Interfacial Tension of Confined Hydrocarbons in Nitrogen-Functionalized Nanopores: Coupled Effects of Surface Chemistry and Confinement.

Interfacial tension (IFT) in nanoscale pores governs multiphase flow, capillary trapping, and hydrocarbon recovery in unconventional reservoirs, yet how pore size and pore-wall chemistry jointly control this behavior remains poorly understood. Because the organic (kerogen) surfaces lining these pores are chemically heterogeneous, and nitrogen is among their most abundant reactive heteroatoms, we investigate how nitrogen-containing functional groups modulate the IFT of confined n-butane and CO2-n-butane mixtures. Using grand canonical Monte Carlo (GCMC) simulations combined with density functional theory (DFT), we model slit-shaped nanopores functionalized with kerogen-relevant nitrogen groups, specifically pyrrolic nitrogen, pyridinic nitrogen, pyridine-N-oxide, and two pyridone tautomers, alongside pristine graphene. IFT is computed from the molecular pressure tensor using the Irving-Kirkwood formalism, and we introduce an interfacial stress transition pressure (ISTP), the pressure at which this computed IFT vanishes, as a pore-scale metric for comparing surfaces and pore geometries. We find that strong confinement (3-5 nm) sustains finite IFT even at elevated pressure, while weaker confinement (10-20 nm) allows pressure to eliminate it. Oxidized pyridinic nitrogen (pyridine-N-oxide) binds hydrocarbons most strongly and consistently produces the highest IFT and ISTP, while pristine graphene and 2-pyridone permit the most rapid IFT suppression; DFT-computed adsorption energies track this ranking closely. In CO2-n-butane mixtures, competitive adsorption between CO2 and n-butane governs interfacial evolution, and pyridine-N-oxide surfaces show unusually weak ISTP sensitivity to CO2 content, implying more efficient CO2-driven interfacial mixing. These results link adsorption chemistry to confinement-modified interfacial thermodynamics, with implications for CO2-enhanced oil recovery and carbon storage in organic-rich nanoporous formations.