Non-equilibrium molecular dynamics of steady-state fluid transport through a 2D membrane driven by a concentration gradient

TL;DR

This study introduces a novel non-equilibrium molecular dynamics algorithm to simulate steady-state fluid transport through a 2D membrane driven by concentration gradients, confirming theoretical predictions and demonstrating efficiency over traditional methods.

Contribution

The paper presents a new MD algorithm for simulating concentration-gradient-driven transport, validated against pressure-driven flow and theoretical models, with improved efficiency.

Findings

Fluxes agree with Onsager reciprocal relations in linear regime

Concentration-gradient flux is more efficiently computed with the new algorithm

Simulated fluxes match continuum theory across various parameters

Abstract

We use a novel non-equilibrium algorithm to simulate steady-state fluid transport through a two-dimensional (2D) membrane due to a concentration gradient by molecular dynamics (MD) for the first time. We confirm that, as required by the Onsager reciprocal relations in the linear-response regime, the solution flux obtained using this algorithm agrees with the excess solute flux obtained from an established non-equilibrium MD algorithm for pressure-driven flow. In addition, we show that the concentration-gradient solution flux in this regime is quantified far more efficiently by explicitly applying a transmembrane concentration difference using our algorithm than by applying Onsager reciprocity to pressure-driven flow. The simulated fluid fluxes are captured with reasonable quantitative accuracy by our previously derived continuum theory of concentration-gradient-driven fluid transport through a 2D membrane [J. Chem. Phys. 151, 044705 (2019)] for a wide range of solution and membrane parameters even though the simulated pore sizes are only several times the size of the fluid particles. The simulations deviate from the theory especially for strong solute--membrane interactions relative to the thermal energy, for which the theoretical approximations break down. Our findings will be beneficial for molecular-level understanding of fluid transport driven by concentration gradients through membranes made from 2D materials, which have diverse applications in energy harvesting, molecular separations, and biosensing.