RELATIONSHIP BETWEEN MICROEMULSION PHASE-BEHAVIOR AND MACROEMULSION TYPE IN SYSTEMS CONTAINING NONIONIC SURFACTANT

In systems containing the nonionic surfactant C12E5, heptane, and 0.01 M aqueous NaCl, the effect of surfactant concentration on emulsion type with respect to temperature has been studied by conductivity, in relation to the equilibrium distribution of both monomeric and aggregated surfactant in microemulsion systems. At high enough concentrations (e.g. 5 wt % or 84 mM initially in oil) emulsions invert from o/w to w/o at temperatures close to where middle-phase microemulsions are formed (Winsor III systems) and the oil-water interfacial tension is minimum. As the surfactant concentration is reduced, emulsions pass progressively from those which invert to those which remain highly conducting at all temperatures. The results are discussed in relation to Bancroft's rule and to the partitioning of both monomeric and aggregated surfactant.