Demixing of binary water-chloroform mixtures containing ionophoric solutes and ion recognition at a liquid-liquid interface: A molecular dynamics study

We report a series of molecular dynamics simulations on the demixing of "homogeneous" binary water-chloroform mixtures containing species involved in the assisted ion extraction process. We consider an ionophore: L (L = 1,3-alternate calix4arene-crown6), uncomplexed salts of Cs+ and the LCs+ and LNa+ cation complexes with a lipophilic (Pic(-)) and a hydrophilic (Cl-) counterion, respectively, as being solutes. In all cases, the liquids separate rapidly, leading to two solvent slabs separated by a well-defined interface. However, the final state is very different, depending on the hydrophilic/hydrophobic balance of the solutes: the Cs+ and NO3- ions of the CsNO3 salt are completely immersed in the aqueous phase, whereas Pic(-) anions display a strong adsorption at the interface. The LCs+ complex and the free ligand L, although more soluble in the organic phase than in water, also display a surfactant like behavior. Similar conclusions are obtained when L, LCs+, Cs+ Pic(-), and Cs+ NO3- ions are simultaneously present in the solution. On the basis of free energy perturbation calculations on LM+ complexes, we calculate a marked Cs+/Na+ recognition by L at the interface. These results have important implications concerning the mechanism of ionophore assisted liquid-liquid ion extraction and recognition processes at the interface.