Ethers, crown ethers and 18-crown-6 K+ complexes at a water/SC-CO2 interface: a molecular dynamics study

Abstract

We present a molecular dynamics study of 18C6, 15C5 crown ethers and their acyclic polyether analogues at the water/supercritical-CO₂ interface. The aqueous and CO₂ components form distinct phases, separated by an interface, where in all systems, the ethers are found to concentrate. Simulations of the inclusive K⁺⊂18C6 Pic⁻ complexes led to decomplexation of K⁺ and accumulation of the crown ethers on the CO₂ side of the interface, while Pic⁻ anions stack in the aqueous phase. When the K⁺ cations are constrained to form inclusive complexes with 18C6, all complexes concentrate at the interface, be the counterions free to move, or constrained to coordinate to K⁺. In the presence of nitric acid, modeled by equimolar mixtures of HNO₃, NO₃⁻ and H₃O⁺ forms, the ethers remain surface active, as does the neutral form of the acid. These simulations demonstrate the importance of interfacial phenomena in assisted ion extraction to supercritical-CO₂.