Solubilization and Microemulsions

Surfactant solutions with concentrations above the CMC can dissolve considerably larger quantities of organic materials than can pure water or surfactant solutions at concentrations below the CMC. This enhanced solubility is important in apphcations ranging from the formulation of pharmaceutical and personal care products, to detergency, to removal of organic contaminants from wastewaters, soils, and ground water aquifers. 

As would be expected, the capacity for solubihzing a compound is proportional to the munber of micelles present and hence to the difference between the total surfactant concentration and the CMC. In this cormection we note that the CMC decreases in the presence of dissolved hydrocarbon, though the effect is usually rather small. 

Molecular theories of solubilization can be rather complex (Nagarajan and Ruckenstein, 1991). However, considerable insight can be gained from a relatively simple model that was suggested some years ago by Mukheqee (1970). Suppose that an aqueous micellar solution has reached its solubilization Umit and is in equilibrium with an excess liquid phase of a pure hydrocarbon or other compound of low polarity. Equating the chemical potentials [Xj and ja of the contaminant in the bulk organic phase and the micelles, we have 

Soap moiecul denotod by (UT 0 denotes o polar group. 

FIGURE 4.22 Schematic view of solubilization. Reprinted from Alexander and Johnson (1949) with permission. 

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K. L. Mittel, Micellization, Solubilization, and Microemulsions, Plenum Press, New York, 1977. 

Martinek K, Yatsimirskii AK, Levashov AV, Berezin IV. In Mittal KL, editor. Micellization, solubilization and microemulsions, vol. 2. New York Plenum 1977. pp. 489-508. 

FIG. 8.4 Determination of the microenvironment of a molecule (a) a portion of the ultraviolet spectrum of benzene in (1) heptane, (2) water, and (3) 0.4 M sodium dodecyl sulfate and (b) ratio of the intensity of the solvent-induced peak to that of the major peak for benzene versus the index of solvent polarity. The relative dielectric constant is also shown versus the index of polarity. (Redrawn, with permission, from P. Mukerjee, J. R. Cardinal, and N. R. Desai, In Micellization, Solubilization and Microemulsions, Vols. 1 and 2 (K. L. Mittal, Ed.), Plenum, New York, 1976.)... 

Muller, N. Errors in micellization enthalpies from temperature dependence of critical micelle concentrations. In Micellization, solubilization, and microemulsions. Vol. l,p. 229, Mittal, K. L. (ed.). New York - London Plenum 1977... 

Micellization, Solubilization, and Microemulsions [Proceedings from the International Symposium on Micellization, Solubilization and Microemulsions 7th Northeast Regional Meeting of the American Chemical Society, Albany, N.Y., Aug. 8-11, 1976) K. L. Mittal, P. Mukerjee, Vol. 1, (Ed. K. L. Mittal), Plenum, New York, 1977]. 

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