Accumulation of host—guest ion complexes with different counterions at the water—supercritical CO2 interface: a molecular dynamics study

Abstract

The behavior of ion complexes at the water—supercritical carbon dioxide interface was considered by molecular dynamics simulations. The following complexes were studied: Cs+calix[4]crown-6, K+222 cryptate with chloride or dicarbollide (CCD–) counterions, the Sr2+18C6 complex with the picrate (Pic–) or perfluorooctanoate (PFO–) counterions, and the Cl–Tet
4+ complex with chloride counterions (Tet
4+ is a tetrahedral tetraammonium cation). The simulations demonstrate the analogy between aqueous interfaces with organic immiscible liquids and the CO2 phase. Water and supercritical CO2 are poorly miscible and form an interface. Most of the complexes are accumulated at the interface, instead of diffusing into the organic phase in which they should be more soluble. In addition, marked counterion effects are observed. The CCD–, Pic–, and PFO– anions are surface active and are concentrated at the interface, but show different relationships with the complexes. The formation of ion pairs is precluded by the very hydrophobic CCD– anions, which promote the extraction of cryptates as separated ion pairs to the CO2 phase. Conversely, the extraction of the Sr2+ ions with 18C6 proceeds via a co-complexation mechanism, including the formation of the Sr18C6(PFO)2, complex having a CO2 affinity. The mechanism of assisted ion transfer to the CO2 phase is discussed.