Thickness and fluctuations of free and adsorbed liquid films

Effective mesoscopic Hamiltonians with the thickness of the adsorbed liquid films as a collective variable have been widely used in the study of adsorbed systems. In the present work, we show that the intrinsic surface of a liquid-vapor interface provides a very accurate way to evaluate the instantaneous film thickness in computer simulations. This film thickness follows with quantitative accuracy the predictions of simple model Hamiltonians, even for films as thin as one monolayer, and the effective interfacial potential has the simplest exponential form with a surprising accuracy, from a single monolayer to very thick films. For both the free liquid slabs and the adsorbed films and despite of the low vapor density, we have found that the fluctuations associated with the evaporation of particles to the vapor and their condensation in the liquid layer give an important contribution to the probability distributions of the liquid film thickness in our canonical (NVT) simulations.