Dynamic properties of confined hydration layers

Prompted by the recent discovery that water and aqueous monovalent Na(+) solutions remain fluid-like when confined to films of a few molecular layers between mica surfaces,[Raviv et al., Nature, 2001, 413. 51-54; and Raviv and Klein, Science, 2002. 297, 1540-1543] we now extend the previous study by comparing the shear- and normal-force properties of 0.1 M Na(+), Cs(+) and Ni(2+) salt solutions which demonstrate a diverse range of behaviors under confinement. In the case of hydrated Na(+) we extend the previous study to higher pressures, up to similar to 10 atmospheres, and record similar fluidity of the hydration layers at these elevated pressures. Aqueous Cs(+) films under confinement between mica sheets have been found to be unable to support an applied load-that is to say they do not demonstrate any hydration repulsion regime as a result of their lower hydration energy [see Goldberg et al., Phys. Chem. Chem. Phys., 2008. 10, 4939-4945] which contrasts with the properties of Na(+). We show that 0.1 M Ni(2+) solution remains close to its bulk viscosity down to nanometre thin films, but does not demonstrate a hydration repulsion. By comparing the properties of this range of cations, with differing valency and hydration, we aim to examine the conditions under which ions serve as a effective lubricants and what we call the 'hydration lubrication' mechanism.