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X-ray Study of Freezing and Melting of Water Confined within SBA-15 Kunimitsu Morishige* and Hiroshi Iwasaki Department of Chemistry, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan Received October 15, 2002. In Final Form: January 13, 2003 To examine the freezing and melting behavior of water in partially filled pores of porous silica, we performed X-ray diffraction measurements of water confined inside the cylindrical pores of SBA-15 with a pore radius of 3.9 nm as a function of temperature and pore filling. The freezing temperature increased continuously with increasing pore filling even in the region of capillary condensation. The results are related to the different states of the pore water depending on the degree of pore filling. On the other hand, the melting of the frozen pore water took place at a well-defined temperature of 256 K, independent of the level of pore filling. The X-ray diffraction patterns show that the freezing of the pore water results in formation of ice microcrystals with almost the same structure and size, irrespective of the different states of the pore water.
I. Introduction There is considerable current interest in the freezing and melting behavior of materials confined in porous media.1 Especially, a water-porous silica system has been extensively explored,2-11 because of its fundamental and technological importance. It is now evident that there are two states of water, free water in the middle of pores and bound water adjacent to the pore wall, in the pores and their freezing and melting temperatures are always depressed compared to those of the bulk. The freezing and melting behavior of the free water differs markedly from that of the bound water. The freezing and melting of the bound water occurs very gradually at lower temperatures than that of the free water. Very recently, Schreiber et al.10 have reported that differential scanning calorimetry (DSC) scans of the freezing of water in partially filled pores of SBA-15 mesoporous molecular sieve reveal a peak pattern depending on the degree of pore filling, whereas only one DSC peak is observed in the melting scans, independent of the pore filling. The different peaks in cooling scans have been attributed to different states of the pore water, namely, the free water in completely filled regions and film water adsorbed on the bound water layer at the pore wall which coexists with the free water below a complete filling. The exact relationship between several freezing peaks in partially filled pores and a single (1) Gelb, L. D.; Gubbins, K. E.; Radhakrishnan, R.; SliwinskaBartkowiak, M. Rep. Prog. Phys. 1999, 62, 1573. (2) Overloop, K.; van Gerven, L. J. Magn. Reson., Ser. A 1993, 101, 179. (3) Bellissent-Funel, M.-C.; Lal, J.; Bosio, L. J. Chem. Phys. 1993, 98, 4246. (4) Schmidt, R.; Hansen, E. W.; Stocker, M.; Akporiaye, D.; Ellestad, O. H. J. Am. Chem. Soc. 1995, 117, 4049. (5) Ishizaki, T.; Maruyama, M.; Furukawa, Y.; Dash, J. D. J. Cryst. Growth 1996, 163, 455. (6) Takamuku, T.; Yamagami, M.; Wakita, H.; Masuda, Y.; Yamaguchi, T. J. Phys. Chem. B 1997, 101, 5370. (7) Baker, J. M.; Dore, J. C.; Behrens, P. J. Phys. Chem. B 1997, 101, 6226. (8) Morishige, K.; Kawano, K. J. Chem. Phys. 1999, 110, 4867. (9) Coulomb, J. P.; Floquet, N.; Grillet, Y.; Llewellyn, P. L.; Kahn, R.; Andre, G. Stud. Surf. Sci. Catal. 2000, 128, 235. (10) Schreiber, A.; Ketelsen, I.; Findenegg, G. H. Phys. Chem. Chem. Phys. 2001, 3, 1185. (11) Denoyel, R.; Pellenq, R. J. M. Langmuir 2002, 18, 2710.
melting peak at a well-defined temperature independent of the level of pore filling is not yet clear however. The main pores of SBA-1512 are cylindrical, and their pore size distributions are sharp compared to those of conventional mesoporous materials such as porous glasses and silica gels. SBA-15 generally possesses rough pore walls with micro- and narrow mesopores that coexist with a regular hexagonal framework of main channels.13,14 Nevertheless, the pore-networking effects for SBA-15 are expected to be negligibly small, because the freezing and melting of the phases confined to the interconnected regions must take place at much lower temperatures than the phase transition temperatures of the phases confined within the main channels of SBA-15. Therefore, the results obtained on SBA-15 may represent the properties of a single cylindrical pore with the pore size corresponding to a mean size of the main pores present. The uniformity of pore size in SBA-15 is one of the reasons that the existence of two or more freezing peaks in DSC scans for water in partially filled mesopores was recognized as a signature of coexisting states of water in the pore.10 This article presents a systematic X-ray diffraction study on water in the cylindrical pores of SBA-15 with a mean radius of 3.9 nm, as a function of temperature and filling. II. Experimental Section The characterization of a SBA-15 sample has been described in detail elsewhere.15 The Brunauer-Emmett-Teller (BET) surface area and the total mesopore volume were 786 m2/g and 1.18 mL/g, respectively. The pore radius was estimated from a comparison of the experimental equilibrium pressure of nitrogen at 77 K with the isotherms due to nonlocal density functional theory.16 The X-ray experiments were performed on a two-circle diffractometer (Rigau, RINT2500) equipped with an incident monochromator of a bent Ge(111) single crystal and a 18 kW (12) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (13) Ryoo, R.; Ko, C. H.; Kruk, M.; Antochshuk, V.; Jaroniec, M. J. Phys. Chem. B 2000, 104, 114657. (14) Imperor-Clerc, M.; Davidson, P.; Davidson, A. J. Am. Chem. Soc. 2000, 122, 11925. (15) Morishige, K.; Ito, M. J. Chem. Phys. 2002, 117, 8036. (16) Neimark, A. V.; Ravikovitch, P. I. Microporous Mesoporous Mater. 2001, 44, 697.
10.1021/la0208474 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/01/2003
Freezing and Melting of Water in Pores of SBA-15
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Figure 1. Adsorption isotherm of water on SBA-15 at 270 K. rotating-anode source, usually operating at 12 kW. The measurements were carried out with Mo KR1 radiation in a symmetrical transmission geometry. A self-supporting disk (∼0.03 g) of 12 mm diameter was attached to a sample holder of Cu. The sample holder was then attached to the cold head of a He closed cycle refrigerator (Iwatani Cryomini) and sealed in a sample cell constructed of a cylindrical Be window and a Cu flange, with an In O-ring. After prolonged evacuation at room temperature, the sample was cooled and then the background spectrum was measured. The SBA-15 was filled with water at 270 K, by adsorption out of the vapor phase. Figure 1 shows the adsorption isotherm of water on the SBA-15 volumetrically measured in the X-ray cryostat. Here f denotes a pore filling. The pore filling is defined as the volume ratio f ) Vw/Vp, where Vp is the mesopore volume of the SBA-15 as determined by nitrogen adsorption at 77 K and Vw is the volume occupied by the adsorbed water at 270 K. After correction for attenuation due to the substrate, the diffraction pattern of pore water was obtained by subtraction of data for charged and empty substrate. The correction for attenuation due to pore water was negligible.
Figure 2. The change of the X-ray diffraction profile of water confined in SBA-15 at f ) 0.4 upon cooling and heating.
III. Results Figure 2 shows some of the diffraction profiles from the pore water when the temperature was successively lowered and then when the temperature was successively increased at f ) 0.4. On cooling, one still obtains pure liquid scattering at 239 K, well below the triple point of bulk water. As the temperature was lowered through 238 K, there was a rather abrupt change in the diffraction profile from liquid to solid form. The transformation was almost completed at 236 K. The profile of a solid was almost symmetrical, and thus the solid takes the form of ordinary three-dimensional microcrystals rather than two-dimensional microcrystals (solid films).17 When the SBA-15 was heated, a profound hysteresis in freezing and melting temperatures was observed. At 256 K, the diffraction profile still preserves the solid form. A rather abrupt change in the diffraction profile from solid to liquid occurred at 257 K. The hysteresis loop between freezing and melting amounts to ∼18 K in width, in good agreement with the DCS results10 on the film water. Figure 3 shows the change of the diffraction profile from the pore water on freezing and melting at f ) 1.3, in excess of a complete (17) Knorr, K. Phys. Rep. 1992, 214, 113.
Figure 3. The change of the X-ray diffraction profile of water confined in SBA-15 at f ) 1.3 upon cooling and heating.
filling. When the SBA-15 was cooled to 265 K, ordinary hexagonal ice microcrystals due to the freezing of external excess water18 formed. The analysis of the peak width by means of the Scherrer formula indicates that the crystallite size is larger than 25 nm. The freezing of the free water in the middle of the pores and subsequent melting occurred around 249 and 257 K, respectively, again in good agreement with the DSC results10 on the free water. To obtain accurate peak parameters (amplitude, position, and width), the observed peak profile was fitted to a Lorentzian line shape with a linearly changing back(18) Morishige, K.; Nobuoka, K. J. Chem. Phys. 1997, 107, 6965.
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Figure 4. Peak width (fwhm) and position as a function of temperature for water confined in SBA-15 at f ) 0.4. Open and closed symbols denote cooling and heating processes, respectively.
ground in a limited 2θ range.19 Figure 4 shows the peak positions and widths [full width at half-maximum (fwhm)] as a function of temperature at f ) 0.4. The peak width decreased on freezing of the film water and increased on melting of the frozen film water. The analysis of the peak width indicates that the crystallite size is ∼4 nm, which is smaller than the pore diameter. The freezing of the film water resulted in the rapid lowering in peak position, which indicates the formation of a crystalline solid with tetrahedral hydrogen bonding. Figure 5 shows the peak intensities as a function of temperature at f ) 0.4, 0.6, 0.8, 1.0, and 1.3. The peak intensities give good measures of a solid component in the pores. Freezing of the pore water always occurred over a wider temperature range than melting of the frozen pore water at each pore filling. This strongly suggests that the freezing of the pore water is very sensitive to the state of water in the pores. On the other hand, melting of the frozen pore water took place at a well-defined temperature of ∼256 K, independent of the level of pore filling. In addition, premelting was observed, as indicated by a gradual decrease in peak intensity with increasing temperature before melting at 256 K. At f ) 0.4, freezing occurred around 237 K. This corresponds to the freezing of the film water at the pore wall. At f ) 0.6, freezing proceeded in two steps, around 242 and 237 K. At this pore filling, the film water seems to coexist with the free water filled in the pores. The freezing temperature of the free water increased with increasing f: 242 K at f ) 0.6, 245 K at f ) 0.8, 248 K at f ) 1.0, and 250 K at f ) 1.3. Figure 6 shows the X-ray diffraction profiles from the frozen pore water as a function of pore filling at 230 K. The peak position and width did not depend on the pore filling. This clearly indicates that the structure and size of the ice microcrystals formed are independent of the (19) Awaya, T. Nucl. Instrum. Methods 1979, 165, 317.
Figure 5. Peak intensity as a function of temperature for water confined inside the cylindrical pores of SBA-15 at various pore fillings. Open and closed symbols denote cooling and heating processes, respectively.
state of water in the pores. The analysis of the peak width indicates that the crystallite size is ∼4 nm, independent of the state of the pore water. IV. Discussion IV-1. Freezing. As Schreiber et al.10 and Wallacher and Knorr20 have clearly shown, freezing of a pore fluid is sensitive to different states of the pore fluid depending on the degree of pore filling: the adsorbed film and capillary condensate freeze at different temperatures, respectively. The water adsorption isotherm for SBA-15 was of type V, in agreement with those for MCM-41.21-23 This is indicative of relatively hydrophobic character in the low-pressure region of the adsorption isotherm. Therefore, water would adsorb initially on surface OH groups with the formation of clusters at these sites. When the pressure is increased, the clusters would grow until at a certain pressure capillary condensation occurs. Although we do not know the exact distribution of water molecules inside the cylindrical pores of SBA-15 at each pore filling, we will adopt the following picture for water adsorption on SBA-15 at 270 K. At f ) 0.2, many clusters of water are formed at the pore wall of the main channels. Since the bound water is interfacial water confined between the surface of the pore wall and the frozen free water, however, this does not necessarily correspond to (20) Wallacher, D.; Knorr, K. Phys. Rev. B 2001, 63, 104202. (21) Llewellyn, P. L.; Schuth, F.; Grillet, Y.; Rouquerol, F.; Rouquerol, J.; Unger, K. K. Langmuir 1995, 11, 574. (22) Takahara, S.; Nakano, M.; Kittaka, S.; Kuroda, Y.; Mori, T.; Hamano, H.; Yamaguchi, T. J. Phys. Chem. B 1999, 103, 5814. (23) Smirnov, P.; Yamaguchi, T.; Kittaka, S.; Takahara, S.; Kuroda, Y. J. Phys. Chem. B 2000, 104, 5498.
Freezing and Melting of Water in Pores of SBA-15
Figure 6. X-ray diffraction profile from the frozen pore water as a function of pore filling at 230 K.
the bound water. This layer does not freeze in the temperature range examined here,22,23 because the growth of the hydrogen-bond network of water is strongly hindered by the interactions with the pore wall. At f ) 0.4, the water clusters grow and thus the water molecules far away from the pore wall appear in the pores. This part of the water corresponds to the film water detected by the DSC measurements, although in fact the film water does not exist in the form of a thin film at the pore wall. When the temperature is lowered, ice nucleation takes place in the clusters around 237 K. Ice clusters are not stable because of their high surface energy. They would be merged in a solid capillary condensate. Capillary condensation starts to occur above f ) 0.4 (see Figure 1). The liquid capillary condensate (the free water) coexists with the film water at f ) 0.6. However, the free water may exist in the form of thin liquid bridges at this pore filling. When the SBA15 is cooled, the thin liquid bridges first freeze around 242 K and then the film water freezes and joins the solid capillary condensate around 237 K. Although at f ) 0.8 a small amount of the film water still coexists, some of the thin liquid bridges grow into liquid domains. On cooling, the liquid domains first freeze at 245 K and then the thin liquid bridges freeze at lower temperatures. The thin liquid bridges will disappear close to a complete filling. A small amount of vapor bubbles still remains close to f ) 1.024 and eventually disappears beyond a complete filling. This
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means that the domain size increases with increasing f and concomitantly the freezing temperature increases. For capillary liquid Ar, such a pore-filling dependence of the freezing temperature has not been observed.20 Therefore, the phenomenon seems to be closely concerned with the properties of water. Our previous study8 strongly suggested that the freezing of the pore water takes place by a homogeneous nucleation and the thermal hysteresis between freezing and melting comes from the metastability inherent to the phase transition between liquid and solid. Molecular dynamics simulation of freezing water has shown that formation of a polyhedral structure composed of long-lasting hydrogen bonds is important for ice nucleation.25 More than 90% of water molecules forming these long-lasting bonds are mostly four-coordinated. The formation of the tetrahedral hydrogen bonds in water is suppressed by confinement.22,23 The suppression is severe for the film water and may be loosened with an increase of the domain size of the free water in the cylindrical pores of the SBA-15. This would explain the increase in freezing temperature with increasing pore filling. Thin-film ice on a solid substrate is unstable and spontaneously transforms into crystallites of ice.26 In this case, it would be very difficult for all pore entrances to be in contact with external ice microcrystals and thus the external ice layer cannot nucleate the freezing of the pore water. IV-2. Melting. Irrespective of the different states of the pore water, the freezing of the pore water resulted in formation of ice microcrystals with almost the same structure and size. Therefore, the melting of the frozen pore water occurred at a well-defined temperature of 256 K, independent of the level of pore filling. This melting temperature is well accounted for by the classical GibbsThomson equation for an idealized cylindrical pore.4,8,10 The main part of the melting occurred in a narrow temperature interval of 2 K, indicating the uniformity of the pore size in the SBA-15. It is known that the thickness of the nonfreezable bound water is ∼0.4 nm.4,10 When the bound water was taken into consideration, the diameter of the frozen free water inside the cylindrical pores of radius 3.9 nm was estimated to be ∼7 nm. On the other hand, the crystallite size of ∼4 nm was obtained from the peak width by means of the Scherrer formula. This suggests that microcrystals do not grow along the pore length but many nuclei are formed in each cylindrical pore. Acknowledgment. This research was supported by Grant-in-Aid for Scientific Research No. 12440201, provided by the Ministry of Education, Culture, Sports, Science, and Technology of Japan. LA0208474 (24) Page, J. H.; Liu, J.; Abeles, B.; Deckman, H. W.; Weitz, D. A. Phys. Rev. Lett. 1993, 71, 1216. (25) Matsumoto, M.; Saito, S.; Ohmine, I. Nature 2002, 416, 409. (26) Sadtchenko, V.; Ewing, G. E.; Nutt, D. R.; Stone, A. J. Langmuir 2002, 18, 4632.
