Ion specificity of confined ion-water structuring and nanoscale surface forces in clays

TL;DR

This study reveals how ion-specific water structuring under nanoscale confinement influences surface forces in clays, affecting their macroscopic behaviors, and highlights the importance of ion-water interactions beyond bulk properties.

Contribution

It demonstrates that ion-water structuring under confinement causes ion-specific surface forces in clays, which are not captured by existing theories, providing new insights into nanoscale interactions.

Findings

Ion-water structures deviate from bulk hydration shells under confinement.

Strong confinement leads to nearly solid-like water-ion assemblies.

Ion-specific interactions significantly influence clay surface forces.

Abstract

Ion specificity and related Hofmeister effects, ubiquitous in aqueous systems, can have spectacular consequences in hydrated clays, where ion-specific nanoscale surface forces can determine large scale cohesive, swelling and shrinkage behaviors of soil and sediments. We have used a semi-atomistic computational approach and examined sodium, calcium and aluminum counterions confined with water between charged surfaces representative of clay materials, to show that ion-water structuring in nanoscale confinement is at the origin of surface forces between clay particles which are intrinsically ion-specific. When charged surfaces strongly confine ions and water, the amplitude and oscillations of the net pressure naturally emerge from the interplay of electrostatics and steric effects, which can not be captured by existing theories. Increasing confinement and surface charge densities promote ion-water structures that increasingly deviate from the ions' bulk hydration shells, being strongly anisotropic and persistent, and self-organizing into optimized, nearly solid-like assemblies where hardly any free water is left. In these conditions, strongly attractive interactions can prevail between charged surfaces, due to the dramatically reduced dielectric screening of water and the highly organized water-ion structures. By unravelling the ion-specific nature of these nanoscale interactions, we provide evidence that ion-specific solvation structures determined by confinement are at the origin of ion specificity in clays and potentially a broader range of confined aqueous systems.