On the molecular origin of high-pressure effects in nanoconfinement: The role of surface chemistry and roughness

Abstract

Experiments and simulations both suggest that the pressure experienced by an adsorbed phase confined within a carbon nanoporous material can be several orders of magnitude larger than the bulk phase pressure in equilibrium with the system. To investigate this pressure enhancement, we report a molecular-simulation study of the pressure tensor of argon confined in slit-shaped nanopores with walls of various models, including carbon and silica materials. We show that the pressure is strongly enhanced by confinement, arising from the effect of strongly attractive wall forces; confinement within purely repulsive walls does not lead to such enhanced pressures. Simulations with both the Lennard-Jones and Barker-Fisher-Watts intermolecular potentials for argon-argon interactions give rise to similar results. We also show that an increase in the wall roughness significantly decreases the in-pore pressure due to its influence on the structure of the adsorbate. Finally, we demonstrate that the pressures calculated from the mechanical (direct pressure tensor calculations) and the thermodynamic (volume perturbation method) routes yield almost identical results, suggesting that both methods can be used to calculate the local pressure tensor components in the case of these planar geometries.

Local Pressure of Supercritical Adsorbed Hydrogen in Nanopores

“…This pressure represents the combined average normal and tangential pressure on the carbon surface. Normal and tangential pressure due to van der Waals force between Argon gas and graphene in slit-shaped pores has been approximated between 10 3 and 10 4 bar [36]. Phases confined in nanospaces exhibit a behavior that is different behavior from that of the bulk phase.…”

On the Gibbs–Thomson equation for the crystallization of confined fluids

“…the mutual influence of the two interface. Even though this second regime is not the main focus of the present work and this is not visible with the considered pore sizes, the disjoining pressure oscillates due to the finite size of the particles and the formation of discrete layers at the interfaces, so that non-trivial effects on the thermodynamic behavior can be observed 8,53,[58][59][60][61] .…”

An EQT-cDFT approach to determine thermodynamic properties of confined fluids

“…3). 14,43,51,57,58 Similar to the normal pressure, the surface tension and solvation force oscillate for the smaller channels and the amplitudes of oscillations decay rapidly with increasing L.…”

“…For example, the pressure experienced by a fluid confined in a nanopore can be orders of magnitude higher than the pressure in the bulk fluid. 14 Such unusual behavior of confined fluids can have practical applications, e.g., a carbon nanotube (CNT) can be used as a super-compressor for the synthesis of valuable high-pressure materials, such as KI nanocrystals. 13 Therefore, study of confined fluids is important to get atomic-level insights into their unusual properties and to enable the design of novel nanofluidic applications.…”