Mass Transfer through Vapor–Liquid Interfaces Studied by Non-Stationary Molecular Dynamics Simulations

Abstract

Molecular dynamics (MD) simulations are highly attractive for studying the influence of interfacial effects, such as the enrichment of components, on the mass transfer through the interface. In a recent work, we have presented a steady-state MD simulation method for investigating this phenomenon and tested it using model mixtures with and without interfacial enrichment. The present study extends this work by introducing a nonstationary MD simulation method. A rectangular simulation box that contains a mixture of two components 1 + 2 with a vapor phase in the middle and two liquid phases on both sides is used. Starting from a vapor−liquid equilibrium state, a non-stationary molar flux of component 2 is induced by inserting particles of component 2 into the center of the vapor phase in a pulse-like manner. During the isothermal relaxation process, particles of component 2 pass through the vapor phase, cross the vapor−liquid interface, and enter the liquid phase. The system thereby relaxes into a new vapor−liquid equilibrium state. During the relaxation process, spatially resolved responses for the component densities, fluxes, and pressure are sampled. To reduce the noise and provide measures for the uncertainty of the observables, a set of replicas of simulations is carried out. The new simulation method was applied to study mass transfer in two binary Lennard-Jones mixtures: one that exhibits a strong enrichment of the low-boiling component 2 at the vapor− liquid interface and one that shows no enrichment. Even though both mixtures have similar transport coefficients in the bulk phases, the results for mass transfer differ significantly, indicating that the interfacial enrichment influences the mass transfer.

Role of Mutual Diffusion in the Dissolution Behavior of One Primary Bulk Gas Nanobubble in Liquid: A Molecular Dynamics Study

“…Furthermore, the mutual diffusion coefficient at the interface prominently lowers than the experimental value, i.e., D O2 = 2.13 × 10 −9 m 2 /s, 20 after the initiation of contraction, and might be ascribed to the unbalanced attraction caused by the bulk gas and bulk liquid. Moreover, this attenuation might also be closely related to the enrichment at the interface reported by Stephan et al, 37 Schaefer et al 39 and Baidakov et al 41,42 Although this hypothesis lacks experimental proofs, it might constitute one of plausible mechanisms. Besides, the S−E equation, derived for large Brownian particles, is not applicable to solute molecules less than approximately 1000 in molecular mass 19 and thereby the mutual diffusion coefficient curves deviate from those plotted based on the S−E equation.…”

“…The interface between two bulk phases also affects the mutual diffusion behavior by acting as an additional mass transfer resistance, and this topic has been intensively discussed. Matsumoto, 35 Stephan et al, 36−38 and Schaefer et al 39 examined the interphase mass transfer in a binary system by employing MD simulations and indicated that interfacial enrichment or adsorption hinders the effective molecular exchange to make the corresponding mutual diffusion coefficient slower compared to the bulk diffusion coefficient.…”

“…The interface between two bulk phases also affects the mutual diffusion behavior by acting as an additional mass transfer resistance, and this topic has been intensively discussed. Matsumoto, Stephan et al, − and Schaefer et al examined the interphase mass transfer in a binary system by employing MD simulations and indicated that interfacial enrichment or adsorption hinders the effective molecular exchange to make the corresponding mutual diffusion coefficient slower compared to the bulk diffusion coefficient. Stephan et al reviewed the enrichment phenomenon that might influence the mass transfer across the interface and discussed a non-monotonicity of vapor–liquid density profiles at the interface, which also constitutes a plausible cause of the low-rate mass transfer. , Braga et al depicted the free-energy profile at the liquid–vapor interface to indicate that a free-energy barrier remarkably exists at the interfacial layer to hinder the effective mass transfer.…”

Quantum Cluster Equilibrium Theory for Multicomponent Liquids

“…These simulations can be applied to study a diverse array of systems, relying on the underlying force field. Calculating thermodynamic properties from MD simulations is a very active research field, with new methods and approaches being developed and benchmarked regularly. − In the standard formulation of MD, the intra- and intermolecular interactions are described by pairwise additive Lennard-Jones (LJ) and Coulomb potentials. In many commonly used force fields, the LJ potential is parameterized to reproduce the experimentally obtained properties of the pure components.…”

Vapor–Liquid Interfacial Properties of the Systems (Toluene + CO2) and (Toluene + N2): Experiments, Molecular Simulation, and Density Gradient Theory

“…This phenomenon is referred to as enrichment and is closely related to the relative adsorption. The enrichment of components at the vapor–liquid interface of mixtures has received some attention in the past decades as it is suspected to influence the mass transfer across the interface, ,− which might have important consequences for fluid separation processes.…”