Adsorption of binary mixtures on two-dimensional surfaces: theory and Monte Carlo simulations
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Adsorption of binary mixtures on two-dimensional surfaces: theory and Monte Carlo simulations F. O. Sanchez-Varretti • G. D. Garcia • P. M. Pasinetti • A. J. Ramirez-Pastor
Received: 17 November 2013 / Revised: 24 June 2014 / Accepted: 27 June 2014 Ó Springer Science+Business Media New York 2014
Abstract The adsorption of binary mixtures on square lattices is studied by combining theoretical modeling and Monte Carlo (MC) simulations in grand canonical ensemble. The adsorption thermodynamics is analyzed through the total and partial isotherms. Two theoretical models have been used in the present study: (i) the first, which we called cluster approximation (CA), is based on exact calculations of configurations on finite cells. An efficient algorithm allows us to calculate the detailed structure of the configuration space for m ¼ l l cells; and (ii) the second is a generalization of the classical quasi-chemical approximation (QCA) in which the adsorbate is a binary mixture of species a and b. Adsorbate–adsorbate lateral interactions are incorporated in the context of the two mentioned approximations. Results from CA and QCA are compared with MC simulations. Close agreement between simulated and theoretical data supports the validity of the theoretical models to describe the adsorption of mixed gases on two-dimensional surfaces. Keywords Adsorption Binary mixtures Monte Carlo simulations Quasi-chemical approximation
F. O. Sanchez-Varretti G. D. Garcia Facultad Regional San Rafael, Universidad Tecnolo´gica Nacional, Gral J. J. De Urquiza 340, C.P. M5602GCH San Rafael, Mendoza, Argentina P. M. Pasinetti (&) A. J. Ramirez-Pastor Departamento de Fı´sica, Instituto de Fı´sica Aplicada, Universidad Nacional de San Luis - CONICET, Eje´rcito de los Andes 950, D5700BWS San Luis, Argentina e-mail: [email protected]
1 Introduction In the context of surface adsorption, the study of the adsorbate–adsorbate interactions in adsorbed films on regular substrates has its own interest (Hill 1960; Clark 1970; Steele 1974) due, particularly, to the emerging adsorbate structures that arise on the surface. These structures have an effect on microelectronics fabrication, catalyst surfaces, chemical sensors and electrodes, surfaces undergoing corrosion (Rupprechter et al. 2004; Matsushima 2004), etc. Various theoretical approaches have been taken to describe monolayer adsorption in the context of the latticegas model (Hill 1960; Clark 1970; Binder and Landau 1980; Landau 1983; Tovbin 1991; Patrykiejew et al. 2000). For ideal, non-interacting particles, the systems are quite simple and usually analytically solved (Hill 1960). Very simple interactions like excluded volume (also called rigid spheres) can also be analytically studied. However for the simplest adsorbate–adsorbate interactions that involve nearest neighbors, complexity grows in such a way that the partition function can only be exactly (and analytically) expressed in very few cases (Ramirez-Pastor et al. 2000; Roma´ and Ramirez-Pastor 2004; Pinto et al. 2011; Phares and
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