We investigate the low density behaviour of fluids that interact through a short-ranged attraction together with a long-ranged repulsion (SALR potential) by developing a molecular thermodynamic model. The SALR potential is a model of effective solute interactions where the solvent degrees of freedom are integrated-out. For this system, we find that clusters form for a range of interaction parameters where attractive and repulsive interactions nearly balance, similar to micelle formation in aqueous surfactant solutions. We focus on systems for which equilibrium behaviour and liquid-like clusters (i.e., droplets) are expected, and find in addition a novel coexistence between a low density cluster phase and a high density cluster phase within a very narrow range of parameters. Moreover, a simple formula for the average cluster size is developed. Based on this formula, we propose a non-classical crystal nucleation pathway whereby macroscopic crystals are formed via crystal nucleation within microscopic precursor droplets. We also perform large-scale Monte Carlo simulations, which demonstrate that the cluster fluid phase is thermodynamically stable for this system.

The adsorption of a Lennard-Jones fluid in an ideal slit pore is studied using weighted density functional theory. The intrinsic Helmholtz free-energy functional is separated into repulsive and attractive contributions. Rosenfeld's accurate fundamental measure functional is employed for the repulsive functional while another weighted density functional method is employed for the attractive functional. This other method requires an accurate equation of state for the bulk fluid and an accurate pair-direct correlation function for a uniform fluid, determined analytically or numerically. The results for this theory are compared against mean-field density functional theory and grand canonical ensemble simulation results, modeling the adsorption of ethane in a graphite slit. The results indicate that the weighted density functional method applied to the attractive functional can offer a significant increase in accuracy over the mean-field theory.

Activated carbons are amorphous microporous graphitic materials formed (or activated) from a variety of organic precursers using high-temperature steam or acids. The possibility of modifying the activation process to create smaller or larger pores, from nanometers to microns in width, tailored to adsorb specific molecules or classes of molecule make activated carbons important industrial adsorbents. For the physical chemist they pose the challenge of understanding how gases adsorb in graphitic nanopores, that is, in restricted geometries, and of using that understanding to improve their characterization. One aim is to make predictions concerning the adsorption properties for a given material, i.e., a specific microstructure. In this paper we use molecular simulation methods, including Gibbs ensemble simulation, to determine new molecular models for nitrogen, methane, and carbon dioxide and grand canonical ensemble simulation (together with new experimental data for the adsorption of these gases on Vulcan at 298 K and up to 20 bar) to generate new adsorption isotherms for model carbon pores. These new data are used to calculate pore-size distributions for typical activated carbons. We find that at these temperatures the high-pressure carbon dioxide measurements reveal more micropore structure than the measurements of nitrogen and methane up to 20 bar, or carbon dioxide measurements up to 1 bar. We also investigate the ability of pore-size distributions (PSDs) obtained from one gas to predict the adsorption of the other gases at the same temperature. We find that carbon dioxide PSDs are the most robust in the sense that they can predict the adsorption of methane and nitrogen with reasonable accuracy.

A general thermodynamic model to investigate responsive adsorption processes in flexible porous materials.

Amorphous materials are usually characterized using nitrogen adsorption isotherms at 77 K taken at pressures up to 1 bar to obtain pore size distributions. Activated carbons are amorphous microporous graphitic materials containing pores which can range from nanometers to microns in width and which can, in principle, be tailored to adsorb specific molecules or classes of molecule by changing the method of preparation (the activation process). For the physical chemist, they pose the challenge of understanding how gases adsorb in graphitic nanopores, that is, in restricted geometries, and of using that understanding to improve their characterization. In this paper, we compare pore size distributions of an ultrahigh surface area activated carbon (AX21) determined from nitrogen adsorption measurements up to 0.6 bar at 77 K with those determined from carbon dioxide adsorption measurements up to 20 bar at 298 K. Our analysis employs grand canonical and Gibbs ensemble Monte Carlo simulations together with accurate site-site interaction models of the adsorbates. We find that the calculated pore size distributions for each adsorbate are quite different, and the adsorption of one gas can be estimated from the adsorption of the other gas to within an error of 25% at the highest pressures only. At lower pressures, we speculate that large errors are due to the behavior of nitrogen in carbon micropores in which diffusion is severely limited. To substantiate this speculation, we have calculated the self-diffusion coefficient for nitrogen at 77 K and carbon dioxide at 298 K in carbon slit pores using equilibrium molecular dynamics. The results suggest that nitrogen is diffusionally limited, and possibly frozen, in such pores whereas carbon dioxide remains mobile. We conclude that room-temperature carbon dioxide adsorption isotherms up to the saturation pressure could provide a more accurate characterization of carbon microstructure than nitrogen isotherms at 77 K up to 1 bar.