MECHANISM OF ULTRALOW INTERFACIAL TENSIONS IN DILUTE SURFACTANT-OIL-BRINE SYSTEMS.

This paper reports on the phase behavior of aqueous preparations of 4-(1 prime -heptylnonyl)benzenesulfonate (SHBS) and sodium di-(2-ethylhexyl(sulfosuccinate (AOT), alone or mixed with sodium dodecylsulfate (SDS), and their interfacial tensions against hydrocarbons. Results, which are interpreted in terms of the various surfactant-rich fluid microstructures that form in these systems, indicate that ultralow tensions are brought about not by surfactant monolayer adsorption but by the formation of a surfactant-rich third phase at the oil-water interface. The third phase consists of the hydrated surfactant liquid crystals which absorb hydrocarbon. Using well-characterized mixtures of SDS and SHBS, the deep tension-concentration minima reported with commercial petroleum sulfonate surfactants was mimicked. It is suggested that the minima are related to various fluid microstructures that form in preparations of surfactant mixtures and vary as a function of surfactant concentration