Confinement-Mediated Phase Behavior of Hydrocarbon Fluids: Insights from Monte Carlo Simulations

Area of Science:

• Physical Chemistry
• Materials Science
• Chemical Engineering

Background:

• Nanopore confinement significantly alters hydrocarbon fluid phase behavior compared to bulk.
• Understanding confinement effects, especially fluid-pore interactions and surface wettability, is crucial but less explored.
• The fundamental physics behind confinement-induced phenomena in porous media requires further elucidation.

Purpose of the Study:

• To investigate the molecular-level phase behavior and capillary condensation of n-hexane under confinement.
• To systematically study the impact of pore size, morphology, and fluid-pore interactions on hydrocarbon phase transitions.
• To elucidate the role of surface wettability and chemistry in mediating confinement effects.

Main Methods:

• Molecular simulations were employed to study n-hexane phase behavior in two distinct confinement models: structureless virtual walls (Steele potential) and all-atom amorphous silica with hydroxyl modification.
• Analysis of van der Waals-type adsorption isotherms was used to predict saturation pressures.
• Systematic variation of fluid-pore interaction strength and pore characteristics was performed.

Main Results:

• Confinement effects, including fluid-pore interaction strength, pore size, and morphology, significantly mediate the pressure-volume-temperature (PVT) properties of hydrocarbons.
• The saturation pressure of confined n-hexane can be elevated or suppressed relative to the bulk phase.
• Decreasing surface energy (weaker fluid-pore interaction) leads to increased isothermal vapor pressure, favoring the vapor state.

Conclusions:

• Fluid-pore interactions are a critical factor governing hydrocarbon phase behavior in nanoporous media.
• The strength of fluid-pore interactions can be tuned to alter saturation pressures, potentially exceeding bulk values.
• This study provides molecular-level insights into capillary condensation and phase transitions under confinement, relevant for applications in porous materials.