LIQUID-LIQUID ION EXTRACTION - INFLUENCE OF THE WATER-CONTENT OF THE ORGANIC-PHASE ON THE ION EXTRACTION SELECTIVITY - AN MD AND FEP STUDY OF CALIX[4]CROWN-6 ALKALI CATION COMPLEXES IN BINARY CHLOROFORM-WATER MIXTURES

In liquid-liquid extraction systems, a large amount of water is dragge d together with the extracted complexed ion into the receiving organic phase. The nature of this co-extracted water and its effect on the io n extraction selectivity are not well characterized. We report a serie s of molecular dynamics simulations on two chloroform-water binary mix tures, corresponding, respectively, to the saturation and supersaturat ion of water in chloroform. In the absence of solute, the most diluted solution contains only dimers of water, whereas in the most concentra ted one, water aggregates in ''droplets'' of 5-9 and 9-12 H2O molecule s. This contrasts with solutions containing the Na+ Pic(-) complexes o f 1,3-alternate dimethorcycalix[4]arene-crown-6, where water aggregate s around the Pic(-) counterion. The role of ''dragged water'' on the N a+/Cs+ extraction selectivity was investigated by MD-FEP studies in th e two binary mixtures. We find that Cs+ is preferred, in agreement wit h experiment, but the selectivity decreases markedly as the humidity o f the organic phase is increased. Based on these data we emphasize the role of the counterion on the water dragging to the organic phase and on the modulation of the ion extraction selectivity.