J. Phys. Chem. B 2009, 113, 14323–14328
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Ordering and Freezing-in Phenomena of Nanochannel Water in Crystalline Organic/ Inorganic Self-Assembled Complex [Cr(H2bim)3](TMA) · 23.5H2O Keisuke Watanabe,*,† Masaharu Oguni,† Makoto Tadokoro,‡ and Chiho Kobayashi‡ Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551, Japan, and Department of Chemistry, Faculty of Science, Tokyo UniVersity of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan ReceiVed: August 11, 2009; ReVised Manuscript ReceiVed: August 27, 2009
The thermal properties of crystalline complex [Cr(H2bim)3](TMA) · 23.5H2O were studied by adiabatic calorimetry to clarify the structural ordering and dynamic freezing-in behaviors of the nanochannel water within the pores possessing crystalline wall structure, where H2bim denotes 2,2′-biimidazole and TMA is 1,3,5-benzenetricarboxylic acid. Phase and glass transitions were found to occur at 233 K with the associated entropy of ∆trsS ) 7.96 J K-1 mol-1 and at Tg ) 100 K, respectively, in the hydrated sample. The phase transition was interpreted as attributed to the crystallization-like formation of the hydrogen-bond network of the channel-water molecules. The glass transition was interpreted as a freezing-in phenomenon on the way of the development of the network, and its presence indicates that the network formation achieves no completion even at 100 K. The Tg value is similar to those found previously in other channel-water systems of [Ni(cyclam)(H2O)2]3(TMA)2 · 24H2O and porous silica. It is noted that the channel water within silica pores with their diameter below 1.8 nm undergoes no structural phase transition while the present one does. The origins of the phase and glass transitions and the implication of their presence are discussed based on the difference in the structures of pore wall interacting with the channel-water molecules. 1. Introduction Water exists ubiquitously on Earth and plays important roles in the meteorological, geological, and biological fields. It is also well-known as a liquid which displays anomalous properties, and its structure and dynamics are still unclear in many aspects. The water molecules tend to form hydrogen bonds: Recently, it has been observed by X-ray emission spectroscopy1 and X-ray absorption spectroscopy2 that part of the liquid water molecules, in subnanosecond time scale, forms strong hydrogen-bonding structure and the others weakly at room temperature. As well as other structural/spectroscopic information, this indicates that the bulk water has a structure with a short-range order through the hydrogen bonds at room temperature. The bulk water crystallizes into ice below 273 K, completing the hydrogenbond network with a long-range order. Water confined in nanopores shows dramatic changes in the structure and properties, and a good deal of elaborate research has been devoted to the investigation on confinement effect of water.3-33 There are two types of confinement systems different in the softness of the pore shapes: soft confinement systems such as micelles,3-5 biological membranes,7,8 hydrated RNA,9 lysozyme,10 myoglobin,11,12 bovine serum albumin,13 and so on; and hard ones such as a glass ampule,14 porous glasses,15 silica gel,16 mesoporous silica SBA-1517,18 and MCM-41,20-24 carbon nanotube,25,26 clay minerals haloysite27 and montmorillonite,28 layered vanadium oxide,29 synthetic silicate, layered30,31 and porous silicon,32 and so on. The water confined in the soft systems serves stabilization of the structural framework so that the structure and pore shape of the system should be changed * To whom correspondence should be addressed. E-mail: kwatanab@ chem.titech.ac.jp. † Tokyo Institute of Technology. ‡ Tokyo University of Science.
by the detail of the hydrogen-bond network of the water. The components such as protein other than water conversely change the water structure in some cases as seen in antifreeze protein;33 in an example of purple membrane, the lamellar distance between water layers also decreases at low temperatures.7 The interfacial water molecules on the pore wall are known to form distorted hydrogen bonds.6 Calorimetric study on bovine serum albumin solution clarified that the dynamic properties are different between the primary hydrate (interfacial) water and the internal water molecules located in the center of the channels formed between the proteins; the respective glass transitions occurred at different temperatures.13 In the hard confinement systems, the pores are formed with rigid framework and no change in the pore shapes is caused by the presence/absence of channel water. The adiabatic calorimetry verified that the water in the pores of silica gel is kept noncrystalline when the pore diameter is below 2 nm.16,19 XRD diffractometry demonstrated that, when confined in the pores whose diameters are 4-70 nm, the channel water crystallizes into, instead of a normal hexagonal ice, a cubic ice or a disordered hexagonal ice, which contains a large amount of crystal-growth faults.17 NMR and neutron diffraction studies of the water in SBA-15 pores of diameter 8.6 nm indicated a reversible conversion between the defective ice and ordered ice.20 The structure and properties of the water confined in such rigid-pore materials are strongly affected by the arrangement of hydrophilic groups located on the pore wall. In a previous article,34 we reported the thermal properties of the channel water in [Ni(cyclam)(H2O)2]3(TMA)2 · 24H2O (hereafter abbreviated as crystal 1) which has cylindrical channels of diameter 1.03 nm. The water channel is formed in the crystalline organic/inorganic self-assembled complex in which the hydrophilic functional groups forming hydrogen bonds with water molecules are arranged with a long-range order. Therefore,
10.1021/jp907736n CCC: $40.75 2009 American Chemical Society Published on Web 10/08/2009
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J. Phys. Chem. B, Vol. 113, No. 43, 2009
Watanabe et al.
TABLE 1: Masses/Molar Ratios of Bulk Water, Anhydride, and Channel Water Contained in the Four Samples Used for Calorimetry
sample A mass/g molar ratioa sample B mass/g molar ratioa sample C mass/g molar ratioa sample D mass/g molar ratioa a
total amount
bulk water
anhydride
channel water
3.750 2.781 2.203 1.333 -
1.498 39.8 0.523 13.9 0.018 0.5 -
1.382 1 1.382 1 1.333 1 1.333 1
0.870 23.1 0.876 23.3 0.852 23.5 -
Molar ratios are given as per mole of anhydride.
the confinement framework is classified as the type of a hard confinement system in the respect that, after the channel has been formed, the channel structure is rigid, but has an aspect of a soft confinement system in the respect that the cylindrical pore structure is maintained only in the coexistence with the channel water. Accordingly, one could refer that the system is between hard and soft confinement systems. In this work, we present an adiabatic-calorimetric study on the structural ordering and dynamics of the channel water in crystalline [Cr(H2bim)3](TMA) · 23.5H2O, where H2bim denotes 2,2′-biimidazole and TMA 1,3,5-benzenetricarboxylic acid. The atoms constituting the pore wall possess a long-range periodicity in their positions as in crystal 1. The diameter of the channel is 1.5 nm35 while that in crystal 1 is 1.03 nm. However, the structure and thermal properties of the present channel water are unknown. In view of the fact that the channel water in crystal 1 revealed its ordering below 196.9 K,34 how the channel water behaves as a function of temperature is intriguing because the channel diameter is larger in the present complex crystal: The clarification of the behaviors in such crystalline organic/ inorganic self-assembled complexes is indispensable for the detailed understanding of the channel water found ubiquitously on Earth. 2. Experiment Powdered crystals of [Cr(H2bim)3](TMA) · nH2O were prepared through the self-assembling process of organic and inorganic components in the aqueous solution according to the procedure described in ref 35. Calorimetry was adapted first to hydrated sample A containing a large quantity of extra bulk water. The second and third sets of measurements were therefore carried out by using the samples B and C decreased in the quantities of the bulk water gradually. The decrease of the bulk water was achieved by drying slowly the sample A and then B, as loaded in a calorimeter cell with its lid opened, in a desiccator where the relative humidity was kept constant in coexistence with a saturated KCl aqueous solution. The quantity of the extra bulk water contained in the sample was estimated from the enthalpy of fusion of the bulk ice at 273.15 K. Calorimetry was also executed with the anhydride, named as sample D, which was obtained by dehydrating the sample C completely under vacuum at 473 K for 28 h. The masses and the molar ratios per mole of the anhydride are tabulated in Table 1 for the four samples used. The total masses of the samples A-D were 3.750, 2.781, 2.20, and 1.333 g, respectively. The molecular formula of the complex was determined as [Cr(H2bim)3](TMA) · (23.5 ( 0.3)H2O from the results of the samples C and D. The calorimetry was carried out with an adiabatic calorimeter reported before.36 Heat capacities of the samples were measured
Figure 1. Heat capacities per mole of [Cr(H2bim)3](TMA) · 23.5H2O with additional bulk water: b, sample A with bulk water whose mass amounts to 40.0% of the total; 0, sample B with less bulk water of 18.9%; O, sample C with bulk water only of 0.8%; +, anhydride sample after the whole water of the sample C was removed by vacuum exhausting at 473 K. Inset shows the DSC curves obtained on cooling and heating a hydrated sample in a range 220 K < T < 245 K.
by an intermittent heating method, namely by repeating energy supply and thermometry alternately, in a temperature range from 11 to 300 K. The inaccuracy and imprecision as the performance of the calorimeter were determined previously to be (0.3% and (0.06%, respectively,36 and thus, considering the masses of the samples used presently, those of the heat capacities obtained in the present work are estimated to be (2% and (0.4%, respectively. Spontaneous enthalpy-relaxation drifts were observed in association with a phase transition or a glass transition. The rates were evaluated by multiplying the spontaneous temperature drift rates, measured in the thermometry period under adiabatic conditions, by the gross heat capacity of the calorimeter cell loaded with the sample. Differential scanning calorimetry (DSC) was performed with a Perkin-Elmer Diamond DSC instrument to characterize the phase transition observed in the present complex crystal. 3. Results and Discussion 3.1. Heat Capacities and Enthalpy-Relaxation Rates as a Function of Temperature. Figure 1 shows the heat capacities per mole of [Cr(H2bim)3](TMA) · 23.5H2O containing extra bulk water of 39.8, 13.9, and 0.5 mol in the ratios for samples A-C, respectively, and per mole of anhydride (sample D). A sharp peak appeared at 273 K for the hydrated samples A and B with rather large quantities of bulk water, and a small peak and no peak for the samples C and D, respectively. This peak is assigned, without doubt, as due to the fusion from bulk ice to water. Another prominent peak was found at 233 K for all the hydrated samples A-C, while no peak for the anhydride one D. The height of the peak was independent of the amounts of the extra bulk water. The peak was therefore judged as originating from a phase transition of the crystal [Cr(H2bim)3](TMA) · 23.5H2O. In view that the [Cr(H2bim)3](TMA) moiety of the hydrated crystal has been reported to show no structural change with temperature below room temperature,35 it is understood that the structural ordering of the channel water proceeds in a cooperative way on cooling below 233 K as found at 196.9 K in crystal 1.34 The inset of Figure 1 shows result of DSC measurements on heating and cooling the hydrated
Phase and Glass Transitions of Nanochannel Water
J. Phys. Chem. B, Vol. 113, No. 43, 2009 14325
Figure 2. Molar heat capacities of [Cr(H2bim)3](TMA) · 23.5H2O. The values were derived by subtracting, from those of sample C in Figure 1, the contribution of extra bulk ice/water of 0.5 mol below/ above 273.15 K using literature values of ref 37. The inset shows the encraty (heat capacity divided by temperature) in the range 70-150 K, indicating the presence of heat-capacity jump in the range 100-120 K.
sample regarding the phase transition. The transition temperatures taken as the rise of the heat capacity curve were 232.4 and 231.0 K, respectively, indicating the presence of a hysteresis effect of 1.4 K. Figure 2 shows the molar heat capacities of [Cr(H2bim)3](TMA) · 23.5H2O. The values were obtained by subtracting the contribution of the extra bulk ice/water with using literature data37 from those of sample C in Figure 1. The inset shows the encraty (heat capacity divided by temperature) in 70-150 K, displaying the presence of a heat-capacity jump in the range 100-120 K on an enlarged scale. Figure 3 shows temperature dependence of the rates of the spontaneous enthalpy drifts observed in a temperature range 67-300 K in the three hydrated samples. A heat-absorption effect was observed at 273 K for the all samples. It is attributed to a slow process of the latent-heat absorption due to fusion of bulk ice. The magnitude of the latent heat depends on the amount of the extra bulk water contained; almost no effect was really observed in the sample C with only a small amount of bulk water. Another heat absorption effect was found clearly at 233 K for all the hydrated samples. The temperature corresponds to that of the heat-capacity peak in Figure 1. This, as well as the presence of the hysteresis in the transition temperatures observed in the above DSC, indicates that the phase transition at 233 K is of the first order. Another small but systematic heat-release/absorption effect was found around 100 K as shown in the inset on an enlarged scale. This effect is considered in next section. 3.2. Glass Transition Due to Freezing-in of the Rearrangement of Water Molecules. Open and filled circles in Figure 3 in the 67-130 K range represent rates of the spontaneous enthalpy drifts observed for the samples cooled rapidly at 2 K min-1 and slowly at 20 mKmin-1, respectively, prior to the measurements. On heating intermittently, the rapidly precooled sample, first a heat release, namely positive s(dHm/ dt), effect was observed around 100 K for all the hydrated samples as shown clearly in the inset of Figure 3. Then, a small heat absorption, namely negative s(dHm/dt), effect was observed for the hydrated samples B and C. On the other hand, in the cases of slowly precooled samples, only a large heat-absorption effect was observed around 100 K for all the hydrated samples. The enthalpy drift returned to the normal one at around 120 K. The behaviors are characteristic of a glass transition.38 A heat-
Figure 3. Rates, per mole of water, of the spontaneous enthalpyrelaxation drifts observed in the course of intermittent-heating calorimetricprocesses of the hydrated samples: (a), sample A; (b), sample B; (c), sample C; O and b, results of the samples cooled rapidly (>2 K min-1) and slowly (
