Characterization of Water Confined between Silica Surfaces Using the

Jun 7, 2013 - Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan. ‡. Toyota Central R&D Labor...
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Characterization of Water Confined between Silica Surfaces Using the Resonance Shear Measurement Motohiro Kasuya,† Masaya Hino,† Hisho Yamada,† Masashi Mizukami,† Hiroyuki Mori, Toshihide Ohmori, ‡ Atsushi Suzuki,§ and Kazue Kurihara*,†,∥

‡

Seiji Kajita,

‡

†

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan Toyota Central R&D Laboratories, Inc., Nagakute, Aichi 480-1192, Japan § Toyota Motor Co., Toyota, Aichi 471-8572, Japan ∥ WPI-Advanced Institute for Material Research, Tohoku University, Sendai 980-8577, Japan ‡

S Supporting Information *

ABSTRACT: We performed the resonance shear measurement (RSM) for evaluating the properties of water confined between silica surfaces with and without water vapor plasma treatment, which was used to increase the density of the silanol groups on the surfaces. We compared the properties of the confined water, such as viscosity and lubricity, by controlling the surface separation at a 0.1 nm resolution. The observed resonance curves for water between the plasma-treated and untreated silica surfaces showed the following results: (1) The viscosity of the water confined between the plasma-treated silica surfaces increased due to water structuring at separations less than 3 nm, while the value for the water between the untreated silica surfaces was 8 nm. (2) The water confined between the plasma-treated surfaces could maintain lubricity under the normal pressure of more than 1.7 MPa; however, the water confined between the untreated surfaces lost lubricity under the normal pressure of more than 0.4 MPa. To discuss these properties in terms of water structures on the silica surfaces, we performed sum frequency generation (SFG) vibrational spectroscopy for water on the plasma-treated and untreated silica surfaces. The main peak of SFG spectra for the water on the plasma-treated silica was around 3200 cm−1, and that for water on the untreated silica was around 3400 cm−1, indicating that the hydrogen bonding network of the water on the plasma-treated silica surface was stronger than that on the untreated one due to the higher silanol density. The strongly networked water could exhibit higher lubricity with the increased silanol density.

1. INTRODUCTION Liquids confined in nanospaces exhibit different properties from those of bulk liquids due to liquid structuring induced by restriction of their molecular motion in nanospaces and by their interactions with solid surfaces. Evaluation of their properties is important in many research fields such as nanofunctional materials, micro- or nanofluidics, and tribology. Confined water is especially interesting for geology and biolubrication because of its general presence in the earth.1−5 The environmentally friendly nature of water makes it an attractive candidate as a future lubricant.6,7 Surface forces measurement can provide a unique tool for studying confined liquids of varying thicknesses (surface separation, D) at a 0.1 nm resolution.8,9 Various shear measurement techniques employing a surface force apparatus (SFA) have been developed for characterizing confined liquids.3,10−16 We developed the resonance shear measurement (RSM) based on SFA and studied the behavior of confined liquids such as viscosity, lubricity, and stick−slip.3,15,16 The resonance shear responses are sensitive to changes in the © XXXX American Chemical Society

properties of confined liquids and insensitive to noise. In our previous study, this sensitivity enabled us to reveal the viscosity increase of water between mica surfaces with decreasing surface separations below 1 nm while maintaining lubricity.3 It is known that silicon-containing materials, such as SiC, SiN, and silicon-containing diamond-like carbon (DLC-Si), exhibit low friction in water.6,7 Fischer has proposed that silanol groups (Si−OH) on such surfaces could be responsible for the low friction.6 We have reported that DLC-Si surfaces exhibited lower friction due to the presence of the silanol groups.7 However, effects of the silanol groups on the low friction have not been clearly understood. It should be important to investigate the role of the silanol groups on the properties of the confined water including low friction and structures. A recent study using the attentuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) revealed that the density of silanol groups on the silicon prism surfaces Received: May 3, 2013

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influenced the structure of the water.17 In this case, water was absorbed at the gas−silica interface by changing the humidity and not at the water−silica interface. Similar effects were studied for the confined water by simulations, which required experimental support.4,5 To our knowledge, there has been no study in which effects of silanol groups on both of the friction and structure of water are investigated. Therefore, in this study, we performed the RSM for investigating the viscosity and nanotribological properties of water confined between silica surfaces. We compared the responses observed for two types of silica surfaces which had different silanol group densities. The silanol density was evaluated using derivatization−X-ray photoelectron spectroscopy (XPS).7,18 RSM results revealed that lubricity of the confined water was higher when the silanol densities on the silica surfaces were increased. In addition, sum frequency generation (SFG) vibrational spectroscopy19−22 was performed for evaluating the structures of the water on silica surfaces. It revealed that ice-like water on the silica surfaces increased when the silanol density increased, which could contribute to the high lubricity, thus low friction.

silanol groups on the surfaces by the FOCS. The derivatized sample was washed and ultrasonically cleaned in chloroform. After the derivatization reaction, the relative silanol density of the surfaces was measured by XPS (ULVAC-PHI Inc., PHI5600) as the fluorine concentration. XPS signals were collected at an angle of 45° to the sample surfaces. It was reported that about 2 atomic % of FOCS was detected using the same derivatization XPS measurement to the H-terminated Si-wafer, which indicated that FOCS was physisorbed on the silicone surfaces without the silanol groups.18 On the other hand, 18 atomic % of florin was detected for the OHterminated Si wafer treated in a piranha acid, indicating that increases in the florin content were clearly due to the increased silanol density. 2.3. Resonance Shear Measurement (RSM). Figure 1 shows the experimental setup of the RSM.3,15,16 The upper

2. EXPERIMENTS 2.1. Materials. Silica sheets for the RSM were prepared with blowing flamed quartz glass using the procedure reported by Horn et al.1,2 The thickness of the sheets was 2−4 μm. The silica sheets were treated with water vapor plasma (Samco, BP1, 20 W, 13.56 MHz radio frequency plasma source in 0.6 Torr of argon and water vapor, 50 mL/min flow rate of argon gas) for 3 min just prior to each experiment to increase the silanol density of the silica surfaces.23 Untreated silica sheets were used as references. The contact angles of water were 52.3° and 26.5° for untreated and plasma-treated silica sheets, respectively. Surface roughness over a 1 μm × 1 μm trace was evaluated by an atomic force microscope (AFM, Seiko, SPI-400). The rootmean-square (RMS) and the peak to valley (PV) roughness of the untreated silica surface were 0.29 ± 0.11 nm and 2.5 ± 0.8 nm, and those of the plasma-treated silica surface were 0.33 ± 0.13 nm and 2.8 ± 1.2 nm, respectively. These values were identical in the range of errors. These sheets were also used for XPS measurements to determine the silanol density. The anhydrous fused quartz prism with a triangular shape was used as the silica surfaces for SFG spectroscopy with and without water vapor plasma treatment similar to the RSM measurement. RMS and PV roughness of the untreated prism surfaces over a 1 μm × 1 μm trace were 1.64 ± 0.21 nm and 17.5 ± 3.6 nm, and those of the plasma-treated prism surface were 1.46 ± 0.41 nm and 14.0 ± 6.4 nm, respectively. The values for the sheets with and without the treatment were identical in the range of errors. The quartz plates treated in the same manner as the prisms were used for the XPS measurements. Ultrapure water (Barnstead, NANOpure DIamond) was used after double distillation. Tridecafluoro-1,1,2,2-tetrahydrooctyl dimethylchlorosilane (FOCS) from Alfa Aesar and chloroform from Nacalai Tesque (guaranteed reagent grade) were used as received. All measurements were done at room temperature (21 ± 1 °C). 2.2. Evaluation of Silanol Groups on Silica Surfaces. Derivatization X-ray photoelectron spectroscopy (XPS) was performed to evaluate the silanol density of the silica surfaces following the previous procedure.7,18 Silica substrates were dipped in a 1% FOCS chloroform solution to derivatize the

Figure 1. Schematic illustration of experimental setups of resonance shear measurement (RSM).

surface was hung by a pair of stiff leaf springs and laterally moved by a four-sectored piezo tube, which was driven by applying a sinusoidal voltage to the two opposite electrodes. The amplitude of the voltage was Uin, and the angular frequency was ω. The deflection of the leaf spring was detected using a capacitance probe as its output voltage (Uout). The amplitude ratio Uout/Uin vs ω was plotted as the resonance curve, in which the Uout/Uin at the resonance peak and its frequency ωres reflected the sample properties, such as viscosity and friction. The lower surface was supported by double cantilever springs. The thickness of a liquid between the sample surfaces (D) and the contact area (A) was determined using fringes of equal chromatic order (FECO).24,25 In this study, we defined the “zero” of separation at the hardwall contact between the untreated silica surfaces in water. This distance was −2.1 ± 0.2 nm relative to the contact in air as shown in Figure S1 of the Supporting Information, which shows the RSM curves at various surface separation D′. Here, the distance D′ was determined using the dry contact as the “zero” separation. Previous studies have reported that freshly cleaved mica in air rapidly accumulates a thin adsorbed layer of a carbon/water-vapor complex, which is, however, soluble in water.3,26,27 The minus separation at the hardwall contact in water relative to the contact in air indicated that a similar phenomenon occurred in the case of the untreated silica surfaces. Therefore, we used this hardwall contact position, −2.1 ± 0.2 nm, relative to the contact in air, as “zero” of separation for the untreated silica surfaces in water, D = D′ + B

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water confined between untreated silica surfaces using RSM. Figure 2 shows the resonance curves at various separations D

2.1 nm, which is reasonable because it is a value similar to that for mica (∼1.6 nm). On the other hand, D′ for the hardwall contact between the plasma-treated silica surfaces was −0.4 ± 0.3 nm, relative to the contact in air as shown in Figure S2 of the Supporting Information. It is difficult to expect a large difference in the D′ shift due to the adsorbed layer of a carbon/ water-vapor complex between untreated and plasma-treated silica. Therefore, we used the same zero distance, −2.1 ± 0.2 nm, relative to the contact in air for plasma-treated silica similarly to the untreated one. The surface forces (F) and normal load (L) were obtained from deflection of the double cantilever springs which supported the lower surfaces using the same method as the conventional SFA.8 The normal pressure (P) on the thin films of water was obtained using P = L/A. 2.4. Sum Frequency Generation (SFG) Spectroscopic Measurement. SFG vibrational spectroscopy was used to monitor the structures of the water molecules on the silica surface.19−22 A picosecond Nd:YAG laser (EKSPLA, PL2143/ SS, 1064 nm, 10 Hz, pulse duration 25 ps) was used to pump an optical parametric generation/difference frequency generation system (EKSPLA, PG501/DFG-10P) which generates tunable infrared radiation from 2.3 to 10 μm. The second harmonic light (λ = 532 nm) of the Nd:YAG laser (λ = 1064 nm) was used as the input visible light. The sets of polarizations for the measurement were s-polarized SF, s-polarized visible, and p-polarized IR light.

Figure 2. Resonance curves obtained from water confined between untreated silica surfaces at various surface separations, D. Number in parentheses indicated normal load L (mN). Two reference states curves for the separation in air (AS) and silica−silica contact (SC) are also plotted for convince.

3. RESULTS AND DISCUSSION 3.1. Effect of Plasma Treatment on the Silica Surfaces. The derivatization XPS measurement was performed to investigate the effects of the plasma treatment on the silanol density of silica surfaces applied to RSM and SFG. Table 1

for water between untreated silica surfaces. Two reference states, the curves for the separation in air (AS) and silica−silica contact (SC), are also plotted for convenience. In the former curves, the peak frequency, ωAS, was characterized by the mass and the spring constant of the upper surface unit. On the other hand, upper and lower surfaces moved identically in a strong adhesion contact when we measured the latter curves. Thus the resonance frequency of the silica−silica contact in air, ωSC, shifted to higher frequency due to contribution of the mass and spring constant of lower surface units. The resonance curve showed the peak at a resonance frequency ωres = 199 rad/s, the same frequency as the AS peak when D = 290 nm. It did not change with the decreasing D from 290 to 15.1 nm. The resonance peak amplitude decreased below D = 8.9 nm, and the peak almost disappeared at D = 5.3 nm. This indicated that the viscosity of the confined water sharply increased due to liquid structuring under confinement at D ≤ 8.9 nm. At 3.0 nm ≤ D ≤ 5.3 nm, the resonance curves shifted toward higher frequencies with the decreasing D, while the peak amplitude remained low. This resonance frequency shift indicated that the viscosity of the confined water further increased. However, the low resonance amplitude indicated that friction between the surfaces was low; i.e., water molecules were mobile under lateral shear. At D = 2.7 nm, a sudden drop in the amplitude appeared in the middle of the resonance curve at ω = 510 rad/s. Similar sudden drops in the amplitudes of the curves were observed at 1.3 nm ≤ D ≤ 2.7 nm. Our previous research has shown that these changes in the curve shapes were due to the stick−slip transition induced by shearing the liquids.16 Details of this transition will be discussed in Section 3.7. The resonance curves returned to symmetric shapes again when 0 nm ≤ D ≤ 0.7 nm. In this D region, the ωres value was 520 rad/s, the same as that of the SC peak, and the peak amplitude was only slightly lower than that of the SC peak. This result suggested that almost no water existed between the silica surfaces at D = 0 nm, which supported our definition of “zero” separations.

Table 1. Atomic % Ratio of Fluorine/Silicon on Various Silica Surfaces (F/Si) Evaluated by Derivatization XPS Measurement samples silica sheets for RSM silica prism for SFG

F/Si for untreated surfaces

F/Si for plasma-treated surfaces

0.187 ± 0.086

0.280 ± 0.087

0.225 ± 0.049

0.320 ± 0.042

summarized the fluorine/silicon concentration ratio (F/Si) of various silica surfaces after the derivatization reaction. The F/Si ratio of the silica sheet surfaces for RSM increased from 0.187 ± 0.086 to 0.280 ± 0.087 with the plasma treatment. The F/Si ratio of the silica prism surfaces for SFG also increased from 0.225 ± 0.049 to 0.320 ± 0.042 with the plasma treatment. These results indicated that the silanol density of the surfaces increased due to the plasma treatment. Surface force profiles between the silica surfaces in water were shown in Figure S3 in the Supporting Information. A long-ranged repulsion of decay length of about 100 nm, ascribed to the electric double layer repulsion,28 and a shortranged repulsion at 2−3 nm, ascribed to the hydration repulsion,1,2 were observed in both cases of the untreated and plasma-treated silica. The force profiles for untreated and plasma-treated silica were identical at short-ranged D, which agreed with the previous report on the surface forces between silica surfaces which showed different hydrophilicity, in water.2 3.2. Resonance Shear Measurement for Water between Untreated Silica Surfaces. We characterized the C

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3.3. Resonance Shear Measurement for Water between Plasma-Treated Silica Surfaces. Figure 3 shows

Figure 4. Plots of normalized resonance amplitude as a function of surface separations, D. Plot for water between plasma-treated silica surfaces, filled symbols, and for untreated surfaces, open symbols.

The resonance peak amplitude for water confined between untreated silica surfaces gradually decreased until 8 nm with the decreasing D and then sharply decreased below 8 nm. This indicated that the viscosity of the confined water sharply increased due to liquid structuring under confinement below 8 nm. On the other hand, the resonance peak amplitude for water confined between plasma-treated silica surfaces slightly decreased when D was above 3.3 nm and sharply decreased below 3.3 nm with the decreasing D. This result indicated that the viscosity of the water between the plasma-treated silica surfaces sharply increased below D = 3.3 nm, which was lower than the value for the untreated silica. At this moment, we cannot interpret the reason why the viscosity of water between plasma-treated surfaces increased at shorter distances than water between the untreated surfaces. Water is known to form various hydrogen-bonded networks.29−32 The water network on the surfaces with different silanol densities should be different and might be responsible for different viscosities of confined water. 3.5. Frictional Properties of Water Confined between Silica Surfaces with and without Plasma Treatment. Figure 5 shows the resonance peak amplitude normalized to the SC peak amplitude when the normal pressure on the thin films of water (P = L/A) increased. Normalized resonance peak amplitude of water between untreated silica surfaces sharply increased and became almost 1 at P = 0.43 MPa. The high normalized amplitude showed that the water between the surfaces became solid-like at P > 0.24 MPa, which was also seen in the stick−slip behaviors of the confined water at these normal pressures. However, for water between the plasmatreated silica surfaces, the normalized peak amplitude remained lower than 0.03 at P ≤ 0.74 MPa and became 0.37 even at P = 1.7 MPa, at which the surfaces were in contact with the hardwall (D = 1.7 nm). The lower normalized peak amplitude indicated that friction between the surfaces was lower; i.e., water mobility under lateral shear was higher. The plasmatreated surfaces could maintain water in larger thickness at higher normal pressures as shown in the inset of the Figure 5. This water film could reduce the friction due to larger mobility of water molecules under shear compared with the untreated surfaces. To discuss the molecular level of mechanisms about these results, we described the structures of water on the silica surfaces in the next section.

Figure 3. Resonance curves obtained from water confined between plasma-treated silica surfaces at various surface separations, D. Number in parentheses indicated normal load L (mN). Two reference states curves for the separation in air (AS) and silica−silica contact (SC) are also plotted for convince.

the resonance curves for various separations (D) for water confined between plasma-treated silica surfaces. The resonance peak did not change with decreasing D from 193 to 3.5 nm. The resonance peak amplitude decreased by 60% at D = 3.1 nm compared to the amplitude of the AS peak and almost disappeared at D = 2.3 nm. This indicated that the viscosity of the confined water sharply increased due to liquid structuring under confinement at D ≤ 3.1 nm. The distance at which the peak amplitude decreased for the plasma-treated silica was smaller than the value for the untreated silica. At 1.9 ≤ D ≤ 2.3 nm, the resonance curves shifted toward higher frequencies with the decreasing D, while the peak amplitude remained low. In this D range, the viscosity of confined water further increased; however, its lubricity was maintained. At D = 1.9 nm and L = 12.38 mN, the resonance amplitude suddenly dropped at ω ≈ 403 rad/s, indicating that the stick−slip transition occurred. At D = 1.7 nm, which was the hardwall contact for the plasma-treated silica surfaces, the resonance curves returned to symmetric shapes again, and a resonance frequency ωres was 410 rad/s, which is lower than that of the SC peak (420 rad/s). The peak amplitude at D = 1.7 nm was ca. 40% of the SC peak amplitude. These lower values of ωres and peak amplitude indicated that water was present between the silica surfaces, which served as a lubricant between the silica surfaces. 3.4. Viscosity of Water Confined between Silica Surfaces with and without Plasma Treatment. When the surface separation between solid surfaces in liquids decreases into the nanometer range, the viscosity of the confined liquid increased due to liquid structuring.3−5,10−16 A sharp decrease in the resonance peak amplitude is observed when a viscosity increase in the confined liquids is induced.3,15,16 For determining the distance at which the viscosity of the confined water started to increase, we plotted the resonance peak amplitude, Uout/Uin, at ωres as a function of D, normalized with that of the AS peak as shown in Figure 4. The peak amplitude was normalized with that of the AS peak. D

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Figure 5. Resonance peak amplitude normalized to the SC peak amplitude under various normal pressures (P = L/A) on thin films of water between silica surfaces. ▲, untreated silica; ■, plasma-treated silica. Inset: Plots of surface separations, D, as a function of P.

3.6. SFG Spectroscopy of Water on Silica Surfaces. The structure of water on the surface of the silica prism was investigated using the SFG spectroscopy. We compared the spectrum of water on the untreated silica with that of water on the plasma-treated silica surfaces, as shown in Figure 6. A broad spectrum at 3000−3700 cm−1 was observed for the untreated silica surfaces. This spectrum is assigned to the main peak of νOH of “liquid-like water” at 3400 cm−1 which is overlapped with a shoulder peak of νOH of “ice-like water” at 3200 cm−1, indicating that “liquid-like water” was dominant on untreated silica surfaces.29−32 On the other hand, two clear peaks at 3200 cm−1 and 3400 cm−1 were observed from water on the plasmatreated silica surfaces. In this case, the former peak is larger than the latter peak. These differences in the νOH peaks indicated that the hydrogen bonding network of the water on the plasmatreated silica surface was stronger than that on the untreated one probably due to the higher silanol density. This result can explain the frictional properties of silica surfaces with different silanol densities in water obtained from RSM as follows: Liquid-like water was a main component in water confined between the untreated silica surfaces with less silanol densities. It was easily squeezed out under high normal pressures (P ≥ 0.43 MPa), resulting in a low lubricity. In contrast, 1.7 nm of water film existed between the plasmatreated silica surfaces even at normal pressure of P = 1.7 MPa probably because a main component of water on the surface with the higher silanol density was ice-like water which has a strong hydrogen bonding network. However, the confined water exhibited lower friction, thus its network was mobile under lateral shear. Different responses in the normal and lateral applied forces are not uncommon. The most popular example is the electric double layer repulsion between charged surfaces, which does not contribute to lateral shear forces between the surfaces.3,27 We could not clearly explain the reasons for the result by RSM that the viscosity of water confined between the plasma-treated silica surfaces with a higher silanol density increased at shorter distances than that confined between the untreated surfaces. A SFG study showed that a more strongly hydrogen-bonded network of water was formed on the surface with a higher silanol density. However,

Figure 6. SFG vibrational spectra obtained from water on free untreated silica surfaces (a) and free plasma-treated silica surfaces (b).

this network could not extend to longer distances as shown in RSM results. Simulation studies by Coridan et al. showed that the thickness of the strong hydration layer between hydrophilic surfaces decreased because of localization of water near the hydrophilic functional groups on the surfaces when the density of the functional groups increased, which agreed with our observation.4 3.7. Stick−Slip Behavior. The stick−slip behavior observed for water confined between untreated silica surfaces was further investigated by varying the input voltage for RSM, Uin, which was proportional to the shear amplitude, i.e., the shear stress. Figure 7 shows the resonance curves of water between the untreated silica surfaces at various Uin’s (= 0.3, 1.0, 3.0 V) when D = ca. 1.7 nm. At Uin = 1 V (identical condition as that in Figure 2), the resonance curve was identical to that of SC (no slip) up to the ωcritical = 685 rad/s, and the amplitude (Uout/Uin) suddenly dropped at ω = 706 rad/s. This means that the stick (no slip) state was kept up to the frequency of ωcritical = 685 rad/s, and the slip occurred at ω = 706 rad/s. The force required to cause the stick−slip motion, F′, is nearly equal to the static friction at the point (ωcritical = 685 rad/s) just before the sudden drop in the amplitude.16 The static friction (Fstatic friction) of confined water at ωcritical = 685 rad/s at D = 1.7 nm can be obtained by calculating the shear force applied to the water layer using eq 1 following the procedure previously reported. E

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friction. Understanding the effect of the silanol density on the confined water structuring and its friction properties will facilitate further development of the low frictional systems of the silicon-containing materials.

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ASSOCIATED CONTENT

S Supporting Information *

Resonance curves obtained from the water confined between the untreated or plasma-treated silica surfaces at various surface separations, in which contact between the silica surfaces in air was defined as “zero” separations and surface force profiles between silica surfaces in water. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

Figure 7. Resonance shear curves measured at the input voltage of 0.1, 1.0, and 3.0 V to the piezo of the shear unit (Uin) at D = 1.7 nm.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the technical assistance by Takahashi N. (Toyota Central R&D Laboratories, Inc.) and Ito M. (IMRAM, Tohoku Univ.) for the derivatization−XPS, and Kudo T. (IMRAM, Tohoku Univ.) for preparing the silica sheets. This work was supported by the CREST program of the Japan Science and Technology Agency (JST) and ″Green Tribology Innovation Network″ Advanced Environmental Materials Area, Green Network of Excellence (GRENE) program sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan in Japan.

F ′ = Fstatic friction = −mUoutCoutω 2 sin(ωt ) + kUoutCout sin(ωt )

(1)

Here, m (= 0.033 kg) is the total effective mass of the oscillating system, and k (= 17700 N/m) is the effective spring constant. They were determined by the analysis of the resonance curve measuring silica−silica contact in air (SC). The Cout (= 5 μm/ V) is the sensitivity of capacitance probe. Fstatic friction was calculated to be 0.56 mN using a Uout value at ωcritical = 685 rad/ s. On the other hand, the resonance curve measured at Uin = 0.3 V was identical to that measured at SC. The force at the peak (ω = 749 rad/s) was calculated to be 0.08 mN using eq 1, which was smaller than Fstatic friction (≑ F′). This means that the stick (no slip) condition was kept at Uin = 0.3 V. At Uin = 3 V, only a broad and weak peak appeared at ω = ca. 425 rad/s, and no sharp increase in the intensity was observed. The force at the peak was calculated to be 1.94 mN using eq 1 and was larger than Fstatic friction (≑ F′). Thus, at Uin = 3 V, the water layer could not be in the stick condition and maintained fluidity of the slip state during the measurement. These results confirmed that the high shear stress induced the transition from the stick state to the slip state.

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REFERENCES

(1) Horn, R. G.; Smith, D. T. Surface Forces and Viscosity of Water Measured between Silica Sheets. Chem. Phys. Lett. 1989, 162, 404− 408. (2) Grabbe, A.; Horn, R. G. Double-Layer and Hydration Forces Measured between Silica Sheets Subjected to Various Surface Treatment. J. Colloid Interface Sci 1993, 157, 375−383. (3) Sakuma, H.; Otsuki, K.; Kurihara, K. Viscosity and Lubricity of Aqueous NaCl Solution Confined between Mica Surfaces Studied by Shear Resonance Measurement. Phys. Rev. Lett. 2006, 96, 046104. (4) Coridan, R. H.; Schmidt, N. W.; Lai, G. H.; Abbamonte, P.; Wong, G. C. Dynamics of Confined Water Reconstructed from Inelastic X-Ray Sctattering Measurements of Bulk Response Function. Phys. Rev. E 2012, 85, 031501. (5) Sendner, C.; Horinek, D.; Bocquet, L.; Netz, R. R. Interfacial Water at Hydrophobic and Hydrophilic Surfaces: Slip, Viscosity and Diffusion. Langmuir 2009, 25, 10768. (6) Tomizawa, H.; Fischer, T. E. Friction and Wear of Silicon Nitride and Silicon Carbide in Water: Hydrodynamic Lubrication at Low Sliding Speed Obtained by Tribochemical Wear. ASLE Trans. 1987, 30, 41−46. (7) Mori, H.; Takahashi, N.; Nakanishi, K.; Tachikawa, H.; Ohmori, T. Low Friction Property and Its Mechanism of DLC-Si Films Under Dry Sliding Conditions. SAE Int. 2007, 1, 1015. (8) Israelachvili, J. N. Intermolecular and Surface Forces, 3rd ed.; Academic Press, Ltd.: New York, 2010. (9) Horn, R. G.; Israelachvili, J. N. Direct Measurement of Structural Forces between Two Surfaces in a Nonpolar Liquid. J. Chem. Phys. 1981, 75, 1400−1411. (10) Israelachvili, J. N.; McGuiggan, P. M.; Homola, A. M. Dynamic Properties of Molecularly Thin Liquid Films. Science 1988, 240, 189− 191.

4. CONCLUSION We studied the properties of water confined between silica surfaces using the surface forces and resonance shear measurements. The measurement showed that water of larger thickness 1.7 nm was maintained between the water plasmatreated silica surfaces, which bore the higher silanol density, even at a higher normal pressure of 1.7 MPa compared to untreated surfaces. The RSM results indicated that water confined between silica surfaces with a higher silanol density exhibited higher lubricity under increasing an applied normal pressure than that between silica surfaces with a lower silanol density. The SFG vibrational spectroscopy revealed that ice-like water on the silica surfaces increased when the silanol density increased. The ice-like network of water between plasmatreated surfaces could remain even under higher pressures. However, such a network could exhibit high lubricity, thus low F

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp404378b | J. Phys. Chem. C XXXX, XXX, XXX−XXX