Mass-action model for solute distribution between aqueous and micellar phases: Mixing enthalpies of alcohols and dodecyltrimethylammonium bromide solutions

Abstract

The mixing enthalpies of aqueous solutions of normal alkanols (from methanol to heptanol) and of dodecyltrimethylammonium bromide were measured taking the enthalpy of dilution of the surfactant solutions as the baseline of the mixing process. The measurements were generally made at a given alcohol concentration by systematically changing the surfactant concentration. In some cases measurements as a function of the alcohol concentration were also carried out. Below the CMC from the mixing enthalpies the pair and the triplet interaction parameters between alcohol and surfactant molecules were calculated. The trend of these parameters as a function of the number of carbon atoms in the alcohol alkyl chain is peculiar in that it is linear for the h RS pair parameter and of an exponential-type for h RSS, whereas the h RRS triplet parameter shows a minimum for butanol. Above the CMC the mixing enthalpies were rationalized using a previously reported model for the alcohol distribution between the aqueous and the micellar phases. From the resulting equation the distribution constant and the transfer enthalpy (and then the standard free energy and entropy) can be obtained at the same time. In the calculation of these quantities the role of the alcohol—surfactant interactions in the aqueous phase and of the displacement of the micellization equilibrium due to the added alcohol is pointed out. As predicted, the additivity rule always holds for the standard free energy of transfer but only holds up to butanol for enthalpy and entropy.