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 π-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.