Hydration forces: Observations, explanations, expectations, questions

The hydration force between large molecules or large surfaces is built on weak perturbation of many solvent molecules. The structure of the surface sets boundary conditions on solvent while structural forces within the solvent set the range. For this collection of essays, we focused on forces between surfaces at nanometer separations. It is instructive to distinguish primary hydration, the binding of water and perturbation within a few layers, from secondary hydration related to redistribution of solutes. The subject is still basically empirical, lacking satisfactory theory and sufficient measurement.