SURFACE-TENSION OF AQUEOUS-SOLUTIONS OF SURFACTANT MIXTURES - COMPOSITION OF MIXED MICELLES

A method for determining the composition of mixed micelles by surface tension measurements is described, and the ideality of mixing in micelles is discussed. The systems investigated were (i) decyl methyl sulfoxide (DeMS)-heptaoxyethylene dodecyl ether (DE7) in water at 30 °C, (ii) DeMS-cetyltrimethylammonium bromide (CTAB) in 1 mM sodium bromide solution at 25 °C, and (iii) dodecyl methyl sulfoxide (DMS)-CTAB in 20 mM potassium bromide solution at 30 °C. Using the observed relationships of critical micellization concentration (cmc) and monomeric composition xb to the surface tension at cmc, one can determine the micellar composition xm from the surface tension of solutions at a higher concentration than cmc. Comparison of the theory of ideal mixing in micelles with observed values of cmc, xb, and xm revealed that the mixing of system i is ideal whereas those of systems ii and iii are not. From values of activity coefficients of each component and the excess free energy of mixing in micelles, the nonideality for the latter two systems may be ascribed to a decrease in electrostatic energy of micelles with the comicellization of the ionic and nonionic surfactants. © 1979 American Chemical Society.