Mass Transfer Through Vapor-Liquid Interfaces From Hydrodynamic Density Functional Theory

Abstract: We assess the capabilities of hydrodynamic density functional theory (DFT) to predict mass transfer across vapor-liquid interfaces by studying the response of an initially equilibrated pure component vapor-liquid system to the localized insertion of a second component. Hydrodynamic DFT captures the effect of interfaces on the dynamics by modeling the chemical potential gradients of an inhomogeneous system based on classical DFT. Hydrodynamic DFT effectively connects molecular models with continuum fluid dynamics. Away from interfaces the framework simplifies to the isothermal Navier-Stokes equations. We employ Maxwell-Stefan diffusion with a generalized driving force to model diffusive molecular transport in inhomogeneous systems. For the considered Lennard--Jones truncated and shifted (LJTS) fluid, we utilize a non-local Helmholtz energy functional based on the perturbed truncated and shifted (PeTS) equation of state. The model provides noise-free partial densities and fluxes for the mass transfer near the interface, as well as profiles of these quantities across the interface. A comparison with non-equilibrium molecular dynamics simulations shows that hydrodynamic DFT accurately predicts mass transfer across the interface, including microscopic phenomena such as the temporary enrichment and repulsion of the light boiling component at the interface. Combining generalized entropy scaling with generalized Maxwell-Stefan diffusion allows for an accurate description of diffusive molecular transport in the system. This approach can accurately predict phase behavior, equilibrium interfaces, and mass transfer across interfaces based on molecular interactions for mixtures of strongly dissimilar components. Our results suggest that hydrodynamic DFT can accurately predict the dynamics of mixtures at vapor-liquid interfaces.