Heat of solution of various alcohols in aqueous micellar solutions of hexadecyltrimethylammonium bromide as a function of surfactant concentration: The preferential solvation phenomenon

Abstract

The standard heat of solution of 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, cyclohexanol, benzylalcohol, and phenol has been determined at 298.15°K by microcalorimetry in aqueous trimethylhexadecylammonium bromide (CTAB) from infinite dilution to 0.12 mole/kg of surfactant concentration. Using the partition coefficient of the alcohols between water and CTAB micelles, the degree of solubilization of the solutes were evaluated and the standard enthalpy of transfer Δ H ∘ t from water to CrAB micelles calculated. The results obtained were compared to the corresponding data in water + sodium dodecyl sulfate (SDS) solutions. At low surfactant concentration above the critical micelle concentration the standard free energy and enthalpy of transfer are equal in both surfactant solutions for 1-pentanol, 1-hexanol, 1-heptanol, cyclohexanol, and benzylalcohol. The more negative Δ G ∘ t and Δ H ∘ t values obtained for phenol are interpreted as an evidence for ion-pair formation between phenoxyde ions and the cationic head groups at the micellar surface. At higher CTAB concentrations (0.05 ⩽ m s ⩽ 0.10 mole/kg) a sudden change of Δ H ∘ t is observed which is equal for all alcohols studied. Within the same CTAB concentration range, the CTAB + water system undergoes an important exothermic change of the heat of dissolution of the surfactant in water. Both phenomena may be interpreted as a consequence of a sphere to rod transformation or to an enhanced counterion binding. Comparing the values of the Δ G ∘ t function for the alcohols in CTAB and SDS solutions, a model of preferential solvation of ionic surfactants is suggested supported by an extrathermodynamic analysis of the salting constants of polar molecules in single-ion aqueous surfactant solutions. It is concluded that the counterions control to a large extent the solubilization capacities of ionic micelles.