarbon by the 

liquid crystalline phase was very slow at this 

temperature below the PIT. At 30ºC, an 

intermediate microemulsion phase was 

observed. Its volume continued to increase until 

the oil phase disappeared. At 40ºC, the drop 

diameter increased with time, the expected 

behavior above the PIT as the oil phase takes up 

surfactant and water. The liquid crystalline 

particles surrounding the drop made it difficult 

to discern whether spontaneous emulsification


in the oil occurred under these conditions, as 

would be expected based on observations above 

the PIT described above for the vertical cell 

experiments. The conclusion reached from these 

experiments is that mixed surfactant systems 

behave in basically the same manner as pure 

surfactant systems for hydrocarbon oils, 

provided that the PIT used to interpret the 

behavior is as defined above. 

Valuable results were also obtained by the oil 

drop technique for the C12E4-water-triolein-TAA system. As discussed in Section 3, the addition 

of TAA reduced the temperature at which the D 

phase formed in this system and also promoted 

formation of the D instead of the D' phase. The 

latter has considerably less ability to solubilize 

triolein than the former. When a drop of pure 

triolein was injected into an alcohol-free 

mixture of C12E4 and water at 50ºC, spontaneous 

emulsification was observed within the drop as 

well as the growth of small myelinic figures at 

the drop surface [52]. The drop volume 

decreased slowly with time, an indication that 

some triolein was being solubilized into the 

liquid crystalline particles present initially. This 

behavior is, as expected, the same as was seen 

with the vertical cell technique for the same 

system and temperature. 

In contrast, with TAA added to the 

surfactantwater mixture in an amount equal to 

20 wt.% of the surfactant, an intermediate liquid 

phase, presumably the D phase, formed when a 

triolein drop was injected at the same 

temperature. As shown in Fig. 31, the drop 

shrank considerably during the first few minutes 

of the experiment as triolein was solubilized into 

the D phase. After about 5 min, the triolein drop 

had disappeared completely. The contacting 

experiments thus confirmed the conclusion 

reached from the phase behavior results that 

TAA promoted the formation of the D phase, 

which is capable of solubilizing considerable 

triolein. 

7.2. Experiments with pure long-chain alcohols 

It can be shown using diffusion path theory 

that when the initial surfactant concentration in 

Fig. 31. Video frames showing the dynamic behavior 

of a drop of triolein contacted with 5 Wt-% C 12E 4 

with added TAA at 50-C (TAA/C12E7=0.20). 

Obtained from Ref. 52. 

an aqueous micellar solution is sufficiently low, 

no intermediate phases form upon initial contact 

of this solution with a long-chain alcohol. 

However, an intermediate phase does form 

when the surfactant mass fraction exceeds a


critical value ws* given by the following 

equation [73] 

w s* = w sB F 1(w oC) + w sC F 2 (w oC) (5) 

Here w sB and w sC are the surfactant mass 

fractions at the L 1 and L 2 ends of the limiting tie 

line forming one boundary of the two-phase 

region for these phases, and w oC is the oil mass 

fraction at the L 2 end. The quantity w sB is often 

called the "limiting association concentration" 

or LAC [56]. The functions F 1 and F 2 are 

defined as follows 

where D o and D s are the diffusivities of the oil 

and the surfactant in the L2 phase, D's is the 

diffusivity of the surfactant in the L1 phase and 

hoC is the (constant) value of the similarity 

parameter [x/(4D.t) 1/2 ] at the L2 end of the 

limiting tie line (see discussion of diffusion path 

analysis above). 

Vertical cell-contacting experiments gave 

results in reasonable agreement with Eq. (3) for 

the sodium octanoate-n-decanol-water system 

[73]. One might expect that detergency would 

be improved when the intermediate phase - in 

this case the lamellar liquid crystal - is formed 

since more of the alcohol is solubilized. Indeed, 

Kielman and van Steen [11] observed such 

behavior in the potassium octanoate-n-decanolwater 

system. 

The focus of this section is the time-dependent 

behavior of the system if a drop of alcohol is 

injected into a solution whose surfactant 

concentration is below the critical value. The 

question to be answered is whether an 

intermediate phase will form at some time after 

initial contact. 

A series of such experiments was performed 

for the C 12E 5-water-oleyl alcohol system at 30ºC 

[74]. The L 1-L 2 coexistence curve and some 

interfacial tensions between these two phases 

are shown in Fig. 32. Note that the coexistence 

curve terminates at a point F corresponding to a 

water content of about 70 wt.%, which is one 

vertex of the L 1-Lα-L 2 three-phase triangle. 

Video frames taken at various times during an 

experiment in which the surfactant 

concentration was 1 wt.% are shown in Fig. 33. 

Note that the drop, initially some 70 mm in 

diameter, swells as it takes up water and 

surfactant. After about 23 min, the lamellar (La) 

phase begins to develop as myelinic figures 

which grow into the aqueous solution. 

Eventually, nearly all the alcohol is converted to 

liquid crystal. 

The order of magnitude of the time required 

for diffusion within the drop is the ratio of the 

square of its radius to the diffusion coefficient. 

As this time is much less than that of the 

experiment, a quasi-steady state scheme may be 

used to model drop behavior. Basically, this 

means that drop composition may be viewed as 

TIE -LINE INTERFACIAL TENSION (dyne/cm) 

1 2.52 

2 1.03 

3 0.25 

4 0.03 

Fig. 32. Partial ternary phase diagram for the 

C 12E 5-water- alcohol system at 30ºC showing the 

L 1-L 2 coexistence curve and the limiting tie line EF. 

The oil drop composition varies as indicated by the 

arrows. The interfacial tensions are between 

pre-equilibrated phases for the four tie lines shown 

[74]. Reprinted with permission of Plenum Press.


Fig. 33. Video frames showing dynamic behavior following contact of 1.0 wt.% solution of C 12E 5 with a drop of 

pure oleyl alcohol at 30ºC [74]: (a) alcohol drop about 2 min after injection; (b) the same, about 20 min later; (c) 

initial formation of lamellar phase about I min later; (d) growth of myclinic figures into the surrounding aqueous 

phase about 3 min later; (e) almost complete conversion of alcohol into liquid crystal about 8 min later. Reprinted 

with permission of Plenum Press. 

moving along the coexistence curve in the 

direction of the arrows in Fig. 32. Since the 

long-chain alcohol has low solubility in the 

aqueous phase, the drop volume increases 

during this process as its contents of water and 

surfactant increase. The result is swellingas


observed during the experiment. When the drop 

composition reaches point F at the end of the 

coexistence curve, the driving force for 

diffusion of the surfactant and water into the 

drop remains. gut because the drop cannot 

remain in the L2 region, according to the phase 

diagram, the lamellar phase begins to form. 

A theory has been developed which predicts 

that the time t from the start of the experiment 

until the liquid crystal starts to form is given by 

the following expression [60] 

2 

t = Ks (R0 /Dsws∞) (6) 

Here R 0 is the initial radius of the drop, D s and 

are the diffusivity and bulk concentration of the 

surfactant in the aqueous solution and K s is a 

constant that depends only on the shape of the 

coexistence curve and the location of point F. 

The predicted proportionality between t and Ro 

2 has been confirmed by experiment (see Fig. 

34), as has the inverse relationship between t and 

the bulk surfactant concentration w s∞. With a 

Fig. 34. Plot of the square root of the time t required 

to initiate liquid crystal formation as a function of 

initial drop size for the system of Fig. 32. The 

surfactant concentration in the aqueous phase is 1.0 

wt.% [74]. Reprinted with permission of Plenum 

Press. 

value of 0.514 for K. calculated using the phase 

behavior of Fig. 32, Eq. (6) and the measured 

values of t were used to estimate Ds. A value of 

about 4 x 10 -11 m 2 s -1 was obtained, which is 

reasonable for a micellar solution. 

A similar equation has been developed for the 

case of a uniform layer of oil on a flat solid 

surface immersed in a stirred aqueous surfactant 

solution [74] 

t = K p (h 0d/D sw s∞) (7) 

where ho is the initial thickness of the oil layer 

and d is the thickness of the diffusion boundary 

layer adjacent to the oil. Like Ks in Eq. (6), Kp depends only on the shape and terminal point of 

the coexistence curve between the L1 and L2 phases. 

For the C12E8-water-n-decanol system at 

temperatures above 14ºC, the three-phase 

triangle bounding the L1-L2 region has D' 

instead of Lα as the additional phase [59]. When 

a contacting experiment was conducted at 27ºC 

in this system, with a drop initially about 60 µm 

in diameter, a liquid intermediate phase 

developed after about 14 min and surrounded 

the initial alcohol drop [55]. As the intermediate 

phase grew during the next 8 min to a diameter 

of about 83 mm, the alcohol drop shrank slightly 

to a diameter of about 83 µm. Presumably, this 

growth process could be analyzed using a 

"shrinking core" model with the quasi-steady 

state approximation. However, as the available 

data on phase behavior [59] do not include 

coexistence curves for the L1-D' and L2-D' two-phase regions, it is not currently possible to 

make quantitative comparisons between 

predictions of the analysis and the experimental 

results. 

7.3. Experiments with mixtures of hydrocarbons 

and long-chain alcohols 

More interesting for detergency applications 

than pure alcohols are mixtures of polar and 

nonpolar oils, which are representative of 

sebum-like soils. Here we discuss a series of 

experiments in which drops of various mixtures 

of n-hexadecane and oleyl alcohol, typically


containing at least 50 wt.% hydrocarbon, were 

injected into dilute aqueous solutions of pure 

non-ionic surfactants at temperatures below 

their cloud points [60]. PIT measurements for 

these systems were given previously (Fig. 15). 

A general pattern of behavior was observed. 

At temperatures below the PIT, as given by Fig. 

15, no intermediate phase was seen at any time 

during the experiment and the drop volume 

decreased very slowly with time owing to 

solubilization of the alcohol and hydrocarbon 

into the micellar solution. Above the PIT, the 

behavior was similar to that described in the 

preceding section for the C12E5-water-oleyl alcohol system. That is, the drop would swell, 

and, at a particular time, the lamellar liquid 

crystalline phase could be seen growing as 

myelinic figures. However, the myelinic figures 

were shorter, smaller in diameter and more 

numerous, and they grew faster than in the pure 

alcohol system. The differences can be seen by 

comparing Fig. 35 for a drop initially containing 

85 wt.% hydrocarbon and 15 wt.% alcohol 

immersed in an aqueous solution containing 

0.05 Wt-% C12E8, with Fig. 33 for the pure 

alcohol system. The low surfactant 

concentration for the mixed oil experiment is 

typical of those used in household laundry 

processes. 

Coexistence curves for the L1-L2 region were 

determined at 30ºC for systems containing 

n-dodecyl heptaoxyethylene monoether (C12E7) and oils with 50% and 75% n-hexadecane, 

respectively [60]. In both cases the curves 

extended to water contents of 75-80 wt.%. 

Contacting experiments for various initial drop 

sizes and surfactant concentrations in the latter 

system confirmed that the dynamic behavior 

was consistent with Eq. (6). Here, too, liquid 

crystalline intermediate phases were seen for 

drops in contact with solutions containing only 

0.05 wt.% surfactant. 

It was also noted that the rate of swelling of 

the drops increased markedly as the limiting tie 

line was approached. Since the coexistence 

curves exhibited nearly constant alcohol-tosurfactant 

ratios near the limiting tie lines, this 

behavior was expected as little surfactant must 

diffuse into the drop for it to experience a 

substantial increase in volume. The basic 

analysis leading to Eq. (6) confirmed 

quantitatively that rapid swelling should occur 

for these conditions. 

Values of the parameter K s were found from 

the phase behavior data to be 0.313 and 0.0516 

for the two systems. That is, for a given initial 

drop size and surfactant concentration, 

formation of the liquid crystal occurred more 

rapidly in the system containing less alcohol, 

presumably because less surfactant had to 

diffuse into the drop to balance hydrophilic and 

lipophilic properties of the surfactant films -to 

the extent that conditions favorable for 

formation of the lamellar phase were created. 

Indeed, a general result of the contacting 

experiments with various surfactants and oils 

was that more time was required for liquid 

crystal formation when the system was made 

less hydrophilic by increasing temperature or 

the alcohol content of the drop or by reducing 

the ethylene oxide chain length of the surfactant 

[60]. 

One may ask why the intermediate phase 

formed during the contacting experiments 

should be the lamellar liquid crystal instead of a 

microemulsion, in cases where the drops 

contained substantial amounts of hydrocarbon. 

After all, diffusion of surfactant into the drop 

should cause the surfactantalcohol films within 

it to become more hydrophilic, as indicated 

above. Eventually, one might expect film 

composition to approach that corresponding to 

the PIT at the experimental temperature and 

thereby cause formation of a middle-phase 

microemulsion. The answer is that the drop 

itself becomes a microemulsion as it takes up 

water and surfactant. The occurrence of low 

interfacial tensions supports this conclusion. For 

instance. a tension of about 0.03 mN m -1 was 

measured by the spinning drop technique about 

50 min after the start of the experiment for an 

oil phase initially containing 75% n-hexadecane 

and 25% oleyl alcohol and an aqueous solution 

containing 0.1 wt% C 12E 7 at 30-C [60]. 

The coexistence curve for this system 

indicates that the water-to-hydrocarbon ratio 

during an oil drop contacting experiment varies


Fig. 35. Video frames showing dynamic behavior following the contact of 0.05 Wt'% C12E8 solution with a drop 

of 5.67/1 n-hexadecane-oleyl alcohol at 50ºC [60]: (a) oil drop about 13 min after injection; (b) the same about 8 

min later; (c) growth of myelinic figures less than 6 s later; (d) further growth of myelinic figures into the aqueous 

phase 12 s later. Reprinted with permission of the American Chemical Society. 

from its initial value of zero to about 5 when 

the drop composition eventually reaches the 

end of the coexistence curve (corresponding to 

point F of Fig. 32). No intermediate phase 

forms until point F is reached because the 

hydrocarbon content does not exceed the 

solubilization limit of the microemulsion. 

However, at point F, where the 

hydrocarbon-to-amphiphile ratio has dropped 

to about 1.5, the microemulsion structure 

apparently cannot be sustained and an 

intermediate lamellar phase begins to form. 

That it would form at such ratios of the various 

components and when the composition of the 

surfactant-alcohol films is approximately 

balanced between the hydrophilic and 

lipophilic properties is generally consistent 

with phase behavior results for other systems 

reported by Ghosh and Miller [46]. Indeed, 

lamellar phases with even higher oil contents 

have been reported near the PIT of the 

C 12E 4-water-n-hexadecane system [75].


When the alcohol content of the oil is low and 

the temperature is near or only slightly above 

the PIT, only a small amount of surfactant need 

diffuse into the drop in order for hydrophilic and 

lipophilic properties of the surfactant-alcohol 

films formed there to be almost balanced. In this 

case, the hydrocarbon solubilization limit is 

exceeded and drops of oil form within the 

original drop, which has become a middle-phase 

microemulsion. Such behavior was observed, 

for example, when a drop containing 90 wt.% 

n-hexadecane was injected into a solution 

containing 0.05 wt.% C12E7 at 40ºC, which is 

near the PIT for an oil of this composition. As 

the oil content of the microemulsion decreased 

during the experiment owing to this spontaneous 

emulsification, and as the surfactant content 

continued to increase, a point was eventually 

reached when the hydrocarbon-to-amphiphile 

ratio was too low to sustain the microemulsion 

structure and the lamellar phase again developed 

as myelinic figures [76]. 

Finally, it is noteworthy that when drops of 

oil containing 75% n-hexadecane and 25% oleyl 

alcohol were contacted with a surfactant 

solution containing 0.05 Wt-% C12E7 at 

temperatures above 40ºC, it appeared that the 

first intermediate phase formed was a liquid, 

presumably D' [76]. That is the system 

apparently experienced between 30. and 400 the 

transition discussed in the phase behavior 

section above which D' replaces La as the third 

phase in the three-phase region bounding the 

region of coexistence between La and L2. 

Detailed phase behavior at 40º was not 

determined, however. The intermediate D' phase 

in the contacting experiment was later converted 

to liquid crystal (myelinic figures). 

7.4. Experiments with mixtures of triolein and 

longchain alcohols 

Drops containing various mixtures of triolein 

and oleyl alcohol were injected into 0.1 wt.% 

solutions of C 12E 6 at 40ºC, which is about 10º 

below the cloud point temperature of dilute 

solutions of this surfactant [61]. As discussed in 

section 3 and shown in Fig. 16, the sequence of 

phases seen with increasing temperature in this 

system is similar to that found with pure triolein, 

the various transition temperatures being lower 

for higher oleyl alcohol contents. For drops 

containing less than 25 wt.% oleyl alcohol, a 

scarcely perceptible change in drop size with 

time was observed, an indication that the oil was 

being slowly solubilized into the surfactant 

solution. As indicated above, similar behavior 

was seen for hydrocarbon- alcohol drops below 

the PIT. Figure 16 confirms that the system is 

quite hydrophilic under these conditions. 

A liquid intermediate phase, presumably D', 

formed after a few minutes for a drop containing 

50 wt.% oleyl alcohol (Fig. 36(b)). Careful 

study of the image at various depths of focus 

revealed that this phase did not surround the 

original drop but instead formed a drop in 

contact with the original drop but larger in 

diameter, as the figure shows. Later, myelinic 

figures began to grow outward into the aqueous 

solution from the D'phase (Fig. 36(c)). 

Ultimately, what was left of the original oil drop 

- its volume was only about a third of the initial 

value - became detached from the intermediate 

liquid phase and showed no further changes 

with time. 

Although this system has four components, 

with the result that its phase behavior cannot be 

adequately described by a triangular diagram 

similar to Fig. 32, an explanation can be given 

for the dynamic behavior observed. As in the 

hydrocarbon-alcohol systems, surfactant 

diffuses into the drop, and the surfactant-alcohol 

films which form there become more 

hydrophilic with time. Eventually, the boundary 

of the L1-L2 coexistence region is reached, and 

an intermediate D' phase forms as surfactant 

continues to diffuse into the drop. As this new 

phase solubilizes little triolein, the 

alcohol-to-triolein ratio in the original drop 

decreases greatly. However, the D' phase is 

itself in contact with the surfactant solution and 

accordingly experiences an increase in 

surfactant content with time. When the 

hydrophilic and lipophilic properties of its 

surfactant - alcohol films are balanced, the D'


Fig. 36. Video frames from an experiment in which a drop containing equal amounts of triolein and oleyl alcohol 

was injected into a 0.1 Wt'% C 12E 6 solution at 40ºC [61]. (a) Shortly after injection; (b) approximately 9 min later 

(the intermediate phase has formed); (c) about 5 min later (the intermediate phase has grown, myelinic figures are 

starting to form); (d) about I min later (a drop of unsolubilized oil separates from the intermediate phase); (e) the 

drop is never completely solubilized. Reprinted with permission of Marcel Dekker publishers.


phase, whose films are known to be slightly 

lipophilic, cannot persist, and the lamellar phase 

forms as myelinic figures. Since the triolein-rich 

drop does not dissolve, it may well be that the 

system enters a four-phase region at the time the 

myelinic figures develop. 

For a drop containing 75% oleyl alcohol 

behavior was more complex. Two intermediate 

liquid phases formed at various times during the 

experiment and greater solubilization was 

observed [61]. Ultimately, the liquid crystalline 

phase was seen as well. 

When the drop is pure oleyl alcohol, no D' 

phase is seen, and the first intermediate phase is 

the lamellar liquid crystal. It develops as many 

short myelinic figures, the appearance being 

more similar to that of Fig. 35 for 

hydrocarbon-oleyl alcohol drops than to that of 

Fig. 33 for the C 12E 5-water-oleyl alcohol system. 

8. Fabric detergency test methods 

Laboratory evaluations of laundry detergency 

can range from use of an actual washing 

machine with a capacity of several gallons to 

use of a Terg-O-Tometer mini-laundry machine. 

The Terg-0-Tometer is a washing machine 

which replicates on a small scale the cleaning 

action of an agitatortype washing machine [77]. 

The capacity of a single Terg-O-Tometer pot is 

approximately 11. The temperature can be 

controlled quite accurately through the use of a 

water bath surrounding a bank of four or six 

pots. The small size of a Terg-O-Tometer allows 

rapid comparative studies of detergent 

formulations under the same washing 

conditions. Also, formulations containing 

expensive ingredients, e.g. pure alcohol 

ethoxylates, or experimental surfactants of 

limited quantity, can be evaluated at reasonable 

costs. For these reasons, the use of 

Terg-O-Tometer machines in detergent research 

laboratories has become quite common. 

The measurement of soil removal from fabrics 

can be performed in several ways [78]. The 

most straightforward technique involves visual 

inspection for determination of cleanliness. 

More sophisticated techniques involve the use 

of spectrophotometry, chemical analyses, and 

radiotracer methods. As with visual inspection, 

the measurement of light reflectance from fabric 

does not quantify actual soil removal but 

depends on factors such as the distribution of 

soil on the fabric, and the particle sizes. 

Analyses of weight gain or loss through the 

chemical extraction of soils from the fabric as 

well as radiotracer methods both yield 

measurements of actual soil removal and do not 

depend on the change of appearance of the 

fabric. Although not representative of real-world 

visualization of cleanliness, both techniques 

provide complementary information to 

reflectance measurements and are more useful 

for mechanistic studies. 

Specific advantages of the radiotracer 

detergency method are given elsewhere [79,80]. 

This method makes use of mildly radiolabeled 

soils for highly sensitive quantitative 

determination of soil removal by simple 

radiochernical techniques. Several radioisotopes 

may be used as labels. Generally, polar oily 

soils such as alcohols and acids are tagged with 

small amounts of carbon-14 labeled material. 

The use of two labels in a soil such as artificial 

sebum, which contains both polar and non-polar 

components, allows the removal of both types of 

soil to be monitored simultaneously. The recipe 

for such a labeled artificial sebum has been 

given elsewhere [80]. Radiolabelled particulate 

and protein soils have also been developed and 

utilized [80]. 

The Shell Development Company has utilized 

a radiotracer detergency method for many years 

to evaluate laundry detergents and to study the 

fundamental nature of oily soil removal [81,82]. 

In the case of oily soils, a 4-in square fabric 

swatch is soiled with 28 mg of radiolabeled oil. 

The soil is applied to the fabric in a toluene 

solution, which is allowed to air dry. The swatch 

is washed under controlled conditions in a 

Terg-0-Tometer. Aliquots of wash water are 

removed for radioactive counting, with the level 

of soil remaining on the swatch determined by 

difference. In addition to the sebum oil 

described above, oily soils which have been 

commonly utilized include tritium-labelled 

hexadecane (cetane), 1/1 hexadecane-squalane


(a C 30 branched hydrocarbon) and triolein. 

Also, hydrocarbon-fatty acid (or alcohol) soils 

of varying composition have been studied in 

which both carbon- 14 and tritium labels are 

utilized to allow the discrimination of removal 

of both components. The following are results 

of fundamental detergency studies using some 

of these soils, which were performed to allow 

correlation to the phase behavior and dynamic 

contacting studies described above. 

9. Fabric detergency - non-polar 

hydrocarbon soils 

Results of radiotracer detergency studies of 

removal of hexadecane from 65/35 permanent 

press polyester-cotton fabric are shown in Fig. 

37 [18]. This soil can be considered a model for 

non-polar hydrocarbon-based soils such as 

lubricating oils. The washing solutions 

contained 0.05 wt.% surfactant, resulting in a 

fabric-to-soil weight ratio of 

Fig. 37. Removal of n-hexadecane from 65/35 

polyester-cotton fabric using 0.05 wt.% aqueous 

solutions of C 12E 4 and C 12E 5 [18]. The arrows show 

the cloud point temperatures of the surfactants. 


40/1 and a surfactant-to-soil ratio of 9/1. 

Triethanolamine (50 ppm) was included as a 

buffer, and no water hardness was present. The 

washing time in the Terg-0-Tometer was 10 

min. 

Of particular interest in these studies was the 

effect of temperature and non-ionic alcohol 

ethoxylate surfactant structure on the levels of 

soil removal. As shown in Fig. 37, a strong 

dependence on temperature was observed with 

the highest levels of soil removal occurring 

almost 200C above the cloud point of the 

washing solution, a temperature regime in which 

the washing solution structure is a liquid 

crystalline dispersion. In fact, the optimum 

detergency temperature (ODT) in each case 

occurred very near the PIT of the 

water-surfactant-hexadecane system. Although 

not shown in Fig. 37, very poor detergency 

occurred in the temperature range of interest 

with C 12E 3 as the surfactant. Also, when a 1/1 by 

weight mixture of hexadecane and squalane was 

used as the soil, a similar peak in performance 

was found for each surfactant near its PIT for 

the mixed soil. These PITs were approximately 

7ºC higher than the corresponding values for 

hexadecane alone, i.e. 37 and 58ºC versus 30 

and 52ºC. The equivalence of the PIT and ODT 

in this type of detergency system has been 

confirmed by the work of Schambil and 

Schwuger [22] as well as Solans et al. [83]. 

Also, the same correspondence has been found 

for the removal of hydrocarbon by the same 

surfactant from 100% polyester fabric [84]. The 

high levels of soil removed near the PIT can be 

attributed to the ultralow interfacial tensions 

achieved near that temperature, and to the high 

rates of oily soil solubilization into 

middle-phase microemulsions, as was visualized 

in the dynamic contacting studies described 

previously. 

When the detergent properties of mixtures of 

C 12E 5 with the hydrophobic additives C 12E 3 and 

n-dodecanol (C 12E 0) were studied, optimum 

detergency was found at temperatures lower 

than the ODT for C 12E 5 alone but somewhat 

higher than the ODT for C12E4. Detergency 

data for the two additive systems, which 

exhibited the same cloud point as C 12E 4, are




solutions of a 90/10 blend of C 12E 5 and n-dodecanol 

[30]. Reprinted with permission of Academic Press. 

shown in Figs. 38 and 39 [30]. The optimum 

temperatures correspond quite closely to the 

phase inversion temperatures extrapolated to the 

low soil-to-surfactant ratios used in the 

detergency studies. In this regard, C 12E 3 was 

somewhat more effective than dodecanol in 

lowering the optimum detergency temperature. 

Of interest here is that the detergency results 



solutions of a 60/40 blend of C 12E 5 and C 12E 3 [30]. 


differed from the behavior observed in the 

verticalcontacting studies in which, as discussed 

above, somewhat higher temperatures were 

required for fastest oil solubilization due to 

partitioning of the additive into the oil. 

Practical detergency applications utilize 

commercial alcohol ethoxylate surfactants 

which contain a broad range of species having 

varying hydrophobe lengths and levels of 

ethylene oxide, Detergency studies were 

performed in the manner described above for the 

single ethoxylate and ethoxylate-additive 

systems using commercial ethoxylates based on 

a blend of predominantly normal C 12-C 13 

alcohols and containing an average of 3, 4 and 5 

mol of ethylene oxide, respectively [85]. These 

materials are denoted N23-3, N23-4 and N23-5. 

Table 2 compares the ODTs for these systems to 

those of the corresponding specific alcohol 

ethoxylate systems having the same average 

structure. The ODTs of the commercial 

materials and their molar-average equivalents 

are the same in all three cases. The ODTs of the 

commercial systems, in fact, match the PITs for 

those systems with hexadecane extrapolated to 

very low levels of oil. The PIT data are shown 

in Fig. 40. Shown for comparison are the PIT 

vs. soil-surfactant ratio plots for C 12E 4 and 

C 12E 5. These are flat, indicative of the fact that 

the PITs for ternary specific alcohol ethoxylatewater-hydrocarbon 

systems are independent of 

the oil-surfactant ratio. Bercovici and Krussman 

Table 2 

Correlation of PIT to optimum detergency 

temperature ODT 

Surfactant PIT (ºC) ODT (ºC) 

Specific ethoxylate 

C 12E 5 52 50 

C 12E 4 31 30 

C 12E 3 < 20 < 20 

Broad-range Ethoxylate 

N23-5 51 a 50 

N23-4 26 a 30 

N23-3 < 20 a < 20 

The data shown are for cetane removal from 65/35 

polyester-cotton with 0.05% surfactant [85]. 

a Evaluated at cetane-surfactant ratio → 0.


Fig. 40. PIT as a function of n-hexadecane(cetane)surfactant 

weight ratio for commercial (broad-range) 

and specific alcohol ethoxylates [85]. 

[86] have shown that the addition of long-chain 

alcohols to commercial alcohol ethoxylates, as 

described above for the specific ethoxylate 

C 12E 5, reduces the PIT and optimum detergency 

temperature for nonpolar soil removal by those 

surfactants. As with the specific ethoxylate and 

surfactant- additive systems, optimum soil 

removal with commercial nonionic surfactants 

occurs near the extrapolated PIT because the 

balanced surfactant system provides high rates 

of oil solubilization and low interfacial tensions. 

The effects of the addition of anionic 

surfactants to non-ionics on the relationship of 

soil removal to washing temperature have also 

been investigated [87]. In this case, the same 

commercial sodium alcohol ethoxysulfate 

mentioned in the phase behavior section 

(Neodol 23-3S) and the sodium salt of C 12 linear 

alkylbenzenesulfonate (denoted C 12LAS) were 

used as the anionic surfactants. As noted 

previously, C 12E 3 itself is not effective at 

removing hexadecane from 65/35 polyestercotton 

at temperatures between 20 and 70ºC. 

This result can be attributed to C 12E 3 being too 

hydrophobic. However, the addition of 

appropriate amounts of hydrophilic anionic 

surfactant was found to improve its performance 

in a 1% NaCl solution, as shown in Figs. 41 and 

42. The total surfactant concentration in the 

Fig. 41. Removal of n-hexadecane (cetane) by 

C 12E 3-N23-3S blends [87]. Reprinted with 

permission of the American Oil Chemists' Society. 

Fig. 42. Removal of n-hexadecane by C 12E 3-C 12LAS 

blends [87]. Reprinted with permission of the 

American Oil Chemists' Society. 

washing solution in these experiments was 

0.05% by weight, the same as in the non-ionic 

surfactant studies discussed above. 

Optimum detergency temperatures were 

increased to approximately 30ºC by the 

substitution for C 12E 3 of 22% and 24% N23-3S 

and LAS, respectively, and to approximately 

50ºC by the substitution of 33% and 38% 

N23-3S and LAS. Consistent with these results 

was the finding of optimum detergency at 35ºC 

with a 76.5/23.5 mixture of C 12E 3 and N23-3S 

[24]. Thus, in contrast to the hydrophobic


additives such as dodecanol described above, 

the addition of anionic surfactants increases the 

temperature for optimum detergency. However, 

as found for the hydrophobic additives, the ODT 

still corresponds closely to the PIT at a low 

oil-surfactant weight ratio. In the case of the 

anionic surfactants, the ratio of anionicto-nonionic 

surfactant yielding phase inversion at a 

given temperature was determined rather than 

the PIT for a given surfactant composition, as 

the oil-surfactant ratio was varied [87]. The 

phase inversion composition plots determined 

by electrical conductivity measurements and 

used for comparison with the detergency data 

are shown in Figs. 43 and 44 [87]. These results 

are consistent with those of other phase behavior 

studies with the same system (see Fig. 7). At a 

given temperature, the extrapolated phase 

inversion composition at a very low 

soil-surfactant ratio matches the detergent 

composition yielding optimum detergency at 

that temperature. The high levels of soil removal 

can be explained by the solubilization of oil into 

a middle-phase microemulsion at the optimum 

conditions, as described in the oil 

dropcontacting section above. 

In summary, for the removal of n-hexadecane 

soils by a variety of pure non-ionic surfactant 

systems, non-ionic surfactant-hydrophobic addi- 

Fig. 43. Phase inversion compositions for 

C 12E 3-N23-3S system [87]. Reprinted with 


Fig.44. Phase inversion compositions for 

C 12E 3-C 12LAS system [87]. Reprinted with 


tive systems, and non-ionic-anionic surfactant 

systems, optimum detergency can be related to 

the PIT. More specifically, the optimum 

detergency temperature is the extrapolated PIT 

at low oil-surfactant ratios where the 

composition of the surfactant films matches the 

composition of detergent in the washing 

solution. In all cases the surfactant solutions at 

the optimum conditions are dispersions of 

surfactant-rich phases, most commonly lamellar 

liquid crystals in water. 

10. Fabric detergency - triglycerides and 

hydrocarbon-polar soil mixtures 

Radiotracer detergency studies of triolein soil 

removal from 65/35 permanent press 

polyester-cotton fabric have been performed 

under the same conditions as. described in the 

previous section [48]. The triolein, which 

models more complex triglyceride mixtures 

such as those found in cooking oils, was tagged 

with tritium-labeled triolein. Figure 45 shows 

the results for triolein removal using 0.05% 

solutionso of C 12E 3, C 12E 4, and C 12E 5. For C 12E 3, 

triolein removal is very low at all temperatures 

studied. This result is consistent with that found 

for the removal of hexadecane by the same 

surfactant. C 12E 4 provides higher and almost 

constant soil removal between 25 and 50 ºC.


Fig. 45. Removal of triolein from 65/35 polyestercotton 

fabric by washing with pure non-ionic 

surfactants [48]. 

Above 50ºC, detergency drops off sharply. C 12E 5 

yielded optimum detergency at still higher 

temperatures, with a rather sharp peak in 

performance occurring at 65ºC. 

The optimum detergency temperature ranges 

for both C 12E 4 and C 12E 5 are higher than the 

ODTs found with hexadecane soil. At 65ºC with 

C 12E 5, optimum detergency corresponds to the 

temperature at which the D phase forms in that 

system, i.e. the PIT. With C 12E 4, no sharp peak 

in performance is noted because the D phase 

forms in that system only for a very narrow 

temperature range [48]. The decrease in 

detergency at high temperatures (above 50ºC for 

C 12E 4 and 65ºC for C 12E 5) corresponds to the 

regime where the surfactant solubility in the 

triolein phase increases significantly and a 

water-in-oil microemulsion forms, i.e. where 

little if any solubilization of oil into the washing 

solution occurs. 

Detergency studies have also been performed 

in which TAA was added to the washing 

solution at levels up to 20% relative to the 

surfactant [52]. This study was performed to 

determine if the addition of TAA could increase 

triolein removal by promoting the formation of 

the D phase during the washing process. The 

levels of soil removal were found by extracting 

the washed fabric swatches (both 100% 

polyester and 65/35 polyester-cotton) with 

hexane. For C 12E 4, triolein removal was poor at 

50ºC and was unaffected by the addition of 

TAA. Soil removal was much greater at 35ºC, 

with the addition of 20% TAA providing only a 

slight improvement when compared with 

detergency with C 12E 4 alone. The lack of 

significant performance enhancement was 

attributed to the high water solubility of TAA, 

which limits its effectiveness in highly dilute 

surfactant solutions. Formation of the D phase 

in the oil drop-contacting studies (see Fig. 31) 

apparently occurred due to the higher 

concentration of surfactant and TAA used in 

that study. 

A similar dependence of soil removal on 

temperature was observed when the same 

surfactants were used to remove a soil mixture 

containing 1/1 hexadecane-triolein. In this 

study, the triolein was labeled with tritium and 

the hexadecane was labeled with carbon-14 to 

allow independent measurement of the removal 

of the two components. The results of that study 

are shown in Fig. 46. When compared with the 

results in Fig. 45, somewhat higher removal 

levels of triolein were found from the soil 

mixture. Also of interest is that removal of hexa- 

Fig. 46. Removal of triolein and n-hexadecane from 

65/35 polyester-cotton fabric by washing with pure 

non-ionic surfactants [48]. Soil contains 50 wt.% 

triolein.


decane was consistently higher than that of 

triolein at all temperatures. The regions of 

optimum detergency occur near the PITs of the 

surfactants with the oil blend. The preferential 

removal of hexadecane can be attributed to the 

higher solubility of hexadecane in the 

middle-phase microemulsion D (see Table 1). 

The removal of blends of hexadecane with 

oleyl alcohol and oleic acid by 0.05% solutions 

of alcohol ethoxylates has also been studied 

[12]. These binary soil mixtures are models of 

sebum-like soils containing both non-polar and 

polar fractions. In general, somewhat more 

hydrophilic surfactants are required to obtain 

high removal of these soils compared to the 

optimum surfactants for nonpolar hexadecane 

removal. Results for the removal of hexadecane 

from 9/1 and 3/1 blends of hexadecane and oleyl 

alcohol are shown in Figs. 47 and 48. N25-9 is a 

broad-range commercial ethoxylate based on a 

predominantly linear C 12-C 15 alcohol and 

containing an average of nine ethylene oxide 

(EO) units. In the case of the 9/1 soil at 20ºC, 

detergency was found to increase with 

decreasing EO content of the pure non-ionic 

surfactants, while the reverse trend was true at 

the higher temperatures. For both soils, 

optimum detergency occurred over a 

temperature range below the cloud points of the 

surfactants, behavior which contrasts markedly 

Fig. 47. Removal of n-hexadecane for mixed 9/1 

n-hexadecane-oleyl alcohol soil [12]. Reprinted with 


Fig. 48. Removal of n-hexadecane for mixed 3/1 

n-hexadecane-oleyl alcohol soil [12]. Reprinted with 

permission of American Oil Chemists' Society. 

markedly with the results discussed above for 

hydrocarbon and triolein soils. The cleaning 

efficiency of the commercial broad-range 

ethoxylate is almost identical at all temperatures 

to that of the specific ethoxylate (C 12E 8) having 

nearly the same cloud point temperature, 

indicating, once again, that surfactant 

partitioning effects in the detergency studies are 

negligible. 

The effect of the soil composition on soil 

removal can be seen more clearly in Fig. 49. 

While the detergency peaks near 80ºC with the 

pure hexadecane soil, good detergency is 

obtained at much lower temperature with the 

Fig. 49. Effect of oleyl alcohol content on 

n-hexadecane removal [12]. Reprinted with 

permission of the American Oil Chemists' Society.


mixed soils. Maximum detergency is reached 

near 40ºC with 10% oleyl alcohol in the soil 

while high levels of cetane removal are attained 

down to 20ºC with 25% oleyl alcohol present. 

The temperatures at the lower limit of the 

plateaus in detergency are the respective phase 

inversion temperatures. For C 12E 7, PIT values 

can be obtained from the curves in Fig. 15. 

Above the PIT, soil removal remains at a high 

level until the cloud point of the surfactant is 

reached. As implied in the oil drop-contacting 

section, the formation of an intermediate liquid 

crystalline phase plays the key role in the 

soilremoval process in the plateau regime. 

The relative amount of polar soil also affects 

the rate of soil removal. Despite being removed 

at a comparable level to that of the 3/1 

cetane-oleyl alcohol soil after 10 min at 50ºC, 

the 9/1 soil is actually removed much more 

quickly, as indicated in Fig. 50. This very fast 

removal can be attributed to that soil having a 

PIT of approximately 50ºC with C 12E 7. A 1/1 

cetane-oleyl alcohol soil is removed very slowly 

at 50ºC, although a high level of removal is 

ultimately attained. Removal of the hexadecane 

soil reaches a plateau at a lower level since the 

washing temperature is well below the optimum 

detergency temperature for its removal by C 12E 7, 

i.e. approximately 80ºC. 

Fig. 50. Kinetics of n-hexadecane removal for mixed 

n-hexadecane-oleyl alcohol blends [12]. Reprinted 

with permission of the American Oil Chemists' 

Society. 

Similar results to those described for 

hexadecane-oleyl alcohol soils were obtained 

for hexadecane-oleic acid soils having the same 

ratio of nonpolar and polar constituents. These 

studies were performed in the absence of 

triethanolarnine to insure that the oleic acid was 

in the non-ionized state. Also, the oleic acid was 

tagged with 14C-labeled material to allow 

measurement of its removal as well as that of 

hexadecane. The results for 3/1 cetane-oleic acid 

soil are shown in Fig. 51. High levels of soil 

removal were found over a wide temperature 

range from 20ºC to the cloud point of the 

surfactant. As with the oleyl alcoholcontaining 

soils, optimum removal was found between the 

PIT and cloud point temperature. Interestingly, 

the data show that the removal of oleic acid 

paralleled that of cetane, although at levels 

10-20% higher. This behavior results from the 

higher ratio of oleic acid to hexadecane found in 

the intermediate liquid crystalline phase 

compared to that in the original soil. 

Presumably, preferential removal of oleyl 

alcohol would have been found in the 

hexadecane-oleyl alcohol experiments if the 

alcohol had been radioactively tagged. 

11. Discussion 

As shown above, maximum removal of liquid 

hydrocarbon soil from synthetic fabrics in non- 

Fig. 51. Oil removal for mixed 3/1 n-hexadecaneoleic 

acid soil [12]. Reprinted with permission of the 

American Oil Chemists' Society.


ionic and anionic-non-ionic surfactant systems 

occurs near an appropriately defined PIT because 

solubilization of hydrocarbon into intermediate 

microemulsion or liquid crystalline phases 

is high and interfacial tensions are low. The low 

tensions facilitate emulsification of the intermediate 

phases into the agitated washing bath. 

Hydrocarbon soil removal is low at 

temperatures significantly below the PIT 

because, as is well known, the amount of oil that 

can be solubilized by microemulsion phases 

under these conditions is well below the 

solubilization capacity of middlephase 

microemulsions near the PIT, and interfacial 

tensions are significantly higher. Moreover, the 

initial rate of solubilization is less than that 

found near the PIT, as shown in Fig. 24 above. 

At temperatures significantly above the PIT, 

the explanation for the poor detergency is 

different. In this case both surfactant and water 

diffuse into the oil phase, so that it actually 

increases in volume instead of being solubilized. 

Moreover, extensive spontaneous emulsification 

of water in the oil occurs, as seen, for example, 

in Fig. 25. Solans and Azemar [19] reported that 

polyester-cotton fabric washed at temperatures 

above the PIT often experienced a net gain in 

weight, a result consistent with the above 

statements. They further observed that 

"aggregates", which apparently contained both 

oil and surfactant, remained on the fabric 

surfaces after washing. They suggested that 

additional water drops might be incorporated 

into the initial spontaneously formed emulsion 

as a result of agitation during the washing 

process, the result being an emulsion with a 

viscosity so high that it is not easily removed. 

Indeed, they proposed that the disperse phase 

content could become so large that a so-called 

high internal phase ratio (HIPR) emulsion could 

be formed on the fabric with the drops of water 

becoming polyhedral and the emulsion 

developing a yield stress. 

Yang and Rathman [88] confirmed that 

increases in weight occurred for polyestercotton 

fabrics washed above the PIT, and they 

found a surfactant-to-oil ratio of about 0.5 in 

material extracted from the washed fabric in one 

system. 

They also found that clean fabric became 

soiled if it was washed with soiled fabric above 

the PIT. Thus, redeposition of soil, probably in 

the form of an oil-continuous emulsion or 

microemulsion, must be considered a significant 

factor in the washing process under these 

conditions. 

In the studies discussed so far, the effects of 

electrolytes on detergency performance were not 

investigated. Recent results have shown that the 

optimum detergency temperature and the PIT 

are equivalent but lower when salts such as 

sodium citrate, a common builder for liquid 

detergent formulations, are present in appreciable 

amounts in the washing solution [89]. Similar 

effects with other salts would be expected. 

For the hydrocarbon soils and surfactants of 

interest for detergency, the PIT is invariably 

above the surfactant cloud point, so that the 

washing bath is a dispersion of the 

surfactant-rich liquid L1 or the lamellar liquid 

crystalline phase La in water. In most contacting 

experiments near the PIT an intermediate 

microemulsion phase was observed near the 

surface of contact. However, as mentioned 

above in connection with the vertical cellcontacting 

experiments for C12E4 and n-hexadecane 

at 30ºC, no microemulsion phase was 

seen when the dispersion of the lamellar phase 

was sufficiently dilute. In this case, small 

particles of liquid crystal striking the oil-water 

interface apparently dissolved in the oil, 

although it is likely that tiny drops of microemulsion 

quickly formed and dissolved during 

the process. However, if only a small quantity of 

oil had been present in this experiment, as in 

practical washing situations, it would have 

rapidly become saturated with surfactant by 

taking up such particles, and subsequent 

particles striking the interface would have 

produced an intermediate microemulsion phase. 

The experiment where the pure lamellar phase 

was contacted with oil for the same system at 

the same temperature is also of interest. Here 

considerable oil was solubilized directly by the 

liquid crystal although, as the dark region in Fig. 

27 and the diffusion path of Fig. 28 indicate, 

some microemulsion did form as a dispersion 

with the liquid crystal at a location away from


the oil-liquid crystal interface. This experiment 

demonstrates that the formation of an 

intermediate phase is not necessary for high 

solubilization if sufficient quantities of a 

surfactant-rich phase are present initially. 

Indeed, such solubilization into the lamellar 

phase was apparently responsible for the 

moderate removal of pure triolein from 

polyester-cotton fabrics by C 12E 4 at temperatures 

between about 25 and 50ºC (Fig. 45). The poor 

detergency performance at high temperatures in 

this case is, as with hydrocarbon soils, the result 

of conversion of the oil phase into a water-in-oil 

microemulsion accompanied by spontaneous 

emulsification of the water there. 

If two different types of hydrocarbon soils are 

present, a useful strategy with a single pure 

surfactant would be to wash near the higher of 

the two PIT values and rinse near the lower. 

Experiments have shown that substantial soil 

removal does take place when washing occurs 

above the PIT and rinsing near the PIT [83,88], 

as would be the case in this example for the soil 

with the lower PIT. One reason this method 

works is that substantial surfactant remains on 

fabric washed above the PIT, as mentioned 

above, much of it probably dissolved in the 

unremoved soil. If there are multiple 

hydrocarbon soils and if a commercial 

surfactant or surfactant mixture is used, washing 

should again start at the highest PIT. In this 

case, however, it would be best to decrease 

temperature with time during the latter stages of 

washing or during rinsing, so that the PIT values 

for the various soils would all be reached at 

some time during the process. Note that during 

rinsing the PIT values will not be the same as 

those for the initial washing bath owing to 

differences in surfactant composition and 

surfactant-to-oil ratio during washing and 

rinsing. The concept of designing a process so 

that the PIT is achieved at some time as the 

system is gradually made more hydrophilic is 

related to certain strategies that have been 

proposed for enhanced oil recovery processes, 

e.g. variation of the injected salinity [90]. The 

concept of washing at high and rinsing at low 

temperatures to improve detergency was 

proposed by Rubingh and Stevens [91] although 

their explanation did not involve the PIT and the 

formation of intermediate phases. 

Achieving excellent detergency for pure 

liquid triglyceride soils and synthetic fabrics is 

difficult because the large triglyceride molecules 

are not readily solubilized. Indeed, substantial 

solubilization apparently requires conditions 

where an intermediate D phase forms. With 

straight-chain ethoxylated alcohols, this seems 

not to be possible except at undesirably high 

temperatures, e.g. about 65ºC with C12E5 and 

triolein (Fig. 10). Adding a short-chain alcohol 

does seem to make the surfactant films more 

disordered and thereby promote D phase 

formation at slightly lower temperatures (Fig. 

11), but the relatively high solubility of the 

alcohols in water makes them ineffective for this 

purpose when the dilute surfactant solutions of 

interest for detergency are used. As indicated 

previously, preliminary results with secondary 

alcohol ethoxylate surfactants, whose 

double-tailed structure with different chain 

lengths also creates disordered films, look more 

promising as a means of forming the D phase at 

temperatures existing during warm and cold 

water washing [55]. Further study of these 

systems is in progress. Of course, the D phase 

could also be formed by using washing baths 

containing solubilized hydrocarbon (see Fig. 

12), but this scheme seems less attractive than 

using the secondary alcohol ethoxylate 

surfactants. 

Results presented above demonstrate that 

liquid mixtures of hydrocarbons and long-chain 

alcohols or undissociated fatty acids can be 

removed effectivcly at temperatures below the 

cloud point of the surfactant but equal to or 

above the PIT of a system whose excess oil 

phase coexisting with the microemulsion has the 

same composition as the soil. Under these 

conditions, the soil takes up surfactant and water 

from the washing bath, as shown in the oil 

drop-contacting experiment of Fig. 35, the 

results being the lowering of the interfacial 

tension between the soil and surfactant solution 

and the eventual formation of an intermediate 

lamellar liquid crystalline phase as filaments or 

myelinic figures. These, presumably, are broken 

off and dispersed into the washing bath as a


result of agitation. Note that Fig. 48 and 49 

show no significant change in detergency 

between 40 and 60ºC for C 12E 7 and the soil 

containing 75% cetane even though the 

contacting experiments mentioned above for this 

system indicated that an intermediate D' phase 

formed first and only later the liquid crystal 

formed. Below the PIT no intermediate phase 

was seen in all the systems, the drops were 

slowly solubilized into micelles in the washing 

bath, and soil removal was lower. 

For all the systems discussed above for which 

both videomicroscopy observations and soil 

removal measurements have been performed, 

intermediate phase formation and low interfacial 

tensions leading to good detergency occurred 

when the hydrophilic and lipophilic properties 

of the surfactant or surfactant -alcohol films of 

the intermediate phase were approximately 

balanced. This balance may be present initially, 

as in the experiments with hydrocarbon soils 

near the PIT, or it may develop during washing 

as a result of mass transfer, as in the 

experiments with mixtures of hydrocarbons and 

long-chain alcohols. Note that if mixed 

surfactants and lower surfactant-to-oil ratios had 

been used in the hydrocarbon washing 

experiments, mass transfer effects would most 

likely have been important there as well, i.e. the 

system could have moved closer to or further 

from the PIT over time. A similar conclusion on 

the need for balance was reached by Malmsten 

and Lindman [92], although without the proviso 

that a system not balanced initially may achieve 

balance during washing. It should also be 

recognized that, even when balance is achieved, 

the intermediate phase formed must have the 

capability of solubilizing considerable soil, a 

property clearly lacking in some of the 

non-ionic surfactant-triolein systems discussed 

above. 

If a system contains soils with several 

different mixtures of hydrocarbons and 

long-chain polar compounds on the fabrics, an 

effective strategy would seem to be to choose a 

washing temperature and surfactant so that the 

temperature is initially above the PIT of all soil 

compositions. The surfactant must be 

hydrophilic enough to be somewhat below its 

cloud point but lipophilic enough to meet the 

above PIT criterion. Mass transfer during the 

washing process will bring the various soils to a 

balanced condition (though at different times) at 

which they can be removed by some 

combination of intermediate phase formation 

and emulsification promoted by low interfacial 

tensions. This strategy is, of course, similar to 

that suggested above for multiple hydrocarbon 

soils except that no variation of temperature is 

required. 

For hydrocarbon soils the phase behavior of 

the surfactant-water mixture in the initial 

washing bath did not seem to have a significant 

influence on detergency. Maximum soil removal 

occurred at the system PIT whether the washing 

bath consisted of a dispersion of the L 1 phase or 

of the Lα phase. In contrast, soil removal 

dropped sharply when the cloud point 

temperature of the surfactant solution was 

reached for the mixed hydrocarbon-alcohol soils 

above their PITs. It is likely that water formed 

as in the intermediate phase for temperatures 

above the cloud point temperature and hindered 

the mass transfer necessary to achieve the 

balanced condition discussed above. That is, the 

diffusion path is most likely similar to that of 

Fig. 26 except that the surfactant-rich phase is 

L 1 instead of La. 

In the oil drop-contacting experiments it was 

not unusual for 20 min or even more to elapse 

before liquid crystal formation began. Such long 

times are unsatisfactory for washing processes. 

It should be kept in mind, however, that the 

contacting experiments were deliberately 

conducted with no imposed mixing to facilitate 

the interpretation of the observations in terms of 

diffusion theory. In actual washing situations, 

mixing would greatly speed up mass transfer, 

and liquid crystal could be expected to form 

after much shorter times. Further discussion, 

including some quantitative estimates of the 

enhancement in mass transfer rates, may be 

found in the original papers [60,74]. When 

mixing occurs, the times required for liquid 

crystal formation were found to be reasonable 

for practical processes, a conclusion confirmed 

by the excellent soil removal obtained in the 

washing experiments described above.


In the preceding section, it was mentioned 

that the time required for removal of mixed 

hydrocarbon-long-chain alcohol soils increases 

when the initial state of the system is further 

above the PIT. This result is consistent with the 

intuitive expectation that surfactant would have 

to diffuse into the oil for a longer time before 

liquid crystal formation occurred for systems 

further above the PIT as well as with the 

quantitative values of the parameter K s in Eq. 

(6) found for soils with different compositions. 

It is also noteworthy that in some of the 

contacting experiments, convection arose 

spontaneously near the interface, an effect that 

would also speed up mass transfer. The 

Marangoni flow mentioned in connection with 

Fig. 19 for the C12E5-water-n-tetradecane 

system at 20ºC is one example, although it 

occurred for a situation when soil removal is 

minimal owing to a low solubilization capacity 

of the surfactant solution. Perhaps more relevant 

to detergency, vigorous convection was 

sometimes seen during the early stages of 

formation of a non-wetting intermediate phase, 

i.e. one that forms preferentially as lenses rather 

than as a continuous layer. Intermediate phase 

formation in the C 12E 5-water-n-tetradecane 

system at temperatures just below and above the 

cloud point is an example. If non-uniformities in 

intermediate phase thickness develop 

immediately following the initial contact when 

the phase is a still a thin liquid film, disjoining 

pressure effects will promote flow from thin to 

thick portions of the film, thus increasing the 

discrepancies in thickness. Diffusion processes 

will continue to form more of the intermediate 

phase in the thin regions, and disjoining 

pressure gradients will continue to drive the 

newly formed material to thicker regions. 

No soil removal experiments are currently 

available for the systems discussed above 

containing mixed triolein-oleyl alcohol soils and 

a pure nonionic surfactant. The contacting 

experiments indicate, however, that the first 

intermediate phase formed is D', which 

solubilizes little triolein although readily 

incorporating alcohol. It may be that some of the 

same ideas suggested above to improve 

detergency with pure triolein, e.g. use of 

secondary alcohol ethoxylate surfactants, will be 

needed to achieve high degrees of removal of 

the triolein portion of these mixed soils. Of 

course, liquid triglyceride soils normally contain 

some fatty acids formed by triglyceride 

hydrolysis. In addition, lipase enzymes are 

incorporated into some detergent formulations 

to promote the breakdown of triglycerides into 

monoglycerides, diglycerides and fatty acids. 

Increasing the pH can convert these acids to 

soaps, which could well improve detergency. 

This behavior is currently being studied by the 

contacting techniques discussed above. 

12. Conclusions 

Solubilization-emulsification is a key 

mechanism in the removal of oily liquid soils 

from polyester and polyester-cotton fabrics. 

Systematic studies, discussed above, utilizing 

several model soils indicate that solubilization 

into an intermediate phase formed during 

washing is generally much more extensive and 

rapid than solubilization into a micellar solution. 

Accordingly, knowledge of the phase behavior 

of surfactant-soil-water systems is needed to 

make a rational choice of optimum surfactant 

compositions and washing conditions. With 

hydrocarbon soils, for instance, washing at 

temperatures near the PIT is best. The 

intermediate phase is a microemulsion although 

the lamellar liquid crystalline phase may form as 

well if it is not already present in the initial 

washing bath. For commercial non-ionic 

surfactants and their mixtures with anionic 

surfactants, the appropriate PIT in the usual case 

of washing with a high surfactantto-oil ratio is 

that for which the composition of the surfactant 

films in the middle-phase microemulsion is that 

of the surfactant mixture in the initial washing 

bath. 

For mixed soils containing hydrocarbons and 

long-chain alcohols or fatty acids, good soil 

removal is achieved at temperatures above the 

PIT but below the cloud point of the surfactant. 

In this case the PIT is evaluated at high 

surfactant-to-oil ratios as above with the 

additional condition that the excess oil phase in 

equilibrium with the microemulsion has the


initial soil composition. The lamellar liquid 

crystal is the intermediate phase. It begins to 

grow as myelinic figures at a time after initial 

contact between surfactant solution and soil 

which can be predicted in simple cases using 

diffusion theory and certain limited information 

on system phase behavior. 

In both these situations, the intermediate 

phase, or phases, start(s) to grow when the films 

of surfactant and long-chain polar compound, if 

present, are roughly balanced with respect to 

hydrophilic and lipophilic properties. This 

balance, which may be present initially or may 

occur at some later time during the washing 

process as a result of mass transfer, facilitates 

emulsification because interfacial tensions, 

including those involving intermediate phases, 

are low. 

Long-chain liquid triglycerides are less 

readily solubilized than the hydrocarbons of 

interest in detergency owing to their higher 

molecular volumes. The best prospect for 

removing such triglycerides seems to be to 

establish conditions under which an 

intermediate D phase will develop during 

washing. This phase, which is closely related to 

the middle-phase microemulsion, can be 

expected to form if sufficient hydrocarbon is 

present with the triglyceride in the initial soil. 

Otherwise, its formation can be promoted by 

using mixtures of surfactants with varying 

hydrocarbon chain lengths, so that surfactant 

films with rather disordered packing in the chain 

region are present. 

Acknowledgments 

The research at Rice University discussed in 

this paper was supported by the National 

Science Foundation, Shell Development 

Company, and the Clorox Corporation. 

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Solubilization-emulsification mechanisms of detergency

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