Molecular Dynamics Reveals Friction’s Key Role in Ultra-Confined Ionic Transport Within Sub-Nanometer Wetting Films

The behaviour of liquids at the nanoscale is fundamental to processes ranging from biological signalling to geological events, and underpins the development of technologies like energy harvesting and water purification. Aymeric Allemand, Anne-Laure Biance, Christophe Ybert, and Laurent Joly, from Université Claude Bernard Lyon 1 and the CNRS Institut Lumière Matière, now reveal a crucial factor governing ionic transport within extremely confined liquid films. Through detailed molecular dynamics simulations, the team demonstrates that ion adsorption at the interface between water and solid surfaces generates frictional forces that significantly impact flow, effectively increasing viscosity by up to four times for ions like potassium. This discovery challenges conventional understandings of nanoscale hydrodynamics and provides a new framework for interpreting experimental observations and designing more efficient nanofluidic devices.

Friction Drives Ionic Transport in Confined Films

Molecular dynamics simulations reveal the crucial role of friction in ultra-confined ionic transport within wetting films, providing new insight into how ions move in these spaces. This research investigates the interplay between ion-wall and ion-ion interactions, and their influence on ionic conductivity, a process vital to natural systems and emerging technologies like energy harvesting. The simulations demonstrate that friction at the interface between ions and confining walls significantly modulates ion mobility and transport efficiency, challenging the conventional understanding that reducing friction always improves transport.

Results show a non-monotonic relationship between wall friction and ionic conductivity, indicating an optimal friction value for maximising transport. The study elucidates the molecular origins of this behaviour, identifying specific frictional mechanisms that either promote or hinder ion movement. By quantifying the contribution of friction to energy dissipation during transport, the research provides a pathway for designing nanofluidic devices with enhanced ionic conductivity and improved energy efficiency for applications such as water desalination. This work offers a new perspective on these complex systems, highlighting that a simple one-dimensional theoretical framework remains valid even at confinement approaching the molecular scale.

Ion Interactions with Amorphous Silica Surfaces

This research uses molecular dynamics simulations to investigate the behaviour of water and ions, such as lithium, sodium, and potassium, at the interface with amorphous silica. The goal is to understand how these interactions influence phenomena like nanofluidics and electrokinetic effects, and to model the structure of water near solid surfaces. Simulations model water and various ions interacting with amorphous silica surfaces, employing specific force fields to accurately represent the interactions between atoms and molecules, providing detailed insights into water structure, ion adsorption, and the behaviour of fluids in nanoscale confinement.

The research focuses on understanding how water molecules arrange themselves near the silica surface, how ions interact with and adsorb onto the surface, and how these interactions affect nanofluidic behaviour and electrokinetic effects, including exploration of surface wettability control. The silica surface is modeled with specific roughness and silanol density, and algorithms are used to control temperature and pressure during the simulations, calculating properties such as density profiles, diffusion coefficients, and forces to provide a comprehensive understanding of the complex interactions at the interface.

Confined Water Films Enhance Ionic Conductance

Simulations reveal that ionic conductance increases with film thickness in extremely confined water films on silica surfaces, aligning with recent experimental observations. Detailed analysis provides molecular-level insights into ion distribution, demonstrating a concentration of ions near the solid-liquid interface, shifted from the ideal dividing surface due to surface roughness, and depletion at the liquid-vapor interface due to electrostatic repulsion. The study highlights the importance of ion dynamics, showing that electro-osmotic flows are reduced compared to predictions based on standard fluid properties.

This reduction arises from increased frictional forces caused by ion adsorption at the surface, leading to an effective viscosity significantly higher than that of bulk water. Potassium ions exhibit lower conductance, stronger adsorption, and a larger effective viscosity compared to sodium and lithium ions, demonstrating the crucial role of ion identity and interfacial structure in nanoscale transport. Continuum models, when corrected with molecular-level information obtained from simulations, effectively describe ionic conductance in these ultrathin films, although more detailed molecular-level descriptions are necessary at thicknesses comparable to molecular size and surface roughness.