Density functional theory predictions of derivative thermodynamic properties of a confined fluid

Fluids in nanopores are of importance for many engineering applications, including energy storage in supercapacitors, hydrocarbon recovery from unconventional sources, or water desalination. Thermodynamic properties of fluids confined in nanopores differ from the properties of the same fluids in bulk. Density functional theory (DFT) has been widely used for modeling the thermodynamics of confined fluids. However, it is rarely used for calculations of derivative thermodynamic properties. Here, we use a rather simple DFT model for argon based on the Percus-Yevick equation and show that with standard parameterization, it fails to predict derivative properties. However, a slight adjustment in parameters leads to quantitative predictions of isothermal compressibility and thermal expansion coefficient at a selected temperature. Using the adjusted parameterization, we performed the calculations of compressibility of argon confined in carbon slit pores of various sizes and demonstrated that the compressibility of argon in confinement is lower than that in bulk and is pore size dependent. We confirmed the DFT predictions using the Monte Carlo molecular simulations. In addition to isothermal compressibility, we calculated the thermal expansion coefficient of confined argon. Our calculations showed that it behaves similarly to compressibility-it is always lower than the bulk value and gradually increases for smaller pore sizes. For several selected pore sizes, we verified the DFT calculations by Monte Carlo simulations. Overall, our results suggest that the classical DFT can be utilized for calculations of derivative thermodynamic properties of confined fluids, which are computationally challenging to predict using molecular simulations.