Understanding adsorption/desorption hysteresis for fluids in mesoporous materials using simple molecular models and classical density functional theory

In this tutorial review, we discuss the use of classical density functional theory (DFT) to understand adsorption/desorption hysteresis phenomena for fluids confined in mesoporous materials. The emphasis is on lattice gas models, for which DFT is especially straightforward when studies of the fluid density distribution in two and three dimensions are required. The theme is to show that much of the experimentally determined hysteresis phenomena can be described using this modeling framework. Several examples have been used to illustrate this, including a simple duct pore, a duct inkbottle and a model of Vycor glass. We discuss hysteresis in single pores in terms of the metastability of the vapor phase in the pore and extend this to systems with pore size distributions. Inkbottle geometries are used to illustrate the phenomena of pore blocking and cavitation. The model of Vycor shows how the DFT formalism can describe systems with disordered interconnected pore structures that lead to type H2 hysteresis. The calculation of scanning curves using DFT is described and the relationship between scanning curves and pore interconnectivity is discussed. By weakening the surface field the DFT approach can be used to describe systems with partial wetting (e.g. water in carbon pores) and partial drying (mercury porosimetry). Finally, a dynamic mean field theory is introduced and used to study the dynamics of capillary condensation in the duct pore and, in particular, the nucleation of the liquid phase via the formation of a liquid bridge between the pore walls. (C) 2012 Elsevier Inc. All rights reserved.