Anisotropic molecular diffusion in confinement II: A model for structurally complex particles applied to transport in thin ionic liquid films

TL;DR

This paper develops a new model based on the 2D Smoluchowski equation to analyze anisotropic, spatially varying diffusion in confined ionic liquids, validated through molecular dynamics simulations, revealing interface-induced structuring effects.

Contribution

It introduces a universal coarse-grained approach for modeling complex, fluctuating particles' diffusion in confinement without assuming time scale separation.

Findings

Demonstrates anisotropic diffusion at interfaces.

Reveals spatial variation of diffusivities due to interface structuring.

Shows different length scales at solid-liquid and liquid-vapour interfaces.

Abstract

Hypothesis:Diffusion in confinement is an important fundamental problem with significant implications for applications of supported liquid phases. However, resolving the spatially dependent diffusion coefficient, parallel and perpendicular to interfaces, has been a standing issue and for objects of nanometric size, which structurally fluctuate on a similar time scale as they diffuse, no methodology has been established so far. We hypothesise that the complex, coupled dynamics can be captured and analysed by using a model built on the $2$-dimensional Smoluchowski equation and systematic coarse-graining. Methods and simulations: For large, flexible species, a universal approach is offered that does not make any assumptions about the separation of time scales between translation and other degrees of freedom. The method is validated on Molecular Dynamics simulations of bulk systems of a family of ionic liquids with increasing cation sizes where internal degrees of freedom have little to major effects. Findings: After validation on bulk liquids, where we provide an interpretation of two diffusion constants for each species found experimentally, we clearly demonstrate the anisotropic nature of diffusion coefficients at interfaces. Spatial variations in the diffusivities relate to interface-induced structuring of the ionic liquids. Notably, the length scales in strongly confined ionic liquids vary consistently but differently at the solid-liquid and liquid-vapour interfaces.