Molecular simulations of disjoining pressures and surface tensions of free liquid films

Molecular-dynamics (MD) simulations of free Lennard-Jones (LJ) films are performed to calculate disjoining-pressure isotherms, pi(h). We compare our MD results with the classical Hamaker theory that is based on dispersion forces but assumes a slab geometry for the density profile and completely neglects fluid structure and entropy. The disjoining pressures obtained from our MD simulations are negative and about an order of magnitude larger than the classical Hamaker theory. A density-functional theory, that relaxes the inherent assumptions of the Hamaker theory and imparts the fluid an approximate structure, results in disjoining pressures that underpredict the MD simulations only by a factor of 3.;Subsequently, we extend our MD simulations to calculate pi( h) for water films. In this case, pi is, again, negative and Lifshitz theory, based on continuum dispersion forces, underpredicts the pi by more than an order of magnitude at an elevated temperature of 479 K. Additionally, we simulate water films surrounded by inert-gas molecules that allow the chemical potential of water in the external liquid reservoir to be kept constant and results in pi values that are about twice as large without the inert gas.;To mimic experiments, we perform Monte-Carlo (MC) simulations of surfactant-stabilized free thin films surrounded by inert-gas molecules using LJ potentials for all the species. In contrast to pure solvent and solvent/inert-gas films, the pi values obtained for surfactant stabilized thin films are positive. Additionally, upon doubling the surfactant concentration in the reservoir, a different isotherm with slightly larger values of pi is obtained. However, the difference between the two isotherms is obscured by the accuracy of the simulations.;Finally, we address the problem of ion specificity in the surface-tension values for aqueous electrolytic solutions using MD simulations. Similar to actual experiments, we find that NaF gives a higher increase in surface tension than NaCl for model ionic potentials with explicit water molecules. On the other hand, approximate theories based on a modified Poisson-Boltzmann approach using primitive water predict an opposite salt-specific behavior. Even an accurate calculation (using MD) of salt solutions with primitive water predicts that NaCl gives a larger increase in surface tension than NaF due to the neglect of the strong water structuring around the solvated ions.