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Langmuir 1999, 15, 3312-3316
Presence of Particles on Melt-Cut Mica Sheets S. Ohnishi and M. Hato Surface Engineering Laboratory, Department of Polymer Physics, National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba Ibaraki 305, Japan
K. Tamada Department of Molecular Engineering, National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba Ibaraki 305, Japan
H. K. Christenson* Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Australian National University, Canberra A.C.T. 0200, Australia Received August 18, 1998. In Final Form: February 18, 1999 We show the presence of numerous particles on mica surfaces melt-cut for use in the surface force apparatus (SFA). The particles, as observed by atomic force microscopy, are typically 20-25 nm in diameter and 2-3 nm high, and cover up to 0.05% of the surface area. They consist of solidified droplets of molten mica, scattered across the surfaces during the cutting of cleaved mica with a white-hot platinum wire. The particles adhere strongly to the mica surfaces in inert atmospheres but become mobile and disappear upon scanning under water and polar liquids. They seem to remain attached to the surfaces in nonpolar liquids. The particles appear to affect to a noticeable extent only experiments involving capillary condensation of water between mica surfaces.
Introduction Experiments involving the cleaved surface of molecularly smooth mica have become very important in many areas of physics, chemistry and biology. In particular, measurements with the surface force apparatus1-4 (SFA) and variants thereof now occupy a central role in research not only in traditional surface science but also in tribology,5-7 rheology,8-10 and contact mechanics.11-13 An SFA experiment starts with the manual cleaving of mica in an atmosphere of filtered laboratory air. Large (typically 10 cm2) areas of constant thickness (usually 1-5 µm) are identified by the clarity and uniformity of interference colors observed in reflected light from, for example, a fluorescent tube. These areas are cut into smaller (1 cm × 1 cm) pieces with a white-hot platinum wire. The mica melts around the platinum wire and each small sheet ends up being considerably thicker over about * Corresponding author. Fax:
[email protected].
61-2-6249 0732. E-mail:
(1) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. (2) Klein, J. J. Chem. Soc., Faraday Trans. 1 1983, 79, 99. (3) Parker, J. L.; Christenson, H. K.; Ninham, B. W. Rev. Sci. Instrum. 1989, 60, 3135. (4) McGuiggan, P. M.; Israelachvili, J. N. J. Mater. Res. 1990, 5, 2223. (5) Homola, A. M.; Israelachvili, J. N.; Gee, M. L.; McGuiggan, P. M. J. Tribol. 1989, 111, 675. (6) Van Alsten, J.; Granick, S. Phys. Rev. Lett. 1988, 61, 2570. (7) Klein, J.; Kumacheva, E.; Mahalu, D.; Perahia, D.; Fetters, L. J. Nature 1994, 370, 634. (8) Chan, D. Y. C.; Horn, R. G. J. Chem. Phys. 1985, 83, 5311. (9) Israelachvili, J. N. J. Colloid Interface Sci. 1986, 110, 263. (10) Israelachvili, J. N.; Kott, S. J.; Fetters, L. J. J. Polym. Sci. Phys. 1989, 27, 489. (11) Horn, R. G.; Israelachvili, J. N.; Pribac, F. J. Colloid Interface Sci. 1987, 115, 480. (12) Maugis, D.; Gauthier-Manuel, B. J. Adhes. Sci. Technol. 1994, 8, 1311. (13) Levins, J. M.; Vanderlick, T. K. J. Phys. Chem. 1995, 99, 5067.
0.5 mm close to the edge. These pieces are then placed face down on a freshly cleaved, thicker piecesthe backing sheetsand silvered by vacuum deposition to a thickness of about 50 nm. The molten edges allow the pieces to be picked up easily with a pair of tweezers before gluing them to silica disks at a later date. Several previous publications have discussed the effect of adsorption from the atmosphere on the adhesion between mica surfaces in nitrogen14-16 and also on the properties of capillary condensates of water formed between mica surfaces in contact.17-19 It has been demonstrated that the initial adhesion of 130-170 mJ m-2 decreases slowly and steadily (noticeable over time scales of hours) with the length of exposure to the nitrogen atmosphere, probably due to adsorption of spurious contaminants. A quick rinse with weakly acidic water gives very high initial values of the adhesion,14,15 most likely because adsorbed contaminants are washed off. These results with the SFA are in good agreement with other estimates of the surface energy of mica, both from direct cleavage experiments20 and from contact angle studies.21 The anomalous properties of capillary condensates of water on mica surfaces have been well documented ever since the inception of the surface force technique.17 Condensed water has a refractive index higher than that of bulk water, evaporates slowly, and often leaves behind a solid residue. In other words, it shows behavior (14) Christenson, H. K. J. Colloid Interface Sci. 1988, 121, 170. (15) Christenson, H. K. J. Phys. Chem. 1994, 97, 12034. (16) Christenson, H. K. Langmuir 1996, 12, 1404. (17) Israelachvili, J. N. J. Colloid Interface Sci. 1973, 44, 259. (18) Christenson, H. K.; Israelachvili, J. N. J. Colloid Interface Sci. 1987, 117, 576. (19) Christenson, H. K.; Yaminsky, V. V. Langmuir 1993, 9, 2448. (20) Wan, K.-T.; Smith, D. T.; Lawn, B. R. J. Am. Ceram. Soc. 1992, 75, 667. (21) Schultz, J.; Tsutsumi, K.; Donnet, J.-B. J. Colloid Interface Sci. 1977, 59, 277.
10.1021/la981049+ CCC: $18.00 © 1999 American Chemical Society Published on Web 04/06/1999
Presence of Particles on Melt-Cut Mica Sheets
reminiscent of that of polywater on silica.22 It has been concluded that this behavior is consistent with the leaching of some solute into the condensed water, just as dissolution of silica is responsible for the anomalous properties of polywater. Obviously, all such information on adhesion and capillary condensation has been obtained with mica pieces that have been cut and silvered according to the procedure outlined above. A recent study of the adhesion of mica surfaces in air and water15 found no evidence for earlier claims23 of the presence and slow growth of organic carbonaceous material on air-cleaved mica surfaces. The presence of soluble material on mica has been proven by experiments where freshly cleaved sheets of mica were dried over phosphorus pentoxide for several weeks, after which shadowed replicas were made and observed by transmission electron microscopy.18 Regular crystallites were found spread over the surface, covering perhaps 0.1% of the area, and containing material equivalent to an average coverage of less than a monolayer. If the mica was acid washed prior to drying, no crystallites were found. The mica samples in these experiments were not cut or silvered. At the time, one of us speculated that some potassium salt, resulting from a reaction between the surface potassium ions of the mica, and carbon dioxide and/or water vapor might be the constituent of the crystallites.18 Mica has more recently become an important substrate for investigations of both forces and surface topography with the atomic force microscope (AFM).24-26 Because it is molecularly smooth and has a high surface energy, it is ideally suited for studies of adsorbed species such as polymers, proteins, and other macromolecules. A highresolution AFM scan of an air-cleaved mica surface shows the expected lack of featuressthe surface is flat to within 0.1 nm. To our knowledge no images of cut and silvered mica samples such as those used in the SFA have been published previously. In conjunction with an ongoing study of surfaces prepared by Langmuir-Blodgett deposition of polymerized fluorosilanes on mica,27 a large numbers of small particles (typically 25 nm in diameter and 2-3 nm in height) on mica surfaces that had not been coated with LB films were found with AFM scans. Further tests showed that these particles were present only on cut and silvered mica pieces, and not on surfaces that had been merely cleaved in the same environment (in a laminar flow cabinet in a clean room). We decided to attempt to determine the origin of these particles, and their behavior on exposure of the surfaces to vapors and liquids. Materials and Methods The mica was brown or green muscovite from various sources (Watanabe Shoko Co, Tokyo; Brown Mica Co., Sydney, Australia, and Mica Supplies Ltd., Dorset, U.K.). It was cleaved and cut in the standard manner, both in Canberra and in Tsukuba. The AFM system used in this study was a commercially available NanoScope IIIa (Digital Instruments, Inc., Santa Barbara, CA). A J-scanner with a scan range of about 150 × 150 µm was used. The D-scanner (12 × 12 µm) was used for highresolution images (Figure 6b). Also, 200 µm long cantilevers with (22) Yaminsky, V. V. Langmuir 1997, 13, 2. (23) McGuiggan, P. M.; Israelachvili, J. N. J. Mater. Res. 1990, 5, 2232. (24) Drake, B.; Prater, C. B.; Weisenhorn, A. L.; Gould, S. A. C.; Albrecht, T. R.; Quate, C. F.; Cannell, D. S.; Hansma, H. G.; Hansma, P. K. Science 1989, 243, 1586. (25) Senden, T. J.; Ducker, W. A. Langmuir 1992, 8, 733. (26) Nishimura, S.; Biggs, S.; Scales, P. J.; Healy, T. W.; Tsunematsu, K.; Tateyama, T. Langmuir 1994, 10, 4554. (27) Christenson, H. K.; Hato, M.; Ohnishi, S.; Tamada, K. Unpublished results.
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Figure 1. Typical AFM images in air of mica sheet cut with Pt wire. The thickness of the mica sheet was about 5.5 µm. Also shown is a height profile scan of the particle in part b.
Figure 2. AFM image in air of mica surfaces from the same cleaved sheet: (a) cut with Pt wire and (b) torn off. The mica thickness was about 5 µm. The image size is 5 µm × 5 µm. a nominal spring constant of 0.12 N/m and Si3N4 tips were employed. AFM images (400 × 400 pixels) were obtained using the height mode, which kept the force constant, at room temperature. Typical AFM parameters were as follows: integral gain ) 3; proportional gain ) 5; two-dimensional gain ) 0.3; scan rate) 19.6 Hz. To obtain the best imaging conditions, the applied force was minimized and stabilized by adjusting the height of the cantilever (set point voltage) during scanning of the sample surface. To investigate the stability of the particles on the mica surfaces in liquids, AFM imaging was carried out using a fluid cell in water and organic solvents (ethanol, cyclohexane, and octamethylcyclotetrasiloxane). The typical force acting on the surfaces during imaging was 1-2 nN in air and 10-50 pN in liquid. The scanning electron microscope (SEM) was a TOPCON DS720 with a field emission electron gun. The observation was done at 20 kV with a magnification of 100× after metal-coating the mica surface. Transmission electron microscopy was performed on a Philips EM430, fitted with a Link Isis energy dispersive spectrometer (EDX).
Results Figure 1a shows an AFM scan in laboratory air of a mica piece cut with a hot Pt wire. Numerous particles on the surface are evident, and a close-up (Figure 1b) shows an amorphous blob with an approximate diameter of 20 nm. The surface density of the particles is about 8 µm-2, and the coverage is less than 0.05%. A topographical scan gives a height of about 3 nm. The particles adhere strongly to the mica surface and are not displaced by repeated scanning. All mica pieces cut with a Pt wire that were examined showed the presence of such particles. No such particles were ever found on uncut mica, i.e., freshly cleaved faces. Figure 2 shows the comparison between a piece that was torn off (b) and one that was cut off (a). Both pieces were taken from the same larger area of uniform thickness.
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Figure 4. AFM images of cut mica sheets under water (top), showing how particles are displaced by successive scans. The bottom shows scans of a mica sheet in cyclohexane, where the particles adhere quite firmly to the surface. The mica thickness was 5-6 µm and the image size 5 µm × 5 µm.
Figure 3. AFM images in air of mica sheets of different thickness. The image size is 5 µm × 5 µm. Key: (a) thickness 5-6 µm; (b) thickness 2-3 µm; (c) thickness less than 2 µm.
The density of particles on the mica surface increased very obviously with the thickness of the mica sheets. Figure 3 shows the comparison between sheets of three different thicknesses, ranging from 6 to less than 2 µm. This range is typical for sheets commonly used in the SFA. It is clear that the particles are caused by the cutting process, and it is likely that they are droplets of fused mica that are ejected from the Pt wire as it cuts through the mica piece. The thicker the mica the more material is molten, and for the thickest sheets there are even unmolten flakes of mica present on the surfaces after cutting. No difference was found between the top and bottom face of the mica (it is mounted horizontally during cutting). If the mica sheets are immersed in water, the particles disappear as the surfaces are scanned with the AFM tip. The particles are evidently very mobile in water, or they dissolve (see Figure 4, top). In contrast, the particles are quite firmly held when the sheets are immersed in nonpolar liquids such as cyclohexane and persist after 20 or more scans (Figure 4, bottom). In liquids of intermediate polarity, such as ethanol, the behavior is between that in water and nonpolar liquids, and the particles disappear slowly on repeated scanning. Exposure to water vapor renders the particles more mobile, and they are easily displaced on scanning in normal laboratory air (humidity ≈ 70%). When exposed to the vapor of cyclohexane, the particle mobility increases slightly, but not as much as after exposure to water vapor. Previous electron microscopy observations of crystalline particles formed on drying of air-cleaved mica over P2O5 were repeated and confirmed with the AFM. Particles with a diameter of 200-350 nm and a height of 30-35 nm were found on (uncut) mica sheets dried over P2O5 for 2 weeks (Figure 5). This correlates well with the earlier results obtained with replicas and TEM.18 These particles are thus quite different in size, surface density and morphology, as a close-up reveals (Figure 6a). Convolution
Figure 5. AFM scan of uncut mica sheet cleaved in air and stored over P2O5 for two weeks. Note the presence of larger particles (diameter about 200-300 nm) on the surface. The top images show close-ups of the particles indicated by arrows in the larger image.
effects that occur between the tip and particles of similar size28 limit the resolution and blur any crystalline facets of the particles. The crystallinity of the surface of the particle is, however, evident in a high-resolution image (Figure 6b). The distance between each row (the arrows) is 0.4 ( 0.01 nm, which is significantly smaller than the oxygen periodicity of the mica lattice (0.52 nm). Scanning electron micrographs of the edge of a sheet of mica cut with platinum wire show a combination of delamination of the layers and melting (Figure 7). Large, micrometer-size flakes are found near the edges. (28) Vesenka, J.; Manne, S.; Giberson, R.; Marsh, T.; Henderson, E. Biophys. J. 1993, 65, 992.
Presence of Particles on Melt-Cut Mica Sheets
a
b Figure 6. (a) AFM scan and section analysis of “crystallites” such as those in Figure 5. (b) High-resolution AFM scan of the surface of a crystallite like the ones in part a.
Figure 7. Scanning electron microscope image of the edge of a piece of mica cut with a Pt wire. Note the combination of melting and tear. Magnification ) 100×. Note scale bar.
We attempted to determine the composition of the particles with EDX by holding a carbon-coated electron microscopy grid next to the platinum wire while cutting a thick (≈5 µm) sheet of mica. While some particles were faintly visible in the electron microscope, X-ray analysis did not provide an intense enough signal to be useful. Discussion There is little doubt that the particles observed on mica sheets that have been cut with a hot platinum wire are solidified droplets of mica ejected during the cutting. From the electron micrographs, it appears that the cutting occurs by a combination of melting and tearing. The thicker the mica, the more of it is molten during the cutting, and
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consequently the number and size of the particles are larger, and occasional flakes of torn rather than molten mica are found. The particles are small enough that they are not resolved by the optical system and would not be visible on the interference fringes. While the surface separation may be resolved with an accuracy of (0.1 nm with the FECO fringes,17 the lateral resolution is limited by the wavelength of visible light and the magnification of the optical system, and features of lateral dimensions smaller than about 1 µm are not visible. Nevertheless, the particles must prevent the mica surfaces from coming into molecular contact at and around the location of the particles. Their presence would thus reduce the effective contact area of the mica surfaces, and one might expect a reduction in the magnitude of the measured adhesion. However, because of the large variability of the adhesion in air and nitrogen it would be difficult to make any quantitative estimate of this effect, and it would certainly be smaller than the standard deviation of the measured adhesion (which is 20-25%).15 Unfortunately, the small size of the particles and the low surface coverage appears to preclude the possibility of obtaining information on their composition and/or structure with surface spectroscopic techniques such as X-ray photoelectron spectroscopy (XPS, or ESCA), Auger, electron or X-ray diffraction, secondary ion mass spectrometry (SIMS), etc. The particles are easily removed on immersion in bulk aqueous solution, whether by dissolution or diffusion, and they would not be expected to influence the results of any force measurements or other experiments in bulk aqueous solution. The disappearance of the particles when the surfaces are immersed in water may be at least part of the reason mica dipped in weakly acidic water (hydrogen mica) shows a larger adhesion than normal potassium mica, as discussed in ref 15. It is also likely that the particles are removed by polar liquids, but they may remain on the surfaces in nonpolar liquids. We can only speculate on the possible consequences, such as a reduction in the range of the solvation force in nonpolar liquids by a smearing out effect. In view of the variability from experiment to experiment commonly encountered in such surface force measurements,29 it is unlikely that even carefully controlled experiments would yield conclusive information on the effects of the presence of the mica particles. By far the most serious implications are likely to be for measurements in water vapor. The anomalous properties of water condensed on mica surfaces are well-known. One of us has previously attributed this chiefly to the dissolution in the condensed water of the material constituting the crystallites.15,18 It appears likely that the effect of the solidified mica droplets may be to further compound the problem. Because of the small size of the particles and possible chemical changes during fusion of the mica they may well dissolve completely in the condensed water, or at the very least considerable amounts of potassium and aluminum could leach into the water. Since the number of particles per unit area varies greatly depending on the thickness of the mica sheets, any such effect would vary from experiment to experiment. In contrast, the amount of material present from the postulated surface reaction would be quite constant from experiment to experiment. Since the particles are not easily removed in nonpolar (29) Yaminsky, V. V.; Christenson, H. K. Langmuir 1995, 11, 5176.
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liquids, they may also contribute to the solute present in water condensed between mica surfaces from nonpolar liquids.30,31 The effect of the particles on experiments in nonpolar vapor would be less serioussthe adhesion is dominated by the Laplace pressure in the condensed annulus and this would not be affected if the particles are insoluble. A small effect on the solid-solid adhesion inside the condensed annulus would be insignificant and unobservable.19 Conclusions We have shown how cutting mica sheets with a hot platinum wire ejects a shower of fused mica particles across (30) Christenson, H. K. J. Colloid Interface Sci. 1985, 104, 234. (31) Christenson, H. K.; Fang J.; Israelachvili, J. N. Phys. Rev. 1989, B39, 11750.
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the surfaces. To minimize the number and density of these particles thin mica sheets should be used. The larger the sheets the better, so that the contact region when the mica is mounted in the SFA is far from the edges. The particles are unlikely to cause any problems for force measurements in aqueous solution, but their presence may effect capillary condensation experiments in water vapor. Their partial dissolution may explain some of the properties of water condensates between mica surfaces in the SFA. Acknowledgment. We thank Dr. K. Yase for carrying out the SEM scans and Dr. T. Senden for help with the EDX. H.K.C. acknowledges a Japanese Government Research Award for Foreign Specialists. LA981049+
