Thermal Characteristics of Channel Water Confined in Nanopores with

Article pubs.acs.org/JPCC

Thermal Characteristics of Channel Water Confined in Nanopores with Crystalline Pore-Wall Structure in [M(H2bim)3](TMA)·nH2O Keisuke Watanabe,*,† Makoto Tadokoto,‡ and Masaharu Oguni§ †

Department of Chemistry, Faculty of Science, Fukuoka University, Nanakuma 8-19-1, Jonan-ku, Fukuoka, 814-0180, Japan Department of Chemistry, Faculty of Science, Tokyo University of Science, Kagurazaka 1-3, Shinjuku-ku, Tokyo 162-8601, Japan § Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551, Japan ‡

ABSTRACT: Thermal properties of channel water within crystalline nanopores of [M(H2bim)3](TMA)·nH2O (M = Co, Ru) were characterized by adiabatic calorimetry, where H2bim and TMA denote 2,2′-biimidazole and 1,3,5-benzenetricarboxylic acid, respectively. Temperature dependence of the heat capacities and spontaneous heat release or absorption rates disclosed the presence of two phase transitions and one glass transition at low temperatures in the both Co- and Ru-complex crystals. Combined with the results of M = Cr crystal, it was suggested that very slight differences in the pore size and shape strongly affect the phase behavior of the channel water. As the n value increases among the three crystals, the entropies of the channel water at 0 °C increased toward the value of pure water and the temperature dependence of the heat capacities around 0 °C seemed to somewhat resemble that of pure water as well. The glass transition was observed to occur at essentially the same temperature around 100 K among the three, indicating that the freezing-in rearrangement motion is related to breaking hydrogen bonds of the same number. The two phase transitions of the partially (88%) pore-filling water in the Ru-complex crystal occurred at appreciably lower temperatures than those of fully filling water. It is concluded that the ordering behavior of water molecules within nanopores with crystalline pore walls is sensitive to the pore structure, such as shape of the pore section and periodicity along the channel, and to the degree of pore filling with water.

1. INTRODUCTION Water shows many peculiar properties that originate from the formation of hydrogen-bond network. The network structure formed, however, depends on the circumstances where water is located. Recently, water confined in small pores whose diameters d are in a range from nanometers to micrometers has been widely studied by calorimetry,1−7 NMR,8−10 spectroscopy,11−13 diffractometry,14−17 molecular dynamics simulation,18−20 and so on. The network structures formed and the phase- and glass-transition behaviors of the water have been found to differ completely depending on whether the pore-wall structure is crystalline or noncrystalline. When water is confined within the pores of silica gel and silica MCM-41 with noncrystalline pore-wall structures and small diameters, the melting point of the crystalline water is lowered.1 The crystal is known to be formed in the center of the pores and is identified as hexagonal or cubic ice. The interfacial water molecules on the pore wall, remaining noncrystalline, constitute a kind of buffer layer between the hydrogen-bond network structure of ice and the pore-wall structure. From the macroscopic viewpoint, the lowering of the melting temperature occurs because the surface Gibbs energy of the interfacial water is higher when the pore-center water molecules are in the crystalline state than in the liquid one. The lowering is well described by the Gibbs−Thomson equation that has been © 2012 American Chemical Society

widely used to express the pore-size dependence of the fusion temperature.1,2 When the pore diameter becomes small to the extent of a few nanometers, the contribution of the surface Gibbs energy gradually dominates even the appearance/ disappearance of ice in the pore center. The previous calorimetric study has disclosed that when water is confined in silica MCM-41 pores with diameter below 2.1 nm, not only the interfacial but also the internal water molecules freeze into a glassy state without crystallization at low temperatures.3 On the other hand, in cases such as porous organic− inorganic-complex crystals4−7 or carbon nanotubes17 with crystalline pore-wall structures, the behavior of water is different from that in the above-stated noncrystalline pores. We have studied the thermal properties of channel water confined in crystalline pores of [Ni(cyclam)(H2O)2]3(TMA)2·24H2O (pore diameter d = 1.0 nm)4 and [Cr(H2bim)3](TMA)·23.5H2O (d = 1.5 nm),5 where cyclam, H2bim, and TMA denote 1,4,8,11-tetraazacyclotetradecane, 2,2′-biimidazole, and 1,3,5-benzenetricarboxylic acid, respectively. The two crystals are synthesized in aqueous solutions at room temperature, and the channel water plays a role of Received: March 7, 2012 Revised: April 17, 2012 Published: April 23, 2012 11768

dx.doi.org/10.1021/jp302222r | J. Phys. Chem. C 2012, 116, 11768−11775

The Journal of Physical Chemistry C

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stabilizing the organic−inorganic-complex crystal structures themselves because dehydration breaks down the crystalline frameworks with channel structure.14 In addition, it is noted that the existence of the shadow of a hydrogen-bond network structure has been indicated among the water molecules neighboring the hydrophilic pore-wall atoms even at room temperature.1,4 The water molecules have been really observed to order at low temperatures with developing the shadow network structure present at room temperature.4,5 However, the ordering temperatures are not predictable. This is because the pore-wall structures are different among those crystals and therefore the network structures developed by the water molecules are naturally different. In order to clarify in more detail the behavioral characteristics of water in crystalline pores, it is indispensable and intriguing to study channel-water systems in the pores formed within essentially the same crystalline framework but with slightly different shapes. Meanwhile, the effect of partial filling of pores with water has been studied intensively. Gallo et al.18 found by a moleculardynamics simulation method that partial filling of silica MCM41 pores decreases the mobility of the water molecules. Tombari et al.21,22 reported, on the basis of calorimetric data, that the fusion temperature of 46%-filled water in Vycor-glass pores is lowered by 5 K compared with that of fully filled water. In this experiment, however, it is not clear whether, when cooled to crystallize, the water keeps on possessing the space layered on the pore wall with a uniform thickness (namely, leaving the pore center vacant) or forms short pore-filling ice particles (namely, leaving cavities intermittently along the channel). It is further noticed that these studies on partially filling water have been restricted so far to pores with noncrystalline wall structures. The partial filling effect in crystalline pores is interesting in connection with the existence of the shadow of a hydrogen-bond network structure adjacent to the pore wall. Water molecules located in the pore center are considered as first taken out of the pores in the partially dehydrated crystals. Thus, knowledge on the effect helps us to understand the role of the pore-center water molecules for development of the hydrogen-bond network. In the present study, two crystals [M(H 2 bim) 3 ](TMA)·nH2O (M = Co, Ru) were taken up and their thermal properties were studied by adiabatic calorimetry. The crystals are composed of the same moieties as the previously reported crystal [Cr(H2bim)3](TMA)·23.5H2O with the exception of the metal species;5 the same one-dimensional pores provide the room for water. The aim is to find any characteristics on how the ordering and dynamics of the water molecules forming hydrogen-bond networks depend on the minute change in the pore structure. The minute change produced by the difference in the metal species of the metal complex potentially yields appreciable effects, and the knowledge would be fundamental to understand the structure and properties of water in such small-pore confinement (d = 1.5 nm) and the role of the water for stabilization of the channel structure. The crystal structure has been determined by X-ray diffraction (XRD) analysis of single crystals by one of our groups.14,15 As shown schematically in Figure 1, the crystals possess a honeycomb structure; the framework structure is constructed through hydrogen bonds between TMA and the metal complex [M(H2bim)3]. The crystal forms one-dimensional pores, filled with water molecules, along the c axis. The pore diameter d = 1.5 nm is essentially the same irrespective of the metal species M. Given that the diameter of water molecule

Figure 1. Schematic structure of [M(H2bim)3](TMA)·nH2O (M = Ru, Co, Cr), determined by XRD analyses of single crystals.14 A honeycomb structure is constructed through hydrogen bonds between TMA and a complex [M(H2bim)3]. The crystalline pores are formed through stacking of the honeycomb sheets. These crystals have the same pore diameter of ca. d = 1.5 nm.

is 0.3 nm, about five water molecules may align along the diameter inside the pore.

2. SAMPLE PREPARATION AND CALORIMETRIC METHOD Crystalline [M(H2bim)3](TMA)·nH2O (M = Co, Ru) was synthesized by self-assembling the organic and inorganic components in water solution as reported before.15 Adiabatic calorimetry was carried out with the homemade calorimeter reported before.23 Heat capacities of the samples were measured by an intermittent heating method, namely, by repeating energy supply and thermometry alternately, in a temperature range from 50 to 300 K. The accuracy and precision of the heat capacities obtained in the present work were estimated to be ±2% and ±0.4%, respectively, as described in the previous report on the M = Cr crystal.5 Spontaneous temperature drift was observed in some temperature ranges when phase or glass transition occurred.23−25 The rates of enthalpy relaxation due to a sample 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 sample. 3. RESULTS AND DISCUSSION 3.1. Determination of Amount, n, of Pore Water. Calorimetry on the Co-complex crystal, [Co(H2bim)3](TMA)·nH2O, was adapted to three hydrated samples, denoted by Co21.4, Co7.4, and Co0.2; the numbers indicate the quantities of bulk water contained. The first sample contained a large quantity of extra bulk water. Then, the bulk water was gradually reduced by keeping the first Co21.4 and then the second Co7.4 sample, as loaded in a calorimeter cell with its lid opened, in a desiccator where the relative humidity was controlled via coexistence with a saturated KCl aqueous solution. The quantities of extra bulk water contained in the respective hydrated samples were estimated from the enthalpies of fusion of the bulk ice at 273 K. The anhydride was obtained by completely dehydrating the Co0.2 sample under vacuum at 473 K for 28 h, as in the case of Cr-complex crystal,4 and weighed to determine its mass. The masses of hydrated samples 11769



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prominent anomalies were found at 220 and 244 K for all the hydrated samples, while no anomaly was found in the Co(anhydride). 4 The peaks were therefore judged as originating from phase transitions of the complex crystal [Co(H2bim)3](TMA)·21.8H2O. It is understood that structural ordering of channel water proceedsnamely, the hydrogenbond network developsin two steps at 244 and 220 K on cooling. Figure 3 shows the molar heat capacities of [Co(H2bim)3](TMA)·21.8H2O without extra bulk water. The values were

and the molar ratios of the bulk and channel water per mole of the anhydride, [Co(H2bim)3](TMA), are tabulated in Table 1. The molecular formula of the fully hydrated crystal was determined as [Co(H2bim)3](TMA)·(21.8 ± 0.3)H2O from the results. Table 1. Masses and Molar Ratios for Crystalline Samples of [Co(H2bim)3](TMA)·nH2O total amount

a

bulk water

sample

m, g

m, g

molar ratioa

Co21.4 Co7.4 Co0.2

2.447 2.019 1.800

0.652 0.224 0.005

21.4 7.4 0.2

anhydride

channel water

m, g

m, g

molar ratioa

1.130 1.130 1.130

0.665 0.665 0.665

21.8 21.8 21.8

Molar ratios are given per mole of anhydride.

Calorimetry on the Ru-complex crystal was carried out on three hydrated samples denoted Ru33.4, Ru8.3, and Ru(−1.5) and the anhydride sample denoted Ru(anhydride). The number −1.5 indicates the deficient quantity up to full channel water. The sample masses and molar ratios of the bulk and channel water per mole of the anhydride are tabulated in Table 2 for the three hydrated samples. The molecular formula of the f u l l y h y d r a t e d c r y s t a l w a s d e t e r m i n e d as [ R u (H2bim)3(TMA)·(20.7 ± 0.3)H2O from the results.

Figure 3. Molar heat capacities of [Co(H2bim)3](TMA)·21.8H2O. The values were derived by subtracting, from those of sample Co0.2 in Figure 2, the contribution of extra bulk ice/water of 0.2 mol below/ above 273.15 K from literature data.26 (Inset) Encraty (heat capacity divided by temperature) in the range 78−142 K, displaying the presence of a small heat-capacity jump around 100 K.

Table 2. Masses and Molar Ratios for Crystalline Samples of [Ru(H2bim)3](TMA)·nH2O total amount

a

bulk water

sample

m, g

m, g

molar ratioa

Ru33.4 Ru8.2 Ru(−1.5)

3.198 2.339 2.007

1.142 0.282 0

33.4 8.3 0

anhydride

channel water

m, g

m, g

molar ratioa

1.348 1.348 1.348

0.709 0.709 0.659

20.7 20.7 19.2

derived by subtracting, from those of sample Co0.2 in Figure 2, the contribution of bulk ice/water from literature data.26 The inset of Figure 3 shows the encraty, Cp·T −1, in the range 70− 150 K on an enlarged scale, displaying the presence of a small heat-capacity jump around 100 K. This jump is attributed to the freezing-in phenomenon of the channel-water molecules as in the case of Cr-complex crystal.4−6,23 Figure 4 shows the heat capacities of two [Ru(H2bim)3](TMA)·nH2O samples containing extra bulk water of 33.4 and

Molar ratios are given per mole of anhydride.

3.2. Phase Transitions Due to Ordering of Channel Water. Figure 2 shows the heat capacities of [Co(H2bim)3](TMA)·nH2O samples containing extra bulk water of 21.4, 7.4, and 0.2 mol/(mol of anhydride) and that of the anhydride. Sharp peaks at 273 K are ascribed to the fusion of extra bulk water, and the magnitude of the peak decreased gradually with reduction of the bulk water. On the other hand, a couple of

Figure 4. Heat capacities of hydrated [Ru(H2bim)3](TMA)·20.7H2O with extra bulk water/ice: (○) Ru33.4; (□) Ru8.3; (+) Ru(anhydride).

8.3 mol/(mol of anhydride) and that of the anhydride. Sharp peaks at 273 K for the two hydrated samples are ascribed to the extra bulk water involved. In addition, a couple of prominent peaks were found at 221 and 200 K for the two hydrated samples while no peak was present for the anhydride. These

Figure 2. Heat capacities of [Co(H2bim)3](TMA)·21.8H2O with extra bulk water/ice: (○) Co21.4; (□) Co7.4; (■) Co0.2; (+) Co(anhydride).4 11770



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peaks were judged as originating from phase transitions of the Ru-complex crystal [Ru(H2bim)3](TMA)·20.7H2O. Structural ordering of the channel water is understood to proceed in two steps at 221 and 200 K on cooling as in the Co-complex crystal, while the phase-transition temperatures are significantly different between the Co- and Ru-complex crystals. Figure 5 shows the molar heat capacities of [Ru(H2bim)3](TMA)·20.7H2O. The values were derived by subtracting, from

Figure 5. Molar heat capacities of [Ru(H2bim)3](TMA)·20.7H2O. The values were derived by subtracting, from those of sample Ru8.3 in Figure 4, the contribution of extra bulk ice/water.26 The lower-right inset shows them in the limited range 190−230 K, and the upper-left inset shows the encraty in the range 78−142 K. Figure 6. Rates, per mole of water, of the spontaneous enthalpyrelaxation drifts observed in the course of intermittent-heating calorimetric processes of hydrated Co-complex samples: (a) Co21.4; (b) Co7.4; (c) Co0.2. (○) Sample precooled rapidly at 2 K·min−1; (●) sample precooled slowly at 30 mK·min−1. (Insets) Rates in the range 65−125 K on enlarged scales.

those of sample Ru8.3 in Figure 4, the contribution of extra bulk ice/water from literature data.26 Open circles represent the data for the sample precooled at ca. 2 K min−1 down to the lowest temperature of the measurements. As described later in reference to the results shown in Figure 7, the sample revealed a spontaneous heat-evolution effect in the range 150−180 K on heating. The effect indicates the progress of a transition to a low-temperature phase in the crystal. Solid circles in Figure 5 represent the heat-capacity data for the sample annealed at 160 K for 24 h and then cooled to the lowest temperature. The lower-right inset shows the heat capacities on an enlarged temperature scale in the range 190−230 K. The annealed sample revealed a relatively bigger peak due to the phase transition at 200 K, corresponding to the annealing at 160 K causing the crystal to transform to the low-temperature phase, which is the most stable one below 200 K. The upper-left inset shows the encraty of the sample in the temperature range between 75 and 145 K. A heat capacity jump is clearly recognized around 100 K for the sample without the long annealing at 160 K. On the other hand, the jump is small for the annealed sample. This indicates that the freezing-in phenomenon of the rearrangement of channel water molecules takes place for both the supercooled intermediate- and stable low-temperature phases in the same temperature range as in the Co-complex crystal. Furthermore, the difference between the magnitudes of the jump around 100 K implies that a considerably ordered network structure of water molecules is formed in the low-temperature phase of the Ru-complex crystal as compared with that of the Co-complex one. 3.3. Spontaneous Enthalpy Relaxation due to the Phase Transitions and Freezing-in Phenomena of Channel Water. Figure 6 panels a−c show temperature dependence of the rates of the spontaneous enthalpy drifts observed for the three hydrated samples of the Co complex

crystals Co21.4, Co7.4, and Co0.2, respectively. Big heatabsorption effects were observed in three temperature ranges around 273, 244, and 220 K. The first one is due to the fusion of bulk ice and was not detected appreciably in the Co0.2 sample. The other two effects were found clearly for all the hydrated samples, and the integrated magnitudes of the respective peaks were independent of the quantities of extra bulk water contained. When it is considered that the two temperatures of the heat-absorption effects correspond to those of the heat-capacity anomalies, the effects are understood as originating from the phase transitions of the channel water and indicating that the transitions are of the first order. Another enthalpy-relaxation phenomenon was observed at low temperatures as due to a glass transition of the rearrangement of water molecules. The effects are shown in the insets of Figure 6 on an enlarged scale in the temperature range between 65 and 125 K. Open and solid circles represent the results for samples precooled rapidly at 2 K·min−1 and slowly at 30 mK·min−1, respectively, prior to the measurements. Depending on the speed of precooling, the spontaneous enthalpy-relaxation rates varied dramatically: The rapidly precooled sample showed a heat-release effect, while on the other hand, the slowly precooled sample showed a heatabsorption effect. These are the effects characteristic of a glass transition as described previously.23−25 The glass transition is accompanied with a heat-capacity jump as found above in the diagram of encraty in Figure 3. The glass transition temperature Tg was determined to be 99 ± 2 K for Co21.4, 100 ± 2 K for 11771



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Co7.4, and 100 ± 2 K for Co0.2, according to the empirical rule that the heat-absorption rate revealed its maximum for the slowly precooled sample at the temperature where the relaxation time becomes 1 ks.23,25 Figure 7 panels a and b show temperature dependence of the rates of spontaneous enthalpy drifts observed in the two

K; This is consistent with the result that the heat capacity jump observed around 100 K for the annealed sample is much smaller than that for the sample without annealing (see upperleft inset of Figure 5). Therefore, the freezing-in phenomenon of the ordering of channel water is judged to occur in both two ordered phases; namely, intermediate- and low-temperature ones. 3.4. Characters of Structural Ordering of Channel Water That Has Self-Aggregated as a Component to Form the Organic−Inorganic-Complex Crystal of [M(H2bim)3](TMA)·nH2O. Figure 8 panels a−c show the molar

Figure 7. Rates, per mole of water, of spontaneous enthalpy relaxation drifts observed in the course of intermittent heating processes of the hydrated Ru-complex samples: (a) Ru33.4 and (b) Ru8.3. (○) Sample precooled rapidly at 2 K·min−1; (●) sample precooled slowly at 30 mK·min−1; (gray circle in lower panel) sample precooled rapidly at 2 K·min−1 after annealing at 160 K for 24 h. (Insets) Rates on enlarged scales in the range 65−125 K.

hydrated samples Ru33.4 and Ru8.3, respectively. A heatabsorption effect observed at 273 K for both samples is attributed to the fusion of bulk ice as in the case of the Cocomplex samples. Two other heat-absorption effects, observed clearly at 221 and 200 K for both hydrated samples, are due to the phase transitions, indicating that the ordering proceeds through first-order phase transitions as in the Co-complex crystal. In addition, a heat-release effect was observed around 160 K. This indicates that the intermediately ordered phase, which is stable in the range between 221 and 200 K, existed below 200 K as the supercooled metastable state. The heatrelease effect disappeared after annealing at 160 K for 24 h, as indicated by gray circles in Figure 7b. It is noted that transformation to the low-temperature stable phase below 200 K requires annealing the sample for a long time. Insets in Figure 7 show the temperature dependence of enthalpy-relaxation rates on an enlarged scale between 65 and 125 K. Open and solid circles represent the rates observed for the samples precooled rapidly at 2 K·min−1 and slowly at 30 mK·min−1, respectively, prior to the measurements. Heatrelease and -absorption effects were found, depending on the precooling speeds of the sample, indicating the presence of a glass transition as in the hydrated Co-complex crystal. The Tg was determined as 105 ± 2 K for Ru33.4 and 100 ± 2 K for Ru8.3. The sample subjected to annealing at 160 K for 24 h followed by rapid precooling at 2 K·min−1 also showed the heat-release effect characteristic of the glass transition around 100 K, but the magnitude of the effect was much smaller than the corresponding one for the sample without annealing at 160

Figure 8. Molar heat capacities of channel water in [M(H2bim)3](TMA)·nH2O: (a) M = Ru; (b) M = Co; (c) M = Cr. The values were derived by subtracting the molar heat capacities of each anhydride from the total ones of the complex hydrate without the extra bulk water. The solid line represents the literature data for pure ice/water.26 The dotted line around 270 K represents the dependence expected by its interpolation in the temperature range of the fusion of bulk ice.

heat capacities of channel water for M = Ru, Co, and Cr complex crystals, respectively. Solid lines in the respective figures represent the literature data for bulk ice and water.26 All three crystals reveal phase transitions due to structural ordering of channel water below 273 K. It is noticed that the ordering proceeds through transitions of two steps in the former two crystals and one step in the Cr crystal. This indicates that the hydrogen-bond network structure formed by the channel water molecules, developing with decreasing temperature, is very sensitive to the pore size and/or pore shape. As stated above, the channel water plays an important role for the formation of the crystal itself in that the exhaustion of water brings the channel structure to collapse. In order to look into how important the role of the water is, it is intriguing to evaluate the excess entropies, ΔexcSm, of water over that of ice at 273 K in the three crystals. The entropy can be assessed by integrating, with respect to temperature below 273 K, the excess molar heat capacity over the value of bulk ice, ΔexcCp,m, 11772



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the temperature is lowered. The above correspondence means that this development, namely structural shrinkage, is remarkable in the Ru crystal as compared to the Cr and Co ones. It is noted that although the differences in the lattice constants are quite small, only below 0.1 nm, the thermal properties and the associated structural changes are appreciably different among the three crystals. This, without doubt, reflects the situation that the hydrogen-bond network structure formed among the water molecules is based on and involves the bonds with the hydrophilic pore-wall atoms. It is noticed, in addition, that the hydrogen-bond network structure developed is not exactly the same as that of bulk water. While the volume increases through the development of the network structure in bulk water with decreasing temperature below 273 K, it decreases in the present channel water. This also reflects that the network structure developed is strongly associated with the arrangement of the hydrophilic pore-wall atoms. The pore diameter of the water channel in the [Ni(cyclam)(H2O)2]3(TMA)2·24H2O (d = 1.0 nm) crystal reported previously6 is considerably smaller than that in the present crystals (d = 1.5 nm). The channel-water molecules in the Ni(cyclam) crystal form a hydrogen-bond network with the hydrophilic pore-wall groups, and the channel structure is broken down without the water molecules as well. The ΔexcSm of water at 273 K in the Ni(cyclam) crystal is estimated to be 5.3 J·K−1·mol−1. The value is much smaller than those obtained above for the present crystals. Furthermore, the heat capacity values around 273 K are much smaller than those of bulk water and increase more drastically with temperature than those in the present crystals. These are quite consistent with the interpretation that the channel-water molecules form the hydrogen-bond network structure to conform to the arrangement of the hydrophilic groups of the pore wall, and as the pore diameter becomes small, the water molecules are restricted more to the pore-wall structure, resulting in the bigger deviation from the behaviors of bulk water. A glass transition was observed in all the crystals at around 100 K as stated above. This means that part of the channelwater molecules fail to form a completely ordered hydrogenbond network at low temperature. How many channel-water molecules are accommodated in the channel is determined essentially by the channel size. What ordered network structure develops at low temperatures is, on the other hand, unknown at room temperature and determined with decreasing temperature. The enforcement of the pore-wall atoms and pore size hardly allows the water molecules to construct a completely ordered network structure within the pores with diameter below 1.5 nm. Accordingly, it is considered that a certain disorder is left in the arrangement of water molecules in the pore center. Taking into account that the Tg is same among the three crystals, the freezing-in rearrangement motion would be related to the breaking of hydrogen bonds of the same number in its activation process.3 3.5. Thermal Behavior of Partially Filling Pore Water. Figure 9 shows the molar heat capacities of water in Ru(−1.5), where a thick dashed line stands for the heat capacity curve of fully hydrated [Ru(H2bim)3](TMA)·20.7H2O. Although the Ru(−1.5) crystal is lacking in channel water by 7% in comparison with the fully filled crystal, two anomalies were observed: a sharp peak at 197 K and a small shoulder at 179 K. These anomalies are considered to correspond to the phase transitions observed at 221 and 200 K, respectively, for the fully filled crystal. Two things should be noted as significant. One is

divided by temperature; here, the excess heat capacities below Tg ≈ 100 K were approximated as given by ΔexcCp,m(T) = (T/ Tg)ΔexcCp,m(Tg). The excess entropy at 273 K was estimated to be 13.5 ± 0.6, 15.8 ± 0.6, and 17.2 ± 0.7 J·K−1·mol−1 for the Ru, Co, and Cr complex crystals, respectively. These values are considerably smaller than 22.0 J·K−1·mol−1 for fusion entropy of bulk ice. Meanwhile, although the heat capacity values around 273 K of the channel water in the three crystals are close to that of bulk water as seen from Figure 8, the temperature dependence is a little different from that of bulk water. The fact that the heat capacities of channel water become large above 273 K as compared with those of bulk water leads to the consequence that the entropic state of channel water approaches that of bulk water at higher temperatures: It is reasonable to expect that the molecules in both channel and bulk water move about rather freely at higher temperatures and those characteristics become quite similar. These results imply that the channel water molecules are really bound in their configuration with the pore-wall atoms at room temperature. This is consistent with the structural result that the water molecules reveal a shadow structure of hydrogenbond network formed with the hydrophilic pore-wall atoms of the [Co(H2bim)3](TMA) moiety.4,14 The water molecules located close to the pore wall are intepreted to thus give the stability to the crystalline channel structure. In the three crystals, the pore diameters are essentially the same, d = 1.5 nm, because the crystals are built up with the same components and the same arrangement among them. Furthermore, the crystals are formed, as shown in Figure 1, by stacking honeycomb sheets principally through van der Waals interaction. This stacking makes it easy to change the lattice parameter along the channel. It is worthwhile to see how the above entropic character is correlated with the lattice constants and their temperature changes. Those structural data are tabulated in Table 3. Among the three lattice constants, c Table 3. Lattice Constants and Their Relative Changes between 298 and 253 K for [M(H2bim)3](TMA)·nH2O Crystalsa T, K

a, nm

b, nm

298 253

1.673 1.681

2.981 2.942

298 253

1.640 1.647

2.947 2.940

298 253

1.654 1.656

2.997 2.986

c, nm M = Ru 1.052 0.995 M = Co 1.095 1.078 M = Cu 1.098 1.073

Δa, %

Δb, %

Δc, %

0.5

−1.3

−5.4

0.4

−0.2

−1.6

0.1

−0.4

−2.3

a Δx (x = a, b, c) was evaluated as 100 × {x(253 K) − x(298 K)}/ x(298 K). All data were taken from ref 14.

reveals the biggest fractional change Δc between 298 and 253 K; the changes are −5.4%, −1.6%, and −2.3% for Ru, Co, and Cr crystals, respectively, and the cell volumes shrink as a whole when the temperature is lowered. The c axis represents exactly the periodicity along the channel. The excess entropies at 273 K deviate gradually from those of bulk water in the order of Cr to Co to Ru crystals. This order corresponds roughly to that of the changes of the lattice constants, the cell volumes, and also the quantities n of water. The hydrogen-bond network structure of channel-water molecules naturally develops when 11773



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for a while, and complex crystals involving water channels with larger diameter than 1.5 nm have been hardly reported to be formed successfully. It is possibly indicated that the effect of instantaneous hydrogen-bond network formation of the water molecules on the pore wall would be too weak in crystalline pores with diameter above 1.5 nm to stabilize the channel structure of the complex crystal at room temperature.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

Figure 9. Molar heat capacities of channel water in sample Ru(−1.5). The dashed line represents the temperature dependence of the channel water in [Ru(H2bim)3](TMA)·20.7H2O. The solid line represents the literature data for pure ice/water.26

The authors declare no competing financial interest. E-mail [email protected] (M.T.), moguni@chem. titech.ac.jp (M.O.).

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ACKNOWLEDGMENTS This work was financially supported partly by Grants-in-Aid for Scientific Research (21340118) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

that the same two transitions appeared clearly; namely, the phase behavior is basically the same as that of the fully filled one. This indicates that the partial dehydration induces no structural changes of either the pore-wall structure of the complex crystal or the shadow hydrogen-bond network structure of interfacial water molecules on the pore wall and therefore that the channel water molecules were removed from the central part of the pore. This is consistent with the fact that the complex crystal collapses without water molecules on the pore wall, as well as confirming again the importance, for the stabilization of the crystalline structure itself, of formation of the shadow network structure of interfacial water molecules. The other thing is that the partial dehydration of channel water destabilizes the more ordered, low-temperature phases as compared with the disordered, higher-temperature ones. This indicates that although the water molecules in the pore center of the fully filled crystal are left in a somewhat disordered state, as evidenced above by observation of the glass transition around 100 K, they contribute importantly to the stable formation of the ordered network structure of water molecules present close to the pore wall.

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REFERENCES

(1) Findenegg, G. H.; Jahnert, S.; Akcakayiran, D.; Schreiber, A. ChemPhysChem 2008, 9, 2651. (2) Oguni, M.; Maruyama, S.; Wakabayashi, K.; Nagoe, A. Chem. Asian J. 2007, 2, 514. (3) Oguni, M.; Kanke, Y.; Nagoe, A.; Namba, S. J. Phys. Chem. B 2011, 115, 14023. (4) Watanabe, K.; Oguni, M.; Tadokoro, M.; Oohata, Y.; Nakamura, R. J. Phys.: Condens. Matter 2006, 18, 8427. (5) Watanabe, K.; Oguni, M.; Tadokoro, M.; Kobayashi, C. J. Phys. Chem. B 2009, 113, 14323. (6) Watanabe, K.; Oguni, M.; Tadokoro, M.; Nakamura, R. J. Phys.: Condens. Matter 2006, 18, 9375. (7) Watanabe, K.; Oguni, M. Bull. Chem. Soc. Jpn. 2007, 80, 1758. (8) Liu, E.; Dore, J. C.; Webber, J. B. W.; Khushalani, D.; Jahnert, S.; Findenegg, G. H.; Hansen, T. J. Phys.: Condens. Matter 2006, 18, 10009. (9) Yoshida, K.; Yamaguchi, T.; Kittaka, S.; Bellissent-Funel, M.; Fouquet, P. J. Chem. Phys. 2008, 129, No. 054702. (10) Vogel, M. Phys. Rev. Lett. 2008, 101, No. 225701. (11) Banys, J.; Kinka, M.; Macutkevic, J.; Völkel, G.; Böhlmann, W.; Umamaheswari, V.; Hartmann, M.; Pöppl, A. J. Phys.: Condens. Matter 2005, 17, 2843. (12) Cerveny, S.; Barroso-Bujans, F.; Alegrıa, A.; Colmenero, J. J. Phys. Chem. C 2010, 114, 2604. (13) Arndt, M.; Stannarius, R.; Gorbatschow, W.; Kremer, F. Phys. Rev. E 1996, 54, 5377. (14) (a) Tadokoro, M.; Fukui, S.; Kitajima, T.; Nagao, Y.; Ishimaru, S.; Kitagawa, H.; Isobe, K.; Nakasuji, K. Chem. Commun. 2006, 12, 1274. (b) The pore diameter is 1.5−1.6 nm: Tadokoro, M.; Ohhara, T.; Ohhata, Y.; Suda, T.; Miyasato, Y.; Yamada, T.; Kikuchi, T.; Tanaka, I.; Kurihara, K.; Oguni., M.; Nakasuji, K.; Yamamuro, O.; Ryota, K. J. Phys. Chem. B 2010, 114, 2091. (15) Kobayashi, C. Bachelor thesis, Tokyo University of Science, 2006. The details are to be published elsewhere. The crystal structure of the Co complex, which is essentially the same as those of Ru and Cr complexess, has been reported previously in ref 14. (16) Morishige, K.; Nobuoka, K. J. Chem. Phys. 1997, 107, 6965. (17) Maniwa, Y.; Kataura, H.; Abe, M.; Udaka, A.; Suzuki, S.; Achiba, Y.; Kira, H.; Matsuda, K.; Kadowaki, H.; Okabe, Y. Chem. Phys. Lett. 2005, 401, 534. (18) Gallo, P.; Rovere, M.; Chen, S.-H. J. Phys.: Condens. Matter 2010, 22, No. 284102. (19) Kastelowitz, N.; Johnston, J. C.; Molinero, V. J. Chem. Phys. 2010, 132, No. 124511.

4. CONCLUDING REMARKS The ordering behaviors characteristic of water confined within the pores with crystalline pore-wall structure were disclosed by adiabatic calorimetry for the complex crystals [M(H2bim)3](TMA)·nH2O, where M = Co and Ru. Two phase transitions and one glass transition were observed for both crystals at low temperatures. With the behaviors in the M = Cr crystal5 taken into account as well, the ordering mechanisms of the water were different among the three crystals with essentially the same size pores. This indicates that the stability of the hydrogen-bond network structures formed is very sensitive to the shape of the pore section and/or the periodicity of the molecular arrangement along the channel. It is also noticed that the stability is strongly affected by how much the water fills the pore. Although the structural ordering of the channel water begins from and is dominated by the molecules adjacent to the pore wall, the network structure formed is stabilized appreciably by the attendance of the water molecules in the pore center. Also noteworthy is that the excess entropy of the channel water at 273 K tends to approach that of bulk water as the quantity n of the water increases in the pore diameter range of 1.5 nm. This is intriguing when it is considered that the present channel structures with pore diameter of 1.5 nm often break down when kept in aqueous solutions above room temperature 11774



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(20) Llave, E.; Molinero, V.; Scherlis, D. A. J. Chem. Phys. 2010, 133, No. 034513. (21) Tombari, E.; Salvetti, G.; Ferrari, C.; Johari, G. P. J. Chem. Phys. 2005, 122, No. 104712. (22) Tombari, E.; Ferrari, C.; Salvetti, G.; Johari, G. P. Therm. Acta 2009, 492, 37. (23) Fujimori, H.; Oguni, M. J. Phys. Chem. Solids 1993, 54, 271. (24) Suga, H; Seki, S. Faraday Discuss. Chem. Soc. 1980, 69, 221. (25) Oguni, M.; Matsuo, T.; Suga, H.; Seki, S. Bull. Chem. Soc. Jpn. 1977, 50, 825. (26) Haida, O.; Matsuo, T.; Suga, H.; Seki, S. J. Chem. Thermodyn. 1974, 6, 815.

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Thermal Characteristics of Channel Water Confined in Nanopores with

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