Wetting behavior of associating binary mixtures at attractive walls: A lattice Monte Carlo study A. Patrykiejew, 1 L. Salamacha, 1 S. Sokolowski, 1 and O. Pizio 2 1 Faculty of Chemistry, MCS University, 20031 Lublin, Poland 2 Instituto de Quimica, UNAM, Mexico Distinto Federal, Mexico Received 2 January 2003; published 20 June 2003 The lattice gas model is used to study the effects of molecular association on the wettability of surfaces with attractive walls by binary symmetric associating mixtures. The model assumes that the adsorbate particles occupy a regular cubic lattice of sites and that the interactions between adsorbate particles involve only the first nearest neighbors. The energies of interaction between the pairs of like particles are the same, while the only interaction between a pair of unlike particles is due to association. Only the formation of dimers is allowed and the energy of association is finite. The particles are subject to the surface, van der Waals-like potential, assumed to be the same for both components. The model is studied with the help of the Monte Carlo simulation method in the grand canonical ensemble. Only the ground state properties are treated analytically. It is demonstrated that, in general, molecular association hinders wetting. In particular, in the systems with nonzero wetting temperature, the increase of the association energy leads to the increase of the wetting temperature and for sufficiently high energy of association the mixture does not wet the surface at all. When the system is expected to exhibit complete wetting at the ground state, the film formed by strongly associating mixtures wets the surface only at sufficiently low temperatures, below the dewetting temperature. It is demonstrated that the dewetting temperature increases with the strength of the surface potential as well as with the increase of the association energy. DOI: 10.1103/PhysRevE.67.061603 PACS numbers: 68.08.Bc, 68.08.De, 67.70.+n, 05.70.Np I. INTRODUCTION The behavior of nonuniform binary mixtures in contact with a wall is a problem of great importance and is not quite well understood 1–7. The main difficulties arising here are connected with a rather large number of parameters that de- fine the interactions in nonuniform mixtures as well as with the multiplicity of thermodynamic paths that can be used. Even in the simplest possible lattice model with only nearest neighbor interactions, there are three parameters, which rep- resent interaction energies between different pairs, and two parameters determining the strength of fluid-solid interaction for each component 4. Under experimental conditions, the interactions between different pairs of adparticles are differ- ent. Also, the interaction of each component of the mixture with the solid substrate is different, and hence, one compo- nent is usually favored. The same assumption is also met in a majority of theoretical and computer simulation studies performed for adsorption of binary mixtures 5,6,8 –11. A particular problem that has recently been studied theo- retically and with help of computer simulations concerns the adsorption of the so-called symmetric binary mixtures, char- acterized by the same interactions between pairs of like par- ticles and by different energy of interaction between a pair of unlike particles, on nonselective, i.e., such that both compo- nents interact with the solid with the same energy, single walls 5,7,12 and in slitlike pores 13,14. The wetting be- havior of symmetric binary mixtures that are in contact with a nonselective wall has been also studied by Dietrich and Schick 5, by Schmid and Wilding 7, and quite recently by Bucior et al. 12. It has been demonstrated that the surface films formed by mixtures, which undergo a demixing transi- tion in the bulk, may exhibit quite complex behavior. In par- ticular, various surface transitions have been found, includ- ing first-order and continuous demixing and prewetting transitions between films of different composition. It is known that bulk associating binary mixtures exhibit quite different phase behavior than ordinary mixtures 15– 19. In particular, molecular association gives rise to the ap- pearance of the so-called close loop liquid-liquid immiscibil- ity and the existence of lower and upper critical solution temperatures. Also, the gas-liquid condensation in associat- ing mixtures has been found to be considerably different 16,20. In the systems with moderate association energies the condensed phase has been found to undergo gradual de- mixing transition at the temperature below the tricritical point, which is the onset of the  line. Thus, at very low temperatures the gas-liquid condensation leads to the mixed condensed phase, while at higher temperatures it leads to a demixed condensed phase. In both cases the transition is of first order. Only above the tricritical point the demixing is a second-order transition. In strongly associating symmetric mixtures, the condensed phase is highly ordered and demix- ing does not occur. In consequence, the  line does not exist and the condensation always occurs between two mixed phases, via a first-order transition 20. The effects of confinement on the behavior of associating binary mixtures has been theoretically studied, but only in the framework of a mean-field-like density functional ap- proach 19. We are not aware of any study aiming at the elucidation of the effects of association on the wetting be- havior of binary mixtures that are in contact with a solid. The studies of wetting at fluid-fluid interfaces, between two liq- uid phases, have demonstrated that the presence of associa- tion, or chemical equilibrium among the components, can lead to reentrant wetting as well as to dewetting transitions 21. In this work we apply a very simple lattice gas model of a PHYSICAL REVIEW E 67, 061603 2003 1063-651X/2003/676/06160311/$20.00 ©2003 The American Physical Society 67 061603-1