Interfacial tension minima in oil + water + surfactant systems. Effects of salt and temperature in systems containing non-ionic surfactants

Abstract

Oil/water interfacial tensions have been determined in systems containing alkane, aqueous NaCl and N-dodecyl polyoxyethylene non-ionic surfactants. The tensions are frequently very low (ca. 10^–3 mN m^–1) and pass through a minimum as the salt concentration, temperature or alkane chain length is varied. By the use of thermodynamics it is shown that the minimum with respect to salt concentration can arise through the competing effects on tension of the positive adsorption of surfactant and the negative adsorption of salt at the oil/water interface. It is assumed that surfactant adsorption is independent of salt concentration; it is not necessary to suppose that the sign of the surface excess of surfactant changes as a tension minimum is traversed. It is shown that a minimum in tension with respect to temperature can occur when the entropy of micelle formation and of formation of the plane oil/water interface (both expressed per mole of surfactant) are equal.