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J. Phys. Chem. C 2008, 112, 9034–9039
Characterization of Nonfreezable Pore Water in Mesoporous Silica by Thermoporometry Akira Endo, Takuji Yamamoto,* Yuki Inagi, Koichi Iwakabe, and Takao Ohmori National Institute of AdVanced Industrial Science and Technology (AIST), AIST Central 5-2, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ReceiVed: February 25, 2008; ReVised Manuscript ReceiVed: April 3, 2008
The freezing/melting behavior of water confined in mesopores was evaluated by differential scanning calorimetry (DSC) using micropore-free SBA-15 materials with different pore sizes as model materials. We determined the mesoporous structure (pore size distribution, specific surface area, and pore volume) by using Ar gas adsorption/desorption measurements, and investigated the thickness dependence of nonfreezable pore water (tnf) on pore size. The tnf value was calculated as the difference between the pore radius, calculated from an Ar adsorption isotherm using the nonlocal density functional theory (NLDFT) analysis, and the radius of ice crystals that formed in the mesopores during the DSC measurement. Several studies have reported tnf values ranging from 0.35 to 1.05 nm depending on the evaluation method, whereas the tnf value estimated for the SBA-15 used in this work was approximately 0.7 nm. The difference between the reported tnf values and the value obtained in this work is mainly due to an underestimation of the pore diameter by the method based on the classical Kelvin equation. After appropriate correction of the pore diameter, the reported tnf value agreed with our results. 1. Introduction The behavior of fluids confined in nanopores has attracted considerable attention especially in the last two decades from both scientific and technological points of view. This is because the behavior of molecules confined in a restricted space can be significantly different from that of the bulk phase owing to the strong interaction between the pore surface and the molecules. It is important to deepen our understanding of the phase behavior in terms of both the fundamentals of physical chemistry and the application of porous materials as catalysts and/or adsorbents. For example, the melting/freezing point of water confined in nanopores (hereafter “pore water”) is lower than that of bulk water due to the strong interaction between the pore wall and water molecules.1–3 Pore water can be classified into two types.4 The first is freezable pore water, which can form ice crystals in the pores, and the other is nonfreezable pore water, which does not undergo a water-ice or ice-water phase transition when the temperature changes around the freezing/melting point of bulk water. This phenomenon can be used to evaluate the porous structure, and the technique is called thermoporometry (TPM), which have an advantage for the characterization of porous solids immersed in a fluid.5 However, TPM has received less attention than the gas adsorption techniques, because certain parameters needed to describe the liquid-solid or solid-liquid phase transition and to determine the structure of pore water in relation to the phase transition, have yet to be well-defined.6 The thickness of nonfreezable pore water, tnf, is one of the most important parameters when the freezing or melting of pore water is discussed. There have been several reports providing estimated tnf values that used such experimental techniques as DSC7–9 and NMR;10,11 however, a method that can measure the tnf value directly has not yet been established. To understand and discuss the phase transition of fluids in nanopores quantitatively, it is important to examine systematically the freezing/ * Corresponding author. Tel: +81-29-861-7896. Fax: +81-29-861-4660. E-mail:
[email protected].
Figure 1. Low angle X-ray diffraction patterns of synthesized micropore-free SBA-15 materials.
melting behavior of the “pore fluids” using well-defined porous materials with different pore sizes. For this purpose, ordered mesoporous materials templated by self-assembled molecules can be used as a model material. After the discovery of MCM-41 in the early 1990s,12 there were many reports concerning the melting/freezing behavior of pore water confined in cylindrical mesopores. Schmidt et al. characterized MCM-41 using 1H NMR spectroscopy, N2 adsorption, and highresolution electron microscopy, and consequently, they determined the tnf value as 0.349 ( 0.036 nm.13 Schreiber et al. reported the melting and freezing behavior of water in MCM41 and SBA-15 with different pore sizes and estimated the tnf value to be 0.38 nm based on an analysis using the Gibbs-Thomson equation.14 However, in both studies the pore size of the mesoporous silica was calculated by the Barrett, Joyner, and
10.1021/jp8016248 CCC: $40.75 2008 American Chemical Society Published on Web 05/21/2008
Nonfreezable Pore Water
J. Phys. Chem. C, Vol. 112, No. 24, 2008 9035
Figure 2. Ar adsorption/desorption isotherms measured at 87 K. (a) Isotherms and (b) corresponding Rs plots. The isotherm and Rs plot of samples SBA-15-P103-500 and SBA-15-P85-500 are shifted by 250 and 500 mL(STP)/g, respectively.
TABLE 1: Porosity Data Determined Based on the Ar Adsorption/Desorption Isotherms materials
D100 spacing/nm
pore diametera/nm
surface areab/ m2g-1
mesopore volumec/mL g-1
micropore volumed/mL g-1
SBA-15-P85-500 SBA-15-P85-600 SBA-15-P85-700 SBA-15-P103-500 SBA-15-P103-600 SBA-15-P103-700 SBA-15-P123-500 SBA-15-P123-600 SBA-15-P123-700
6.68 6.49 6.25 6.82 6.56 6.48 7.83 7.68 7.55
5.33 4.76 4.43 5.70 5.70 5.51 7.09 6.83 6.83
382 321 269 418 340 316 390 341 298
0.49 0.42 0.33 0.61 0.49 0.44 0.71 0.64 0.58
0.01< 0.01< 0.01< 0.01< 0.01< 0.01< 0.01< 0.01< 0.01
