Capillary Condensation and Freezing of Simple Fluids Confined in Cylindrical Nanopores

Abstract

We present a molecular simulation study aimed at understanding the phase behavior of pure simple fluids, when they are confined inside nanopores of cylindrical geometry. In this situation, new surface-driven phases can appear, and phase transitions typical of bulk systems (gas-liquid, freezing) can be shifted to different conditions. A fundamental understanding of these phenomena is necessary for applications in separations, catalysis and nanotechnology. Studies of these phenomena can also provide important insights on the effect of surface forces, confinement and reduced dimensionality on the phase behavior of host molecules. We have performed two independent, but directly related studies: (1) freezing of carbon tetrachloride within multi-walled carbon nanotubes (MWCNT) of different diameters, and (2) capillary condensation and freezing of krypton within templated mesoporous silica materials (MCM-41). MWCNT and MCM-41 are representative of materials with strongly and weakly attractive walls, respectively. In the first part of this project, the structure and thermodynamic stability of the confined phases, as well as the temperatures and the order of the phase transitions were determined using dielectric relaxation spectroscopy measurements and Monte Carlo simulations in the grand canonical ensemble. A rich phase behavior with multiple transition temperatures was observed for such systems. In the second part of this project we developed realistic, atomistic models of MCM-41 type materials that include pore surface roughness and morphological defects in agreement with experimental results. Grand Canonical Monte Carlo simulations show that these variables have a profound influence on gas-liquid and freezing transitions in confinement.