Critical Phenomena in a Surfactant/Water/Oil System. Basic Study on the Correlation between Solubilization, Microemulsion, and Ultralow Interfacial Tensions

The phase equilibrium in C8H17(OCH2CH2)3OH/water/decane system was studied at 13.6, 15.8, 21.5, 26.0, and 30.0 °C. An upper critical solution point for the surfactant and oil phases, and a lower critical solution point for the surfactant and water phases were found. Strong critical opalescence was observed in the vicinity of the critical points. In a temperature range between two critical points, a three-phase region appeared due to the superposition of three miscibility gaps, i.e., 1) surfactant+water; 2) surfactant+oil, and 3) oil+water phases. The correlations between the phase equilibria and the ultralow interfacial tensions were straightforward. An ionic surfactant/cosurfactant/brine/oil system gave similar phase equilibria as the nonionic surfactant system, but the effect of temperature was the reverse.