A chemical equilibrium model is proposed to describe the effects of acid content, pH and sodium ion concentration on the interfacial activity of crude oil in aqueous alkaline solutions. The model is based on an equilibrium among the dissociation of acids in the crude, the dissociation of the sodium salt containing the active species and the dissociation of water. It is shown that once the pH of the interface reaches the pKa of the acids, the interfacial tension (IFT) drops steeply and that further addition of sodium ions increases the IFT by shifting the equilibrium to form undissociated soap.
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Chan et al. (1982) studied this question.
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