Effect of hydration ratio on the degree of counterion binding and pH distribution in reverse micelles with aqueous core

Abstract

The effects of the hydration ratio, the addition of an electrolyte, and the pH of the stock aqueous solution on the degree of counterion binding, the surface charge density, the concentrations, and the pH distributions in the water pool solubilized by reverse micelles of ionic surfactants in an organic solvent are theoretically investigated. The Poisson-Boltzmann equation along with a site binding model based on the dissociation equilibria of surfactant molecules is employed to determine the electrical potential distribution inside the water pool. Two cases are considered: In Case I, the water pool is assumed to be present only in the interior of the reverse micelles; in this case the Poisson-Boltzmann equation combined with the mass balance equations for the ionic species does not have a unique solution in the potential but has unique solutions in the surface charge density, the degree of counterion binding, and the ionic species concentrations distributions. In Case II, the water pool is assumed to be in equilibrium with a large reservoir of the aqueous stock solution; in this case the solution in the potential is shown to be unique. In the absence of a buffer, the results obtained in Cases I and II for the surface charge density, the degree of counterion binding, and the ionic species concentrations are found to be marginally different from each other. The degree of counterion binding slightly increases, while the absolute surface charge density slightly decreases with increasing hydration ratio and increasing electrolyte concentration. The decrease of the dielectric constant with decreasing hydration ratio leads to an additional increase in the degree of counterion binding. However, in the presence of a buffer, the pH distributions for Cases I and II differ significantly. The calculated results for Case II of the local pH at the reverse micellar surface show qualitative and quantitative agreement with the few experimental results available in literature.