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 superc ritical CO2, we present a series of molecular dynamics (MD) simulations on the "interfacial'' behaviour of ions, of uncomplexed extractant molecules a nd of their complexes: the tri-n-butyl phosphate (TBP) and calixarene ligan ds and the calixarene . Cs(+)Pic(-), UO2(NO3)(2)TBP2 and Eu(NO3)(3)TBP3 com plexes. The simulations demonstrate the importance of interfacial phenomena in ion extraction to CO2. Water and CO2 do not mix and display an interfac e where all free calixarene and about 65% of the TBP ligands adsorb. Monoch arged calixarene . Cs(+)Pic(-) complexes are also surface active and form a n unsaturated monolayer. This contrasts with the more hydrophilic di- and t rivalent cation complexes of TBP, which move from the "interface'' to the a queous phase and induce important solvent mixing. Simulations on the uncomp lexed Cs(+)Pic(-) salt reveal the interfacial activity of the Pic(-) anion, widely used in extraction experiments. Some systems are also compared at t he chloroform/water interface simulated in the same conditions.