Ion-specific nanoscale forces in charged soft matter

In charged soft matter, surface forces are often ion-specific and cohesive in ways that cannot be captured by existing theories such as DLVO. We use semi-atomistic Monte Carlo and Molecular Dynamics simulations of counterions and explicit water confined between charged surfaces in order to unravel the origin of ion specificity and nanoscale cohesion relevant to systems ranging from biological membranes to clays or hydrated cement. We find that both effects are the consequence of the formation of hydrated-ion structures that are induced by the confinement and dramatically reduce water dielectric screening. Our results for different counterions allow us to disentangle the distinct effects and interplay of ion size and valency in the water-ion structuring, which then naturally lead to ion-specific nanoscale forces.