Charge Distribution in the Electrical Double Layer at the Oil–Water Interface

Interactions between dissolved ions in water and acidic, basic, and nonpolar crude oil components give rise to an electrical double layer at the oil–water interface. The properties of this electrical double layer, including distribution of ions, surface charge density, and surface conductance, exert a strong control over the wettability, emulsion flow, and frequency-dependent electrical conductivity of oil-bearing rocks. Several surface complexation models have been developed to describe the association of dissolved ions with surface groups on the oil–water interface, but these models do not capture the structure or properties of the electrical double layer. We used a surface complexation model developed for ice–water and silica–water interfaces and modified it to model the charge distribution within the electrical double layer at the oil–water interface. We validated the model by comparing predicted zeta potentials to measurements of oils with different acid and base numbers. We then computed the fractions of counterions in the Stern and diffuse layers for different salinities, pH levels, and acid and base numbers. Our results show that 96% of the counterions are in the diffuse layer in all cases. The low surface area of the oil–water interface likely prevents the conductivity of the diffuse layer from making any appreciable contribution to the conductivity of the oil-bearing rocks.