Langmuir 2000, 16, 7285-7288
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Capillary Condensation of Water between Rinsed Mica Surfaces Mika M. Kohonen* and Hugo K. Christenson Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Australian National University, Canberra ACT 0200, Australia Received October 26, 1999. In Final Form: April 27, 2000 In previous studies of capillary condensation of water between mica surfaces using the surface force apparatus (SFA) it has not been possible to obtain agreement with the Kelvin equation. The condensates in these studies exhibited refractive index values larger than that of bulk water and often deposited residues of involatile material upon evaporation. We report a study of capillary condensation of water between mica surfaces which have been rinsed in dilute acid solution prior to mounting in the SFA. The refractive indices of water condensates between such surfaces are equal to that of bulk water, and no visible deposits of involatile material remain upon evaporation. The equilibrium meniscus curvatures of the condensates agree with theoretical values calculated from the Kelvin equation.
Introduction The vapor pressure of a liquid is affected by curvature of the liquid-vapor interface, as described by the Kelvin equation,1
(RT/Vm) ln(p/p0) ) γ/r - (p - p0)
(1)
where p is the vapor pressure above a liquid-vapor interface with mean radius of curvature r, p0 is the vapor pressure above a flat interface, and γ and Vm are the surface tension and molar volume of the liquid, respectively. For wetting liquids confined in small pores r is negative and so the vapor pressure of the liquid is lowered and coexistence between the liquid phase and an undersaturated vapor is possible. This phenomenon, referred to as capillary condensation, is an important feature of the study of fluids in porous media. The Kelvin equation, as applied to capillary condensation, forms the basis for the determination of pore-size distributions from adsorption isotherms2,3 and is also used in interpreting the adhesion between surfaces due to capillary-condensed liquid bridges.4 The validity of eq 1 has been the subject of numerous experimental studies, the results of which are discussed in several reviews.5-7 Although many of the early studies apparently suffered from problems with contamination and/or lack of equilibrium, Melrose7 concludes that there is no reason to doubt the applicability of the Kelvin equation to capillary-condensed liquids with meniscus radii of the order of 1 µm or larger. In many practical applications, however, the interest is in liquids with |r| , * Corresponding author. E-mail:
[email protected]. Fax: 61-2-6249 0732. (1) See, e.g.: Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; Wiley-Interscience: New York, 1997. Defay, R.; Prigogine, I.; Bellemans, A.; Everett, D. H. Surface Tension and Adsorption; Green and Co.: London, 1966. (2) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (3) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: New York, 1982. (4) Zimon, A. Adhesion of Dust and Particles; Plenum: New York, 1982. (5) Skinner, L. M.; Sambles, J. R. Aerosol. Sci. 1972, 3, 199. (6) Fisher, L. R.; Israelachvili, J. N. J. Colloid Interface Sci. 1981, 80, 528. (7) Melrose, J. C. Langmuir 1989, 5, 290.
1 µm and there have been relatively few direct measurements on condensates in this regime. Fisher and Israelachvili6 have established the validity of eq 1 for -r in the range 4-20 nm for the case of cyclohexane condensed between mica surfaces. For water, an important liquid in many systems, the situation is less satisfactory. To our knowledge there are only three studies presenting direct measurements of the sizes of water condensates with nanometric radii of curvature. Fisher et al.8 studied the condensation of water between fused silica surfaces and found a small but significant difference between theory and experiment. In a study of capillary condensation of water from undersaturated nonpolar liquids, Christenson9 found that the measured values of |r| were significantly larger than predicted by the Kelvin equation. In another study an environmental scanning electron microscope was used to image water condensates with -r values in the range 45-150 nm.10 However, due to limitations in the determination of the temperature only very qualitative agreement with the Kelvin equation could be asserted. The measurements of Fisher and Israelachvili6 and Christenson9 were obtained using the surface force apparatus (SFA), the design and use of which are described in numerous references.6,11-13 Two back-silvered muscovite mica surfaces are mounted in a crossed-cylinders geometry and when brought into contact form a pore in which capillary condensation can take place. Information such as the refractive index and interface radius of curvature of the condensate is obtained from analysis of the interference fringes produced when white light is passed through the surfaces. Fisher and Israelachvili attempted to study the condensation of water but noted that the water condensates in their experiments deposited substantial amounts of involatile material upon evaporation. Such behavior has (8) Fisher, L. R.; Gamble, R. A.; Middlehurst, J. Nature 1981, 290, 575. (9) Christenson, H. K. J. Colloid Interface Sci. 1985, 104, 234. (10) Schenk, M.; Futing, M.; Reichelt, R. J. Appl. Phys. 1998, 84, 4881. (11) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. Parker, J. L.; Christenson, H. K.; Ninham, B. W. Rev. Sci. Instrum. 1989, 60, 3135. (12) Israelachvili, J. N. J. Colloid Interface Sci. 1973, 44, 259. (13) Christenson, H. K. Colloids Surf. 1997, 123, 355.
10.1021/la991404b CCC: $19.00 © 2000 American Chemical Society Published on Web 08/03/2000
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also been observed in much earlier SFA experiments.12 The refractive index of water condensed between mica surfaces used in the SFA has also been observed to be higher than that of bulk water,9,12 the measured values ranging up to as high as n ) 1.46. These observations, together with the lack of agreement with the Kelvin equation which was reported by Christenson, are consistent with the presence of a surface contaminant which is soluble and/or mobile in water. In fact, there is evidence which suggests the presence of at least two different types of water-labile contaminant on mica surfaces used in the SFA. Christenson and Israelachvili14 demonstrated the presence of a watersoluble compound on mica surfaces cleaved in laboratory air. They showed that this compound could be removed by rinsing the mica in dilute acid solution and suggested that it might consist of a potassium salt formed by reaction between water, carbon dioxide, and the surface potassium ions on native mica surfaces. Immersing the surfaces in acid solution would result in the exchange of potassium ions for hydrogen ions, thus preventing the formation of any potassium salts. Recently it has also been shown that mica surfaces, as prepared in the standard manner, are covered with small particles.15 The mica surfaces used in the SFA are prepared by cleaving large areas of mica of uniform thickness which are then cut into approximately 1 cm × 1 cm pieces using a white-hot platinum wire. It was suggested that the particles observed on surfaces prepared in this manner may be produced by sputtering of molten mica during the melt-cutting process.15 These particles were also shown to be mobile in water. Motivated by the observation that rinsing the mica surfaces in dilute acid solution appears to clean the surface of these adsorbed contaminants,15,16 we have studied the capillary condensation of water between rinsed mica surfaces. The results suggest that the surfaces prepared in this manner are relatively free of water-labile contaminants, and the measured values of r are found to agree with the Kelvin equation within the range -5 to -50 nm. Materials and Methods The measurements were carried out with a simplified surface force apparatus17 in which the lower mica surface is mounted on a rigid support. The surfaces were prepared in the standard manner15 from brown or green muscovite mica obtained from various sources (Brown Mica Co., Sydney, Australia, and S&J Trading, New York) and glued onto supporting silica disks with an epoxy resin (Epon 1004, Shell Chemical Co.). Prior to mounting the surfaces in the SFA they were immersed in dilute aqueous solutions of HCl or HNO3 (pH ∼ 3) for between 1 and 2 min and then dried with a stream of nitrogen. The sealed SFA chamber was flushed with dry nitrogen for approximately 90 min before recording the contact wavelengths. All measurements were carried out at 25.0 ( 0.05 °C. The assembly of the apparatus, the preparation of solutions, and the rinsing of the surfaces were all carried out in a laminar flow cabinet. The vapor pressure of water in the sealed SFA chamber was controlled by introducing solutions of known concentrations of LiCl or Na2SO4 onto the bottom of the chamber. The relative water vapor pressure (p/p0) was then calculated from tables of osmotic coefficients.18 (14) Christenson, H. K.; Israelachvili, J. N. J. Colloid Interface Sci. 1987, 117, 576. (15) Ohnishi, S.; Hato, M.; Tamada, K.; Christenson, H. K. Langmuir 1999, 15, 3312. (16) Christenson, H. K. J. Phys. Chem. 1993, 97, 12034. (17) Christenson, H. K.; Yaminsky, V. V. Langmuir 1993, 9, 2448. (18) Robinson, R. A.; Stokes, R. H. Electrolyte Solutions, 2nd ed.; Butterworth: London, 1959.
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Figure 1. Film thickness t as a function of relative vapor pressure p/p0 for water adsorbed from vapor onto mica. The symbols represent interferometrically determined values for water on rinsed mica surfaces at 25.0 °C. Different symbols are from different experiments, and each point was obtained after several evaporation/condensation cycles. The dotted curves represent the range in values obtained from ellipsometric measurements on air-cleaved mica at 18.0-20.0 °C.19 All chemicals were of analytical grade, used as received. The water was distilled and then processed through a Millipore UHQ unit. All glassware was treated with dichromate-sulfuric acid cleaning solution and then rinsed with large quantities of purified water. The equilibrium radius of curvature of the water-vapor interface of a condensate was determined using the same method as in refs 9 and 17. The contact angle of water on mica is very small19 and so the meniscus radius of curvature is, to a very good approximation, given by half the surface separation at the meniscus. This separation was determined from the position of the discontinuities in the even-order interference fringes at the location of the liquid/vapor interface (see Figure 3c in ref 6 or Figure 4b in ref 9). The value of |r| calculated in this manner was corrected for the presence of adsorbed films (see below) by subtracting the measured film thickness.6 The smallest value of |r| which can be measured with confidence is around 5 nm.
Results and Discussion In contrast to previous SFA studies of the capillary condensation of water, in our experiments with rinsed mica surfaces we did not observe the deposition of substantial amounts of involatile material upon evaporation of water condensates. In a control experiment with surfaces which had not been rinsed the deposition of involatile material was very evident from the observed deformation in the interference fringes. Adsorption from the vapor phase gives rise to thin films of water on the mica surfaces,19 and the thickness of these films can be calculated from the measured refractive index when the surfaces are not in contact.6 The interferometrically determined thicknesses of water films on rinsed mica surfaces, measured after several condensation/evaporation cycles, are shown in Figure 1. The fact that the measured values fall within the range reported in a previous study of water adsorption on mica19 is consistent with the lack of a significant accumulation of material on the surfaces. There is some indirect evidence, however, that rinsing the surfaces is not completely effective in removing the material which gives rise to the involatile deposits. Specifically, the minimum separation to which the mica surfaces could be brought before they jumped into contact (the “jump-in distance”, Dj) was sometimes observed to increase with the number of water condensation/evaporation cycles. The value of Dj when the surfaces were brought together for the first time in water vapor (i.e. prior to the (19) Beaglehole, D.; Radlinska, E. Z.; Ninham, B. W.; Christenson, H. K. Phys. Rev. Lett. 1991, 66, 2084.
Condensation of Water between Mica Surfaces
Figure 2. Refractive index n as a function of surface separation D for water condensed between rinsed mica surfaces. Different symbols represent measurements in different condensates, all from one experiment. Approximately 15 condensation/evaporation cycles were carried out between the first and last set of measurements. The error in the measured refractive index depends on the surface separation: typical errors are shown by the error bars on the points represented by filled diamonds. The dashed line indicates the refractive index of bulk water (n ) 1.33).
Figure 3. Measured meniscus radii of curvature of capillary condensates of water between rinsed mica surfaces. r is the mean radius of curvature of the water-vapor interface, and p/p0 is the relative vapor pressure. Different symbols are from different experiments. The solid line is the theoretical curve obtained from the Kelvin equation (eq 1 in text). Inset: the same data presented in the format used in refs 6, 8, and 9.
first condensation/evaporation cycle) was 10 ( 2 nm (average of values from five separate experiments). The jump-in distance subsequently increased with the number of condensation/evaporation cycles up to a maximum of 20-30 nm. It is plausible that a very small amount of involatile material, too small to be observable in the interference fringes, could locally decrease the separation between the surfaces and give rise to a larger value of Dj. The refractive indices of water condensates between rinsed mica surfaces were found to be equal, within experimental error, to that of bulk water (nwater ) 1.33). Figure 2 shows the results of measurements of refractive index versus surface separation for a series of condensates from one particular experiment. Approximately 15 condensation/evaporation cycles were carried out between the first and last set of measurements. Figure 3 shows the experimentally measured meniscus radii of curvature of water condensates between rinsed
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mica surfaces as a function of the relative vapor pressure, p/p0. Each point is the average of at least five measurements, and the error was taken to be the maximum deviation from the mean. The errors in the p/p0 values represent an estimated (0.1 °C uncertainty in the temperature at the surfaces. The solid line in Figure 3 shows the values of the radii of curvature calculated from the Kelvin equation: the measured radii of curvature are in reasonable agreement with theory in the range -5 to -50 nm. (Note that the scatter in the measured values of r is too large to allow any conclusions to be drawn concerning the possible effect of curvature on surface tension. Using standard expressions for the curvature dependence of the surface tension of water1 only results in a change of a few 0.1 nm in the predicted values of r in the range -5 to -50 nm.) The reversibility of the measurements was checked in two separate experiments, and agreement with theory was obtained with both decreasing and increasing values of p/p0. The increase in jump-in distance observed after several condensation/evaporation cycles and the small scatter in the measured values of r may be due to a small amount of contaminants remaining even after rinsing. In the study by Christenson and Israelachvili no trace of the compound which forms on air-cleaved mica surfaces was observed after rinsing the surfaces. This observation is consistent with the high efficiency of ion exchange of potassium on mica surfaces which has been observed in previous studies.20,21 We speculate that the rinsing process is probably not completely effective in removing the particles which were recently shown to be present on mica surfaces prepared in the standard manner. In one experiment we cleaved a piece of mica approximately 3 cm × 5 cm in size in which only one of the short edges was melt-cut. After silvering, and before removal from the backing sheet, thin strips of mica were cut parallel to the long edge with a scalpel blade. The ends of the strips furthest from the melt-cut edge were then glued to silica disks and the remainder of the strips torn off. Capillary condensates of water between these (unrinsed) surfaces were observed to be significantly larger than predicted on the basis of the Kelvin equation, but no significant deposits of involatile material were observed upon evaporation. Also, the jump-in distance did not increase with the number of condensation/evaporation cycles. This suggests that the lack of agreement with the Kelvin equation observed in previous SFA experiments was probably due largely to the presence of the compound formed on cleavage of mica in air, while the involatile deposits probably arise from accumulation of the particles present on melt-cut mica. Further investigation of these issues is underway. In a recent study of wetting and capillary phenomena of water on air-cleaved mica, Xu et al.22 inferred that the water condensates which formed between an atomic force microscope tip and the mica surface were larger than theoretically predicted. The formation of large condensates on air-cleaved mica is consistent with the presence of the compound described by Christenson and Israelachvili. Interesting structures were also observed in the thin films of water on the mica surface, and some aspects of the behavior of these films were attributed to the presence of hydrated potassium ions. It would be interesting to see if rinsing the mica surface in dilute acid solution would have any effect on the structure of adsorbed water films. (20) Pashley, R. M. J. Colloid Interface Sci. 1981, 80, 153. (21) Xu, L.; Salmeron, M. Langmuir 1998, 14, 2187. (22) Xu, L.; Lio, A.; Hu, J.; Ogletree, D. F.; Salmeron, M. J. Phys. Chem. B 1998, 102, 540.
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Conclusion The anomalous behavior of water condensates observed in previous SFA studies can be attributed to the presence of water-labile surface contaminants which can be removed by rinsing the surfaces. In particular, the experimentally
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measured meniscus radii of curvature of capillary condensates of water between rinsed mica surfaces are in reasonable agreement with the Kelvin equation. LA991404B
