Oil Chain-Length Effect on Wetting Transitions in Ternary Water + Oil + Surfactant Mixtures

Abstract

In the three-liquid-phase coexisting region of the system water + tetradecane + diethylene glycol monohexyl ether (C 6 E 2 ), the middle surfactant-rich phase undergoes a sequence of wetting transitions comprising nonwetting → partial wetting → complete wetting at the interface of oil-rich and water-rich phases as the temperature is raised from its lower critical consolute temperature toward its upper critical consolute temperature (J. Chem. Phys. 1993, 98, 4830). In this study, the effect of oil chain length on the wetting transition in the system water + C 6 E 2 + oil (dodecane, decane, and octane) is investigated by interfacial tension measurements. A sequence of wetting transitions, nonwetting → partial wetting → complete wetting, of the middle C 6 E 2 -rich phase is observed in all three systems while the system temperature increases. A decrease of the oil chain length in the system water + C 6 E 2 + oil would make the three-liquid-phase body shrink and lead the system to approach a tricritical point. The effect of the system water + C 6 E 2 + oil approaching its tricritical point on the wetting behavior of the middle C 6 E 2 -rich phase is discussed.