Retrograde transition in the phase behaviour of surfactant-oil-water systems produced by an oil equivalent alkane carbon number scan

Abstract

The phase behaviour of surfactant-oil-water systems is affected by the so-called formulation variables, i.e. by the nature of the components or their physicochemical characteristics. One of the formulation variables is the nature of the oil, often rendered through the alkane carbon number (ACN), or the equivalent alkane carbon number (EACN) when the oil phase is not an alkane. In most cases the decrease in the oil ACN contributes to the increase in the interfacial interaction between the surfactant and the oil, which results in a WI→WIII→WII transition, similar to that observed when the water salinity is increased. However, in some cases the Winsor II phase behaviour is never reached with the EACN decrease, and a WI→WIII→WI so-called retrograde transition is exhibited instead. Such an anomalous case is analysed here for a system containing a commercial non-ionic polyethoxylated surfactant, a mixture of n-heptane (ACN 7) and benzene (EACN 0), and water. The high performance liquid chromatography analysis of the different phases indicates that the surfactant oligomer partitioning between phases is affected by the alkane benzene mixture. The retrograde transition due to the increase in benzene is shown to arise from the strong increase in the partitioning of lipophilic and balanced oligomers into the more aromatic oil phase, with the remaining surfactant, in particular the interfacial mixture, becoming more hydrophilic. A phase diagram bidimensional (EACN-water-to-oil ratio) mapping indicates that this phenomenon occurs in very limited situations.