Solubilization-emulsification mechanisms of detergency

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174 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 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
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 175 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].
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Solubilization-emulsification mechanisms of detergency

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