Nanoconfined Methane Thermodynamic Behavior below Critical Temperature: Liquid–Vapor Coexistence Curve under Wettability Effect

The nanoconfinement effect, induced by strong surface–molecule interactions at the nanoscale, is correlated only to the pore size from a traditional perspective. However, when the pore size is comparable to the molecular diameter, the impact of surface wettability on the surface–molecule interaction strength cannot be overlooked. In order to bridge the knowledge gap, in this article, the nanoconfinement effect is described as a function of not only pore size shrinkage but also surface wettability. The Peng–Robinson equation of state is modified by adding a fluid–surface attraction term, incorporating the shift of critical properties induced by surface affinity. Particularly, the wettability effect is described as a function of contact angle, an easily accessed macroscopic parameter, facilitating the model application. Reliability of this research is verified against shifted fluid critical properties, collected from existing reports, focusing on fluid behavior in graphite or mica nanopores. The results show that (a) reduction of methane critical temperature takes place when methane molecules are confined in hydrocarbon-wet nanopores; (b) both the enhancement of surface affinity and decline of pore size will result in the shrinkage of the methane liquid–vapor coexistence curve; and (c) the phase diagram of nanoconfined methane suggests an upward trend, attributed to the shifted critical properties. In this article, the wettability effect on nanoconfined methane behavior is revealed, expecting to enrich the theoretical background about methane behavior inside nanopores.