Interaction of enhanced oil recovery surfactants with model crude oil surfactants

Abstract Transient minima are often observed in the interfacial tension of enhanced oil recovery surfactants against crude oil. One possible explanation for the time-dependence is the interaction of surfactants from the crude oil with those from the aqueous phase at the interface. The suggested mechanism involves the diffusion of crude oil surfactants initially into the interface forming a mixed layer responsible for the low tension followed by solubilisation in the bulk aqueous solution. This mechanism has been investigated for a carboxybetaine surfactant with decane and decanoic acid as model oil and oil surfactant, respectively. Rheological properties and micellar sizes have been determined for two regimes corresponding to different degrees of enhancement of surfactant concentration at the interface. The lowest tensions are observed close to the boundary between viscous surfactant solutions containing large micelles and a two-liquid-phase region. The low tensions extend into the two-phase-composition range. The occurrence of low interfacial tensions is postulated to correspond to the compositions for which hydrocarbon solubilisation in the surfactant phase is a maximum.