PHASE-BEHAVIOR OF NONIONIC SURFACTANT OIL-WATER SYSTEMS CONTAINING LIGHT ALKANES

The phase behavior of nonionic ethoxylate surfactant/light alkane/water systems is reported in detail. In compressible liquids such as propane, phase transitions which are normally induced by changing temperature or salinity can also be accomplished with pressure A complete transition from a lower to middle to upper phase microemulsion with pressure is reported for the first time in propane. Widely accepted trends in surfactant phase behavior as a function of the alkane carbon number (ACN) of the oil component reverse themselves in the light alkanes butane, propane, and ethane. This pattern occurs in both reverse micelle systems (small water-to-oil ratio) and Winsor microemulsion systems (water-to-oil ratio near unity). The observed pressure and ACN effects can be explained qualitatively in terms of the miscibility gaps of the binary phase diagrams and quantitatively in terms of enthalpic and entropic interactions between the surfactant tails and the compressible solvent.