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 where p is the vapor pressure above a liquid-vapor interface with mean radius of curvature r, p 0 is the vapor pressure above a flat interface, and γ and V m are the surface tension and molar volume of the liquid, respec- tively. 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 under- saturated 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 adsorp- tion isotherms 2,3 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, Melrose 7 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| , 1 μm and there have been relatively few direct measure- ments on condensates in this regime. Fisher and Israelachvili 6 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, Christenson 9 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 Israelachvili 6 and Christenson 9 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 con- densation of water but noted that the water condensates in their experiments deposited substantial amounts of involatile material upon evaporation. Such behavior has * Corresponding author. E-mail: mmk110@rsphysse.anu.edu.au. 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. (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. (RT/V m ) ln(p/p 0 ) ) γ/r - (p - p 0 ) (1) 7285 Langmuir 2000, 16, 7285-7288 10.1021/la991404b CCC: $19.00 © 2000 American Chemical Society Published on Web 08/03/2000