J. L. Riccardo

Capillary condensation in heterogeneous mesoporous networks consisting of variable connectivity and pore-size correlation

Heterogeneous three-dimensional mesoporous networks (A. J. Ramirez-Cuesta, S. Cordero, F. Rojas, R. J. Faccio and J. L. Riccardo, J. Porous Mater., 2001, 8, 61, ) constructed under the premises of the dual site–bond model have been used as probe substrates to study the effects of variable connectivity and pore-size correlation on the aspects of both hysteresis loops and primary sorption scanning curves. The shapes of the hysteresis loops obtained from sorption simulation in networks of diverse morphologies are compared and discussed. It is found that the precursor structural parameters of the Monte Carlo simulated networks together with the sorption algorithm used in this work, can lead to IUPAC types H1, H2 and H3-like hysteresis loops, depending on the values chosen for the pore-size distribution parameters and mean connectivity. Network morphology also influences greatly the mechanisms of sorption processes in poorly or highly size correlated porous substrates. Sorption results on these 3-D porous specimens help to visualize the extents of pore blocking (vapour percolation) and delayed adsorption (liquid percolation) phenomena and also to foresee the most appropriate methods to ascertain the structure of porous materials.

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.

Simulation of three-dimensional porous networks

Abstract Simulation of porous networks, with characteristics similar to those of real media, is essential for the study of capillary processes that take place within these substrata. The dual site-bond model (DSBM) provides a theoretical basis from which it is possible to adequately describe and simulate porous networks of diverse structural properties. Following the DSBM principles, heterogeneous 3-D cubic porous networks have been built by a Monte Carlo method. The desired topological properties of these substrata have been introduced by considering: (i) different sizes of the void entities (sites or cavities and bonds or throats); (ii) different connectivities ( C ) of the pore elements with their neighbours, i.e. the number of throats (bonds) that surround and connect a pore cavity (site) with its homologous entities is not constant throughout the network; (iii) geometrical restrictions, in the sense that the sizes of the bonds that meet into a site must be of such values as to avoid any mutual interference. The overlapping ( Ω ) between the site and bond distribution functions, the connectivity ( C ) and the geometrical restrictions ( G ), are the three fundamental factors that promote segregation effects in the substrate. For regular networks (i.e. those of constant C ) subjected to G and high Ω , it is found that big sites: (i) prefer big bonds as neighbours, and (ii) are less affected by geometrical restrictions than small ones. In turn, for irregular networks of varying C subjected to G and large Ω it is found that: (i) the smallest sites are linked to the biggest possible bonds thus acquiring a low connectivity, and (ii) the biggest sites adopt the maximum possible connectivity and allocate small and medium size bonds rather than large ones. All these particularities strongly influence the topology of a porous network and hence the repartition of fluids inside the pores during a capillary process.

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.

The diffusion-controlled annihilation reaction in random adsorptive fields

The kinetics of the A+A to 0 reaction on a correlated heterogeneous one-dimensional chain is studied. As a novel result it is found that the temporal behaviour of the density of A particles depends on the energetic topography of the surface when A particles are initially located preferentially on the more energetic sites, as happens in the case of thermodynamical equilibrium for the absorbate.

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.

Pore-level modelling of wetting on correlated porous media

The displacement of a non-wetting fluid by a wetting one in a porous medium is influenced, among other things, by two competing mechanisms: the flow of the wetting phase along crevices, giving the possibility of snapping off in throats; and its advance through the centres of the pore space under various pore- and throat-filling conditions, leading to a cooperative filling. The percolation process associated with these two mechanisms on porous networks in two and three dimensions is well understood; it is a classic bond percolation problem competing with an invasion percolation one. We present a three-dimensional pore-level model that describes these effects on a site and bond network representing pores and throats, respectively. The network elements may have various degrees of correlation among their sizes. Site and bond-size distributions may be any kind of function representing a real pore space. In this work, we are able to predict various kinds of patterns that arise when the two aforementioned mechanisms compete and to study the effect of the correlations strength on the onset of each pattern, revealing the strong influence of the topology of the network in determining which process will dominate. Buoyancy forces are not taken into account in the present work.

Temperature behavior of tracer diffusion on heterogeneous surfaces

The behavior of a tagged particle on a generalized heterogeneous surface is studied by Monte Carlo simulation. Using the dual site-bond model (SBM), the energetic properties of the heterogeneous substrates can be appropriately described through the site and bond energy distributions and the overlapping degree between them. The effect of the adsorptive energy topography on the tracer diffusion coefficient, as well as on the time behavior of the mean-square displacement of the adparticle, are analyzed for different temperatures and energy correlation degrees. The short-time behavior of tracer diffusion is highly sensitive to the surface energy structure. The possibility of using tracer diffusion analysis for a better characterization of the energetic topography of heterogeneous surfaces is discussed.

ADSORPTION OF POLYATOMICS: THEORETICAL APPROACHES IN MODEL SYSTEMS AND APPLICATIONS

The adsorption of polyatomics on one- and two-dimensional lattices is studied by combining theoretical modeling, Monte-Carlo (MC), simulations and their correspondence with experimental results. In one dimension, the rigorous statistical thermodynamics of interacting chains has been presented. With respect to two-dimensional adsorption, six different models to study non-interacting adsorbates have been discussed: (i) an extension to two dimensions of the exact thermodynamic functions obtained in one dimension; (ii) the Flory–Hugginss approximation and its modification to address linear adsorbates; (iii) the well-known Guggenheim–DiMarzio approximation; (iv) the fourth one is a new description of adsorption phenomena, based on Haldanes fractional statistics; (v) the so-called Occupation Balance, based on the expansion of the reciprocal of the fugacity; and (vi) a simple semi-empirical model obtained by combining exact one-dimensional calculations and Guggenheim–DiMarzio approach. In addition, the statistical thermodynamics of interacting polyatomics has been developed on a generalization in the spirit of the Bragg–Williams and the quasi-chemical approximations. Comparison with MC simulations and experimental adsorption isotherms are used to test the accuracy and reliability of the proposed models. Finally, applications to heterogeneous systems and multilayer adsorption are discussed.