Molecular insight on ultra-confined ionic transport in wetting films: the key role of friction

TL;DR

This study uses molecular dynamics simulations and a simple theoretical model to reveal how ion adsorption and interfacial friction influence ionic transport in ultra-confined water films on silica, challenging traditional theories.

Contribution

It demonstrates the validity of a one-dimensional model at molecular scales and uncovers the impact of ion adsorption on nanoscale hydrodynamics and apparent viscosity.

Findings

Ion adsorption at water-silica interfaces affects ionic conduction.

Adsorbed cations increase interfacial friction and apparent viscosity.

The simple model remains valid even at sub-nanometer confinement.

Abstract

Nanofluidic transport is ubiquitous in natural systems from extra-cellular communication in biology to geological phenomena, and promotes the emergence of new technologies such as energy harvesting and water desalination. While experimental access to ultraconfined fluids has advanced rapidly, their behavior challenges conventional theoretical descriptions based on Poisson-Boltzmann theory or the Stokes equation whose possible extension remains an open question. In this work, we use molecular dynamics simulations to investigate ionic transport within wetting films of water confined on silica surfaces down to the sub-nanometer scale. We then analyze these results using a simple one-dimensional theoretical framework. Remarkably, we show that this model remains valid even at confinement close to the molecular scale. Our results reveal that the dynamic of ion plays a key role in ionic transport, through ion adsorption at the water-silica interface. Adsorbed cations do not participate in ionic conduction, but instead generate molecular-scale roughness and transmit additional frictional forces to the substrate. This mechanism produces an apparent viscosity increase in electrostatically driven flows, reaching up to four times the bulk value in the case of potassium. Our findings highlight the critical role of interfacial ion adsorption in nanoscale hydrodynamics and provide new insights for interpreting experiments and designing nanofluidic systems.