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The critical properties of the Lennard-Jones fluid in slitlike pores of different widths have been studied by the Gibbs ensemble Monte Carlo method and the lattice gas model. Graphite pores up to 10 molecular diameters in width and similar pores with weaker solid−fluid interactions have been considered. Strong layering of the adsorbate in the graphite pores confirmed the applicability of the lattice gas model to such systems. The vapor−liquid phase diagrams for the confined fluids obtained with the two methods are in reasonable agreement with each other. Linear dependence of the critical temperature on the inverse pore width was found. It is demonstrated that the critical temperature may depend strongly on the strength of the sold−fluid interactions. The lattice gas model showed nonmonotonic dependence of the critical density on the pore width.

A proper analysis of thermal desorption spectra (TDS) can give important information about molecular properties of the adsorption system studied (bond energies of heterogeneous adsorption sites and their composition and structure, strengths of adspecies lateral interactions, role of adspecies mobility, etc.); however, this is complicated because all these factors are distributed over the total spectrum. To solve this problem, it is convenient to use a lattice model since this enables us to extend an approach of traditional chemical kinetics for nonideal reactionary systems in such a way that it jointly takes care of surface heterogeneity and adspecies lateral interactions. In this paper, this model is taken as a base to consider fundamental issues of TDS interpretation dealing with a heterogeneous distribution function of adsorption centers over bond energy, on one hand, and the number of peaks and shape of TDS, on the other hand. Moreover, we discuss the validity of phenomenological approaches to interpr...

In this work the two phase coexistence region for simple fluids confined within slit and cylindrical pores is investigated. The format of this study involves a comparison of the results obtained with the aid of lattice models and molecular dynamics simulations in the near-critical region, and it is shown that the shape of the phase diagram which may be observed depends not only on pore geometry but also on the relative strengths of the fluid-wall and fluid-fluid interaction potentials. In comparison of the results predicted by lattice-gas theory and molecular dynamics simulation, it is found that the lattice model correctly accounts for all of the general features of the phase diagram arising from fluid-wall interaction potential, pore size, and pore shape. In the critical region however there are quantitative disagreements between lattice theory and molecular simulation due primarily to the mean-field nature of the quasichemical approximation employed in the lattice-gas calculations. Modifications of the lattice model computations which improve the agreement with simulation are presented and discussed.

Real adsorbents are heterogeneous, therefore, the computation of the adsorption characteristics for any concentration of adsorbate must take account of the joint effect of the lateral interaction between adsorbed molecules and surface heterogeneity. The local coverage of any adsorption center depends on the adsorption capacity of this center and upon the local coverage of the neighboring centers. As a result, the distribution of the different types of adsorption centers plays an essential role. A comparison of the different approaches based on the fragment, cluster, and pair description of the surface structure is given here. A technique for increasing the precision of calculating adsorption characteristics is found. It uses cluster distribution functions and takes into account the mutual disposition of the different types of sites. As an example of a distributed system, multilayer adsorption is considered. A lattice-gas model for the case of capillary condensation in slitlike micropores gives results in accord with those obtained by molecular dynamics and Monte Carlo techniques. The possibility of using a simplified equation for multilayer adsorption on a heterogeneous surface is analyzed. This equation gives a good description of the experimental data. However, the model parameters are outside the region of their definition in the lattice-gas model. For an exact calculation ofthe adsorption characteristics on a small fragment of surface, a new method is proposed which allows us to consider fragments that are twice as large as in the well-known matrix technique.