(437f) Pressure Enhancement in Nanopores: Effect of Pore Shape

Abundant experimental evidence suggests that adsorbates confined in nanoporous materials can exhibit high pressures. Examples include ice nanocrystals (ice VIII and ice IX, only existing at pressures of ~1 GPa for bulk water) in carbon nanotubes at 0.1 MPa bulk pressure¹, a bcc crystalline form of KI, found in the bulk phase only at pressures above 1.9 GPa, in carbon nanohorns at 0.1 MPa bulk pressure², chemical reactions that occur at low bulk pressures in nano-pores, but only at very high pressures in the bulk phase³, and the deformation of pore walls confining the adsorbate in the course of adsorption⁴.

We report molecular simulation studies of the pressure tensor for simple adsorbates (e.g. argon, water) in carbon nanopores of slit, cylindrical and spherical geometries.  We calculate the pressure tensor by both the mechanical route and the thermodynamic route. Both routes give comparable results (especially for the slit geometry, the results from both routes agree perfectly). For modest bulk phase pressures of 1 bar or less, the pressures parallel to the pore walls (tangential pressure) were found to be positive and of the order 1 GPa, and because of the degree of confinement the tangential pressure in spherical pore is larger than that in the cylindrical pore, while the tangential pressure in the slit pore is the smallest. In slit pores, the pressure normal to the wall was constant throughout the pore and of the order of 0.1 GPa, and could be positive or negative depending on the pore size. For the cylindrical and spherical pores, the pressure normal to the wall is not constant over the radial direction, but the peak value is also ~ 0.1 GPa.  Moreover, the tangential pressure is very sensitive to small changes in the bulk pressure, indicating a way to experimentally control the in-pore pressure.

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