Viscoelasticity and apparent molar heat capacities of aqueous micellar solutions of hexadecyltrimethylammonium hydroxybenzoates

Abstract

Heat capacity measurements have been made on solutions containing the hexadecyltrimethylammonium cation with the ortho- and para-substituted hydroxybenzoate ions as counterions. The molality varied between 0.04 and 0.125 mole kg −1 and measured heat capacities refer to 30.0°C. The apparent molar heat capacities C p, ϕ of the two salts, denoted o-OHBzCTA and p-OHBzCTA, were derived. The found values of C p, ϕ were compared with the corresponding values for the sodium salts. The difference ΔC p, ϕ = C p, ϕ (OHBzCTA) − C p, ϕ (OHBzNa) should be nearly the same for the two anions in the absence of specific counterion interactions in the micellar solutions. However, this difference is significantly lower for the o-OHBz − ion than for the para-substituted ion. This indicates that the o-OHBz ion penetrates into the head group region, where it will be less hydrated than the p-OHBz remaining outside the palisade layer. The inclusion of the aromatic ring into the head group region would then favor the growth of micelles and accompanying viscoelasticity which is observed in o-OHBzCTA solutions from very low concentrations but not in solutions containing the para-substituted ion as counterion.