A. J. Ramirez-Pastor

Monte Carlo study of dimer adsorption at monolayer on square lattices

The localized monolayer adsorption of interacting homonuclear dimers (AA) on square lattices is studied using a lattice-gas model. The effect of lateral interactions on the behavior of different thermodynamic quantities is considered. Phase diagrams (i.e. critical temperature versus coverage) are calculated using Monte Carlo simulation and finite-size scaling for both attractive and repulsive nearest-neighbors lateral interactions. Of special interest is the repulsive case where different ordered structures are observed, confirming the results given by Phares et al. [J. Phys. A: Math. Gen. 26 (1993) 6847] based upon exact transfer-matrix method for dimers on a semi-infinite square lattice.

Dimer physisorption on heterogeneous substrates

Abstract Physisorption of dimers on heterogeneous surfaces is studied by combining theoretical modelling, Monte Carlo simulation and experimental results. Monte Carlo simulation of dimer physisorption is carried out for substrates modelled in such a way that random, patchwise and intermediate adsorption site topographies can be generated. Simulated adsorption isotherms are used to test a model for adsorption of polyatomic molecules on heterogeneous surfaces, recently developed by Nitta et al.. Experimental adsorption isotherms for O2 and N2 adsorbed on zeolites 5A and 10X, as well as adsorption heats, are used to test the reliability of the simulation model. A simple theoretical form for the adsorption isotherm, based on the Fermi-Dirac approach, is finally proposed. Close agreement between simulated, theoretical and experimental results supports the validity of the proposed equation to describe interactions of diatomic gases with zeolites.

Dependence of the percolation threshold on the size of the percolating species

Site and bond percolation of k-mers of different structures and forms deposited on 2-D regular lattices is studied. In addition, the percolation threshold for percolating k-mers on a Bethe lattice is analytically obtained. By using finite-size scaling theory, the analysis of the results is performed in order to determine the behavior of the percolation threshold which exhibits an exponential decrease with the k-mer size. Characteristic parameters of that function are dependent not only on the form and structure of k-mers but also on the properties of the lattice where they are deposited. An expression for the percolation threshold as a function of the parameters of the problem is proposed and discussed.

Configurational entropy for adsorbed linear species (k-mers)

The configurational entropy of interacting linear molecules (k-mers) absorbed on a regular lattice is addressed through analytical as well as numerical methods. The general definitions for computational exact calculations of k-mers lattice-gas entropy are presented. In addition, theoretical basis for accurate analytical estimations of the entropy of reference states are given. The coverage and temperature dependence of the configurational entropy of interacting adsorbed dimers on one and two-dimensional lattices are obtained. A novel phase behavior of k-mers lattice-gas is shown and discussed.

Study of adsorption of binary mixtures on disordered substrates

The adsorption of binary mixtures on solid heterogeneous substrates is studied by Monte Carlo simulation in the framework of the lattice gas model. The energy of the surface has been modeled by considering two kind of adsorption sites, deep and shallow traps, forming square homogeneous patches of different sizes; these adsorption domains have been distributed either at random or in chessboard-like lattice to obtain simple heterogeneous topographies. The adsorption process has been monitored through total and partial isotherms and differential heats of adsorption corresponding to both species of the mixture, for different values of the parameters involved in the model (lateral interactions, energy gap between deep and shallow patches) and different topographies. A rich variety of behaviors is found and analyzed in the context of the lattice gas theory.

Scaling behavior in adsorption on bivariate surfaces and the determination of energetic topography

Adsorption of particles with repulsive nearest-neighbor interactions is studied through Monte Carlo simulation on bivariate surfaces characterized by patches of weak and strong adsorbing sites of size l. Patches can be either arranged in a deterministic chessboard structure or in a random way. Quantities are identified which scale obeying power laws as a function of the scale length l. Consequences of this finding are discussed for the determination of the energetic topography of the surface from adsorption measurements.

Surface diffusion of dimers: I repulsive interactions

We analyze the diffusion process of rigid homonuclear dimers (AA) adsorbed on a simple cubic (sc(100)) surface. The coverage dependence of the collective diffusion coefficient is obtained by means of Monte Carlo simulations in the framework of the Kubo-Green formalism. Different microscopic diffusion mechanisms are introduced and their influence in the collective motion have been investigated. Repulsive adsorbate-adsorbate interaction, JAA, is considered in order to analyze the influence of such parameter on the diffusion process. The behavior of the diffusion coefficient in the critical region is studied, where several ordered adsorbate structures appear depending on the values of JAA.

Influence of surface heterogeneities on the formation of diffusion-limited aggregates

Abstract In the present paper, we study the influence of surface energetic heterogeneities on the main features of fractal aggregates generated through a diffusional mechanism. The diffusion-limited aggregation (DLA) model was introduced by Witten and Sander (Phys. Rev. Lett. 47 (1981) 1400) and has stimulated growing interest in the study of a variety of nucleation and growth processes since then. In the DLA model, growth begins with a seed particle in the center of a two-dimensional lattice. Then, individual particles are launched uniformly from a launching circle, sufficiently big, and they perform activated random walk until they stick to the growing cluster. The DLA clusters formed in this way are fractal objects with a well determined fractal dimension df=1.72±0.02. In order to include surface heterogeneities, we have used a square lattice with two kinds of sites which are assembled in such a way that the resulting structures have patchwise topography. Lattices formed by collections of orderly localized patches of different sizes are generated. DLA clusters are clearly affected in their morphology due to the presence of the surface heterogeneities which is analyzed and explained in terms of the different energetic topographies.

Differential heat of adsorption in the presence of an order–disorder phase transition

We studied the coverage dependence of the differential heat of adsorption in the presence of an order–disorder phase transition, by employing theoretical mean-field and quasi-chemical approaches and Monte Carlo simulation, in the framework of the lattice-gas model. For a square lattice, with repulsive interactions between the nearest-neighbor adatoms, Monte Carlo reveals the existence of singularities in the differential heat at critical coverages, which are not observed through the theoretical approaches. The differential heat of adsorption appears as a sensitive quantity to the phase transition, allowing a very accurate determination of the phase diagram.

Critical behavior of long straight rigid rods on two-dimensional lattices: Theory and Monte Carlo simulations

The critical behavior of long straight rigid rods of length k (k-mers) on square and triangular lattices at intermediate density has been studied. A nematic phase, characterized by a big domain of parallel k-mers, was found. This ordered phase is separated from the isotropic state by a continuous transition occurring at an intermediate density theta(c). Two analytical techniques were combined with Monte Carlo simulations to predict the dependence of theta(c) on k, being theta(c)(k) proportional to k(-1). The first involves simple geometrical arguments, while the second is based on entropy considerations. Our analysis allowed us also to determine the minimum value of k (k(min) = 7), which allows the formation of a nematic phase on a triangular lattice.