(242f) High Pressure Effects In Nanopores

       There is an abundance of anecdotal evidence that nanophases adsorbed within nanoporous materials exhibit high pressures as a result of the confinement.  For example, phase changes and chemical reactions that only occur at high pressures in the bulk phase occur in the confined phase at bulk phase pressures that are orders of magnitude lower.¹ The structure of confined ice has been studied in carbon nanotubes using molecular simulation² and experiment,³ and provides convincing evidence for the formation of different kinds of ice nanocrystals, including ice VIII and ice IX, phases that only occur at pressures of GPa and above in bulk water.  Examples of chemical reactions that occur at low bulk pressures in nano-pores, but only at very high pressures in the bulk phase, have also been frequently observed in experiments⁴ and molecular simulations⁵.

        Our previous simulation studies of the pressure tensor in slit pores (pore widths from 2 to 8 molecular diameters) found that the tangential pressure can be locally very high, tens of thousands of bars, in the pore, even though the bulk phase in equilibrium with the pore is at pressures of one bar or less.  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.  These very high in-pore pressures result from the strong interaction with the pore walls, which compress the confined nanophase  This leads to strong repulsive intermolecular forces in the tangential direction and  large positive tangential pressures.    In contrast to the simple slit-pore model, real porous materials, such as activated carbons, have highly complex structural and energetic topologies that strongly influence the behavior of confined phases. We examine the impact of these features on the in-pore pressure tensor using modified versions of the slit-pore model that include structural and energetic surface heterogeneities.  We also calculate the pressure tensor in the absence of attractive solid-fluid interactions. The results indicate that the high tangential pressure arises from the strong repulsive interaction between compressed fluid molecules due to the geometric confinement of the wall, while the attractive force from the wall determines the degree of compression of the fluid molecules, and thus determines the pressure enhancement factor. Adsorption studies in a silica slit pore also indicate that the atomically detailed geometry of the wall surface has a large effect on the arrangement of in-pore fluid molecules and the pressure tensor.

1. Gelb, L. D.; Gubbins, K. E.; Radhakrishnan, R.; Sliwinska-Bartkowiak, M. Rep. Progr. Phys., 1999, 62, 1573.

2. Takaiwa, D.;Hatano, I.; Koga, K.; Tanaka, H. Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 39.

3. Matsuda, K.; Hibi, T.; Kadowaki, H.; Katara, H.; Maniwa, Y. Phys. Rev. B, 2006 ,74, 073415.

4. e.g. K. Kaneko, N. Fukuzaki, K. Kakei, T. Suzuki and S. Ozeki, Langmuir, 5, 960 (1989); Byl, O.; Kondratyuk, P.; Yates, J.T., Jr., J. Phys. Chem. B, 2003, 107, 4277.

5. e.g. Turner, C.H.; Johnson, J.K.; Gubbins, K.E., J. Chem. Phys., 2001, 114, 1851.