Interfacial tension minima in oil + water + surfactant systems. Effects of salt, temperature and alkane in systems containing ionic surfactants
In systems consisting of an n-alkane and a dilute ionic micelle-forming surfactant in aqueous electrolyte it is possible to attain very low oil/water interfacial tensions. These tensions frequently pass through a minimum as the salt concentration, alkane chain length or temperature is varied. We present a thermodynamic treatment of the tension variation and show under what conditions minima can arise. The minimum with respect to salt concentration is seen to result when the effective degree of dissociation of surfactant in micelle and at the oil/water interface are equal (and probably close to zero). At the temperature corresponding to a minimum tension the entropy change on transferring a mole of surfactant (and other associated species) from solution to the oil/water interface is approximately equal to the entropy of formation of micelles containing a mole of surfactant.