Microemulsion Systems Based On A C-8/10 Alkyl Polyglucoside: A Reentrant Phase Inversion Induced By Alcohols?

The phase behavior of quaternary microemulsion systems with a technical-grade nonionic surfactant (C-8/10 alkyl polyglucoside) and different medium-chain alcohols (including the monoterpene geraniol) as cosurfactants is studied at 25 degrees C. Contour diagrams of the three-phase body at constant oil-to-water ratio have been mapped in terms of the mass Fraction variables gamma (surfactant + alcohol) and delta (relative amount of alcohol in the surfactant + alcohol mixture). Significant differences in the phase behavior of systems based on the pure alkyl glucoside beta-C(8)G(1) and those with the technical-grade C(8/10)APG are observed. Specifically, at high delta an anomalous shape of the three-phase region, or a second narrow three-phase region appears in systems with C(8/10)APG, which is absent in systems based on beta-C(8)G(1) Whereas at delta = 0.5 all systems exhibit a phase inversion (2) under bar-3-(2) over bar on increasing gamma, a retrograde transition (2) under bar-3-(2) under bar is observed at high delta. The anomalous shape of the three-phase body can lead to a phase sequence (2) under bar-3-(2) over bar-3-(2) under bar* with a reentrant (2) under bar* state, on increasing delta at constant gamma. It is argued that the reentrant (2) under bar* state is due to a preferential extraction of the more lipophilic oligomers of the technical-grade surfactant into the oil phase, causing the interfacial film to become more hydrophilic again, as delta is increased.