(227b) Confinement-Induced Compression and High Pressure Phases in Nanopores

There is an abundance of anecdotal evidence that nanophases adsorbed within nanoporous materials can exhibit high pressures as a result of the confinement^1,2. The pressure in the pore is a second order tensor, and for simple pore geometries has both a normal pressure component (normal to the walls) and one or more tangential components (parallel to the walls).

For simple fluids in pores that are up to a few nanometers in width, molecular simulations show that both the normal and tangential pressures can be locally very high (thousands or tens of thousands of bars) in the pore, even though the bulk phase in equilibrium with the pore is at a pressure of one bar or less. The cause of these high in-pore pressures will be discussed.

When the molecules in the confined nanophase react with each other chemically it may be possible to achieve even higher tangential pressures, in the megabar range. Thus, Kaneko et al.³ have shown that when sulfur atoms are confined within a narrow carbon nanotube they covalently bond to form a one-dimensional phase that is metallic. In the bulk phase sulfur forms a metallic phase only at pressures above 95 GPa. In our recent molecular dynamics simulations of this system we find that the sulfur atoms are covalently bonded in the pore and that they experience tangential pressures in excess of 100 GPa as a result of the strong confinement⁴. Recently, Medeiros et al.⁵ have reported experiments in which they observe a similar covalently bonded one-dimensional phase, and an insulator-metal transition, for tellurium in single-walled carbon nanotubes.

1. Yun Long, Jeremy C. Palmer, Benoit Coasne, Małgorzata Śliwinska-Bartkowiak and Keith E. Gubbins, MACROBUTTON MTEditEquationSection2 Equation Chapter 1 Section 1 SEQ MTEqn \r \h \* MERGEFORMAT SEQ MTSec \r 1 \h \* MERGEFORMAT SEQ MTChap \r 1 \h \* MERGEFORMAT “Pressure enhancement in carbon nanopores: A major confinement effect”, Physical Chemistry Chemical Physics, 13, 17163-17170 (2011).
2. Yun Long, Jeremy C. Palmer, Benoit Coasne, Małgorzata Śliwinska-Bartkowiak, George Jackson, Erich A. Müller and Keith E. Gubbins, “On the Molecular Origin of High Pressure Effects in Nanoconfinement: Effects of Surface Chemistry and Roughness”, Journal of Chemical Physics, 139, 144701 (2013)
3. Y. Fujimori, A. Morelos-Gómez, Z. Zhu, et al., “Conducting Linear Chains of Sulphur Inside Carbon Nanotubes”, Nature Comm., 4, 2162 (2013).
4. K.E. Gubbins, C.A Addington and J.M. Mansell, “Computer Simulation of Conductive Linear Sulfur Chains Confined in Carbon Nanotubes”, Molecular Simulation, 43, 519-525 (2017).
5. P. V. C. Medeiros et al., “Single-Atom Scale Structural Selectivity in Te Nanowires Encapsulated Inside Ultranarrow, Single-Walled Carbon Nanotubes,” ACS Nano, 11, 6178–6185 (2017).