Structural interactions in the wetting and spreading of van der Waals fluids

Abstract

The structural interactions for the submonolayer of a van der Waals (vdW) fluid have been modeled with the two-dimensional vdW equation of state (Hill-deBoer isotherm). The cohesion and co-area parameters can be estimated from the critical temperature and pressure of the bulk fluid. The difference in the standard state chemical potentials is estimated from the substrate/ fluid/vapor Hamaker constant calculated with the Lifshitz theory and an interaction distance of a single molecule with the substrate. For the systems studied, this interaction distance was approximately equal to the distance from a flat substrate to the middle of a molecule lying flat on the substrate. The resulting adsorption or disjoining pressure isotherm is assumed to describe the submonolayer part of the isotherm until it intersects the isotherm predicted from the Hamaker-Lifshitz theory. The composite isotherm is used to predict the equilibrium film thickness, film pressure, initial spreading coefficient, equilibrium spreading coefficient, and contact angle. Good agreement was observed for all systems with polytetrafluoroethylene (PTFE) as the substrate and for fluids which do not hydrogen-bond with water as the substrate. This agreement gives confidence that this model represents the structural interations of fluids that have only vdW interactions. It is possible to model the interaction of a monopolar liquid with a polar substrate. The acid-base component of the interaction energy from the van Oss, Chaudhury, Good theory was added to the model. With this addition, the model can predict the film pressure and spreading of benzene and chloroform on water.