Dynamics of water, hydrated-ions and charged polymers in highly-confined films, and their role in friction modification
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Dynamics of water, hydrated-ions and charged polymers in highly-confined films, and their role in friction modification 1,2,*
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Jacob Klein , Uri Raviv , Susan Perkin , Nir Kampf , Suzanne Giasson 1 Weizmann Institute of Science, Rehovot 76100, Israel 2 P.T.C.L., University of Oxford, Oxford OX1 3QZ, U.K. 3 Present address: MRL, Univ. California at Santa Barbara, CA 93106, U.S.A. 4 Département de Chimie, Université de Montréal C.P. 6128, Montréal, Qué., H3C 3J7, Canada * (- [email protected] or [email protected]). ABSTRACT 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 [1,2]. 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 [3]. This behaviour is attributed to the tenacity of the hydration sheaths together with their rapid relaxation time. Finally, a very recent study [4] 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. REVIEW AND DISCUSSION OF RECENT RESULTS The effective viscosity of oils and other non-associating liquids, such as organic solvents, when compressed between solid surfaces to film thicknesses of less than some 5-8 molecular layers, tends to increase sharply as the film thickness decreases [5]. This can happen either gradually if the molecules are asymmetric, as for the case of liquids consisting of chain-like molecules, or more abruptly [6] – at some given confinement – for liquids consisting of simple quasi-symmetric molecules (such as cyclohexane or OMCTS). In such cases the confined liquid, upon being sheared, becomes capable of sustaining a yield stress for macroscopic periods, thereby behaving as a solid (albeit one whose yield stress may be much weaker than for the corresponding bulk crystal). Recently, using a surface force balance (SFB) the fluidity of pure water (no added salt) was studied, when it was confined to films of thickness in the range 3 – 0±0.3 nm, i.e. from about ten down to at most one or two molecular layers [1,2]. This was done both by observing the shear forces transmitted across the water as one of the confining surfaces slid past the other, and particularly by analyzing the time over which the two surfaces jumped into contact under attractive van der Waals forces. Such jumps occur as the su
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