Ion exchange in reverse micelles

Abstract

The distribution and dynamics of alkali cations inside Na-AOT reverse micelles have been investigated using Monte Carlo and molecular dynamics simulations. Water is modeled using the extended simple point charge (SPC/E) model. Simulations were carried out for alkali salts of Li⁺, Na⁺, K⁺, and Cs⁺ placed into the aqueous core of the reverse micelle, for situations corresponding to one and three molecules of added salt. In all cases, we observe that the larger K⁺ and Cs⁺ ions exchange with the Na⁺ counterion; however, the smaller Li⁺ ion prefers to remains solvated within the core of the reverse micelle. Our study reveals that the oil-water interface of the Na-AOT reverse micelle has the greatest selectivity toward Cs⁺ followed by K⁺ and Li⁺. A model based on enthalpic contributions illustrates that the solvation energies of the different cations in water control the ion-exchange process. The hydration number of the first water shell for Li⁺ situated in the aqueous core of the reverse micelle with radius R = 14.1 Å was similar to that observed at infinite dilution in bulk water.