  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.
