Importance of counter-ions in alkali and alkaline-earth cation extraction by 18-crown-6:: molecular dynamics studies at the water/sc-CO2 interface

We present a molecular dynamics study of concentrated solutions of K+ and Sr2+ cations and their complexes with 18C6 at the water/supercritical-CO2 interface, and compare picrate "Pic(-)'' vs. perfluorooctanoate "PFO-'' counter-ions at 305 and 350 K. The results allow to understand why K+ is poorly extracted in conditions where Sr2+ is transferred to the supercritical fluid. In all cases, the solutes are found to concentrate near the interface, with interesting differences, depending on the cation, counter-ion and extractant. With the two anions, the SrX2 salts are surface active, as a result of cation-anion pairing, while the distribution of the K+ ions depends on X-. The inclusive K+ subset of18C6 complexes partially decomplex, and decomplexation is more effective at 350 K than at 305 K. The most remarkable result is the extraction of Sr2+ subset of18C6,2PFO(-) complexes in which the anions axially co-complex the cation, while retaining CO2-philic interactions via their perfluorinated chain. This Sr2+ extraction is confirmed by several computational tests which point to the importance of concentration effects and agree with experiment. It contrasts with the formation of an interfacial film with Sr2+ subset of18C6,2Pic(-) complexes, assembled via bridging Pic(-)...Sr2+...Pic(-) and stacking Pic-...Pic(-) interactions.