Dynamics of water, hydrated-ions and charged polymers in highly-confined films, and their role in friction modification

Recent studies have revealed that, in contrast to non-associating liquids such as oils or organic solvents, salt-free water retains a viscosity close to its bulk value even when confined to films thinner than some 3 nm, indeed down to only one or two monolayers thick. For the case of high concentration aqueous salt solution compressed down to subnanometer films between charged surfaces, the trapped hydrated ions serve to act as molecular ball-bearings, sustaining a large load while remaining very fluid under shear. This behaviour is attributed to the tenacity of the hydration sheaths together with their rapid relaxation time. Finally, a very recent study has shown that when charged polymer brushes in aqueous media are compressed and slid past each other, they provide a lubrication that is considerably superior to that afforded by neutral brushes: This is attributed on the one hand to the resistance to mutual interpenetration of the chains due to entropic barriers in the good-solvent conditions, and, on the other hand, to the hydration-sheaths on the charged polymer segments which can act - as noted above - as molecular ball-bearings.