10/20/2026
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Illinois researchers show that both pore size and surface chemistry shape hydrocarbon behavior under nanoscale confinement.
Much of the oil and gas in shale formations is trapped inside pores only a few nanometers in diameter. At that scale, molecules are always close to a surface, and the properties of the pore wall can become just as important as those of the fluid itself.
A new study led by professor Jiajun He and mechanical engineering graduate student Jahid Emon shows that the chemistry of shale pore walls can significantly affect the interfacial behavior of confined hydrocarbons.
The study addresses a limitation in many molecular models of shale. While reservoir fluids have been modeled with increasing chemical complexity, pore walls are often represented by simple, chemically uniform carbon surfaces. Real shale contains kerogen, an organic material with heterogeneous surface chemistry, including several nitrogen-containing groups.
Working with electrical and computer engineering professor Shaloo Rakheja, graduate student Sheikh Mohd Ta-Seen Afrid, and Texas A&M professor Birol Dindoruk, the researchers used molecular simulations and density functional theory to study hydrocarbons in pores measuring 3 to 20 nanometers with different nitrogen-containing surface groups. They examined interfacial tension, or IFT, which describes the energetic cost of maintaining an interface between fluid phases.
The work builds on an earlier study by Emon, Dindoruk, and He in The Journal of Physical Chemistry C. (10.1021/acs.jpcc.5c07664) That study showed that confinement alone can suppress IFT by as much as 70% in ultra-tight 2- to 4-nanometer pores. The new study asks the next question: Does the chemistry of the confining surface matter, too?
The answer is yes. Strong confinement remained the dominant effect: in 3- to 5-nanometer pores, finite IFT could persist even at elevated pressures, while in wider 10- to 20-nanometer pores, pressure more readily eliminated the interface. But pores of the same size behaved differently depending on their surface chemistry. Pyridine-N-oxide surfaces, for example, bound hydrocarbons particularly strongly and generally maintained higher IFT. Quantum-mechanical calculations showed a similar ordering in hydrocarbon adsorption strength, supporting the connection between surface chemistry and interfacial behavior.
To quantify these differences, the researchers introduced the interfacial stress transition pressure (ISTP), the pressure at which the calculated IFT vanishes. Unlike the conventional minimum miscibility pressure used for bulk CO2-oil displacement, ISTP is designed to describe the mechanical disappearance of an interface under nanoscale confinement.
When CO2 was added, the researchers found that competitive adsorption of CO2 and hydrocarbons on the pore walls further changed the interfacial response. Pyridine-N-oxide showed relatively little increase in ISTP as CO2 concentration rose, suggesting that surface chemistry can influence the pressure conditions associated with CO2-driven interfacial mixing.
The findings could help improve molecular descriptions of CO2-enhanced oil recovery and carbon storage in organic-rich shale. They also show why pore size alone may not be enough to predict confined-fluid behavior: what a nanopore is made of can matter alongside how small it is.
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Jiajun He is a teaching assistant professor of mechanical science and engineering in The Grainger College of Engineering.
The paper, “Interfacial Tension of Confined Hydrocarbons in Nitrogen-Functionalized Nanopores: Coupled Effects of Surface Chemistry and Confinement,” by Emon, Afrid, Dindoruk, Rakheja, and He, was published in Langmuir. It can be found online at 10.1021/acs.langmuir.6c03154.
The related earlier study, “Interfacial Tension Suppression of Confined Hydrocarbons in Nanopores: Insights from Grand Canonical Monte Carlo Simulations,” by Emon, Dindoruk, and He, appeared in The Journal of Physical Chemistry C 130(8), 3102–3110 (2026).