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Fluidity of Bound Hydration Layers

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Science  30 Aug 2002:
Vol. 297, Issue 5586, pp. 1540-1543
DOI: 10.1126/science.1074481

Abstract

We have measured the shear forces between solid surfaces sliding past each other across aqueous salt solutions, at pressures and concentrations typical of naturally occurring systems. In such systems the surface-attached hydration layers keep the compressed surfaces apart as a result of strongly repulsive hydration forces. We find, however, that the bound water molecules retain a shear fluidity characteristic of the bulk liquid, even when compressed down to films 1.0 ± 0.3 nanometer thick. We attribute this to the ready exchange (as opposed to loss) of water molecules within the hydration layers as they rub past each other under strong compression.

The presence of water molecules tightly bound to ions or ionized surfaces in aqueous electrolytes leads to strong repulsion when they approach each other to within a few nanometers or less (1–4). This effect is thought to arise from the reluctance of the ions or surfaces to shed their hydration sheath (3–6). It can dominate the double-layer repulsion/van der Waals attraction mechanisms [accounted for in the classic DLVO (Derjaguin, Landau, Verwey, and Overbeek) theory (7)] and is particularly important at the high salt concentrations (∼0.1 M salt) found in nature. The way in which the properties of such hydration layers differ from those of bulk water has for decades excited much debate (8–11). At issue here is a simple question: Is the hydration layer surrounding such highly confined bound ions fluid, or is it highly viscous? The difference is crucial and is directly implicated in areas ranging from clay plasticity (12) and biolubrication (13) to gating of charge migration in DNA (14). In addition, many biological processes require shear and displacement of the final subnanometer layers of bound hydration layers before molecular contact or passage. These include interactions between ligands and receptors, transport within the very crowded intracellular environment (15) or through ion channels (16), and protein folding (17).

Extensive direct measurements, as well as modeling (1–4, 6), have shed much light on equilibrium interactions of such bound hydration layers. In contrast, few direct measurements have been reported concerning their fluidity (18–22), particularly in the regime of the hydration sheaths, i.e., films of thickness D = 7 to 10 Å (23–25). We used a surface force balance (SFB) with extreme sensitivity in measuring shear interactions to probe directly the fluidity of aqueous electrolytes compressed and sheared between molecularly smooth mica surfaces. While our results confirm the long-established equilibrium hydration repulsion, they reveal at the same time that the bound water in the hydration layers remains extremely fluid under shear. This fluidity persists down to films in the range D = D c = 1.0 ± 0.3 nm, a thickness comparable to the size of hydrated ions in solution. Within such films, most of the confined water molecules are expected to be in bound hydration layers.

The SFB used has been described in detail (26). Its main features are schematically outlined in Fig. 1. We focus here on investigations of NaCl solutions, although preliminary studies on KNO3solutions (in the hydration repulsion regime) reveal very similar findings. The following results are based on several different experiments (different pairs of mica sheets), as well as on different contact positions within an experiment. Initial measurements in salt-free water were found to be crucial to establish the integrity and purity of the system. They were made to ensure the removal of any contaminant layers that adsorb on the mica while exposed to air (2, 27, 28), so as to attain true (mica-lattice/mica-lattice) contact between the surfaces (29). When the above behavior (29) could not be reproduced, as in our own earlier studies (30), subsequent measurements at high salt concentrations revealed high effective viscosities already at values of D as low as <2 to 3 nm, and these were attributed to contamination. A previous observation (31, 32) of comparably high viscosities in salt solutions (∼0.03 M) confined to D < 2 to 2.5 nm may have been related to this. Improvements in water purification and handling in the present study resolved this problem, as shown in figure 1 of (33) and in Fig. 1 of this study.

Figure 1

Interactions across low-concentration aqueous salt solution. The normalized forcesF n/R as a function of closest surface separation D are shown on approach of curved muscovite mica surfaces across (1 ± 0.1) × 10−3 M NaCl (Mica grade I; S & J Trading Inc., New York, is glued, using Shell EPON 1004F, onto cylindrical quartz lenses with a mean radius of curvatureR = ∼1 cm). The top left inset shows schematically the main features of the SFB used (26), indicating the two orthogonal springs Ks and Kn whose bending measures directly the shear and normal forces between the surfaces as they are moved laterally parallel (±Δx 0) or normal to each other, respectively. The solid line is the theoretical DLVO fit (7) to theF n(D)/R plot,F n/ 2πR = 64Ck B Tκ−1tanh 2(eψ0/k B T) exp(-κD) –A H/12πD 2, whereC = 10−3 is the electrolyte concentration in mol dm−3, T is the temperature (296 ± 1 K), k B is Boltzmann's constant,A H is the Hamaker constant of mica across water (2 × 10−20 J), and ψ0 = 78 mV and κ−1 = 9.5 nm are, respectively, the effective (large-separation) surface potential and the Debye length corresponding to the salt concentration used. The broken line is from the study on 1.4 × 10−3 M NaCl (3). The right inset shows the applied motion (upper trace) and shear force transmitted between the surfaces (lower trace) as they approach each other under slow thermal drift before jumping (arrow J) from D =D j = 2.3 ± 0.3 nm into adhesive contact at D = D 0 = 0 ± 0.4 nm, after which they are rigidly adhered and move in tandem (all traces taken directly from the oscilloscope). On separation, the surfaces jump out to D = 2.8 ± 0.2 μm, corresponding to a surface energy γ = −9.8 ± 0.5 mN/m (determined from the pull-off force), comparable to earlier reports (40, 41). Before adding the NaCl (Fluka Certified Standard 99.886%) to the required concentration, the zero of the surface separation axis, D =D 0 = 0, is determined in conductivity water [for purification details, see (33)] and is at a position –0.8 ± 0.3 nm with respect to air contact between the surfaces, based on measurements from four different experiments (29).

High-purity NaCl was then added to a concentration of 10−3M (±10%), and normal and shear forces were measured as a further control. Mica loses K+ ions to solution, leaving a net negative surface charge, and the resultant distribution of ions in the intersurface gap leads to a long-ranged osmotic repulsion followed by a jump-in to adhesive mica/mica contact, in close agreement with earlier studies (3, 4) and in accord with DLVO theory (7) (Fig. 1). Such total extrusion of the low-salt electrolyte from between the adhering surfaces resembles that observed in salt-free water (33) and is thought to arise when the predominant hydroxonium H3O+ ions condense into the charged mica, retaining no hydrated layers at the surface (3, 4).

Shear forces were measured as described recently (33). With the surfaces compressed down to separations of a few nm, the upper mica surface is made to move laterally back and forth, exactly parallel to the lower one at velocity v s (Fig. 1inset, upper trace). The shear forces F stransmitted across the intersurface gap are simultaneously recorded as the surfaces further approach under slow thermal drift. No shear forces greater than the noise-limited sensitivity δF s (±30 nN) are detected between the surfaces down to adhesive contact at D =D 0 = 0.0 ± 0.4 nm. Analysis similar to that of (33) reveals that, as for the case of salt-free water (33), the effective viscosity of the 10−3M salt solution remains close to its bulk value down to subnanometer-thick films and during the jump into contact atD 0. In all cases, the behavior described was fully reproducible on subsequent separations and approaches and consistent with earlier reports (19, 20,22) on the viscosity of aqueous salt solutions confined to thicker films (D ≥ 1.8 nm).

Following this, the surfaces were taken apart and the NaCl concentration increased to (0.7 ± 0.2) × 10−2M and subsequently to (0.8 ± 0.1) × 10−1 M. At these salt concentrations hydration layers are bound at the surfaces, ensuring that on normal approach repulsive hydration forces overcome the van der Waals attraction at all surface separations (3, 4). At both these concentrations we observed very similar shear force behavior. Figure 2 summarizes the normal force profiles and the shear measurements for both NaCl concentrations [(0.7 ± 2) × 10−2 and (0.8 ± 0.1) × 10−1 M]. The normalF n(D) profiles (Fig. 2A) confirm the results of earlier studies (3, 20) showing the strong repulsion due to the confined surface-attached hydration layers at separations D < ∼2 nm. Figure 2B [upper trace (a)] shows the back-and-forth motion of the upper mica surface. The lower traces (typical of many experiments) show shear force responses both at very large surface separations [trace (b), D = ∼9.6 μm] as a control, and when strongly compressed down toD = D c in the range 1.0 ± 0.3 nm [traces (c) to (e)]. Within the scatter, traces (b) to (e) are very similar. The frequency (ν) dependenceF s(ν) of the shear force responses is shown inFig. 2B on the right, with the arrows indicating the drive frequency of the upper mica surface. At this drive frequency the magnitude ofF s at D c is within the noise level δF s (±30 nN) of its magnitude at macroscopic separations. In other words, the resistance to sliding even at these strongest compressions across the thinnest films is extremely weak, and within our sensitivity is indistinguishable from its value when the surfaces are far apart. This behavior was reversible and reproducible. The surfaces could be separated from D =D c to large separations, then recompressed to show again the absence of any detectable frictional force (34, 35).

Figure 2

Interactions across high-concentration aqueous salt solutions. (A) Normal forces measured on approach of the surfaces at two different concentrations: (0.7 ± 0.2) × 10−2 M, empty symbols; and (0.8 ± 0.1) × 10−1 M NaCl solution, filled symbols, corresponding to the shear traces in (B). The solid lines are a fit to a DLVO expression together with a short-ranged exponential term representing the hydration forces:F n/2πR = 64Ck B Tκ−1tanh 2(eψ0/k B T) exp(−κD) –A H/12πD 2 +E hexp(−D/D h). The fit values are ψ0 = 58 mV, κ−1 = 4.0 nm, E h = 0.1 J/m2, andD h = 0.35 nm; and ψ0 = 35 mV, κ−1 = 1.8 nm, E h = 0.1 J/m2, and D h = 0.3 nm at the lower and higher NaCl concentrations, respectively. The broken lines are from previous studies for the corresponding lower (3) and higher (20) NaCl concentrations. (B) Variation of the shear force responseF s(D) [traces (b) to (e)] with lateral motion of the top surface [trace (a)] for curved mica surfaces sliding past each other across the salt solutions (see schematic in fig. 1), at theD values indicated. The respective salt concentrations for the different traces are as follows: (b), (d) – 0.8 × 10−1 M NaCl; (c), (e) – 0.7 × 10−2 M NaCl. The frequency components of the shear forces corresponding to the traces are shown to the right in (B), with the drive frequency of the top surface (0.5 Hz) indicated by arrows. The peaks around 2.4 Hz correspond to the flexural motion of the building. The cartoon in (C) shows, approximately to scale relative to the mica surface separation D = 1 nm, the spacing of the ionizable mica surface lattice sites (∼0.7 nm) and the size of a free hydrated Na+ ion [roughly 0.75-nm diameter from bulk studies (5), or somewhat smaller for surface-attached ions, as suggested in (4)]. Water molecules (diameter ∼0.25 nm) are indicated for comparison.

To examine the generality of our findings, we varied several parameters. Various (high) salt concentrations and different monovalent salts (NaCl and KNO3) were used. The similarity of the results shows that for these two pairs, neither the different binding (monovalent) metal counterion nor the different co-ion affected the results. Shear rates were varied over the range 6 to 1200 s−1. The mica sheets were mounted in different relative orientations, and the close similarity of results in all instances thus shows that this does not—within the scatter—affect the fluidity of the bound hydration layers. In two of the experiments mica pairs from the identical cleaved facet were used, and the results in those instances did not differ (within the scatter) either among themselves or from those where a pair of surfaces from a completely different mica sheet were used, suggesting a lack of sensitivity to the precise mica surface composition. Within the resolution and range of our experimental conditions, the fluidity of the bound hydration layers appears to be a general feature. In addition, stopping the sliding motion (at D = D c) for periods up to 5 min and then restarting did not change the behavior. This indicates that any rearrangement of the layers over these time scales did not lead to significant differences.

We estimate an upper limit on the effective viscosity (ηeff) of the bound hydration layers (36) to be ≤ 0.08 Pa·s. Although this value is higher than the viscosity of the bulk electrolyte (∼10−3 Pa·s), we stress that it is resolution-limited by F s ≤ δF s (∼30 nN) and is therefore an upper limit, so that the actual value may be much closer to the bulk viscosity. The mean pressure in the flattened contact region of areaA isF n/A = ∼4 atm at the typical normalized loadsF n/R = 104 μN/m associated with the high-compression regime (36).

These findings have interesting implications. The surface potential associated with the mica surfaces, deduced from theF n(D) profile, corresponds to a net surface charge of ∼1e/10 nm2. Although the precise state of the hydrated sodium ions in the gap is not known, this value (which applies for the two surfaces far apart), together with estimates of the density of the ions on the mica surfaces at these high salt concentrations (3), suggests that hydrated Na+ions had condensed over nearly all of the ionizable lattice sites (Fig. 2C). Given the size of the hydration sheath surrounding the ions (0.7 to 0.8 nm in the bulk) (5), this result shows that at surface separations of 1 nm the gap between the surfaces is occupied mostly by charge-bound water molecules, and that on sliding of the surfaces these bound layers must rub past each other. As shown by our data, the resulting shear forces are characteristic of a fluidity that is not too far removed from that of bulk water. This raises the following question: if the hydrating water layers are so tenaciously bound as to render the film almost solidlike in its reluctance to being squeezed out, thereby preventing approach of the surfaces, how can they at the same time be so fluid, providing striking lubrication as they slide past each other? [In contrast, free or unbound water, which also retains its bulk fluidity down to subnanometer confinement, is readily squeezed out by the confining surfaces as they come into adhesive contact) (Fig. 1) (33)].

The answer is unlikely to be related to the mobility of the surface-bound ions themselves (37). Rather, it may be attributed to the following. Water molecules are difficult to remove outright from the surface-attached ions to which they are bound, owing to the large energy penalty associated with the increase in the ion self-energy (5). However, a much lower energy barrier may be associated with exchange of water molecules at the outer surfaces of the two bound hydration layers. During such an exchange, the remaining bound molecules could rearrange at little entropy cost to minimize the binding charge's self-energy (5). This would also be consistent with the rapid exchange of hydration water surrounding monovalent ions in bulk solution (38). A corollary is that the hydrated layers act as highly efficient lubricants (39). They are capable of supporting a large normal load because on average, the bound water tenaciously adheres to the underlying surface or ions, but the rapid exchange of molecules [which in the bulk liquid occurs at ∼109 s−1 (38)] ensures that the surface-bound hydration layer remains very fluid at the shear rates (up to ∼103 s−1) used in our study.

Finally, this fluidity of bound water at salt concentrations and pressures typical of those in living systems has implications for electrolytes under extreme confinement in biological and other naturally occurring systems. Our results indicate that such aqueous species, irrespective of the structural origin of their equilibrium interactions (6, 9, 10), will have mobilities that do not differ greatly from those in the bulk solution. This is because the conditions of our experiments—confining the ions by hard, smooth, parallel, high-energy surfaces—present an extreme of confinement. This suggests that the fluidity we observe is more than likely to carry over to bound hydration layers in the less stringent, softer environment of biological systems.

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