(242a) Use of the Transition Matrix Monte Carlo Simulation Method to Study Metastability and Phase Transitions In Gas Adsorption Processes

In light gas adsorption in porous materials, an understanding of the key features of the adsorption isotherm and any engineering exploitation of those features is often reliant on identification of the stable and metastable parts of the phase diagram. For example, in the quintessential gas adsorption process where capillary phase transitions are present in the adsorption isotherm, pore-filling occurs at a limit of stability and pore-evacuation occurs at a stable coexistence state point. Identification of these stability limits and, more importantly, the location of phase transitions is problematic in molecular simulation since it requires simulation of state points in the metastable region. In simple Monte Carlo simulations, such as those in the grand canonical ensemble with normal Boltzmann sampling, it is difficult to probe far into the metastable region. More sophisticated techniques are necessary to probe these regions of the phase diagram.

Recently, the Transition Matrix Monte Carlo (TMMC) technique [1] has come to prominence as a method for simulating gases and liquids and directly identifying phase equilibria for the simulated fluid. TMMC differs from normal Monte Carlo simulation in that it utilizes information from all attempted transitions between microstates, even those that are rejected, to construct the probability distribution for an entire set of microstates. When used in the grand canonical ensemble, TMMC provides the probability of observing a certain density at a given temperature and pressure. From this probability distribution, one can compute properties of the macrostate such as total density, pressure, the grand free energy and identify stable and metastable states within the macrostate [2]. When combined with histogram reweighting [3], one can reconstruct an entire density-pressure isotherm from TMMC simulation at a single pressure-temperature state point.

We apply TMMC to light gas adsorption in porous materials such as graphitic carbon in both infinite-length and finite-length configurations[4]. In doing so, we demonstrate the usefulness of the technique in the computation of adsorption isotherms and the identification of limits of stability, metastable regions, and the equilibrium phase transition for the confined fluid. Additionally, we use TMMC to examine prewetting phase transitions [5] sometimes found for adsorbents of intermediate-strength. Overall, we aim to introduce TMMC as a useful tool for studying fluid behavior in porous materials.

[1] Fitzgerald, M., et al., Europhys. Lett., 46, 282 (1999), Errington, J.R., J. Chem. Phys., 118, 9915 (2003)

[2] Shen, V.K. and Errington, J.R., J. Phys. Chem. B, 108, 19595 (2004)

[3] Ferrenberg, A.M. and Swendsen, R.H., Phys. Rev. Lett., 61, 2635 (1988)

[4] Siderius, D.W. and Gelb, L.D., Langmuir, 25, 1296 (2009)

[5] Ebner, C. and Saam, W.F., Phys Rev. Lett., 38, 1486 (1977), Errington, J.R., Langmuir, 20, 3798 (2004), Errington, J.R. And Wilbert, D.W., Phys. Rev. Lett., 95, 226107 (2005)