Properties of adsorbed water layers and the effect of adsorbed layers on interparticle forces by liquid bridging

The potential of molecular dynamics (MD) simulation for the study and prediction of particle/particle and particle/wall interaction in the wide context of technology has been explored. The present study concerns the nature of adsorbed water and its effect on the interaction between two surfaces. Computer models of two opposing (1,0,-1) crystal surfaces of a-quartz (dimensions 5.49 x 4.91 nm) were constructed and up to 1500 water molecules positioned between the surfaces. The simulations were performed in the NVT ensemble in "math mode" at a temperature of 300 K. The axial profiles of density and mobility (the latter resolved in planar and axial components) in the adsorbed layers were studied. The separation between the crystal surfaces was varied, monitoring the adsorbed layer morphology and the forces acting on the crystals. Most of the simulations shown are with 1500 molecules between the plates, giving around 3.1 adsorbed monolayers, corresponding to a relative saturation (humidity) of 67% according to the BET isotherm. The density profiles show an ordered packing of molecules in the first two adsorbed layers with density peaks considerably higher than in bulk water and a low molecular mobility. The density tails off to zero, and the mobility rises to above that of bulk water at the surface of the adsorbed layer, which was clearly defined but undulating. Determination of the forces acting on the crystals was difficult due to strong fluctuations on a short time scale, so only simulations for long times yielded statistically significant average forces. At a surface separation of 3 nm, spontaneous bridge forming took place, paired with significant attractive forces between the crystals. The nature of the bridge is discussed. The observed bridging and resulting surface/surface force are shown to be roughly consistent with expectations based on macroscopic theory represented by the BET isotherm, the Kelvin equation (using the surface tension of bulk water), and a bridging force calculated from pressure-deficiency and surface tension contributions.