Adsorption Of Lennard-Jones Fluid Research Articles

Constructing a new closure theory based on the third-order Ornstein-Zernike equation and a study of the adsorption of simple fluids

The third-order Ornstein-Zernike equation (OZ3) is used in the construction of a bridge functional that improves over conventional liquid-theory closures (for example, the hypernetted chain or the Percus-Yevick equations). The OZ3 connects the triplet direct correlation C((3)) to the triplet total correlation h((3)). By invoking the convolution approximation of Jackson and Feenberg, we are able to express the third-order bridge function B(3) as a functional of the indirect correlation γ. The resulting expression is generalized to higher-order bridge terms. This new closure is tested on the adsorption of Lennard-Jones fluid on planar hard surfaces by calculating the density profiles and comparing with Monte Carlo simulations. Particular attention is paid to the cases where molecular depletion on the substrate is evident. The results prove to be highly accurate and improve over conventional closures.

Effects of Graphene Layer Size on the Adsorption of Fluids on Graphitized Thermal Carbon Black. A Computer Simulation Study

The adsorption of Lennard-Jones fluids (argon and nitrogen) onto a graphitized thermal carbon black surface was studied with a Grand Canonical Monte Carlo Simulation (GCMC). The surface was assumed to be finite in length and composed of three graphene layers. When the GCMC simulation was used to describe adsorption on a graphite surface, an over-prediction of the isotherm was consistently observed in the pressure regions where the first and second layers are formed. To remove this over-prediction, surface mediation was accounted for to reduce the fluid–fluid interaction. Do and co-workers have introduced the so-called surface-mediation damping factor to correct the over-prediction for the case of a graphite surface of infinite extent, and this approach has yielded a good description of the adsorption isotherm. In this paper, the effects of the finite size of the graphene layer on the adsorption isotherm and how these would affect the extent of the surface mediation were studied. It was found that this finite-surface model provides a better description of the experimental data for graphitized thermal carbon black of high surface area (i.e. small crystallite size) while the infinite-surface model describes data for carbon black of very low surface area (i.e. large crystallite size).

Adsorption of Lennard-Jones Fluids in Carbon Slit Pores of a Finite Length. A Computer Simulation Study

The adsorption of simple Lennard-Jones fluids in a carbon slit pore of finite length was studied with Canonical Ensemble (NVT) and Gibbs Ensemble Monte Carlo Simulations (GEMC). The Canonical Ensemble was a collection of cubic simulation boxes in which a finite pore resides, while the Gibbs Ensemble was that of the pore space of the finite pore. Argon was used as a model for Lennard-Jones fluids, while the adsorbent was modelled as a finite carbon slit pore whose two walls were composed of three graphene layers with carbon atoms arranged in a hexagonal pattern. The Lennard-Jones (LJ) 12–6 potential model was used to compute the interaction energy between two fluid particles, and also between a fluid particle and a carbon atom. Argon adsorption isotherms were obtained at 87.3 K for pore widths of 1.0, 1.5 and 2.0 nm using both Canonical and Gibbs Ensembles. These results were compared with isotherms obtained with corresponding infinite pores using Grand Canonical Ensembles. The effects of the number of cycles necessary to reach equilibrium, the initial allocation of particles, the displacement step and the simulation box size were particularly investigated in the Monte Carlo simulation with Canonical Ensembles. Of these parameters, the displacement step had the most significant effect on the performance of the Monte Carlo simulation. The simulation box size was also important, especially at low pressures at which the size must be sufficiently large to have a statistically acceptable number of particles in the bulk phase. Finally, it was found that the Canonical Ensemble and the Gibbs Ensemble both yielded the same isotherm (within statistical error); however, the computation time for GEMC was shorter than that for canonical ensemble simulation. However, the latter method described the proper interface between the reservoir and the adsorbed phase (and hence the meniscus).

Isotherms for Monolayer Adsorption of Lennard-Jones Fluids onto Flat Surfaces

Using computer-simulated results for the radial distribution function of two-dimensional Lennard-Jones fluids, with intermolecular potential separated as in the Weeks-Chandler-Andersen perturbation theory, and the two-dimensional Steele approximation, we propose a very simple equation for the isotherms for adsorption of simple fluids onto flat surfaces. Previous theoretical equations and computer simulation results are reviewed and compared. A first attempt to compare experimental and theoretical results is made in order to show the influence of the choice of the intermolecular potential parameters.

Density-functional theory for inhomogeneous fluids: Adsorption of binary mixtures.

The nonlocal-density-functional theory for hard-sphere mixtures proposed in an earlier paper [Phys. Rev. A 42, 3382, (1990)] is used to investigate the structure and the thermodynamics of binary simple liquid mixtures adsorbed on substrates. We study (i) the adsorption of Lennard-Jones fluids in slit-shaped pores with varying pore size, (ii) the selective adsorption of liquid argon-methane mixtures onto a planar graphite surface at constant pressure, (iii) the density profiles of charged hard spheres in the vicinity of a highly charged hard wall (primitive model of the electrical double layer). We compare our calculations to computer-simulation data and to the predictions of other recent theories. In all cases, packing effects are very well described by the theory, and most discrepancies can be ascribed to the mean-field treatment of attractive forces.