Mechanisms for lowering of interfacial tension in alkali/acidic oil systems 1. Experimental studies

Experimental studies are conducted in order to determine the physicochemical mechanisms responsible for lowering of interfacial tension in alkali/acidic oil systems. A well-defined model acidic oil is selected for controlled design of experiments, thus enhancing verification of known and unknown mechanisms. Ionic strength, pH, alkalinity, acid concentration and acid type were investigated for their effect on interfacial and surface tension. We show that interfacial tension goes through an ultralow minimum with NaOH (initial pH) at constant ionic strength. Unionized acid is shown to be present in the system at alkali concentrations where interfacial tension is ultralow. The results presented here suggest that the unionized acid adsorbs onto the interface together with the ionized acid, resulting in ultralow transient and equilibrium interfacial tension. Current theories do not consider adsorption of unionized acid on the interface, and, as a result, they do not adequately describe the effect of pH. A preliminary investigation of buffers shows that buffering the aqueous pH against dilution will also buffer the interfacial tension against dilution.