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

The
phase behavior
of hydrocarbon fluids confined in porous media has been reported to
deviate significantly from that in the bulk environment due to the
existence of sub-10 nm pores. Though experiments and simulations have
measured the bubble/dew points and sorption isotherms of hydrocarbons
confined in both natural and synthetic nanopores, the confinement
effects in terms of the strength of fluid–pore interactions
tuned by surface wettability and chemistry have received comparably
less discussion. More importantly, the underlying physics of confinement-induced
phenomena remain obfuscated. In this work, we studied the phase behavior
and capillary condensation of n-hexane to understand
the effects of confinement at the molecular level. To systematically
investigate the pore effects, we constructed two types of wall confinements;
one is a structureless virtual wall described by the Steele potential
and the other one is an all-atom amorphous silica structure with surface
modified by hydroxyl groups. Our numerical results demonstrated the
importance of fluid–pore interaction, pore size, and pore morphology
effects in mediating the pressure–volume–temperature
(PVT) properties of hydrocarbons. The most remarkable finding of this
work was that the saturation pressure predicted from the van der Waals-type
adsorption isothermal loop could be elevated or suppressed relative
to the bulk phase, as illustrated in the graphical abstract. As the
surface energy (i.e., fluid–pore interaction) decreased, the
isothermal vapor pressure increased, indicating a greater preference
for the fluid to exist in the vapor state. Sufficient reduction of
the fluid–pore interactions could even elevate the vapor pressure
above that of the bulk fluid.