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Low-Temperature Surface Forces Apparatus to Determine the Interactions between Ice and Silica Surfaces Florian Lecadre,† Motohiro Kasuya,‡ Aya Harano,§ Yuji Kanno,§ and Kazue Kurihara*,† †
New Industry Creation Hatchery Center (NICHe) and ‡Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Sendai 980-8577, Japan § Nihon Michelin Tire Co., Ltd., 3-7-1, Nishishinjuku, Shinjuku-ku, Tokyo 160-0023, Japan
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S Supporting Information *
ABSTRACT: We have developed a low-temperature surface forces apparatus (SFA) using a thermoelectric Peltier module inserted below the bottom surface of the lower sample holder, giving easy access to the samples and allowing quick temperature changes. In air, the temperature can be decreased to ca. −20 °C. To demonstrate the performance of the apparatus, we measured the interactions between ice and a silica surface at −11.5 ± 0.5 °C. An exponentially decaying repulsion of the decay length, 11.2 ± 1.0 nm, was observed, and attributed to the electric double layer (EDL) repulsion. The surface potential of the ice was calculated to be −35 mV by fitting the data to the EDL model.
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INTRODUCTION The surface forces measurement employing a surface forces apparatus (SFA) has been essential to understanding the interactions between surfaces in liquid media, including the electrical double layer (EDL) force.1,2 Various SFAs have been developed to obtain fundamental information about phenomena at the solid−liquid interfaces and for confined liquids between solid surfaces. They include the SFA for nontransparent samples,3 the electrochemical SFA for characterizing electrodes,4,5 and various shear devices for nanotribology and rheology.6−10 The temperature dependence of interactions is important since the temperature typically affects interactions, adsorption, molecular conformations, and so forth. However, temperaturecontrolled SFA measurements have been limited possibly because it was considered difficult to develop a system that avoids the thermal drift of the surfaces. Therefore, early temperature-variation measurements were performed by changing the room temperature.11 The studied temperature range was between 15 and 35 °C. Extending this approach, Heuberger et al. placed the SFA in a temperature-controlled box, which provided a temperature range below 70 °C.12 Our group has developed a high-temperature control device for controlling the temperature up to 100 °C with a stability of less than 0.1 °C using a local heater.13 However, low-temperature measurements are still missing. Characterization of the ice surface is important in many fields and thus is attracting attention. The surface charge of ice is considered to play a role in the environment, for example, in thunderstorms14,15 and pollution transportation in snow.16,17 Other examples include designing anti-icing surfaces.18 The © XXXX American Chemical Society
surface forces measurement should provide essential information about the ice surfaces. However, the conventional SFA employing FECO (fringes equal chromatic order) for the distance determination requires transparent samples and thus is not suitable for characterizing ice that is composed of cracks and bubbles.1 The twin-path surface forces apparatus we developed for opaque samples can be used for measuring interactions involving ice.3 In this study, we installed a thermoelectric plate (a Peltier module) in the twin-path SFA and cooled the studied interface to −11.5 ± 0.5 °C. To demonstrate the performance of this SFA, the interactions between the ice and silica surface were measured, for the first time, in an ethylene glycol and water mixture containing 1.0 mM KBr.
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EXPERIMENTAL SECTION
Low-Temperature SFA. The twin-path SFA was chosen for its ability to study nontransparent samples. It uses a red laser reflected on the back of the lower sample disk to monitor the displacement of the lower sample mounted on a pair of leaf springs and driven by a pulse motor. The details of the instrument were previously reported.3 A cooling unit was installed in the SFA chamber as shown in Figure 1. It was composed of a Peltier module (9506/023/040B, Hayashi Tokei Kogyo), and it is a PEEK-carved cooling block. Cold ethylene glycol was pumped through the PEEK block, and an aluminum socket was placed at the Peltier location to transfer the generated heat to the ethylene glycol. The Peltier module was a flat square (14 × 14 mm2) with a 5-mm-diameter hole in its center to let the light from the twinReceived: June 7, 2018 Revised: July 12, 2018
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DOI: 10.1021/acs.langmuir.8b01902 Langmuir XXXX, XXX, XXX−XXX
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Figure 2. (a) Schematic of ice preparation. (1) Liquid nitrogen reservoir. (2) Bent mica. (3) Growing ice. (4 and 5) Peltier module and cooling block. (6) Invar sample holder. (b) Ice sample after preparation. The mica layer has been peeled, and the silica disk with a glued silica film is hanging above ice.
Figure 1. Schematic of the integration of the samples and the cooling unit inside the SFA chamber. (1) Kel-F holder. (2) Samples (silica disks shown). (3) Leaf springs (connected to the pulse motor). (4) Lower sample holder. (5) Peltier module. (6) PEEK cooling block with an Al socket for the Peltier module (not visible). path laser located below the chamber reach the sample. The PEEK block also had a similar hole. The cooling unit was connected to the SFA chamber to place the Peltier module directly below the lower sample. The temperature was controlled by adjusting the electric power supplied to the Peltier unit. Since only the back side of the lower sample was cooled, a temperature gradient existed between the samples, and the precise value of the temperature at the contact point was unknown. To estimate the temperature, two platinum resistors (cylindrical, 3.9 × 0.8 mm2, Netsushin Co) were embedded inside each sample as close as possible to the sample surface. For ice, the probe was inserted in water before freezing. For the silica or mica surface, a probe was inserted into a hole in the supporting silica disk. We also used a thin thermocouple (20 μm thickness, Anbe SMT) for calibration experiments to estimate the interface temperature. The reported temperature is that detected by the probe inside the lower sample. At a constant Peltier module power, the temperature of the lower sample decreased with the decreasing sample−Peltier separation. Shorter separations allowed lower temperature but enhanced the thermal drift. A micrometer screw was used to move the cooling unit up or down, and the relative Peltier−sample separation could be adjusted by the twin-path unit. The shortest separation was about 10 μm, while we usually worked between 20 and 50 μm. Since the Peltier unit and the samples were much colder than the rest of the SFA chamber, a flow of dry nitrogen gas was necessary to avoid any water condensation on them. A flow rate of 0.5−1.0 l/min was enough to keep the moisture level below 5%. Material Preparation. The silica sheets were prepared using a blowing method,19 attached to a freshly cleaved mica support, and kept in a desiccator. Before the experiments, the silica sheets were lifted from the mica support to be glued on a cylindrical silica disk with an epoxy resin following the common procedure. Once glued, the silica surfaces were treated for 30 min in water-vapor plasma20 (Samco, BP-1, 20 W, 13.56 MHz radio frequency plasma source in 0.6 Torr argon and water vapor with a 50 mL/min flow rate of argon gas) to ensure a contact angle of silica with water of close to 5°. For the temperature measurement, as already mentioned, the silica disks have a hole just below their surface in which to insert the platinum temperature probe. The ice samples were grown from ultrapure water directly in the chamber. To work in the cross-cylinder configuration, the ice samples were grown under a freshly cleaved mica sheet bent into a cylinder (Figure 2a). Water was injected below the mica sheet and cooled to form ice using a narrow glass reservoir filled with liquid nitrogen and placed 1 to 2 cm above the mica sheet. The ice grew under the mica template from the top to the bottom within several minutes. The topdown cooling helped to keep bubbles and eventual impurities away from the ice/mica interface. During ice growth, the temperature just below the mica reached −15 °C. Once the ice growth was completed, the glass reservoir was removed. The temperature of the ice was set to −5 °C using the Peltier module, and the mica template was manually lifted with tweezers. Figure 2b shows an example of the ice prepared using this procedure. The roughness of ice was measured using in situ confocal microscope (Shimadzu, SFT3500) and was typically found to be RMS = 11.5 nm over a 100 μm cross section (Figure S1). The
platinum resistor probe was set in water close to the mica stripe prior to freezing, typically about 0.1 mm away from the surface. Ultrapure water (Barnstead, NANOpure DIamond) was used after double distillation. Ethylene glycol (99.0%, Nicalai Tesque) and KBr (99.999%, Merck KGaA) were used as received and mixed with water to prepare a 10% mole fraction solution of ethylene glycol. The freezing point of the 10% ethylene glycol solution was −12.8 °C.20 The KBr concentration used was 1 mM to observe the extended EDL repulsion since the characteristic decay length (Debye length) of the EDL force is 9.6 nm.1 SFA Measurements. Surface forces measurements were conducted in the crossed cylinder configuration following the established procedure.1,3 The curvature (R) of the silica disk was 20 mm, and the initial curvature of the ice was about 12−15 mm. After the sample surfaces were placed in the SFA, about 30 μL of the solution was injected between the two surfaces. The SFA measurements were made using an approach speed of 15 nm/s. Approach and retraction curves were identical in the force measurements in this study (silica−silica and silica−ice, see Figure S2 for the interactions between ice and silica surfaces in a 10% mole fraction of aqueous ethylene glycol with 1 mM KBr); therefore, only the approach curves are shown. Nitrogen was allowed to flow during the entire measurement, starting during the ice preparation. The obtained force was normalized by the radius R of the surface curvature using the Derjaguin approximation,1 F/R = 2πGf, where Gf is the interaction free energy per unit area between the two flat surfaces. Since the curvature of ice increased in the liquids, making it difficult to determine the change, the ice−silica force curves were simply normalized by the radius of the silica disk.
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RESULTS AND DISCUSSION Performance of the Low-Temperature Apparatus. The temperature stability was monitored both in air and in a liquid (10% aqueous ethylene glycol), as shown in Figure 3a,b. In both cases, the Peltier unit was used at its maximum power (∼20 W). In both cases, the temperature was stable with only a slow drift toward high temperature occurring at a rate of 0.1° per 30 min. However, the temperature difference between the bottom and upper surfaces was strongly affected by the air or liquid medium. In air, the difference was significant, about 14 °C, with the ice sample reaching −22 °C and the upper silica disk reaching −8 °C. On the other hand, when a liquid was present between the two silica disks, the temperature difference was significantly reduced to only 3 to 4 °C due to much better heat transfer, but the lowest temperature reachable on the lower sample was only −8 °C. The reason that the former performed better for the lowest reachable temperature could be essentially due to the height difference in the lower sample. The ice was 1.9 mm in height, while the glass disk was 6 mm. With ice, both the lower and upper samples were thus much closer to the Peltier unit, resulting in the lower reachable temperature. This showed that the sample preparation plays a crucial role in determining the working temperature range. B
DOI: 10.1021/acs.langmuir.8b01902 Langmuir XXXX, XXX, XXX−XXX
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Figure 3. Evolution over time of the temperatures obtained with (a) ice and a silica disk in air (best values). (b) Two glass disks in a 10% mole fraction ethylene glycol−water mixture.
Figure 5. Displacement trace of the lower sample (black) for (a) ice in air and (b) glass disks in water. The pulse motor trace (red) triggers steps of 15 nm.
In order to more precisely monitor the interface temperature, we conducted experiments with a thin thermocouple (10 μm thick) that was sandwiched between the ice sample and the silica surface. The results are shown in Figure 4. Three traces
driven by the step motors (red line). In water, the damping of the liquid environment decreased the amplitude of the shortperiod vibrations by half to about 5 nm. However, parasite displacements of the lower sample sometime appeared, resulting in a distortion of the stepped displacement trace. A steady drift of ca. 10 nm/min was observed in both cases (it was subtracted in Figure 5). We do not know the exact sources of error, but the presence of thermal drift and a temperature gradient can be sources, although we know that the drift which could be caused by the temperature gradient can easily be accounted for by simply adding the drift speed to the driven motor speed if steady. Thus, the drift can be of no issue for the forces measurement since it was steady, so it could be subtracted as shown in Figure 5. Surface Forces between Silica Surfaces at Various Temperatures. A KBr concentration of 1 mM was chosen because its EDL force decay length was 9.6 nm (at 25 °C, in water), which is above the peak-to-peak noise amplitude. Using this electrolyte, the apparatus was tested by monitoring the interactions between the silica surfaces at various temperatures from −5.5 to 29 °C (Figure 6). At negative temperature, the ethylene glycol−water mixture was used; otherwise, the measurements were performed using an aqueous KBr solution. A practically identical exponentially decaying repulsion was observed at all temperatures. The van der Waals forces were not observed between silica surfaces in the literature and in our own experiments. This reason is generally considered to be hydration forces, which was reported previously (ref 19). The decay length of the repulsion was (12.0 ± 2.0), in agreement with the Debye length of 9.6 nm, for the EDL force for 1 mM KBr at room temperature. We obtained the surface potential Ψ0 of the silica surfaces from the measured force profiles by fitting the data with the exponential equation associated with the EDL forces.1 Since the deviation in the decay lengths was significant, the calculated Debye length value was used instead of the measured values. The calculated surface potential values
Figure 4. Temperatures measured over time: (black) in ice close to the surface, (red) at the ice−silica interface, and (green) in the upper silica disk. The blue trace is the current flowing in the Peltier module.
were recorded, i.e., the temperature inside the ice, at the sample interface and inside the silica disk. The Peltier power was increased or decreased in a stepwise manner as shown in the same figure. The interface temperature was much closer to the ice temperature rather than the silica disk temperature. When the lowest temperature was reached in the ice (−20 °C), the interface temperature was −16 °C, while under much warmer conditions, close to 0 °C, the difference in the temperature between the ice and the interface was about 2 °C. After each Peltier power step, the temperature stabilized in less than 10 min, although on a longer time scale the temperatures continued to drift. It was difficult to completely avoid the influence of vibrational noise arising from the flow of the cooling water. In air, the noises produced short periods of vibrations around 10 nm peak-to-peak (black line Figure 5a). This did not disturb the 15 nm stepwise movement of the bottom surface C
DOI: 10.1021/acs.langmuir.8b01902 Langmuir XXXX, XXX, XXX−XXX
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This approach that used antifreeze successfully allowed us to observe the hard wall where ice and silica came in contact and thus to determine the distance between the silica and ice. The repulsive exponentially decaying force profile was ascribed to the EDL forces between two similarly charged surfaces, hence the ice had a negative surface potential. To extract the surface potential of ice, we used the equation for forces between dissimilar surfaces1,21 F=
2πRεε0 [2ψaψbe−D / λ − (ψa 2 + ψb 2)e−2D / λ] λ
(1)
where ψa and ψb are the surface potentials of the two surfaces. We used this equation to fit the ice−silica force curves by setting λ = 9.6 nm and by using the silica surface potential of −25 mV. This yields an ice surface potential of −35 ± 5 mV. Considering the altered geometry of the silica−ice contact, this value should be regarded as the upper limit value. If the radius of curvature of ice was 200 mm (10 times that of silica), the force magnitudes are expected to increase by a factor of 3. This is in contrast to the values reported by Kallay et al.22 using ice as an electrode. They reported the surface potential being between −60 and −75 mV in the 5.5−5.8 pH range for ice at 0 °C in a mixture of ca. 0.9 mM HCl and 1.65 mM NaOH. Alternatively, for D2O ice, Drzymala et al.23 reported a zeta potential of about −70 mV in 1 mM NaCl at pH 5 to 6. Our value is thus significantly lower than previously reported results. However, the experimental setups used for these studies are very different, and since the surface potential depends on the salt concentration, electrolyte composition, and temperature, it is not straightforward to directly compare these values.
Figure 6. Force curves between two silica surfaces at different temperatures in 1 mM KBr. At −5.5 °C, the solution was the 10% mole fraction ethylene glycol−water mixture with 1 mM KBr. For the other temperatures (the positive ones), the solution contained only KBr and water.
were contained between −25 and −20 mV with no correlation to the temperature. When the 1 mM KBr solution contained 10% ethylene glycol (Figure 6, black triangles plot), the force profile between the silica surfaces remained unaltered with a decay length and a repulsion magnitude similar to those for the KBr solution without the ethylene glycol. The Debye length depends on the square root of the temperature and the dielectric constant of the medium. From +25 to −5 °C, it decreases by 5.2%, while adding 10% ethylene glycol to the 1 mM KBr solution at 25 °C results in a decrease of 2.2%. (According to ref 24, the relative dielectric constant of a 10% w/w water−ethylene glycol mixture is about 75.) Combining both the temperature and the dielectric constant effect leads to a decrease of 7.0% of the Debye length, which is small but significant. However, it is still within the measurement errors here, and it is not surprising that we cannot detect changes. Surface Forces between Ice and Silica Surfaces. The ice and silica interactions were measured in the 10% ethylene glycol−water mixture. Since the silica−silica force profile obtained in the ethylene glycol−water mixture proved to be similar to those in water, we used the same solution for this measurement. Figure 7 shows a profile of the interaction between the ice and the silica surfaces at −11.5 °C. An exponentially decaying repulsion was observed between the two surfaces. The decay length was 11.2 nm, and the magnitude of the forces was also similar to the silica−silica symmetric system.
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CONCLUSIONS We developed a low-temperature surface forces apparatus using the Peltier module as the cooling unit. The apparatus could be operated at a temperature of −22 °C in air and −11.5 °C in solution. The temperature became stable within 10 min upon tuning the Peltier module due to a local cooling system. This system has the advantage of minimizing the influence of the thermal drift due to the different parts of the SFA. The concept of local cooling also allows very good access by the operator to the sample, and simple things such as changing the contact spot or injecting a solution can be quickly done during the course of the experiment. To demonstrate the performance of the SFA, the EDL repulsion between silica−silica and ice− silica surfaces was measured in water (for silica−silica) and in 10% aqueous ethylene glycol. On the basis of the observed repulsion, the surface potential of ice was determined to be −35 mV. Such an apparatus could prove useful for the study of nonionic surfactants and their polar head hydration and other phenomena such as the adhesion or friction of ice.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b01902. In situ confocal microscope images of the ice surface in Figure 7. Force profile at the ice surface in a 10% ethylene glycol− water mixture containing 1 mM KBr.
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DOI: 10.1021/acs.langmuir.8b01902 Langmuir XXXX, XXX, XXX−XXX
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(18) Meuler, A. J.; Smith, J. D.; Varanasi, K. K.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E. Relationships between Water Wettability and Ice Adhesion. ACS Appl. Mater. Interfaces 2010, 2, 3100−3110. (19) Horn, R. G.; Smith, D. T. Surface Forces and Viscosity of Water Measured between Silica Sheets. Chem. Phys. Lett. 1989, 162, 404−408. (20) Bevan Ott, J.; Rex Goates, J.; Lamb, J. D. Solid-liquid phase equilibria in water + ethylene glycol. J. Chem. Thermodyn. 1972, 4, 123−126. (21) Butt, H.-J. Measuring Electrostatic, van der Waals, and Hydration Forces in Electrolyte Solutions with an Atomic Force Microscope. Biophys. J. 1991, 60, 1438−1444. (22) Kallay, N.; Cakara, D. Reversible Charging of the Ice−Water Interface. J. Colloid Interface Sci. 2000, 232, 81−85. (23) Drzymala, J.; Sadowski, Z.; Holysz, L.; Chibowski, E. Ice/Water Interface: Zeta Potential, Point of Zero Charge, and Hydrophobicity. J. Colloid Interface Sci. 1999, 220, 229−234. (24) Zahn, M.; Ohki, Y.; Fenneman, D. B.; Gripshover, R. J.; Gehman, V. H. Dielectric properties of water and water/ethylene glycol mixtures for use in pulsed power system design. Proc. IEEE 1986, 74, 1182−1221.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Motohiro Kasuya: 0000-0002-2324-6121 Kazue Kurihara: 0000-0002-7299-1371 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by a collaboration research grant from Nihon Michelin Tire Co. Ltd and JSPS KAKENHI (grant no. 17K05740).
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REFERENCES
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DOI: 10.1021/acs.langmuir.8b01902 Langmuir XXXX, XXX, XXX−XXX
