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.

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► The hydration force is built on weak perturbation of many solvent molecules.
► Surfaces set solvent boundary conditions, forces within solvent set range.
► Distinguish primary from secondary hydration forces by solute redistribution.
► Deformable interfaces also require a lateral equation of state, pressure vs. area.