Molecular processes of ion effects on aqueous nanofilm rupture

Abstract

Much progress has been made in the understanding and application of specific ion effects; however, a detailed interpretation of film stability regarding the ion effect has not yet been achieved. This research aims at developing an understanding of the physico-chemical colloidal aspects in an endeavor to acquire an ability to predetermine specific ion effects on nanofilm rupture. Despite the series of reports published regarding the surfactant-free films, the experimental technique proved nearly to be unable to provide further convincing interpretation towards this issue. Molecular dynamics (MD) technique was applied to investigate the effect of salt ions on film stability in various concentrations with pair-additive and many-body polarizable water–ion potentials to validate with certainty the rupture process of aqueous nanofilms in the absence and presence of salts. Properties of the liquid film including dipole moment, molecular distribution profiles, together with a detailed quantitative analysis of film rupture, potential energy, evidences e.g. ion–water binding energy, Dipole Autocorrelation Functions (DAFs) and disjoining pressure isotherm data were examined. The DAFs of water within the film were observed to be stronger in salt films than in a pure water film. The results show that salt ions destabilize the nanofilms at low concentrations, while the ability of salt to break the films depends on the strength of the ion–water interaction and the molecular partition at the film surfaces.