Effect of pressure on the adsorption and micelle formation of aqueous dodecyltrimethylammonium chloride-cyclohexane system

The interfacial tension of aqueous dodecyltrimethylammonium chloride (DTAC) solution-cyclohexane system was measured as a function of pressure at concentrations around the critical micelle concentration (cmc). The derivative of interfacial tension with respect to pressure was observed to change abruptly at the cmc. The volume changes associated with the adsorption of DTAC from its monomeric and micellar states and the volume of micelle formation were calculated and compared with the corresponding ones of the aqueous DTAC solution-hexane system. It was found that the molar interfacial excess volume of the adsorbed film is slightly larger than the molar volume of micelle in the case of the cyclohexane system, while in the case of the hexane system the former is approximately equal to the latter. Further, it was concluded that the micellar particles solubilize molecules of the oil coexisting in equilibrium with the aqueous solution. It was also proved in the case of the present system that the micelle formation can be treated thermodynamically as the appearance of a macroscopic bulk phase in the solution.