SOLUBILITY OF WATER IN WATER-IN-OIL MICROEMULSIONS STABILIZED BY CETYLTRIMETHYLAMMONIUM - EFFECTS OF THE SURFACTANT COUNTERION, THE NATURE AND COMPOSITION OF THE OIL, AND THE SALINITY OF THE DROPLETS

The solubility of water in water-in-oil microemulsions stabilized by cetyltrimethylammonium (CTA) has been investigated as a function of the nature of the surfactant counterion (bromide; CTAB/chloride; CTAC), the composition of the oil (chloroform and chloroform/heptane mixtures), the salinity of the droplets, and the nature of the salt employed to modify it. Changing the composition of the oil from neat chloroform to a 50% (v/v) mixture with heptane produces an increase of the water solubility in the microemulsions stabilized by CATB and decreases it in the CTAC microemulsions. Strong alterations of the solubility of water were found to occur upon mixing of the surfactants. Solubility increases abruptly on going from either CTAC- or CTAB-stabilized microemulsions to mixtures of both surfactants, reaching a maximum at CTAC mole fractions of 0.5 and 0.4 for solutions of the surfactants in chloroform and chloroform/heptane (3/1 v/v), respectively. Mixtures of CTAC and CATB in a solvent comprising 50% chloroform/heptane (v/v) show a different behavior. In this system, water solubility continuously decreases when the CTAC/CTAB ratio increases. Increasing the salinity of the droplets affects in an opposite fashion the solubility of water in the CTAC- and in the CTAB-stabilized microemulsions, irrespective of the nature of the salt employed. Solubility increases with salinity in the CTAC-stabilized microemulsions and decreases in the CTAB-stabilized microemulsions. Nevertheless, the magnitude of the effects observed is dependent on the nature of the salt employed to modify the salinity of the droplets. Results obtained from experiments in which equivalent salinities were reached by addition of different salts make it possible to differentiate salt effects from counterion-specific effects. The results obtained are explained in terms of the model of Shah and co-workers (Langmuir 3, 1086 (1987); J. Colloid Interface Sci. 120, 320 and 330 (1987)) by considering that, due to the stronger binding of bromide than of chloride to the surfactant heads at the micellar interface, water solubility in solutions of CTAB or CTAC in chloroform is determined by different factors: the curvature of the surfactant film (CTAB) and the interaction between droplets (CTAC). © 1993 Academic Press. All rights reserved.