Molecular Simulation of Capillary Phase Transitions in Flexible Porous Materials
Vincent K. Shen, Daniel W. Siderius, Nathan Mahynski
We used at-histogram sampling Monte Carlo to study capillary phase transitions in deformable adsorbent materials. Specifically, we considered a pure adsorbate fluid below its bulk critical temperature within a slit pore of variable pore width. The instantaneous pore width is dictated by a number of factors, such as adsorbate loading, reservoir pressure, fluid-wall interaction, and bare adsorbent properties. In the slit pores studied here, the bare adsorbent free energy was assumed to be biparabolic, consisting of two preferential pore configurations, namely the narrow pore (NP) and large pore (LP) configurations. Four distinct phases could be found in the adsorption isotherms. We found a low-pressure phase transition, driven primarily by capillary condensation/evaporation and accompanied by adsorbent deformation in response. The deformation can be a relatively small contraction/expansion as seen in elastic materials, or a large-scale structural transformation of the adsorbent. We also found a high-pressure transition driven by packing effects, which tend to expand the material. The adsorption isotherms and osmotic free energies can be rationalized by considering the relative free energy differences between the basins of the bare adsorbent free energy.
, Siderius, D.
and Mahynski, N.
Molecular Simulation of Capillary Phase Transitions in Flexible Porous Materials, The Journal of Chemical Physics, [online], https://doi.org/10.1063/1.5022171
(Accessed July 27, 2021)