(109d) Quasi-High Pressure Effects in Nanopores
Recent experimental studies have shown that some phenomena, which only occur in bulk phases under high-pressure conditions, occur in porous materials under near ambient conditions. Such confinement or surface driven phenomena include high pressure phases [Alba-Simonesco, et al, J. Phys. Condens. Matter, 2006, 18, R14.] and high pressure chemical reactions [Kaneko, et al, Langmuir, 1989, 5, 960]). For example, ices VII, VIII and IX, which only occur at pressures above 1 GPa in bulk water, are readily observed in porous carbons at temperature about 250 K and atmospheric (bulk phase) pressure [M. Sliwinska-Bartkowiak et al., Phys. Chem. Chem. Phys., 2008, 10, 4909].
In order to understand this behavior, we have examined the relationship between confinement and pressure, and the impact of both on the phase behavior of confined Lennard-Jones fluids in slit shaped pores. The key quantities examined are the two non-zero components of the in-pore pressure tensor, the normal and tangential components, of the confined fluid. Using semi-grand canonical Monte Carlo simulations, we have studied the behavior of the pressure tensor along the sorption isotherms in both infinite and finite length rigid slit pores. We discuss the impact of variables such as pore width, geometry, guest-host phase interactions and temperature on the in-pore pressure tensor and illustrate that under many circumstances the confining geometry leads to a large enhancement of the effective in-pore pressure. We also discuss the impact that this enhancement has on the thermodynamic and structural properties of the confined guest phases. Finally, we present results paralleling our discussion for rigid pores for the case of flexible adsorbents.