Anomalous three-phase behavior in a water/octaethyleneglycol dodecyl ether/decanol system

Abstract

In water/nonionic surfactant/saturated hydrocarbon and brine/ionic surfactant/cosurfactant/saturated hydrocarbon systems, a W + D + O (excess water, surfactant, and excess oil phases) type three-phase region appears in a wide range of water/oil ratios when the hydrophile-lipophile property of surfactant is just balanced in a given system. On the other hand, a W + D′ + Om type three-phase region forms in a water/octaethyleneglycol dodecyl ether/decanol system as well as in a brine/ionic surfactant/cosurfactant system, where Om is an isotropic phase extended from an oil-surfactant axis. The D′ is a surfactant phase whose composition remains in the vicinity of the water-surfactant axis. The phase behavior of D′ is very different from that of an ordinary D phase which is known as a middle-phase microemulsion. AW + L.C. + O(Om) type three-phase region is present at lower temperature, where L.C. denotes lamellar liquid crystal. With a rise in temperature, it splits into two three-phase regions, W + D′ + Om and D′ + L.C. + Om via the W + D′ + L.C. + Om region. With a further increase in temperature, the composition of Om phase shifts toward the water-surfactant axis and at last D′ and Om phases merge. This anomalous three-phase behavior may be very common in a water/surfactant/amphiphilic oil system.