Selected biologically relevant ions at the air/water interface: a comparative molecular dynamics study

Interfacial behavior of selected biologically and technologically relevant ions is studied using molecular dynamics simulations employing polarizable potentials. Propensities of choline, tetraalkylammonium (TAA), and sodium cations, and sulfate and chloride anions for the air/water interface are analyzed by means of density profiles. Affinity of TAA ions for the interface increases with their increasing hydrophobicity. Tetramethylammonium favors bulk solvation, whereas cations with propyl and butyl chains behave as surfactants. The choice of counter-anions has only a weak effect on the behavior of these cations. For choline, sodium, chloride and sulfate, the behavior at the air/water interface was compared to the results of our recent study on the segregation of these ions at protein surfaces. No analogy between these two interfaces in terms of ion segregation is found.