A multiscale theory to determine thermodynamic properties of confined fluids

Abstract

Empirical potential-based quasi-continuum theory (EQT) provides a route to incorporate atomistic detail into a continuum framework such as the Nernst- Planck equation. EQT is a simple and fast approach to predict inhomogeneous density and potential profiles of confined fluids. EQT potentials can be used to construct a grand potential functional for classical density functional theory (cDFT). The combination of EQT and cDFT provides a robust and accurate approach to predict the structure and thermodynamic properties of confined fluids at multiple length-scales, ranging from few Angstroms to macro meters. In this work, first, we demonstrate the EQT-cDFT approach by simulating sin- gle component Lennard-Jones (LJ) fluids, namely, methane and argon, confined inside slit-like channels of graphene. For these systems, we show that the EQT- cDFT can accurately predict the structure and thermodynamic properties, such as density profiles, adsorption, local pressure tensor, surface tension, and solva- tion force of confined fluids as compared to the MD simulation results. Next, we extend the EQT-cDFT approach to confined fluid mixtures and demonstrate it by simulating a mixture of methane and hydrogen inside slit-like channels of graphene. We show that the EQT-cDFT predictions for the structure of the confined fluid mixture compare well with the MD simulations results. In addi- tion, our results show that graphene slit nanopores exhibit a selective adsorption of methane over hydrogen