THERMODYNAMICS OF SOLUBILIZATION OF PENTANOL IN SODIUM DODECYL-SULFATE - DODECYLDIMETHYLAMINE OXIDE MIXED MICELLES

Heat capacity and density measurements of pentanol (PeOH)-sodium dodec yl sulfate (NaDS)-dodecyldimethylamine oxide-water mixtures were carri ed out at 0.03 m PeOH as a function of the total surfactants concentra tion (m(t)) at different ratios (X(NaDS)). From experimental data, the apparent molar volumes (V(PHI,R)) and heat capacities (C(PHI,R)) of P eOH in the surfactants mixture solutions were calculated. As a general feature, at a given mixture composition, V(PHI,R)) increases monotoni cally with m(t) as observed in pure surfactants. In the cases of X(NaD S) = 0.1 and 0.3, V(PHI,R)) drops at about 0.1 and 0.15 m, respectivel y. The decreasing C(PHI,R)) vs m(t) curve shows peculiarities which we re ascribed to the presence of a liquid crystalline phase for X(NaDS) = 0.1 and 0.3 and of a structural micellar transition for X(NaDS) = 0. 6 and 0.9. At a given X(NaDS), the dependence of the apparent molar pr operty of PeOH on m(t) was rationalized in terms of the distribution c onstant of alcohol between the aqueous and the micellar phases (K) and its partial molar property in the aqueous and the micellar phases. Th e standard free energy, calculated from the K values, and the partial molar properties of PeOH in the surfactant-surfactant micelles change linearly with X(NaDS) according to the ideal behavior. (C) 1994 Academ ic Press, Inc.