Phase Behavior Studies of Two Model Surfactant Systems

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Abstract
Phase behavior studies were carried out for two systems containing pure surfactants but exhibiting behavior similar to that of commercial petroleum sulfonates. One system contained the isomerically pure surfactant sodium-8-phenyl-n-hexadecyl-n-sulfonate (Texas 1). The other contained sodium dodecyl sulfate (SDS). Additional components used in both systems were various pure short-chain alcohols, NaCl brine and n-decane. Aqueous solutions containing surfactant, cosurfactant, and NaCl were studied over a wide range of compositions with polarizing and modulation contrast microscopy as well as the polarized light screening technique. Viscosity measurements were conducted on selected scans of the Texas 1 system. Maxima and minima of the scans were correlated with textural changes observed with microscopy. The aqueous solutions were contacted with equal volumes of n-decane, and phase behavior and interfacial tensions were determined. The middle microemulsion phase was found to be oil continuous close to the upper phase boundary and water continuous close to the lower phase boundary. Both the Texas 1 and SDS systems showed similar behavior in that the middle microemulsion phase was observed over the entire range of surfactant concentrations studied.

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