Computer Simulation Study of Adsorption, Isosteric Heat and Phase Transitions of Methane on Graphite
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COMPUTER SIMULATION STUDY OF ADSORPTION, ISOSTERIC HEAT AND PHASE TRANSITIONS OF METHANE ON GRAPHITE SHAOYI JIANG AND KEITH E. GUBBINS School of Chemical Engineering, Cornell University, Ithaca, NY 14853 ABSTRACT We report Canonical (NVTMC), Grand Canonical (GOMC) and Gibbs Ensemble Monte Carlo (GEMC) simulations for adsorption of methane on graphite. Lennard-Jones (LJ) potentials are used for the intermolecular interactions, and both structured and structureless (10-4-3) solid-fluid potentials are considered. Several sets of methane-methane L.I parameters have been used in the literature, and we compare results obtained with these sets throughout our simulations. The adsorption isotherm and isosteric heat curve are obtained at 77.5 K and found in good agreement with experiments. The commensurateincommensurate transition (CIT) of methane on a graphite substrate with periodically varying adsorbate-adsorbent potential at 40.0 K is studied and is in qualitative agreement with experiment. The effect of varying the corrugation of the fluid-wall potential on the commensurate and incommensurate phases is explored. The GEMC simulations have been carried out to study the vapor-liquid equilibrium (VLE) of a two-dimensional (2D) LJ fluid with system sizes up to 3000 particles. The effect of system sizes on the critical behavior is investigated. The GEMC method has also been successfully applied to study the VLE in 2D adsorbed films for the first time. INTRODUCTION The behavior of methane in carbons is of practical interest in connection with gas separation and purification, the development of storage media for natural gases, and in the behavior of methane in underground reservoirs. This system, particularly monolayer methane on graphite, is also of great interest from a theoretical point of view, since there are many interesting 2D phase transitions, such as 2D freezing transitions, commensurateincommensurate transitions and 2D VLE, all taking place at convenient temperatures. A considerable amount of experimental data exists. The adsorption of methane on graphite at temperatures around 77.5 K has been extensively studied experimentally [1]. Besides adsorption isotherms, isothermal calorimctric measurements of the differential enthalpy of adsorption were also carried out and shown as curves of isosteric heat against coverage [2]. The work described here was undertaken to achieve a better understanding of the way in which the model system relates to the observed phenomena and to determine the effect of the LJ parameters on reproducing the experimental results. The GCMC simulation method is well suited to studying adsorptions and is employed in this work to study the adsorption isotherm and isosteric heat curve for methane on graphite at 77.5 K. The detailed 2D phase diagram of this system [3] has been reported at temperatures between 20 and 100 K and up to monolayer coverage. It has been shown that there is a CIT between commensurate solid (CS) and dense solid (DS) phases below 50 K, and the DS always locks into a commensurate
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