Enthalpies of Mixing of Some Primary Hydrogenated and Fluorinated Alcohols and Sodium Dodecanoate Aqueous Solutions

Abstract

The enthalpies of transfer from water to the surfactant solutions ΔH(W → W + S) of some hydrogenated and fiuorinated alkanols and of sodium dodecanoate NaL were determined. In the premicellar region the measurements were carried out 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 is shown that in this region, pair, triplet, and quadruplet interaction parameters between unlike solute molecules contribute to ΔH(W → W + S). Within the large uncertainty with which these parameters are determined because of the very narrow surfactant and alcohol concentration interval investigated, these parameters change regularly with the number of carbon atoms in the alkanols alkyl chain. In the post-micellar region the measurements were made at a given additive concentration by systematically changing the surfactant concentration. As expected, ΔH(W → W + S) increases with the surfactant concentration for all alcohols with the exception of pentanol and hexanol for which at higher surfactant concentrations ΔH(W → W + S) decreases. In addition, for the investigated systems a minimum, which becomes more important the longer the alkyl chain length, is present at NaL 0.08 m. Experimental data (with the exception of perfluoroethanol and propanol for which all experimental points have been considered) ranging between this concentration and the CMC were rationalized using a previously reported model for the additive distribution between the aqueous and the micellar phases. From this model the distribution constant and the standard transfer enthalpy (and, then, the standard free energy and entropy) are simultaneously obtained. The absolute values of all properties are smaller for the hydrogenated alcohols with respect to the fluorinated ones.