2025 AIChE Annual Meeting

(366e) Derivative Thermodynamic Properties of Confined Fluids Based on Density Functional Theory

Authors

Gennady Gor - Presenter, New Jersey Institute of Technology
Andrei Kolesnikov, Institut fur Nichtklassische Chemie
Geordy Jomon, New Jersey Institute of Technology
Thermodynamic properties of fluids confined in nanopores differ from the properties of the same fluids in bulk [1]. Most prominent examples of such differences include shifts of the phase transitions - capillary condensation, or freezing in the pores. Recent experimental and molecular simulation studies showed that isothermal compressibility of a fluid confined in a nanopore differs from the compressibility of the same fluid in bulk [2]. Density functional theory (DFT) has been widely used for modeling thermodynamics of confined fluids, and e.g. is capable of quantitatively predicting capillary condensation for simple fluids, such as argon or nitrogen, in nanopores [3,4,5]. However, the same DFT models failed to reproduce compressibility for even bulk fluids [6].

Here we use a rather simple DFT model for argon based on the Percus-Yevick equation, and consider the temperature at which the vapor and liquid densities match the experimental values. We show that the isothermal compressibility of bulk liquid argon at this temperature matches the experimental value as well. 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. The bulk modulus (1/compressibility) appears to be a linear function of the 1/pore size, consistent with the molecular simulation results [2]. In addition to isothermal compressibility, we calculated another derivative thermodynamic property - thermal expansion coefficient of confined argon. Our calculations showed that it behaves similar to compressibility - it is always lower than the bulk value and gradually increases for the smaller pore sizes. Our study contributes to the fundamental knowledge in thermodynamics of confined fluids. It also demonstrates the potential of DFT for calculation of properties which are very challenging to calculate using molecular simulations.

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