Heat capacities and volumes of the mixed micellar system cetyltrimethylammonium bromide and 2-butoxyethanol in water

Abstract

The active mixture cetyltrimethylammonium bromide (CTAB) and 2-butoxyethanol (BE) is very effective in cosolubilizing water and light hydrocarbons. This mixed system was studied through measurements of the apparent molar volumes and heat capacities of both solutes in water. Each solute undergoes micellization or some microphase transition in water, which is enhanced in the presence of the third component. CTAB in water also undergoes a post-micellar transition at about 0.15 mole kg −1, which is reflected in heat capacities but not in volumes, and which has been attributed to enhanced counterion binding. In the presence of BE at 0.1 mole kg −1 this transition is shifted to lower CTAB concentrations (0.06 mole kg −1). These observations are consistent with small-angle neutron-scattering studies. The reciprocity theorem for thermodynamic functions and a chemical equilibrium model for mixed micellar systems have been extended to mixtures of CTAB micelles and BE and predict correctly some of the observed trends in the transfer functions. In experiments where both solutes are kept at a fixed molar ratio (BE/CTAB = 5, 10, and 20), depending on the overall concentration, the present results show a predominance of CTAB micelles containing BE and BE aggregates containing CTAB.