Phase Behavior of Two Types of Isotropic Surfactant Phases in a Brine/Sodium Dodecyl Sulfate/Hexanol/Hexadecane System

Abstract

Two types of isotropic surfactant phases (microemulsions) were found in a brine/sodium dodecyl sulfate (SDS) /hexanol/hexadecane system, and their phase behavior was systematically investigated.In the system of water/SDS/hexanol, lamellar liquid crystals (L.C.) coexisted with excess water (W) and oil (alcohol; Om) phases under the condition of an exact balance between hydrophile-lipophile constituents of the mixed surfactant (SDS+hexanol). The addition of salt caused the L.C. to turn into an isotropic surfactant phase (D') coexisting with W and Om, a reversed micellar solution phase. With the addition of hexadecane, the Om phase split into an excess oil phase (O) and surfactant phase (D) in which large amounts of oil and water, were solubilized; the latter is known as a middle-phase microemulsion. The solubilization of hexadecane in the D' phase was only slight, and its composition remained in the vicinity of brine-SDS-hexanol plane. Two three-phase regions, W+D+O and W+D'+Om (or D) were formed independently at higher temperature, but they overlapped and four coexisting phases (W+D'+D+O) were observed in a certain range of lower temperature.L.C., D', and D phases appeared at an exact balance of hydrophile-lipophile constituents of the surfactant in a diluted region, but the order of surfactant molecular assembly reverted from the L.C. to D phase by way of the D' phase.