usually require a cosurfactant to form reverse micelles (6-The method used to form a reverse micellar phase determines 18). Cosurfactants are some additives that are generally the amount of water solubilized in the organic phase. The effect understood to be amphiphilic molecules assisting the emulsiof cosurfactant and salt on the water solubilization capacity per fying property of surfactants . Based on this definition, mole of surfactant was determined for two methods of solubilizadifferent compounds such as alcohols with relatively long tion: contact with an excess aqueous phase and the titration chains, benzene, carbon tetrachloride, and nitrobenzene are method. The two methods are intrinsically different. While in the considered cosurfactants (19).
titration method, there is a transition from a Winsor IV system Reverse micelles can be formed in an organic phase by the to either Winsor I or II system, in the contact method, the expericontact method or by the titration method. The experimental ments are always in a Winsor II system. When the titration method was used at low salt concentrations, or even without salt, the water procedure for the contact method is depicted in Fig. and solubilized increased with the concentration of cosurfactant, at that for the titration method in Fig. .
low cosurfactant concentrations. When the contact method was
Contact method. A bulk excess aqueous phase is conused, the water solubilized per mole of surfactant decreased with tacted with an organic solvent containing the surfactant and an increase in the cosurfactant concentration. When titration was cosurfactant. The excess aqueous phase must contain an used, at a fixed cosurfactant-to-surfactant molar ratio, several solelectrolyte to prevent the transfer of the surfactant from the ubilization capacities were obtained at low salt concentrations, organic phase to the aqueous phase (20). After mixing and whereas a single solubilization capacity was found when the titrant settling, the organic phase is in equilibrium with an excess had a higher salt content. With the contact method, a single solubilization capacity was observed for all salt concentrations at which aqueous phase, thus forming a Winsor II system (1). The reverse micelles were formed. When the same salt solution was amount of water solubilized is obtained by measuring the used as the titrant in the titration method and the initial excess water content of the organic phase, usually by Karl Fischer aqueous phase in the contact method, a larger amount of water titration. The composition of the water pool is determined was solubilized by the latter method. α§ 1997 Academic Press by the exchange equilibrium of ions and solutes between the Key Words: reverse micelles; microemulsions; water uptake; phases.
surfactants.
Titration method. An aqueous solution is titrated into an organic solvent containing a surfactant and a cosurfactant. The maximum amount of aqueous solution that can be soluinside the reverse micelles (5). The contact method, on the