RETROGRADE TRANSITION IN THE PHASE-BEHAVIOR OF SURFACTANT OIL-WATER SYSTEMS PRODUCED BY AN ALCOHOL SCAN

The phase behavior 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 alcohol effect, which accounts for the type and concentra tion of alcohol. In most cases the addition of a lipophilic alcohol co ntributes to the increase of the amphiphile mixture lipophilicity at t he interface, which results in a WI --> WIII --> WII transition, simil ar to the-one observed when the water salinity is increased. However, in some cases the Winsor II phase behavior is never reached with the a lcohol content increase, and a WI --> WIII --> WI so-called retrograde transition is exhibited instead. Such an anomalous case is analyzed h ere for a system containing a commercial nonionic polyethoxylated surf actant, n-heptane, water, and n-pentanol, the concentration of the lat ter playing the role of the formulation variable. HPLC analysis of the different phases indicates that the surfactant oligomer partitioning between phases is affected by the alcohol content. The retrograde tran sition due to the increase in alcohol content is shown to come from th e strong increase in the partitioning of lipophilic and balanced oligo mers into the oil phase, with the remaining surfactant, in particular the interfacial mixture, becoming more hydrophilic.