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

Abstract

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⁺⊂18C6 complexes partially decomplex, and decomplexation is more effective at 350 K than at 305 K. The most remarkable result is the extraction of Sr²⁺⊂18C6,2PFO⁻ 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⁺⊂18C6,2Pic⁻ complexes, assembled via bridging Pic⁻⋯Sr²⁺⋯Pic⁻ and stacking Pic⁻⋯Pic⁻ interactions.