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

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(+)subset of18C6 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.