Effect of coexistent hydrocarbon phase on the volume behavior of adsorbed film and micelle of dodecyltrimethylammonium chloride

Interfacial tension (γ) between aqueous dodecyltrimethylammonium chloride (DTAC) solution and benzene was measured as a function of pressure (p) and concentration. The ∂γ/∂p was observed to change discontinuously at the critical micelle concentration; this indicates that the micelle formation of DTAC in the aqueous solution coexisting with benzene can be treated like the appearance of a macroscopic phase. It was shown by drawing the π vs. A curves that hydrocarbon, such as benzene, cyclohexane, and hexane, make the adsorbed film of DTAC expand. The volume behavior of the micelle with benzene molecules solubilized was found to bear a strong resemblance to that of the adsorbed film at the water/benzene interface. The difference in the molar volume value of adsorbed DTAC among the coexistent hydrocarbon phases was attributed to the difference in the contribution of the hydrocarbon molecules to the interfacial excess volume; the number of the solubilized hydrocarbon molecules was evaluated to be one or two a micelle.