Dynamics of water and ions confined between charged surfaces

Confined water and ions mediate the interactions between charged surfaces in a wide range of contexts, from biological membranes and DNA to clays and cements. We use numerical simulations with a semi-atomistic model to study electrolyte solutions (ions, molecular water) confined by uniformly charged planar surfaces. By investigating sodium, calcium, and aluminum ions we find that with increasing ion valency the ion-water coupling grows, changing the structure and dynamics of the ion-water assemblies, which in turn affects the net pressure between the charged surfaces. However, this link is neither linear nor monotonic: instead controlled by the complex interplay of electrostatic, hydration, and entropic effects. In certain conditions, the formation of a highly organized, nearly frozen assembly of water and ions minimizes energy and produces a strong cohesion between surfaces, while in others the presence of large and very stable hydrated structures creates repulsive barriers in the effective interaction potential. By sampling a broad parameter space of ion type, surface charge, and confinement, we determine how these parameters control the structure and dynamics of the confined water/ions and determine the net interactions between the charged surfaces.