Model for Interfacial Tension of Nanoconfined Lennard-Jones Fluid

Abstract

Understanding and controlling the interfacial tension (IFT) of nanoconfined fluids has tremendous implications in scientific research and engineering applications. On the basis of the physical meaning of the equimolar dividing surface and the density distribution characteristic at the interface, we propose a simple model for the density profile at the vapor–liquid interface. The equimolar dividing surface and the surface of tension are assumed to coincide with each other in this work since the inhomogeneity of the interface is characterized by the proposed density profile model. Then, on the basis of the density distribution model, the density function theory (DFT) and the extended Peng–Robinson equation of state (PR EOS, which considers the effects of critical properties shift and capillary pressure) are used to estimate the confined interfacial properties and phase behavior in nanopores. Besides, the influences of temperature, pore radius, and wettability on IFT and capillarity are addressed. The developed model is validated as reliable for IFT calculation through comparison with the experimental data in the literature. Results show the following: (i) The IFT decreases with the reduction of pore size and the increase of temperature, and the decreasing rate is larger in smaller pores and higher temperatures. (ii) Capillary pressure increases with the decreasing pore size and the decreasing temperature, and it is more sensitive to pore size compared with the IFT. (iii) In the liquid phase-wet condition, as the contact angle decreases, the IFT decreases, while the capillary force increases, and the change rate is more obvious in smaller pores. Compared with other methods, the model for nanoconfined IFT proposed in this work, which is derived from the underlying physics mechanism, has a simper formulation with the similar reliability. Particularly, this work sheds light on the variation of IFT of hydrocarbons confined in nanopores of shale gas reservoirs, which provides an insight into the means of enhanced gas recovery in petroleum engineering.