On the thermodynamics, structure and phase stability of the nonuniform fluid state

Abstract

This study is based on a simple and straightforward generalization of the van der Waals theory for the thermodynamics and structure of a nonuniform fluid postulated almost one century ago. The relationship of this generalized van der Waals theory to the currently popular perturbation theory for liquid-vapor surfaces is demonstrated. An application of this theory to the description of the Lennard-Jones fluid interface near the triple point is reported, and comparisons of the predictions with numerical Monte Carlo experiments are made. The generalized van der Waals theory is then used to study infinitesimal density fluctuations and their relation to the stability of the single phase fluid state and critical opalescence. Theories for describing nonequilibrium density variations in a fluid are discussed with particular emphasis on the diffusion approximation. Theoretical predictions using the generalized diffusion equation are presented and compared with a numerical molecular dynamics simulation of a Lennard-Jones fluid quenched into the unstable region of the phase diagram (spinodal decomposition). Finally, a formal statistical mechanical theory for nonuniform fluids is presented and related to the van der Waals type theories.