THEORETICAL CALCULATIONS OF EXTRACTION SELECTIVITY - ALKALI CATION COMPLEXES OF CALIX[4]-BIS-CROWN6 IN PURE WATER, CHLOROFORM, AND AT A WATER CHLOROFORM INTERFACE/

We report theoretical calculations of ion extraction selectivity by io nophores, based on molecular dynamics simulations coupled with the fre e 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 wi th the experiment; This high Cs+ ionophoricity is accounted for mostly by differential solvation effects, with standard 1-6-12 pairwise pote ntials without need of ''special pi interactions'' with the ionophore. The effect of a picrate (Pic(-)) counterion on structures and selecti vities is investigated. Finally, we report simulations on the L ionoph ore free and on the LCs(+) and LCs(+)Pic(-) complexes at the water/chl oroform interface. We find that all these species are ''adsorbed'' at the interface like surfactants instead of diffusing spontaneously to t he organic phase. (C) 1996 by John Wiley & Sons, Inc.