Wetting transitions at solid-gas interfaces

The nature of the wetting transition at solid-gas interfaces in systems interacting with Lennard-Jones forces is investigated. When the adsorbed film is liquid, it is found that the wetting transition is quite generally first order within the mean-field or van der Waals approximation. Excluded-volume effects which force the adsorbate density to zero near the substrate, effectively increasing the range of the fluid-substrate interaction, are shown to be responsible for this effect. In fact, when excluded-volume effects are neglected, second-order transitions are also found to be possible. Generally, it is found that the wetting transition is second order if the effective range of the fluid-substrate interaction is smaller than that of the fluid-fluid interaction and can be first order when it is of longer range.