Distribution of hydrophilic, amphiphilic and hydrophobic ions at a liquid/liquid interface: a molecular dynamics investigation

We report molecular dynamics studies on the interfacial distribution of ionic species of different size, shape and topology at a water/chloroform interface: hydrophilic K+ Cl-, K+ SCN- and K+ Pic(-) ions, amphiphilic ammonium NTMA(+) cations and farnesylphosphate FPH- anions, tetrahedral hydrophobic AsPh4+ and BPh4- ions, with different counterions. Contrasted distributions are observed. The K+ Cl- and K+ SCN- ions sit almost exclusively in the water phase, but SCN- is less 'repelled' than Cl- by the interface. The Pic(-) anions are partly adsorbed at the interface and dissolved in the water phase where they display remarkable pi-stacking interactions. Amphiphilic NTMA(+) cations or FPH- anions adsorb and dilute at the interface. Less expected is the high surface activity of symmetrical AsPh4+ and BPh4- ions, with marked counterion effects. The two ions fully adsorb at the interface in the AsPh4+ BPh4- salt, while in the Na+ BPh4- or AsPh4+ Cl- salts, they display an equilibrium between the organic phase and the interface. Crossed comparisons between the different solutions reveal the important role of counterions on the distribution of a given ionic species. These results are discussed in relation to experimental data. (C) 2000 Elsevier Science S.A. All rights reserved.