Three-phase behavior in a mixed nonionic surfactant system

Abstract

The effect of monodisperse solubilities of each surfactant in an excess oil phase on the three-phase behavior was investigated in a water/octaethyleneglycol dodecyl ether (R12EO8)/tetraethyleneglycol dodecyl ether (R12EO4)/heptane system. The mid temperature of the three-phase region is defined as the HLB temperature. The HLB temperature is largely skewed to higher temperature in a dilute region due to the difference in the distribution of each surfactant between excess oil and microemulsion (surfactant) phases forming the three-phase body. Taking account of the monodisperse solubilities, the equation for the HLB temperature was obtained on the basis of geometrical calculation of a particular three-phase triangle. The equation well describes the three-phase behavior for a mixed surfactant system in a space of compositions and temperature. In the mixed surfactant system, the monodisperse solubility of R12EO8 in oil phase forming a three-phase body is monotonously increased with the rise in temperature, whereas that of R12EO4 is first increased and then is decreased. Consequently, the sum of both solubilities does not change greatly in a wide range of temperature.