pubs.acs.org/Langmuir © 2009 American Chemical Society
Interactions between Molecularly Smooth Gold and Mica Surfaces across Aqueous Solutions Liraz Chai and Jacob Klein* Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel Received April 23, 2009. Revised Manuscript Received June 20, 2009 Using a surface force balance, we measured the forces between an ultrasmooth (0.2 nm rms roughness) templatestripped gold surface and a molecularly smooth mica surface. Comparison of these forces in both low salt (conductivity water, equivalent to 10-6-10-5 M 1:1 salt) and high salt (10 mM KClO4) regimes enabled us to examine the properties of water layers confined between a metal and a dielectric to films of a few nanometers or less in thickness. We find that the long-range forces between gold and mica are similar to those between two mica surfaces, indicating a net effective negative charge density on the gold similar to that on the mica. Differences were more pronounced at small separations, manifested by the larger jump-in distance in pure water and the weaker hydration repulsion in high salt between a gold and a mica surface compared with two mica surfaces. However, despite these short-ranged differences, replacing one mica surface with gold does not measurably alter the viscosity of nanoconfined water layers, either as free molecules or as bound hydration layers, relative to their confinement by two mica sheets.
Introduction Interactions between surfaces across liquid media are a fundamental issue in science and technology. Stabilization of colloids by electrostatic forces1 or by adsorbed polymers,2,3 the interaction of cells with surfaces,4 and the crystallization of globular proteins due to the interaction of protein segments5 are all manifestations of surface interactions. The most direct techniques for measuring surface forces are the surface force balance (SFB)6-8 and scanning probe techniques such as the atomic force microscope (AFM).9 The latter enables a larger choice of interacting surfaces; the SFB, on the other hand, is strongly advantageous in terms of the sensitivity of the force per unit area8 and especially in its ability to determine absolute surface separations, which is essential when examining nanometer and subnanometer films. However, the SFB is often limited to use of molecularly smooth mica substrates as the test surface.7,10 Indeed, with a few exceptions where silica,11 platinum,12 mercury,13 or gold14 were used in surface force measurements, mica has been used as a model surface for direct measurements of surface interactions for the past 40 years. In particular, the smoothness of the surface of mica;a single *Corresponding author: e-mail
[email protected]; Tel +972-8934-3823; Fax +972-8-934-41-38. (1) Derjaguin, B. V.; Churaev, N. V.; Muller, V. M. Surface Forces; Consultants Bureau: New York, 1987. (2) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983. (3) Klein, J.; Luckham, P. F. Macromolecules 1984, 17, 1041. (4) Cohen, M.; Klein, E.; Geiger, B.; Addadi, L. Biophys. J. 2003, 85, 1996. (5) Pellicane, G.; Costa, D.; Caccamo, C. J. Phys.: Condens. Matter 2003, 15, 375. (6) Tabor, D.; Winterton, R. H. Nature (London) 1968, 219, 1120. (7) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 79, 975. (8) Klein, J.; Kumacheva, E. J. Chem. Phys. 1998, 108, 6996. (9) Binning, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930. (10) Perkin, S.; Chai, L.; Kampf, N.; Raviv, U.; Briscoe, W.; Dunlop, I.; Titmuss, S.; Seo, M.; Kumacheva, E.; Klein, J. Langmuir 2006, 22, 6142. (11) Vigil, G.; Xu, Z.; Steinberg, S.; Israelachvili, J. J. Colloid Interface Sci. 1994, 165, 367. (12) Smith, C. P.; Maeda, M.; Atanasoska, L.; White, H. S. J. Phys. Chem. 1988, 92, 199. (13) Connor, J.; Horn, R. Langmuir 2001, 17, 7194. (14) Frechette, J.; Vanderlick, T. K. Langmuir 2001, 17, 7620.
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crystallographic plane;enabled the study of the properties of water under nanometer and subnanometer confinement.15,16 Mica becomes negatively charged in pure water due to the release of potassium ions from the surface lattice. The forces between such negatively charged, bare mica surfaces in aqueous solutions therefore follow the Derjaguin-Landau-VerweyOverbeek (DLVO) theory:17,18 repulsive at large separation due to the osmotic repulsion of trapped counterions and attractive at small separations where van der Waals forces dominate. In measuring instruments where one surface is mounted on a spring (as in the SFB, Figure 1), the surfaces jump-in to contact due to the instability of the spring when ∂F/∂D > Kn, where Kn is the normal spring constant. Using the SFB to measure the jump-into-contact time, the viscosity of water layers confined between two mica surfaces was found to be of the order of bulk water, even when confined to a few molecular layers or less.15 In aqueous salt solutions above some critical hydration concentration (CHC), the DLVO theory fails to describe the forces below separations of a few nanometers. In this hydration-repulsion regime, hydrated ions trapped between the surfaces prevent them from reaching atomic contact or jumping-in to a primary minimum.19,20 Rather, the water molecules of hydration are so strongly held by the counterions that they can withstand a pressure of at least several atmospheres. At the same time, the friction between the surfaces sliding in this hydration repulsion regime remains very low due to the fluidity of the tenaciously held hydrating water molecules.16 In order to generalize the behavior of water confined between mica surfaces to different confining surfaces at nanometer-scale separations, we used smooth gold that was prepared using the template-stripping method described earlier.21 In a previous study (15) Raviv, U.; Klein, J. Nature (London) 2001, 413, 51. (16) Raviv, U.; Klein, J. Science 2002, 297, 1540. (17) Verwey, E. J. W.; Overbeek, J. T. G. Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. (18) Israelachvili, J. N. Intermolecular and Surface Forces, 9th ed.; Elsevier Science Ltd.: Amsterdam, 2002. (19) Pashley, R. M. J. Colloid Interface Sci. 1981, 80, 153. (20) Pashley, R. M. J. Colloid Interface Sci. 1981, 83, 531. (21) Chai, L.; Klein, J. Langmuir 2007, 23, 7777.
Published on Web 07/22/2009
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Figure 1. Schematic of the surface force balance (SFB).8 The fused silica lenses on which the mica and gold films are glued are mounted in a crossed-cylinder configuration. The bottom surface (usually mica in gold-mica experiments) is mounted on a leaf spring of spring constant Kn. In the measurement of normal profiles we approach with the top surface and measure the separation between the surfaces, D, relative to the absolute D = 0 in adhesive contact in water. Changes in the intersurface separation, D, are translated into increments of normal force, ΔFn, through the bending ΔD of the spring of constant, Kn: ΔFn=KnΔD. To measure shear forces, a motion ΔX0 is applied to the top surface, and the shear force Fs transmitted to the bottom surface is monitored via the bending ΔX* 0 of the shear spring Ks (of spring constant Ks) measured with a sensitive capacitance probe as Fs=KsΔX0*.
where forces between gold and mica surfaces were measured across an aqueous solution,14 the average roughness of the gold surface was 2 nm, an order of magnitude rougher than in the present study and some 10-fold larger than the size of a water molecule. Furthermore, since the gold was exposed to ambient air prior to insertion into the solution, it may have additionally collected a thin layer of contaminants.21 As a result, the properties of water confined to D e 2 nm could not be studied. In contrast, the rms roughness (ca. 0.2 nm) of the template-stripped gold in our present study21 is similar to or slightly smaller than the size of a single water molecule. This, together with the cleanliness of the as-prepared gold surface,21 enabled the measurement of normal forces between gold and mica down to within a few molecular layers or less. Here we extend significantly our earlier preliminary results21 to examine comprehensively both the normal and, especially, shear forces between a sub-nanometer-smooth gold surface and a mica surface, across water with no added salt and across an aqueous salt solution. In particular, via comparison with the corresponding interactions between two mica surfaces, we examine the effect of replacing mica by gold on long-ranged forces as well as on previously measured characteristics of water confined to nanometric films. These include the bulklike viscosity of pure water in ultrathin films and the fluidity of trapped hydration water molecules under shear.
Experiment and Materials Materials. Conductivity water was prepared by passing tap water twice through a reverse osmosis system and then passing through mechanical filters of mesh size 5 and 2 μm or an ionexchange column, before processing in a Milli-Q A-10 water purification system or a Barnsted Nanopure Diamond UV/UF system. This treatment resulted in water with a total organic content (TOC) of 3-4 ppb (Milli-Q) or less than 1 ppb (Barnsted), a resistivity of 18.2 MΩ cm (so-called conductivity water), and pH ∼ 5.5. Gold (99.99%) was purchased from Kurt J. Lesker and evaporated as previously described.21 Mica was ruby muscovite, grade I or V-2 special grade, from S&J Trading Inc. (New York). Potassium perchlorate, KClO4, 99.99+% pure, was purchased from Aldrich and used as received. The perchlorate was chosen as a counterion because it does not adsorb specifically 11534 DOI: 10.1021/la9014527
onto gold.22,23 The pH of the 10 mM potassium perchlorate solutions was in the range 5-5.5. Experiment. The SFB used to measure the normal and shear forces between the surfaces has been described in detail earlier.6-8 Figure 1 shows a schematic of the SFB where two plano-cylindrical fused silica lenses are mounted in a cross-cylinder configuration. The closest distance between the surfaces, D, was measured via the separation between interference fringes, as detailed below. Changes in the intersurface separation, D, in response to an applied normal motion of the top surface, were translated into increments of normal force, ΔFn, through the bending ΔD of the normal spring of constant Kn = 150 N/m: ΔFn = KnΔD. Shear forces, Fs, were monitored via the deflection, ΔX0*, of a lateral spring of constant Ks =300 N/m, Fs =KsΔX0*, in response to an applied lateral motion, ΔX0, of the top surface. The sensitivity in the shear force measurement was δFs = ( 40 nN, as extracted from a fast Fourier transform (FFT) analysis of the data and comparison with systematic signals at large separations. In addition to any signals at the applied frequency, the FFT traces show prominently the noise at the vibration frequency of the building at ca. 2 Hz (see e.g. Figure 5). We measured the interaction between a gold film and a mica surface that were glued on opposing lenses. Gold was prepared and glued onto the lens as described in detail earlier.21 However, it was not always annealed; therefore, its surface domains could be other than the (111) crystallographic planes obtained on annealing.21 Nonetheless, the gold surface rms roughness remains at ca. 0.2 nm. Mica was either downstream melt-cut or torn-off,10 back-silvered, and glued onto a second lens (we used an epoxy glue, EPON 1004, for both surfaces) with the silver facing the glue. In most of the experiments the gold was mounted on the top lens, but similar results (particularly in shear) were obtained for the opposite configuration. In an SFB the separation between the surfaces corresponds to the position of interference fringes that are formed by multiple reflections between reflecting layers. Earlier21 we showed that the roughness of the gold surface (∼0.2 rms) does not contribute either to the peak broadening δλ or to shifting of the fringe tip positions (relative to the theoretical values for a given mica thickness). However, the scatter in our absolute D=0 measurements (measured by the scatter of the measured spacing between adjacent fringes in different contact positions) was (0.7 nm in the gold-mica configuration relative to the optimal resolution of ca. (0.2-0.3 nm in a mica-mica configuration. This decrease of the resolution of the gold-mica separation is intrinsic to the system and can be explained as follows. The mica-mica interferometer is composed of three layers confined between silver films: silver/mica/medium/mica/silver. In contact, the condition for the nth-order constructive interference fringe is24 2kμY=nπ, k = 2π/λ, where μ is the refractive index of mica and Y is the thickness of the mica sheet. To a first approximation and since D , Y (D is of the order of nanometers and Y of the order of micrometers), we may use the same condition for constructive interference at a separation, D (where for simplicity we assume some average μ for the system), and therefore 2kμ(Y + D) = nπ, so that δD=nδλ/4μ. In contrast, the gold-mica system is composed of two layers confined between silver and gold: gold/medium/mica/ silver. Therefore, for the same mica thickness 2kμ(Y + D) = 2nπ, and δD = nδλ/2μ. As a result, similar δλ values;the uncertainty or scatter in the measured wavelength;would result in a larger surface-separation uncertainty δD in the gold-mica system relative to the mica-mica one (note: the relation between D and λ in the exact expression for constructive interference is not linear, and therefore δD is not constant throughout the measured D space). We also note that in our previous study we analyzed the (22) Clavilier, J.; Huong, C. N. V. J. Electroanal. Chem. 1977, 80, 101. (23) Hamelin, A.; Vitanov, T.; Sevastyanov, E.; Popov, A. J. Electroanal. Chem. 1983, 145, 225. (24) Israelachvili, J. N. J. Colloid Interface Sci. 1973, 44, 259.
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Figure 2. Normal force vs surface separation profiles Fn(D)/R measured in water with no added salt (pH ∼ 5.5) between gold and mica. (In the Derjaguin approximation, such normalization by the mean surface curvature radius R yields the interaction energies per unit area of flat parallel surfaces obeying the same force laws.) Different symbols show results from different contact positions in different experiments from the present study. The black arrow marked J indicates the position Dj of jump-in to contact (mean value Dj=6.6 ( 2 nm). The inset shows the forces between two mica surfaces under similar conditions as reported in different earlier studies: open triangles, from ref 10; diamonds, from ref 15; circles, from ref 19; stars, from ref 26. Empty squares show the force between silver and mica in pure water, taken from Parker et al.27 Solid symbols correspond to the gold-mica forces in our study, taken from the main plot. photographed fringes and the positions of the fringe tips were determined by fitting the light intensity at the fringe tip to a Gaussian distribution. In other experiments, where we did not use image analysis for measuring the fringe positions, the scatter in the absolute D = 0 could be as high as (1 nm (depending on the thickness of the mica used in each experiment). In the mica-mica experiments we first measured the fringe positions in air contact. After adding water, cleanliness was confirmed by jump-in of the surfaces into adhesive contact which also determined the absolute D=0 fringe position. (This D=0 is 1 ( 0.5 nm closer in relative to the mica-mica contact in air due to dissolution of a thin water-soluble layer adsorbed to mica under ambient conditions.10) To change salt concentrations, water was drawn out of the chamber, leaving a meniscus between the surfaces to prevent their drying, and a different salt solution was introduced to the chamber through a precleaned injection system; this was then repeated. In contrast to the mica-mica experiments, when we worked in a gold-mica configuration, we did not calibrate the fringe positions in air in order to minimize air exposure and contamination of the gold surface by carbonaceous (and other) contaminants. Instead, we glued a mica piece onto the lens and placed it in a SFB bath. The gold-on-mica piece was then glued onto a second lens, and after filling the SFB bath with water, the mica was stripped off the gold, which was immediately (within t < 1 min) mounted in the water-filled bath. For such short air exposure time the gold remains relatively hydrophilic.21,25 The absolute D=0 was determined as the fringe position after the gold and mica surfaces jumped-in to adhesive contact in water. Salt was introduced to the system in a similar way to that described above for the mica-mica configuration. The normal force profiles are measured only on a first or second approach in a given contact position in order to prevent any influence of asperity flattening on roughness or cleanliness of the gold surface. As earlier reported, the force profiles on first and second approach to the same contact position are similar.21 (25) Smith, T. J. Colloid Interface Sci. 1980, 75, 51.
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Figure 3. Shear forces transmitted between upper and lower surfaces, as recorded by a capacitance probe (see Figure 1 caption). In trace a the surfaces are approaching in the normal direction and the vibrations due to ambient noise are recorded. After the surfaces spontaneously jump-in to contact (black arrow), the noise decreases due to the adhesion between the surfaces. The marked area in (a), on an expanded time scale on the right-hand side, shows that the noise level within the jump-in period (between the arrowheads), just prior to the surfaces making contact, is similar to that measured at larger separations, suggesting little increase in the viscous coupling during the jump itself. Traces b and c illustrate a different procedure used to measure shear forces between the surfaces during their approach into contact from D > Dj. A back and forth motion is applied to the top surface, as shown in trace b (in the specific experiments we show here the shear velocity was 330 and 180 nm/s for the mica-mica and gold-mica configurations, respectively). The corresponding shear forces transmitted between the surfaces are shown in traces c and d for the mica-mica and gold-mica configuration, respectively. The mica-mica contact interfacial friction is larger than the applied shear forces and does not yield (trace c, right, where the surfaces move in tandem as they are rigidly coupled). In contrast, in the gold-mica system the applied force exceeds the static friction force at the yield point (marked as Fy) and the top surface slides over the bottom (right, horizontal parts of trace d).
Results Conductivity Water (No Added Salt). Figure 2 shows the force profiles between template-stripped (TS) gold and mica in pure water, measured in different contact positions and within different experiments (independent pairs of surfaces). As briefly indicated earlier,21 the forces between gold and mica resemble those between similarly charged mica surfaces in water, qualitatively obeying the DLVO model:17,18 At large separations, there is a long-ranged osmotic repulsion between the surfaces, resulting from the confinement of counterions between the surfaces. At short separations, this repulsion is overcome by van der Waals forces, and the overall interaction becomes attractive with the surfaces jumping into contact (marked with an arrow J in the figure) due to the instability of the spring when ∂F/∂D > Kn. The inset shows the forces between two mica surfaces in pure water (empty symbols), published in four different studies,10,15,19,26 together with the forces from the main figure. Clearly the scatter in magnitudes is similar for both mica-mica and mica-gold profiles. (The forces between silver and mica in pure water, measured by Parker and Christenson,27 shown as empty squares, also resemble those between gold and mica.) The large scatter in (26) Chai, L.; Klein, J. J. Am. Chem. Soc. 2005, 127, 1104. (27) Parker, J. L.; Christenson, H. K. J. Chem. Phys. 1988, 88, 8013.
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Figure 4. Normal forces in 10 mM KClO4 between gold and mica (full symbols) and between two mica surfaces (empty symbols). The solid line is a best (far-field) fit of the mica-mica data to eq 1 with κ-1 =2.9 ( 0.7 nm and ψ0 =66 ( 15 mV (the determination coefficient, R2 = 0.8) together with an exponential term, Eh exp(-D/Dh), describing the hydration repulsion. The fitting parameters for the hydration repulsion are Eh = 7 ( 2 mJ/m2 and Dh = 0.6 ( 0.3 nm. The dashed line is a best (far-field) fit of the gold-mica data to eq 1 (R2=0.86). The Debye length and surface potentials extracted from the far-field fit are κ-1=4.0 ( 0.5 nm and ψ0 = -47 ( 3 mV, assuming gold and mica have similar surface potentials. Putting in the value of the mica surface potential ψ0(mica) =66 ( 15 mV in eq 1, we find that the best fit (dashed line) yields ψ0(gold)=-35 ( 10 mV. The horizontal scale bars show the scatter in D for gold-mica and the mica-mica systems, respectively (the negative D values are within this scatter in D). The inset shows an expanded version of the main figure at smaller separations.
the data is characteristic of both gold-mica and mica-mica systems, and it may result from differences in the effective ionic concentration due to ions leaching from the glassware and different water purification procedures.10 The similarity in the long-ranged (or far-field) profiles suggests also a similar effective surface potential for the gold and the mica, ranging between ψ0= 110 and 150 mV.10,15,19,26 The Debye length corresponding to the range of far-field profiles is κ-1 ≈ 10015-20021 nm, corresponding to effective 1:1 electrolyte salt concentrations 9.4 10-6-2.4 10-6 M, probably originating in dissolved CO2 and ions leached from glassware. To measure the jump-in separation Dj more accurately, we positioned the surfaces at a separation close to Dj and let them approach by thermal drift sufficiently slowly (ca. 0.4-0.7 nm/s) to enable us to monitor the fringe positions continuously until the jump-in to contact. This measurement of Dj is more accurate than step-by-step profiles where steps of ca. 7-10 nm are used. Following the fringe position motion with time also enables us to set an upper bound to the jumping-in time: τj < 1 s (still more accurate measurements of τj can be made using a fast recording camera). From many such measurements of the jump distance we found that the average Dj for gold-mica was Dj = 6.6 ( 2 nm, while for mica-mica, Dj = 3.8 ( 0.5 nm. At the same time as tracking the fringe positions during approach, we record the lateral forces between the surfaces. A typical example is shown in trace a of Figure 3: the noise in the shear signal arises from lateral vibrations of the shear springs due to ambient noise. After the jump-in at Dj the surfaces reach contact (marked with an arrow), and the noise decreases sharply because the adhesion between the surfaces suppresses their relative lateral motion. The frame to the right of trace a on an expanded time scale shows that the noise level throughout the jump-in (range Dj shown by the double headed arrow) is similar to the ambient noise just before the jump-in. It was also possible to apply a back-and-forth motion to the top surface while the surfaces were drifting into contact, as shown in Figure 3b, and to monitor the corresponding shear forces transmitted between the surfaces, as shown in Figure 3c for 11536 DOI: 10.1021/la9014527
Figure 5. Traces of the applied shear motion (trace a in parts A and B) with an amplitude ΔX0 and the corresponding shear force, Fs, transmitted between the surfaces (traces b-f) for the micamica (A) and gold-mica (B) configurations. The shear traces were measured at decreasing surface separations D (from top to bottom) as shown above each trace together with the corresponding (normalized) normal force, Fn/R, for each D, taken from Figure 4. The fast Fourier transform (FFT) of each trace is given to the righthand side with an arrow pointing at f = 0.5 Hz, which is the frequency of the applied motion (top traces). The peaks around 2 Hz correspond to the motion of the building. The FFT of the noise at a large separation (D = 430 and 180 nm in parts A and B, respectively) is superimposed to the FFT signals (as broken line) showing that the shear force signal in traces b-f in part A and traces b-e in part B is within the scatter δFs of the systematic largeseparation signal.
mica-mica. After jump-in to contact (marked with a black arrow), the motion of the bottom surface tracks that of the top surface to which it is adhesively coupled. This is because the frictional force between them due to their strong adhesion overcomes the highest shear forces applied, so that the mica-mica interface does not yield; this was true even when applying a 6-fold larger shear force (Fs ≈ 165 μN, not shown) than that (Fs ≈ 25 μN) shown in Figure 3c. In contrast, the gold-mica case shown in Figure 3d behaves rather differently: In this case the surfaces also jump into an adhesive contact (arrow), but unlike mica-mica, the mica-gold adhesive coupling is much weaker and is overcome by a much weaker shear force (Fs ≈ 9 μN, the magnitude at the yield point Fy). The top surface then slides over the bottom one. On separation of mica from the gold, the surfaces jumped out of contact to a separation in the range Djo = 160 ( 80 nm, corresponding to a pull-off force, Fpull-off = 24 ( 12 mN. From the Johnson-Kendall-Roberts (JKR) model28 for the adhesion (28) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London A 1971, 324, 301.
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energy, γ = Fpull-off/3πR, we find that γ(gold-mica) = -0.3 ( 0.15 mN/m. This value is an order of magnitude smaller than the adhesion energy between two mica surfaces in water, γ(micamica)=-4 ( 1 mN/m. 10 mM KClO4. Hydrated ions trapped between two mica surfaces in a high salt solution (c > CHC) provide a strong lubricating effect.16 To examine this effect between gold and mica, we increased the salt concentration to a value, 10 mM KClO4, larger than the CHC and compared to the mica-mica at the same salt concentration.29 Figure 4 shows the normal force profiles in 10 mM in the two systems: mica-mica (empty symbols) and gold-mica (full symbols). For mica-mica the force profiles follow the DLVO description of the osmotic repulsion at large surface separation (D > ca. 3 nm), as shown by the straight solid line. This fits the data to the solution of the linearized PoissonBoltzmann equation under constant surface potential30,31and using the Derjaguin approximation,1 as eq 1: Fn ¼ 2π 3 64 3 kB TCK -1 tanh γ1 tanh γ2 e -KD , R ZeΨ1, 2 γ1, 2 ¼ 4kB T
ð1Þ
Here kB is the Boltzmann constant, T is the temperature, C is the bulk concentration in (ions/unit volume), R is the mean radius of curvature of the√surfaces, κ-1 is the Debye length18 (and is given by κ-1 =0.304/ C nm where C is in M), and ψ1,2 0 is the surface potential of either surface 1 or surface 2 (in the case of different surface potentials). From a fit of eq 1 to the data at D > 2.5 nm (with γ1=γ2) for mica-mica, we find that the mica surface potential, ψm 0 =-66 ( 15 mV, while κ-1 = 2.9 ( 0.7 nm (corresponding to C = 11 ( 4 mM). However, at a separation D < 2 nm the forces deviate from the DLVO prediction and the repulsion increases further due to the onset of hydration repulsion. This strong repulsion is expected16,32 above the CHC, in line with earlier studies of hydrated potassium ions between mica surfaces, with chloride counterions,16,20 as opposed to perchlorate ions in the present study. A better description of the forces is therefore given by adding an exponential term to account for the hydration repulsion: Fn ¼ 2π 3 64 3 kB TCK -1 tanh γ1 tanh γ2 e -KD þ Eh e -D=Dh ð2Þ R
A fit to the small D data (using the above values of κ-1 and ψm 0 extracted from the far-field fits) yields Eh=7 ( 2 mJ/m2 and Dh= 0.6 ( 0.3 nm. These values are indicative of a smaller hydration repulsion than those reported for 10 mM NaCl,16 which may be accounted for by the loss of (part of) the hydration shell of potassium when it is adsorbed onto mica.33 The force profiles between gold and mica are qualitatively similar to the mica-mica configuration at larger separations, D > 3 nm,34 and eq 1 may be used to extract the Debye length, κ-1, and the surface potentials of gold and of mica. The dashed line in Figure 4 is the fit of eq 1 to the gold-mica data (29) The CHC for potassium salts has been reported as ∼10-4 M,19 but we observed hydration effects with mica-mica only at concentrations larger than ∼10-3 M (data not shown). (30) Gregory, J. J. Colloid Interface Sci. 1975, 51, 44. (31) Parsegian, V. A.; Gingell, D. Biophys. J. 1972, 12, 1192. (32) Pashley, R. M. Adv. Colloid Interface Sci. 1982, 16, 57. (33) Goldberg, R.; Chai, L.; Perkin, S.; Kampf, N.; Klein, J. Phys. Chem. Chem. Phys. 2008, 10, 4939. (34) In a few cases there was a long-range attraction both in the gold-mica and the mica-mica systems, probably due to air bubbles that were stabilized by the perchlorate anion.
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with κ-1 =4.0 ( 0.5 nm (corresponding to C=6 ( 2 mM) and assuming similar surface potentials of both mica and gold, for which we obtain ψm,g 0 =-47 ( 3 mV. We may also fix the mica surface potential to that found above for mica-mica, ψm 0 (mica)= 6 γ2.18,30 The fit to the data in -66 ( 15 mV, and use eq 1 for γ1 ¼ this case yields ψg0(gold)=-35 ( 10 mV. At smaller separations the forces between gold and mica deviate significantly from those between two mica surfaces, as shown more clearly in the inset to Figure 4. The surfaces do not jump-in to adhesive contact, which is suggestive of the presence of hydration layers between them; however, the repulsion is much smaller than that in the micamica system, and in fact, it has a magnitude similar to that predicted by DLVO right down to contact (within the uncertainty in D). The presence of a trapped counterion layer between mica surfaces was previously associated with a hydration lubrication effect.16 We therefore compare the shear forces in the two systems on decreasing separations, particularly for the regime where the hydration repulsion is observed in the mica-mica case. Such traces are shown for mica-mica in Figure 5A and for gold-mica in Figure 5B. In both parts A and B trace a is the back-and-forth motion applied to the top surface. Subsequent traces b-f show the corresponding shear forces transmitted between the surfaces, measured at decreasing D values. The fast Fourier transform (FFT) analysis of the traces (right-hand side to each trace) gives a more sensitive measure of the shear force at the 0.5 Hz applied lateral motion frequency (marked with an arrow above the FFT plots). The FFT of the shear data when the surfaces are far apart (D=180 nm for gold-mica and D=430 nm for mica-mica) is superimposed (broken line) on traces b-f. This shows;in line with earlier studies16;that in the mica-mica case (Figure 5A) any frictional force is within the noise level ((40 nN) at the applied frequency, even at the highest applied loads, Fn/R= 69.3 mN/m (trace f). Similar results were obtained when raising the shear velocity up to Vs = 5400 nm/s (mean shear rate, γ = 18 000 s-1 at D=0.3 ( 0.5 nm). However, the shear (or friction) forces transmitted between the surfaces in the gold-mica case (Figure 5B), though within the noise level up to Fn/R=3 mN/m, trace e, clearly increase beyond that at higher loads, as seen in trace f of Figure 5B. This may be due to contact between the gold (strictly, the gold asperities) and the mica (we note that the Fn/R value at this separation corresponds to the highest Fn/R point reached in the gold-mica normal force profiles in Figure 4). Here, too, similar results were obtained at larger shear velocities (up to Vs=600 nm/s, corresponding to mean shear rates, γ=1500 s-1 in trace f).
Discussion The main new findings of this study concern the normal and shear forces between a very smooth gold surface ((0.2 nm rms roughness) and mica, both in pure water (with no added salt, corresponding to 10-5-10-6 M 1:1 electrolyte concentration) and in 10-2 M aqueous KClO4 solutions. Comparison with the corresponding forces between two mica surfaces highlights the effect of the surface properties of gold (as opposed to mica) both on the surface interactions and in particular on the properties of water under confinement when one mica surface is replaced with gold. Adsorption of Ions onto Gold in Water and 10 mM KClO4. The long-range normal forces between gold and mica in pure water resemble those between two mica surfaces, showing that both gold and mica bear a similar, negative, surface charge, both in conductivity water and in 10 mM KClO4. This observation DOI: 10.1021/la9014527
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is consistent with other force measurements, using an AFM35-37 between two gold surfaces35 (measured in 4 10-5 M-2 10-6 M NaCl solutions at pH = 6.3) and between gold and silica36 (measured in 10-2-10-3 M KNO3 at pH=6.4).38 Furthermore, an earlier SFB study shows a similar repulsive interaction of silver and mica across water27 (see inset to Figure 2). The negative surface charge of gold may be most readily explained by specific adsorption of anions from an aqueous solution35,39-41 (even at the expense of losing their hydration sheaths41), among which hydroxide is a common potential determining ion for gold.36,42,43 As noted in the Results section, an estimate of the effective surface potential ψ0 of the gold surface may be extracted from the normal force plots in Figure 2, by extrapolation from the far-field (large D) regime, giving ψ0=100-150 mV, similar to the effective potential on mica in water. This value may be used to estimate the effective (negative) charge density σeff on the gold. From Grahame’s equation, σeff = (8εε0CkBT)1/2 sinh(eψ0/kBT), where ε and ε0 are the dielectric constant of water and permittivity of free space, respectively, we estimate σeff =1-2.6 mC/m2 [or (0.61.6) 1016 charged sites/m2]. This effective surface charge density is consistent with a study of the ζ (zeta) potential of gold,36 where 0.8 1016 sites/m2 was evaluated as the maximum surface density of proton binding sites. Using the average (calculated) atomic surface density of gold (having equal proportions of gold atoms in three orientations: (110), (100), and (111)),44 σa = 1.15 1019 atoms/m2, we find that only 0.05-0.14% (or 0.04-0.12% if all gold atoms are (111) oriented) of the gold surface atoms bear a negative charge. It is important to bear in mind that the effective charge density obtained from extrapolating the far-field (large D) Fn(D)/R profile to D = 0 does not correspond to the density of negative -OH atoms actually adsorbed on the gold. This is because there will be a significant cancellation of this negative charge by counterions localized near the surface, so that the effective charge density;as reflected in the effective potential ψ0;will be substantially reduced, as can readily be shown.45 A similar consideration applies to the effective surface potential and effective surface charge density estimated for mica in water: the value of the latter, as obtained from mica-mica interactions (e.g., Figure 2) is always much lower than the density of ionizable potassium sites on the mica surface. The picture for both gold and mica in water, therefore, is that of a surface covered with negative charges (adsorbed -OH- for gold and negatively ionized surface lattice sites for mica) with some intrinsic surface charge density σ0. This is largely neutralized;as far as extrapolation of the far field Fn(D)/R profile to obtain ψ0 goes;by localized positive counterions. The fact that the reduced effective surface charge density σeff is similar for both mica and gold may be rationalized as follows. In both cases, even if the intrinsic values σ0 differ, the extent of neutralization by localized counterions is determined by (35) Biggs, S.; Mulvaney, P.; Zukoski, C. F.; Grieser, F. J. Am. Chem. Soc. 1994, 116, 9150. (36) Barten, D.; Kleijn, J. M.; Duval, J.; van Leeuwen, H. P.; Lyklema, J.; Cohen Stuart, M. A. Langmuir 2003, 19, 1133. (37) Hillier, A. C.; Kim, S.; Bard, A. J. J. Phys. Chem. 1996, 100, 18808. (38) The study in ref 37 is exceptional to this rule. (39) Lipkowski, J.; Shi, Z.; Chen, A.; Pettinger, B.; Bilger, C. Electrochim. Acta 1998, 43, 2875. (40) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons: New York, 1980. (41) Bockris, J. O. M.; Reddy, A. K. N. Modern Electrochemistry; Plenum Press: New York, 1970; Vol. II. (42) Chen, A.; Lipkowski, J. J. Phys. Chem. B 1999, 103, 682. (43) Giesbers, M.; Kleijn, J. M.; Stuart, M. A. C. J. Colloid Interface Sci. 2002, 248, 88. (44) Kirk, D. W.; Foulkes, F. R.; Graydon, W. F. J. Electrochem. Soc. 1980, 127, 1069. (45) Pincus, P. Private communication.
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entropic factors related to the salt concentration in the surrounding medium, so that the magnitude of σeff;as measured by extrapolation from the far field Fn(D)/R profile;remains similar for both gold and mica. Properties of Confined Water (No Added Salt). Normal Forces. The main difference between the normal forces of the two configurations is in the jump-into-contact region, where van der Waals forces exceed the osmotic repulsion. In the gold-mica system the surfaces jump-in to contact from Dj = 6.6 ( 2 nm compared to Dj=3.8 ( 0.5 nm in the mica-mica system. A larger Dj is expected for the former system since gold is more polarizable than mica: the gold/water/gold Hamaker constant is18 Ah =(34) 10-19 J, and that of the mica/water/mica is18 Ah=2.2 10-20 J. If only van der Waals forces were active, we could estimate Dj using the relation46 Dj < (AhR/3Kn)1/3, derived by combining the Euler instability condition for jump-in, ∂F/∂D > Kn, with the van der Waals term, FvdW/R=-Ah/6D2. Putting in the relevant values (taking R=1 cm and the Hamaker constant for the gold/water/ mica system as Ah = 9 10-20 J, the geometric mean of the gold/ water/gold and mica/water/mica values), we find that for micamica Dj < 7.9 nm and for the gold-mica system Dj < 12.6 nm. The presence of the double-layer osmotic repulsion, which is similar in both couples, would decrease this value by a similar factor for both systems, as indeed observed: the calculated ratio of jump-in separations (12.6 nm/7.9 nm) is equal, within the scatter, to the observed ratio (6.6 ( 2 nm/3.8 ( 0.5). From the observed approximate jumping-in time, τj < 1 s, we can estimate the upper limit of the effective viscosity of the confined water using the expression for the jump time from Dj to D0:47 τj ¼
18πRηeff ðDj 2 - D0 2 Þ Ah
ð3Þ
Putting R=0.01 m, the measured Dj = 6.6 ( 2 nm, and the gold/ water/mica Hamaker constant, Ah = 9 10-20 J, we find that ηeff < 3.6 mPa 3 s, which is comparable with the bulk viscosity of water, η=0.89 mPa 3 s at 25 °C. This result is consistent with a previous estimate of the viscosity of confined water between two mica surfaces,15,47 and it shows that water retains its bulk viscosity even under nanometer and subnanometer confinement between a metal (gold) and a dielectric surface. This observation implies that bulklike fluidity under nano- or subnanometer confinement is a general property of water that is not related to the confining substrate. We emphasize that this conclusion is, in particular, enabled by the extreme smoothness (0.2 nm rms) of the gold surface. Adhesion and Shear Forces. Following jump-in to contact in water, the force needed to separate the surfaces (corresponding to the adhesion energy via the JKR model) is smaller for the goldmica system compared with the mica-mica one. This difference is suggestive of a smaller effective contact area between gold and mica, which may result from the smaller contact between the asperities of gold and the smooth mica. In such a scenario water molecules will be trapped between the surfaces, leading to a weaker interfacial tension. This appears likely also to be related to the smaller yield shear forces between contacting gold and mica relative to those between two contacting mica surfaces. A more detailed consideration of the different response to shear between mica-mica compared to gold-mica will appear in a forthcoming paper (in preparation). (46) Raviv, U.; Laurat, P.; Klein, J. J. Chem. Phys. 2002, 116, 5167. (47) Raviv, U.; Perkin, S.; Laurat, P.; Klein, J. Langmuir 2004, 20, 5322.
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Properties of Confined Aqueous 10 mM KClO4. Normal Forces. At short range, in the hydration repulsion regime (D < 2 nm), the normal forces across 10 mM KClO4 are much weaker;by well over an order of magnitude;in the gold-mica couple compared with the mica-mica one (Figure 4). With two mica surfaces the forces increase sharply beyond the DLVO prediction, but such an increase is not observed between gold and mica. However, the fact that the surfaces do not jump-in (or even approach smoothly) to adhesive contact;as they do in pure water or in 10-4 M KClO4 (data not shown);shows that this salt concentration is indeed above the CHC for KClO4 and that hydrated potassium ions are trapped as the surfaces approach. We may attribute the weaker effective hydration repulsion (relative to mica-mica) in the gold-mica system to two factors: 1. Hydrated potassium ions probably populate the mica surface. However, the approaching gold surface may penetrate through these layers as the pressure at the gold asperities tips is much larger than in the micamica configuration. 2. It has been previously shown that a necessary condition for hydration repulsion to take place is that the hydrated ions remain attached to the surface.20,48 This is indeed the case with potassium ions which “fit in” into the mica lattice at its built-in negatively ionized lattice sites, possibly after some of their hydration shell is removed. Such “commensurability” is lacking at the gold surface, and in addition the negative charges due to the adsorbed -OH- ions are in principle removable. The hydrated potassium ions may therefore have larger lateral mobility so their squeeze-out from the gold-mica gap to the solution;together with -OH- ions stripped away from the gold to maintain electroneutrality;is more probable. These two characteristics of the gold surface result in a smaller hydration repulsion between gold and mica. Shear Forces. An additional property of highly confined hydration layers is that they remain fluid when sheared, even under pressures as high as a few atmospheres, because of their rapid relaxation. This is shown for example by trace f in Figure 5A (mica-mica couple), where the magnitude of the shear forces during sliding remains similar to the noise level, and the pressure, Fn/A, is ca. 10 atm (A is the area of the contact position, either measured from the flattened region of the fringes or by using the Hertzian model of contact mechanics,18 A ∼ π(RFn/K)2/3, where K = (1 ( 0.3) 109 N/m2 is taken as the effective elastic modulus of the mica/glue/mica configuration).8 The direct measurement gives A = (6.7 ( 0.4) 10-10 m2 while the Hertz calculation gives a similar value (A = (7.1 ( 2) 10-10 m2). This extends (by some 3-fold) an earlier observation of the shear forces between two mica surfaces in another 10-2 M salt solution (NaCl16) where hydration layers remained fluid under shear up to ca. 3 atm of pressure. Our higher pressure is comparable, however, to that recently applied across 0.1 M NaCl48 where fluidity was maintained. For the gold-mica couple, even though the slight roughness of the gold surface results in weaker effective hydration repulsion, the shear forces are nonetheless very weak and within the noise level at all D separations down to contact. This shows that the thin hydration layers remain fluid when sheared at the gold surface as well, though under smaller pressures: The pressure (48) Perkin, S.; Goldberg, R.; Chai, L.; Kampf, N.; Klein, J. Faraday Discuss. 2009, 141, 399.
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between the surfaces just before they reach contact (e.g., trace e in Figure 5B) is ca. 1 atm, an order of magnitude smaller than the pressure reached in the mica-mica configuration for similar D separations. From traces such as (f) in Figure 5A and (e) in Figure 5B, we can determine an upper limit to the effective viscosity, ηeff, of the confined hydration water layers (trace f in Figure 5B shows a higher sliding friction force, probably due to gold-mica asperity contact). We do this by assuming a Newtonian relation σ s ¼ ηeff γ_
ð4Þ
_ where σs =Fs/A is the shear stress during sliding and γ=V s/D is the shear rate (Vs is the sliding velocity). The shear force resolution in our measurements is δFs =( 40 nN (see Experiment and Materials section), and the shear rate in Figure 5 is ca. γ=700 s-1; however, we obtained a similar level of the shear force even when _ applying higher shear rates: up to γ=1500 and 18 000 s-1 for the gold-mica and mica-mica systems, respectively. Putting these numbers in eq 4, we find that the upper limit for the effective viscosity of the confined water in the mica-mica system is ηeff e 3.3 mPa 3 s, which is close to the viscosity of bulk water (η = 0.89 mPa 3 s at 25 °C). This value is considerably smaller than the upper limit ηeff e 30 mPa 3 s estimated earlier for hydration layers between mica sheets across 0.1 M NaCl.48 The corresponding upper limit for the gold-mica system is ηeff e 100 mPa 3 s. We emphasize again that all these values are limited by the shear rates attained in this study and that the actual measured sliding shear stress is always within the uncertainty in the signal.
Conclusions Replacing one mica surface with smooth gold in the conventional “mica vs mica” SFB experiments enables the study, for the first time, of the effects of a confining metal surface on the properties of water confined to a few nanometers and below, by comparing with corresponding results for the mica-mica case. In pure (no added salt) water, gold becomes negatively charged, probably due to adsorption of hydroxide ions, OH-, which is a known property of gold under an externally applied (positive) potential42,44,49,50 and is now seen also for gold at open circuit. The similarity of the effective surface potential of gold (and also silver)27 to that of mica in pure water, under low salt conditions, implies that their effective surface charge (once counterion localization near the surface is accounted for) is similar, though, as discussed, it does not directly reveal the extent of -OH- adsorption on the gold. In concentrated salt, even though our TS gold is almost molecularly smooth, hydration repulsion and hydration lubrication between gold and mica are both reduced relative to mica-mica. This is attributed respectively to the larger lateral mobility of the trapped hydrated potassium ions and to the weaker nature of the negative charge attachment to gold (enabling the K+ to escape), together with the large pressure at the gold asperity tips. However, despite these differences in the surface forces due to having one mica surface replaced by gold, the properties of the confined water itself are not affected, within the sensitivity of our measurements, either in pure water or in salt solutions. In the former, the viscosity of the confined water layers between gold and mica resembles bulk viscosity, reproducing the mica-mica behavior. In salt solutions, bound hydration water molecules (49) Hamelin, A.; Sottomayor, M. J.; silva, F.; Chang, S.-C.; Weaver, M. J. Electroanal. Chem. 1990, 295, 291. (50) Arai, T.; Fujihira, M. J. Vac. Sci. Technol. B 1996, 14, 1378.
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remain fluid under shear down to the gold-mica contact. This again is similar to previous results obtained between two mica surfaces. Acknowledgment. We particularly thank Philip Pincus for illuminating discussions and correspondence, David Andelman
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and Dan Ben-Yaakov for useful correspondence, and Gilad Silbert and Sam Safran for useful discussions. This work was supported by the Israel Science Foundation and the Minerva Foundation at the Weizmann Institute. J.K. holds the Hermann Mark Chair of Polymer Physics. This research was made possible in part by the historic generosity of the Harold Perlman Family.
Langmuir 2009, 25(19), 11533–11540
