pubs.acs.org/Langmuir Transport of Water in Small Pores Shuangyan Xu, †, ) Gregory C . Simmons, †,^ T. S. Mahadevan, ‡ George W. Scherer,* ,† Stephen H. Garofalini, ‡ and Carlos Pacheco § † Department of Civil and Environmental Engineering/PRISM, Princeton University, Engineering Quad E-319, Princeton, New Jersey 08544, ‡ Interfacial Molecular Science Laboratory, Department of Materials Science and Engineering, Rutgers University, Piscataway, New Jersey 08855 and § Department of Chemistry, Princeton University, Princeton, New Jersey 08544. ) Present address: Applied Materials Xi’an, 28 Xinxi Road, Xi’an Hi-Tech Industrial Development Zone, Xi’an, Shaanxi, P.R. China 710119. ^ Present address: 401 E. 34th Street, Apt. S23E, New York, New York 10016-4963. Received December 10, 2008. Revised Manuscript Received February 11, 2009 Experimental measurements of the thermal expansion coefficient (R), permeability (k), and diffusivity (D) of water and 1 M solutions of NaCl and CaCl 2 are interpreted with the aid of molecular dynamics (MD) simulations of water in a 3 nm gap between glass plates. MD shows that there is a layer ∼6A ˚ thick near the glass surface that has R ∼2.3 times higher and D about an order of magnitude lower than bulk water. The measured D is ∼5 times lower than that for bulk water. However, when the MD results are averaged over the thickness of the 3 nm gap, D is only reduced by ∼30% relative to the bulk, so the measured reduction is attributed primarily to tortuosity of the pore space, not to the reduced mobility near the pore wall. The measured R can be quantitatively explained by a volume-weighted average of the properties of the high-expansion layer and the “normal” water in the middle of the pore. The permeability of the porous glass can be quantitatively predicted by the Carman-Kozeny equation, if 6 A ˚ of water near the pore wall is assumed to be immobile, which is consistent with the MD results. The properties and thickness of the surface-affected layer are not affected significantly by the presence of the dissolved salts. 1. Introduction Transport of aqueous solutions in porous media with nanometric pores is of interest for applications ranging from filtration of water to predicting the durability of concrete. Very good data for the permeability, k, of Vycor glass 1,2 show that k decreases as the molecular size of the liquid increases; the results could be quantitatively explained by assuming that a monolayer of liquid is immobilized against the pore wall. Recently, molecular dynamics (MD) simulations have been done using a dissociative potential for water that allows reaction with silica to form silanols. 3,4 When water is intro- duced into a 3 nm gap between plates of silica glass, the calculated pair distribution functions for oxygen indicate that the structure of water is altered only within about 0.6 nm of the silica surface; 5 the existence of that layer was shown to account for the anomalously high thermal expansion coeffi- cient of water in porous glasses. 5,6 If the mobility in that layer is low, then it could also account for the dependence of k on pore size in porous glasses. In the present study, we provide additional data for the mobility of water and aqueous solutions in porous glasses, including measurements of per- meability and diffusivity, and interpret the results in light of MD simulations. 2. Experimental Procedure The porous hosts included rods of Vycor glass (kindly provided by Corning, Inc.) with diameters of ∼7.2 and 3.4 mm and a silica xerogel with a diameter of ∼3.4 mm; these materials are identified in ref 6 as “7 nm Vycor”, “5 nm Vycor”, and “Xerogel 3 nm”. (The 5 nm Vycor rods proved to be unsatisfactory for beam bending, because they were geometrically imperfect, but they were used for measurement of diffusivity and thermal expansion.) To remove organic impurities from the pores, the Vycor rods were boiled in a 30% solution of hydrogen peroxide for several hours until the rods turned clear, then rinsed in deionized (DI) water and stored in ethanol. Before measurements, the samples were exchanged into water by submerging them in a volume at least 15 times greater than the volume of rods for at least 12 h. The xerogel rods, which had been prepared from tetramethoxysilane, 7 were obtained dry and were rehydrated slowly by condensation of water vapor over a period of ∼4 h (following the method described in ref 8), after which they were stored in DI water. DI water, 1 M NaCl, 1 M CaCl 2 , and 1 M MgSO 4 solutions were prepared for both sample types, and a 1 M NaNO 3 solution was prepared exclusively for the Vycor rods. The pH of the MgSO 4 solution was 7, and that of the other solutions was ∼5.5; for some tests, the 1 M CaCl 2 solution was adjusted to pH 2 using hydrochloric acid. That pH was chosen because it is close to the isoelectric point of silica, where the charge on the pore wall is zero; this is expected to reduce the interaction of the wall with dissolved ions. The three point beam-bending method has been developed to measure the permeability in materials where k is low (e10 -18 m 2 ≈ 1 microdarcy). 2,9,10 The principle of the measurement is that *Author to whom correspondence should be addressed. E-mail: scherer@princeton.edu. (1) Debye, P.; Cleland, R. L. J. Appl. Phys. 1959, 30(6), 843–849. (2) Vichit-Vadakan, W.; Scherer, G. W. J. Am. Ceram. Soc. 2000, 83(9), 2240–2245. (3) Mahadavan, T. S.; Garofalini J. Phys. Chem. B 2007, 111, 8919–8927. (4) Mahadavan, T. S.; Garofalini J. Phys. Chem. C 2008, 112, 1507–1515. (5) Garofalini, S. H.; Mahadevan, T. S.; Xu, S.; Scherer, G. W. Chem. Phys. Chem. 2008, 9, 1997–2001. (6) Xu, S.; Scherer, G. W.; Mahadevan, T. S.; Garofalini, S. H.Thermal Expansion of Confined Water. Langmuir, submitted for publication, 2008. (7) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: New York, 1990. (8) Scherer, G. W. J. Non-Cryst. Solids 1997, 212, 268–280. (9) Scherer, G. W. J. Non-Cryst. Solids 1992, 142(1-2), 18–35. (10) Scherer, G. W. J. Am. Ceram. Soc. 2000, 83(9), 2231–2239.; Erratum, J. Am. Ceram. Soc. 2004, 87 (8), 1612-1613. Published on Web 4/6/2009 © 2009 American Chemical Society DOI: 10.1021/la804062e Langmuir 2009, 25(9), 5084–5090 5084 Downloaded by RUTGERS UNIV on July 6, 2009 Published on April 6, 2009 on http://pubs.acs.org | doi: 10.1021/la804062e