Under PRESSURE: Quasi-High PRESSURE EFFECTS IN NANO-Pores

It is frequently observed that 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 For example, 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 VII 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 experiments5 and molecular simulations^4,5.

We report a study, using semi-grand canonical Monte Carlo and molecular dynamics simulations, of the pressure tensor in nano-pores of simple slit-shaped geometry. We show that for nano-scale pores (pore widths from 0 to 8 molecular diameters) 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 pressure of the bulk phase in equilibrium with the pore phase, 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 results in compression of the molecules in the confined nanophase, resulting in strong repulsive intermolecular forces in the tangential direction. This gives rise to large and positive tangential pressures. The pressure normal to the pore walls can also be large, and oscillates between positive and negative values, depending on the pore width. The normal pressure (approximately equal to the salvation pressure) causes changes to the pore width and interlayer spacing on adsorption. Such changes have been observed in recent x-ray and neutron diffraction experiments.

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. 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.