Theory of interfacial tensions in multicomponent systems

A quantitative theory of interfacial tension in multicomponent systems has been derived from the scaling theory of critical phenomena. This theory is closely related to that previously presented for the quantitative interpretation of phase-volume behavior of such systems near critical points. The interfacial tension theory involves only one additional undetermined parameter once the phase-volume behavior has been determined. It should facilitate the organization and interpretation of interfacial tension data and can be used to interpolate between interfacial tension measurements. The theory has been applied, with good agreement, to experimental results in a simple, ternary, two-phase liquid mixture, as well as to multicomponent three-phase surfactant systems of the type employed in tertiary oil recovery. For the surfactant systems, a combination of the theory for the phase-volume behavior and that for the interfacial tension behavior provides a strong theoretical basis for the heretofore empirical relationship between interfacial tension and volume uptake of oil or water into the microemulsion phase. This empirical volume uptake function has been used extensively in the literature to characterize surfactant systems for enhanced oil recovery. More importantly, the comparison of the results for the surfactant systems with those of the ternary system separates the two important functions of surfactant action. One is associated directly with the surface active nature of the molecule and the other is associated with the solvent character of the material which makes critical points accessible at moderate temperatures and pressures.