Density functional theories and molecular simulations of adsorption and phase transitions in nanopores

Abstract

The nonlocal density functional theory (NLDFT) of confined fluids is tested against Monte Carlo simulations by using the example of Lennard-Jones (LJ) fluid sorption in slit-shaped and cylindrical nanopores ranging from 0.3 to 10 nm in width. The fluid-fluid and solid-fluid parameters of the LJ potentials were chosen to represent several experimentally important adsorption systems: nitrogen and carbon dioxide in activated carbons, zeolites, and mesoporous molecular sieves of the MCM-41 type. Freezing in nanopores is discussed using the example of methane sorption in carbon at 111 K. Comparison with reference experiments is given when available. Two versions of NLDFT, the smoothed density approximation and the fundamental measure theory, are considered. It is shown that NLDFT approaches with properly chosen parameters provide quantitative agreement with the results of Monte Carlo simulations and reference experiments. Appreciable deviations are found in extremely narrow pores of less than two molecular diameters in width. In wider pores, NLDFT models can be used for quantitative predictions of reversible and hysteretic adsorption isotherms and analyses of the specifics of phase transformations, including the equilibrium and spinodal phase transitions.

Nitrogen Gas on Graphene: Pairwise Interaction Potentials

“…Ravikovitch et al have reported values of 0.202 kcal/mol for and 4.058Å for r 0 for a LJ CM-CM model without charges [44]. The is higher than ours, while the r 0 is smaller, but in reasonable agreement taking into account that they were acquired via very different methods.…”

Adsorption equilibria of single gas and gas mixture on homogeneous surfaces: a unified approach based on statistical thermodynamics developments. Part I: single gas adsorption

“…Some theoretical approaches are able to deal with the complexity of the adsorption phenomenon: molecular simulation methods like the Grand Canonical Monte-Carlo method (GCMC) [3] which is based on an equilibrium statistical treatment of the system or the density functional theory (DFT) [4] which is an approximate statistical mechanics method.…”

GCMC Simulations of Gas Adsorption in Carbon Pore Structures

“…In general, the gas-liquid phase transition in pores (capillary condensation) is a very interesting feature, which is observed when condensable gases are confined in small cavities. The phenomenon has been extensively studied in pores of various geometries both experimentally and theoretically (Evans & Tarazona, 1984 ;Ravikovitch et al, 2001 ;Nilson & Griffiths, 1999 ;Neimark et al, 2003). Capillary condensation takes place at pressures below the bulk condensation pressure at a given temperature.…”