Molecular simulation of disjoining-pressure isotherms for free aqueous thin films

Abstract

We present canonical-ensemble molecular-dynamics (MD) simulations of disjoining-pressure isotherms for symmetric, free aqueous thin films. For such symmetric films, the disjoining pressure is purely attractive. Lifshitz's theory, based on continuum dispersion forces, predicts that the disjoining pressure varies as an inverse cube of film thickness, with a constant of proportionality that can be calculated within the framework of this theory. Our MD results indicate that Lifshitz theory, which assumes a slab geometry for the water density profile and neglects the fluid structure, underpredicts the disjoining pressure by about 50 times for films ranging from about I to 2 nm at 479 K. To investigate more closely actual experimental conditions, we also perform simulations of water films surrounded by inert gas molecules. The additional gas component adds an extra thermodynamic degree of freedom to the system, allowing for the chemical potential of the water in the external liquid reservoir to be maintained constant, thus mimicking actual experimental conditions more closely. Inclusion of inert gas at high pressure leads to a disjoining-pressure isotherm that is about twice as large as that without the inert gas for pressures in the liquid reservoir about 2 orders of magnitude larger than the vapor pressure of water at 479 K. Finally, we qualitatively show that upon decreasing the added inert-gas pressure, we obtain a different set of thin films with a smaller magnitude of the disjoining pressures