Solubilization-emulsification mechanisms of detergency

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176 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 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
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 177 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.
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Solubilization-emulsification mechanisms of detergency

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