ENTHALPIES OF MIXING OF SOME PRIMARY HYDROGENATED AND FLUORINATED ALCOHOLS AND SODIUM PERFLUOROOCTANOATE AQUEOUS-SOLUTIONS

The enthalpies of transfer DELTAH(W-->W+S) of some hydrogenated and fluorinated alcohols from water to sodium perfluorooctanoate solutions were measured. In the premicellar region DELTAH(W-->W+S) were determined as functions of both the additive and the surfactant concentrations in order to evaluate the interaction parameters between the additive and the surfactant molecules. It has been observed that pair, triplet, quadruplet, and quintuplet interaction parameters between unlike solute molecules contribute to DELTAH(W-->W+S). Because of the narrow range of the surfactant and the alcohol concentrations, the values of the parameters are rather uncertain. However, they change regularly with the number of carbon atoms in the alkanol alkyl chain. In the postmicellar region the measurements were made at a given additive concentration as functions of the surfactant concentration. Generally, DELTAH(W-->W+S) changes with the surfactant concentration according to the distribution of the additive between the aqueous and the micellar phases. Whenever possible, the experimental data were rationalized using a previously reported model for the additive distribution between the aqueous and the micellar phases which permits simultaneous determination of the distribution constant and of the standard enthalpy of transfer; from these, the standard free energy and entropy of transfer and, therefore, the thermodynamics of solvation in the micellar phase were derived. The present data are compared to those in hydrogenated surfactants, and the effect of the nature of the hydrophobic apolar moiety of both the additive and the surfactant is discussed.