RETROGRADE TRANSITION IN THE PHASE-BEHAVIOR OF SURFACTANT-OIL-WATER SYSTEMS PRODUCED BY AN OIL EQUIVALENT ALKANE CARBON NUMBER SCAN

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 vari ables is the nature df the oil, often rendered through the alkane carb on number (ACN), or the equivalent alkane carbon number (EACN) when th e oil phase is not an alkane. In most cases the decrease in the oil AC N contributes to the increase in the interfacial interaction between t he surfactant and the oil, which results in a WI-->WIII-->WII transiti on, similar to that observed when the water salinity is increased. How ever, in some cases the Winsor II phase behaviour is never reached wit h the EACN decrease, and a WI-->WIII-->WI so-called retrograde transit ion 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 h igh 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 mor e aromatic oil phase, with the remaining surfactant, in particular the interfacial mixture, becoming more hydrophilic. A phase diagram bidim ensional (EACN-water-to-oil ratio) mapping indicates that this phenome non occurs in very limited situations.