Theoretical calculations of extraction selectivity: Alkali cation complexes of calix[4]-bis-crown6 in pure water, chloroform, and at a water/chloroform interface

Abstract

We report theoretical calculations of ion extraction selectivity by ionophores, based on molecular dynamics simulations coupled with the free energy perturbation technique. This method is applied to the Calix[4]-bis-crown6 (L) ionophore, which displays remarkable selectivity for Cs+ over Na+ extraction from an aqueous to a chloroform phase. Using a thermodynamic cycle, we model the cation extraction selectivity of L from water to chloroform and calculate a peak for Cs+, in agreement with the experiment. This high Cs+ ionophoricity is accounted for mostly by differential solvation effects, with standard 1–6–12 pairwise potentials without need of “special π interactions” with the ionophore. The effect of a picrate (Pic−) counterion on structures and selectivities is investigated. Finally, we report simulations on the L ionophore free and on the LCs+ and LCs+Pic− complexes at the water/chloroform interface. We find that all these species are “adsorbed” at the interface like surfactants instead of diffusing spontaneously to the organic phase. © 1996 by John Wiley & Sons, Inc.