Solubilization surfactant mixtures

In some systems containing surfactant mixtures, a synergistic effect on the water solubilization capacity has been observed [50]. 

The partitioning can be revealed by different patterns that are found in experimental data. The first one is a reduction in solubilization because a part of the surfactant mixture is no longer at interface. For the same amount of surfactant in the system at optimum formulation, the volume of microemul-son middle phase is lower, just because a certain proportion of the surfactant is no longer in the microemulsion, but has partitioned into one of the excess phases. However, it is worth noting that there are other reasons for the solubilization to decrease, hence this is only a hint. 

The effect of using mixtures of surfactants on micelle formation, monolayer formation, solubilization, adsorption, precipitation, and cloud point phenomena is discussed. Mechanisms of surfactant interaction and some models useful in describing these phenomena are outlined. The use of surfactant mixtures to solve technological problems is also considered. 

This overview will outline surfactant mixture properties and behavior in selected phenomena. Because of space limitations, not all of the many physical processes involving surfactant mixtures can be considered here, but some which are important and illustrative will be discussed these are micelle formation, monolayer formation, solubilization, surfactant precipitation, surfactant adsorption on solids, and cloud point Mechanisms of surfactant interaction will be as well as mathematical models which have been be useful in describing these systems,... 

This brief review has attempted to discuss some of the important phenomena in which surfactant mixtures can be involved. Mechanistic aspects of surfactant interactions and some mathematical models to describe the processes have been outlined. The application of these principles to practical problems has been considered. For example, enhancement of solubilization or surface tension depression using mixtures has been discussed. However, in many cases, the various processes in which surfactants interact generally cannot be considered by themselves, because they occur simultaneously. The surfactant technologist can use this to advantage to accomplish certain objectives. For example, the enhancement of mixed micelle formation can lead to a reduced tendency for surfactant precipitation, reduced adsorption, and a reduced tendency for coacervate formation. The solution to a particular practical problem involving surfactants is rarely obvious because often the surfactants are involved in multiple steps in a process and optimization of a number of simultaneous properties may be involved. An example of this is detergency, where adsorption, solubilization, foaming, emulsion formation, and other phenomena are all important. In enhanced oil recovery. 

The trough itself measured 20 x 12 cm, was milled from a block of Teflon, and held approximately 750 ml of liquid. Monomolecular films were prepared on a subphase of (ultrapure) distilled water or on aqueous subsolutions containing varying concentrations of either NaF, HC1, NaOH, thiourea, or dimethyl sulfoxide (DMSO). Aliquots, between 50 and 250 pi, of the ethanol-solubilized microbubble-surfactant mixture were applied slowly to the surface of the subsolution from a Hamilton microsyringe. It was found unnecessary to allow the films to stand for more than 2 min after spreading before taking measurements. Furthermore, following compression or expansion, the surface pressure was observed to remain constant for periods of up to at least 10 min. All measurements were made at 20.0 0.5°C. 

Most surfactant mixtures deviate negatively from Raoult s Law. The extent of deviation and thus the solubilization is largest for mixtures containing surfactants with different headgroup charges (Scamehorn, 1986) or with significant headgroup size differences (Abe et al., 1992). 

Butler, E. C Hayes, K. F. Micellar Solubilization of Non-Aqueous Phase Liquid Contaminants by Nonionic Surfactant Mixtures Effects of Sorption, Partitioning and Mixing, Water Research, 1998, 32, 1345-1354. 

The solubility of either acrylamide or water in ethane/propane mixtures is extremely low (8.9). The solubility of the surfactant mixture B52/B30 is also quite low (21). Adding acrylamide to the B52/B30 blend allows significantly larger amounts of both components to be solubilized in the alkane continuous phase, suggesting that acrylamide is a co-surfactant in this system. However, the B52/B30 mixture will solubilize acrylamide only up to an [acrylamide] [surfactant] molar ratio of 1 4 larger amounts of acrylamide lead to precipitation of a solid phase. 

Another possible extension is to consider an excess oil phase which is a mixtnre of two or more species. Provided that mixing within the micelle can still be considered ideal and that activity coefficients for all species in the bulk oil mixture are known, an expression for for each solnte is readily obtained. Micelles formed from surfactant mixtures can be treated provided that micelle composition is known or can be calculated from theories of mixed micelles such as regular solution theory and that solubilization is low enough not to affect micelle shape or composition. Finally, nonideal mixing in the micelles can be included if some model for the nonideality is available as well as data for evaluating the relevant parameters. Perhaps the simplest scheme for incorporating nonideality with nonpolar solutes is to use volume fractions instead of mole fractions in the spirit of Flory-Huggins theory. 

Interaction between the surface-active components in surfactant mixtures and with the solubilizate can both increase and decrease solubilization into the mixed micelles. Thus, the addition of small quantities of sodium dodecyl sulfate sharply decreases the solubilization of Butobarbitone by micellar solutions of a commercial POE nonionic, Ci2H2s(0C2H4)230H. The competitive interaction of the sodium dodecyl sulfate with the oxyethylene groups on the surface of the micelles of the nonionic surfactant is believed to be the cause of this phenomenon (Treiner, 1985). On the other hand, a mixture of sodium dodecyl sulfate and sorbitan monopalmitate in aqueous solution (Span 40) solubilized dimethylaminoazobenzene more than either surfactant by itself, with maximum solubilization observed at a 9 1 molar ratio of the anionic to the nonionic (Fukuda, 1958). 

The thermodynamic modeling of microemulsions has taken various lines and gave conflicting results in the period before the thermodynamic stability and microstructure were established. It was early realized that a maximal solubilization of oil and water simultaneously could be discussed in terms of a balance between hydrophilic and lipophilic interactions the surfactant (surfactant mixture) must be balanced. This can be expressed in terms of the HLB balance of Shinoda,Winsor s R value, and a critical packing parameter (or surfactant number), as introduced to microemulsions by Israelachvili et al. [37], Mitchell and Ninham [38], and others. The last has become very popular and useful for an understanding of surfactant aggregate structures in general. 

In summary, NMR studies can deal with a wide range of problems in surfactant science. These include, e.g., molecular transport, phase diagrams, phase structure, self-association, micelle size and shape, counterion binding and hydration, solubilization, and polymer-micelle interactions. NMR is fruitfully applied to isotropic or liquid crystalline bulk phases, to dispersions (vesicles, emulsions, etc.), to polymer-surfactant mixtures, and to surfactant molecules at solid surfaces. In all cases NMR can provide information on molecular interactions and dynamics as well as on microstructure. 

M.A. (1995) Effect of oil on the solubilization in microemulsion systems including non-ionic surfactant mixtures. Langmuir, 11, 3302-3305. 

Figure 20.3. Solubilization experiments support the notion of a polymer-induced micellization. The amount of a dye, Orange OT, solubilized in mixtures of sodium alkyl sulfates of different chain lengths (C10-C16) and poly(vinyl pyrrolidone) (PVP) is given as a function of the surfactant concentration. (Redrawn from H. Lange, Kolloid-Z. Z. Polym., 243 (1971 101)...

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