Molecular simulation of capillary phase transitions in flexible porous materials

Abstract

We used flat-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 and the large pore 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 excluded volume effects, which tends to expand the material and thus results in a large-scale structural transformation of the adsorbent. 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.

Coupling between adsorption and mechanics (and vice versa)

“…The osmotic framework was adapted to such cases [45], under the assumption that each configuration is rigid. For the same family of materials, by considering a free energy of the dry porous material which exhibits two wells, the features of the energy landscape (i.e., the depth of the two wells) were shown to impact the transitions between the two configurations and the phase transitions of the pore fluid [46].…”

In Situ Small-Angle Neutron Scattering Investigation of Adsorption-Induced Deformation in Silica with Hierarchical Porosity

“…The explicit calculation of the stresses and corresponding strains as a function of p / p 0 requires the combination of the thermodynamics of the system represented by the adsorption isotherm and solid mechanics depending on the pore geometry and the mechanical properties of the pore walls. While many theoretical and/or computational studies treat the detailed adsorbate–adsorbent interactions, and numerous works have been published on the deformation of the pore space as a result of internal and external pressures (poromechanics), the combined treatment of both aspects is more recent. − We have lately developed a general theoretical framework to describe the adsorption- and the strain isotherms for cylindrical mesopores by combining the Derjaguin–Broekhoff–de Boer (DBdB) theory, − the adsorption stress model, and the mechanical model of a cylindrical tube . The model delivered analytical equations for the axial and radial adsorption stresses and corresponding strains, and allowed the quantitative comparison of the calculated strains during adsorption of nitrogen at 77 K with experimental strain isotherms .…”

Relationship between pore-size distribution and flexibility of adsorbent materials: statistical mechanics and future material characterization techniques

“…Modifications to Eq. 7 (specifically, the bounds of the summation over N 1 -states) are necessary to account for coexisting phases at subcritical conditions [36, 37], such as in an isotherm that displays adsorption-desorption hysteresis. Lastly, we note that the adsorption isotherm in Eq.…”