Interfacial tension of oil—brine systems in the presence of surfactant and cosurfactant

Abstract

The interfacial tension between bulk oil and brine phases containing a surfactant and a cosurfactant has been calculated using a multicomponent adsorption isotherm model that accounts for the different sizes of the solute molecules as well as for the solute—solvent and solute—solute interactions in the surface phase. The presence of appreciable amounts of electrolyte in these systems alters the equilibrium distribution of a solute between the two bulk phases because of the salting-out effect, and the calculations also account for this effect. The interfacial tension is found to decrease with increasing concentration for both ionic and nonionic surfactants. However, for fixed values of total concentrations of surfactant, the interfacial tension is found to be much smaller for nonionic surfactants than for ionic surfactants, since the head group is larger in the former case. At fixed values of the surfactant concentration, increasing amounts of cosurfactant and salt have the effect of lowering the interfacial tension. Other important parameters studied are the chain length of oil and the relative amounts of water and oil. The calculations for both ionic and nonionic surfactants indicate that the interfacial tension decreases as the chain length of surfactant increases, while it goes through a minimum as a function of cosurfactant chain length.