Fluidity of water and of hydrated ions confined between solid surfaces to molecularly thin films

Abstract

In contrast to non-associating liquids such as oils or organic solvents, whose viscosity diverges when they are confined by solid surfaces to films thinner than about ten molecular diameters, recent studies reveal that salt-free water remains fluid, with a viscosity close to its bulk value, even when confined to films down to only one or two monolayers thick. For the case of high concentration aqueous salt solutions compressed down to subnanometre films between confining planar surfaces, the hydration sheaths about the ions (trapped between the oppositely charged surfaces) also remain extremely fluid: this behaviour is attributed to the tenacity of water molecules in the hydration layers together with their rapid relaxation/exchange time. Related experiments on highly compressed, polyelectrolyte brushes in aqueous media reveal a remarkable lubricity which is in large measure attributed to similar hydration layers about the charged segments: this water of hydration strongly resists being squeezed out, but at the same time it may rapidly exchange with adjacent water molecules, thereby remaining quite fluid and acting as a molecular lubricant.

Progress in experimental study of aqueous lubrication

“…On the other hand, Horn et al [64] found that water retains a fluidity characteristic of the bulk liquid, although the structure was modified. This was supported by the SFA results between bare mica surfaces from Klein et al [65]. Klein et al [65] studied the fluidity of highly purified deionized water confined in a gap less than 3.5 nm.…”

“…This was supported by the SFA results between bare mica surfaces from Klein et al [65]. Klein et al [65] studied the fluidity of highly purified deionized water confined in a gap less than 3.5 nm. The study suggested that the effective viscosity of water remains at the same order of its bulk even when confined in the range of 3.5±1 to 0.0±0.4 nm, which markedly differs from non-associative liquids [66].…”

“…Lee et al [75] showed that poly(L-lysine)-g-poly (ethylene glycol) (PLL-g-PEG) improves the lubrication capability of water for metal-oxide-based tribo-systems. A relatively low friction coefficient (about 0.0006-0.001) even at low sliding speeds and at pressures of up to several atmospheres was reported by Raviv et al [65] for the polyelectrolytes (PMMA-b-PSGMA) using the SFA. The results show that brushes of charged polymers attached to surfaces result in superior lubrication ability.…”

“…The study suggested that the effective viscosity of water remains at the same order of its bulk even when confined in the range of 3.5±1 to 0.0±0.4 nm, which markedly differs from non-associative liquids [66]. As shown in Figure 1, confinement for water molecules seems primarily to suppress the formation of the highly directional hydrogen-bonded networks associated with freezing [65]. In the case of water with low ion concentration (~10 3 mol/L), the effective viscosity of a water film confined between two bare mica surfaces reportedly ranged from 10 3 to 10 7 Pa s within 0.2±0.2 nm confinement [63].…”

“…The results show that brushes of charged polymers attached to surfaces result in superior lubrication ability. Figure 6 shows the results of Raviv and his co-workers [65]. The good lubrication performance is attributed to the strong resistance to interpenetration of compressed brushes, combined with the fluidity of the hydration layers surrounding the charged polymer segments.…”