Solubilization-emulsification mechanisms of detergency

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172 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 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
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 173 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
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Solubilization-emulsification mechanisms of detergency

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