Pressure Tensor of Adsobate in Nanoporous Materials: Molecular Simulation Studies.

摘要

Abundant experimental evidence suggests that adsorbates confined in nanoporous materials can exhibit high pressures, even if the systems are in equilibrium with a bulk phase of 1 bar pressure. Examples include the occurrence in the confined nanophase of high pressure chemical reactions, high pressure solid phases, high pressure effects in solid-liquid equilibria, effects on spectral properties, and the deformation of the solid materials.;With the aim of providing fundamental understanding of the high pressure effects in confined nanophases, we report Monte Carlo simulation studies of the pressure tensor of adsorbates confined in microporous materials. The pressure tensor components are calculated by both the mechanical and thermodynamic routes with the Irving-Kirkwood and Harasima definitions. We calculate the microscopic pressures as functions of spatial position. Although they cannot be measured experimentally, these local pressures contain the information of maximum value, and can be readily used to calculate various average pressures that are appropriate for different applications.;We find that the in-pore pressure is either enhanced or reduced depending on the nature of the interaction between adsorbate and wall atoms. For a well adsorbate-wall system, the in-pore tangential pressure, the pressure tensor component which is parallel to the wall, is significantly enhanced (e.g., by a factor of 104 ∼ 106). This enhancement effect arises from the strong attraction from the wall, which compresses the adsorbate phase in the direction parallel to the wall, leading to repulsive forces between the molecules. On the contrary, for a non-wetting system this enhancement is much weaker; in some cases the tangential pressure is reduced. This is because the attraction from the wall is weak (even weaker than the attraction between the adsorbate molecules), and thus the wall does not help to compress the adsorbate phase. A very large bulk pressure is needed (e.g., ∼ 4000 bar for mercury into a carbon pore) to push the adsorbate into the pore. For a wetting system with a cylindrical or spherical pore, the surface curvature strengthens the adsorbate-wall interaction, so that the tangential pressure is more enhanced than that in a slit pore. Moreover, the in-pore tangential pressure is very sensitive to small changes in the bulk pressure, indicating a way to experimentally control the in-pore pressure. Studies for silica pores show similar pressure enhancement effects, but these are smaller by about one order of magnitude due to the roughness of the pore wall surfaces, which causes a looser structure of the adsorbate molecules in contact with the wall surface. The normal pressure of adsorbate, the pressure tensor component which is perpendicular to the wall, is also enhanced (by a factor of ∼ 103), but the value can be positive or negative depending on the pore size. This large normal pressure causes deformation of the porous material: a positive normal pressure (strong repulsive force) expands the pore, while a negative normal pressure (strong attractive force) contracts the pore.