Phase behavior of associating two- and four-bonding sites Lennard-Jones fluid in contact with solid surfaces

We have studied the adsorption of associating fluids on solid substrates, i n the framework of the models with two and four bonding sites and with nona ssociative Lennard-Jones (LJ) attraction between fluid particles. The fluid -substrate interaction is chosen in the form of the 9-3 LJ-type potential. Our analysis involves the strongly adsorbing, graphitelike substrate, as we ll as several weaker model surfaces. Special attention is paid to the layer ing transitions, their relevance to the prewetting transition, and the prew etting transition on its own. We have evaluated the surface critical temper ature and made estimates for the critical temperatures of the layering tran sitions and of the wetting temperature, for each model in question. An insi ght into thermodynamic behavior of the models is given in terms of the phas e diagrams. Our calculations reveal that the details of the surface phase b ehavior of strongly associating fluids differ from the phase behavior for w eakly associating fluids and of; adsorbed, simple nonassociating fluid. Als o, we have analyzed structural changes in the adsorbed films during transit ions in question, considering the entire distribution of fluid particles an d a set of distributions for fractions of differently bonded species.