Importance of interfacial phenomena in assisted ion extraction by supercritical CO2: a molecular dynamics investigation

In relation to the liquid–liquid extraction of cations from water to supercritical CO2, we present a series of molecular dynamics (MD) simulations on the " interfacial" behaviour of ions, of uncomplexed extractant molecules and of their complexes: the tri-n-butyl phosphate (TBP) and calixarene ligands and the calixarene·Cs+Pic−, UO2(NO3)2TBP2 and Eu(NO3)3TBP3 complexes. The simulations demonstrate the importance of interfacial phenomena in ion extraction to CO2. Water and CO2 do not mix and display an interface where all free calixarene and about 65% of the TBP ligands adsorb. Monocharged calixarene·Cs+Pic− complexes are also surface active and form an unsaturated monolayer. This contrasts with the more hydrophilic di- and trivalent cation complexes of TBP, which move from the "interface" to the aqueous phase and induce important solvent mixing. Simulations on the uncomplexed Cs+Pic− salt reveal the interfacial activity of the Pic− anion, widely used in extraction experiments. Some systems are also compared at the chloroform/water interface simulated in the same conditions.