Water Dynamics in Bulk and Dispersed in Silica CaCl2 Hydrates

Jul 26, 2008 - Daniil I. Kolokolov , Maxim S. Kazantsev , Mikhail V. Luzgin , Hervé Jobic , and ... Daniil I. Kolokolov , Alexander G. Stepanov , and ...
0 downloads 0 Views 880KB Size
Download PDF
J. Phys. Chem. C 2008, 112, 12853–12860

12853

Water Dynamics in Bulk and Dispersed in Silica CaCl2 Hydrates Studied by 2H NMR Daniil I. Kolokolov,†,‡ Ivan S. Glaznev,† Yurii I. Aristov,† Alexander G. Stepanov,*,† and Herve´ Jobic‡ BoreskoV Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Prospekt Akademika LaVrentieVa 5, NoVosibirsk 630090, Russia, and Institut de Recherches sur la Catalyse et l’EnVironnement de Lyon, CNRS, UniVersite´ de Lyon, 2 aVenue Albert Einstein, 69626 Villeurbanne, France ReceiVed: February 11, 2008; ReVised Manuscript ReceiVed: May 8, 2008

The mobility of water in deuterated analogues of CaCl2 · nH2O (n ) 2, 4, 6) hydrates has been studied by solid-state 2H NMR spectroscopy. Dynamics of water molecules in hydrates dispersed in the mesopores of silica are compared with those in the bulk state. Analysis of the 2H NMR line shape and T1 and T2 relaxation times allowed us to characterize the water mobility in a wide temperature range (120-493 K). In both crystalline and melted hydrates, the mobility of water molecules is governed by O-D · · · Cl hydrogen bonding. Both bulk and dispersed hydrates have been found to exhibit three types of molecular motion. Two of these represent fast internal and local motions performed on a time scale of 10-10-10-11 s. The third, slow isotropic reorientation occurs on a time of 10-6-10-7 s. Dispersed hydrates become involved in the slow isotropic motion at temperatures 50-130 K lower than the corresponding bulk hydrates. The temperature TNMR at which dispersed hydrates are involved in isotropic motion represents the melting point of the hydrates located in the silica pores. The decrease of the melting point for the dispersed hydrates is in good accordance with the Gibbs-Thompson effect for dispersed materials. In dispersed hydrates, water molecules reorient isotropically 1 order of magnitude faster in the temperature range 230-490 K; that is, water is more mobile in the dispersed hydrates. The slow isotropic reorientation of water molecules is influenced by both the quantity of water in the hydrate and the dispersibility of the hydrates. In the case of the hydrate with n ) 4, the activation energy of this motion decreases by ca. 3 times when the hydrate becomes dispersed in the silica pores. 1. Introduction Although water and water-containing systems are widely distributed in nature, the properties of water in biological systems, minerals, porous solids, etc., are still subjects of numerous studies because the behavior of water in a bound state is complex. Hydrates of inorganic salts constitute convenient objects for studying the bound states of water because they contain a certain number of water molecules which are better ordered so that their properties are easier to comprehend. Indeed, the measurement of unusually well-defined vibrational modes of water was reported for calcium chloride-water complexes containing different amounts of water, namely 1/3, 2, or 4 waters per CaCl2 molecule.1 Based on the crystallographic structure, normal-mode calculations from first principles were performed, and inelastic neutron scattering spectra were simulated.1 A very good agreement between the calculation and the experiment, without any parameter refinement, confirmed and completed the intuitive assignment of the vibrational excitations of water. Moreover, a closer look at the calculation enabled the investigation of the anharmonicity of the system and the dispersion of the excitations along the hydrogen bonds.1 Another important target is to find out how the state and properties of hydration water vary when the dimensions of the hydrate crystals reduce to a nanometer scale. One of the ways to produce nanodimensional crystals of hydrates of an inorganic salt is its confinement within the pores of a host matrix * Corresponding author. Fax: +7 383 330 80 56. E-mail: stepanov@ catalysis.ru. † Boreskov Institute of Catalysis. ‡ Universite ´ de Lyon.

(dispersion), which is chemically inert and acts mainly as a dispersant. In such a way, a new family of composite sorbents of water (the so-called selective water sorbents, SWS) has been synthesized to be applied for gas drying, adsorptive heat pumps and chillers, humidity control, etc.2 The study of water sorption properties of these materials revealed that the formation of salt hydrates in the dispersed state occurred at a lower relative pressure η of water vapor with respect to the bulk state.3,4 For instance, calcium chloride dihydrate, CaCl2 · 2H2O, forms at ηd ) 0.02-0.03 in silica pores of 15 nm size, whereas ηb ) 0.07-0.09 for the corresponding bulk salt.3 This makes a desiccant containing the salt dispersed on silica much more efficient than a common bulk salt. A possible reason for this relative pressure reduction could be a higher mobility of water molecules in the hydrate confined in silica with regard to the bulk state, so that the transition of the water molecules from the gas to the condensed state in the course of the reversible reaction of the salt hydration-dehydration in the silica pores results in a smaller entropy reduction compared with the transition to the bulk state. In this paper, we have performed a detailed study of the water dynamics in the bulk hydrates of calcium chloride as well as in the corresponding hydrates confined to the pores of silica, using deuterium solid-state NMR (2H NMR) spectroscopic techniques. 2H NMR spectroscopy has been shown to be a powerful technique to probe the dynamics of water molecules.5–8 Moreover, 2H NMR also provides information on the structure of crystalline hydrates.9 The line shape of 2H NMR signals, being completely defined by intramolecular quadrupole interaction, is especially sensitive to the nature of molecular motion and to its rate. Spin-lattice (T1) and spin-spin (T2) relaxation times

10.1021/jp801223c CCC: $40.75  2008 American Chemical Society Published on Web 07/26/2008

12854 J. Phys. Chem. C, Vol. 112, No. 33, 2008

Kolokolov et al.

also bring information on the energy and the rate of different inter- and intramolecular motions. Here we report the results of 2H NMR studies on the dynamic behavior of water in deuterated analogues of CaCl2 · 2H2O, CaCl2 · 4H2O, and CaCl2 · 6H2O hydrates, in both bulk and dispersed states. 2. Experimental Section 2.1. Materials. Anhydrous CaCl2 salt of chemical grade purity, mesoporous silica (KSK) (pore diameter dj ) 10-30 nm, specific area SBET ) 176 m2/g, specific volume Vp ) 0.82 cm3/ g), and deuterated water (99.9% 2H isotope enrichment), commercially available, were used without further purification. The polycrystalline (bulk) hydrates CaCl2 · 2D2O and CaCl2 · 4D2O were prepared by exposing the granules of CaCl2 (0.1-0.2 mm diameter) to surroundings preliminarily degassed under vacuum heavy water vapor with fixed PD2O in a nitrogen atmosphere. The CaCl2 · 6D2O was prepared by recrystallization from saturated solution in degassed D2O. The content of water in all bulk hydrates was additionally checked by X-ray diffraction analysis.10–12 CaCl2 · nD2O (n ) 2, 4, 6) hydrates dispersed in silica, all representing the composite adsorbents “CaCl2 in nanoporous matrix” (SWS-1L) were prepared by impregnating the silica KSK with a saturated solution of CaCl2 at 298 K under conditions preventing the access of air to the sorbent. The detailed procedure of SWS-1L preparation was similar to that described elsewhere,3 except the procedure was realized in a glovebox under nitrogen atmosphere. Homogeneity and the number of water molecules per Ca cation in dispersed hydrates was controlled by analysis of neutron diffraction intensities, based on the procedure described in ref 13. For both bulk and dispersed hydrates, the final D2O content was also controlled by measuring the sample weight. The accuracy of final n values was estimated as (0.2. 2.2. Sample Preparation. To prepare CaCl2 · nD2O (n ) 2, 4, 6) hydrate samples for the NMR experiments, approximately 0.3 g of bulk or dispersed in silica KSK (SWS-1L) hydrates taken from the desiccator was loaded in a 5 mm (o.d.) glass tube of 2.5 cm length under air atmosphere and immediately sealed. An additional test experiment has shown that 2 min exposure of the samples to air before sealing provided no noticeable influence on the relaxation parameters T1 and T2; i.e., the potential effect of oxygen on these parameters is negligible. For differential scanning calorimetry (DSC) measurements and X-ray diffraction experiments with bulk hydrates, no additional treatments of the samples were performed. 2.3. NMR Measurements. 2H NMR experiments were performed at 61.432 MHz on a Bruker Avance-400 spectrometer, using a high-power probe with a 5 mm horizontal solenoid coil. All 2H NMR spectra were obtained by Fourier transformation of the quadrature detected quadrupole echo, arising in the pulse sequence14,15

( π2 )

(X

( π2 ) - τ -acquisition-t

- τ1-

Y

2

(i)

where τ1 ) 20 µs, τ2 ) 22 µs, and t is a repetition time for sequence i during the accumulation of NMR signal. The duration of the π/2 pulses was 3.0-4.0 µs. Excess of τ2 over τ1 by 2 µs was chosen to compensate a finite π/2 pulse duration.16 Spectra were typically obtained with 500-5000 scans and a repetition time of t ) 4 s. Inversion-recovery experiments, to derive spin-lattice relaxation times (T1), were carried out using the pulse sequence17

( π2 )

(π)X - tv-

(X

( π2 ) - τ -acquisition-t

- τ1-

Y

2

(ii)

where tv was a variable delay between the 180° (π)X inverting pulse (as in standard inversion-recovery pulse sequence) and the quadrupole echo sequence i. T1 values were calculated on the basis of the time (τo ) 0.693T1).17 τo is the time tv for which the intensity of the NMR line changes from the inverted negativepositiontothenormalpositiveoneintheinversion-recovery experiment (sequence ii). This simple procedure of T1 determination by finding τo values was used after we ensured that monitoring the full recovery of magnetization with sequence ii gives actually one-exponential recovery and the discrepancy in T1 values estimated by two ways does not exceed 5-12%. For our experimental conditions, T2* ≈ T2 was a good approximation.18 Therefore, the values of T2 were derived from the Lorentzian-type line shape according to the well-known relation T2 ) 1/π∆ν1/2, where ∆ν1/2 is the width at half-height of the Lorentzian. The temperature of the samples was controlled with a flow of nitrogen gas, stabilized with a variable-temperature unit (Bruker Model BVT-3000) with a precision of ∼1 K; the sample was allowed to equilibrate at least 15 min at a given temperature before the NMR signal was acquired. 2.4. Calorimetric Measurements. The differential scanning calorimetry (DSC) measurements were performed for CaCl2 · nD2O (n ) 2, 4, 6) bulk hydrates on the NETZSCHDSC 404C Pegasus device with an aluminum crucible and a standard heating rate of 3 K min-1. 3. Results 3.1. 2H NMR Spectra of Bulk and Dispersed Hydrates. Analysis of the 2H NMR data has shown that all studied hydrates exhibit similar temperature dependences of 2H NMR spectra line shapes. Only small differences can be found between the bulk and dispersed hydrate samples. A similar comment can be made for the temperature dependences of 2H NMR spinlattice (T1) and spin-spin (T2) relaxation times. Such similarity points out that the water molecule motions have a common characteristic pattern for all hydrates. Therefore, its main features can be demonstrated through detailed consideration of only one of the hydrates. We provide below a detailed analysis of 2H NMR spectra for CaCl2 · 6D2O hydrate in the bulk and dispersed states. Figure 1 shows the temperature dependences of 2H NMR spectra for bulk and dispersed in silica CaCl2 · 6D2O hydrate. The temperature range of spectra variation can be conventionally divided into three regions. In the low-temperature region, the spectra exhibit a 2H NMR broad powder pattern, typical for solid or immobile water. Increase of temperature results in the appearance of an additional narrow peak (typical for liquids) at the center of the solidlike spectrum, arising from the mobile (rapidly and isotropically reorienting) water molecules. The region where the narrow liquidlike and the broad solidlike signals coexist characterize the intermediate-temperature region of spectral variation. At temperatures above TNMR the experimental spectrum consists only of a single isotropic signal with a Lorentzian line shape. TNMR marks the beginning of the hightemperature region of spectra variation. The dynamics of water and the structure of the hydrates in the polycrystalline state can be derived from the analysis of 2H NMR line shapes in the low-temperature region. In this region, the bulk and dispersed hydrate samples show almost identical 2H NMR spectral line shapes with similar principal features: each spectrum represents a superposition of two Pake-type

Water Dynamics in Bulk and Dispersed in CaCl2 · nH2O

J. Phys. Chem. C, Vol. 112, No. 33, 2008 12855

Figure 1. Temperature dependence of 2H NMR spectrum line shapes of bulk (A) and dispersed in silica (B) CaCl2 · 6D2O hydrate.

Figure 2. (A) 2H NMR spectra and their line shape simulations for bulk CaCl2 · 6D2O hydrate in the low-temperature region. (B) Simulation of 2H NMR spectrum for bulk CaCl2 · 6D2O hydrate at 223 K as a sum of two signals from static molecules (dashed line) with Q1, η1 and molecules involved in fast 180° flips about the D-O-D bisector (solid line) with Q2, η2 (see Table 1).

powder patterns. Simulations of the spectra based on classical theory for 2H NMR solid-state spectral line shapes19 (Figure 2) show that one of the signals has a large quadrupole constant Q1 ∼ 250 kHz and a small asymmetry parameter η1 ∼ 0.1, whereas the other one exhibits a smaller quadrupole constant Q2 ∼ 125 kHz and a much larger asymmetry parameter η2 ∼ 0.85. The parameters of the spectrum, Q1 ∼ 250 and η1 ∼ 0.1, are typical for hydrogen-bonded water molecules in crystalline hydrates6,9 or ice.5,8 In other words, the spectra are typical for water, static on the NMR time scale. If we denote τC as the characteristic time of molecular motion, then τC. τNMR; τNMR ) Q-1 ∼ 4 × 10-6 s for static water. For the second signal, the large asymmetry parameter η2 and the smaller quadrupole constant Q2 indicate that it corresponds to water involved in some fast anisotropic motion (τC , τNMR). In the lowtemperature region, hydrates are purely in crystalline state, and the only reasonable type of motion not prohibited by the hydrate’s lattice symmetry is the 2-fold flips about the D-O-D bisector. Such behavior is typical for crystalline hydrates.6,7,9

TABLE 1: 2H NMR Spectra Line Shape Simulation Results for CaCl2 · nD2O Hydrates in the Low-Temperature Regiona bulk hydrate Q1, kHz η1 Q2, kHz η2 θ, deg rD · · · Cl,b nm rD · · · Cl,c nm

dispersed hydrate

n)2

n)4

n)6

n)2

n)4

n)6

250 0.08 125 0.85 53.2 0.21 0.22

250 0.09 125 0.83 53.2 0.20 0.23

250 0.09 125 0.83 53.2 0.21 0.22

260 0.09 130 0.84 53.2 0.22

240 0.12 120 0.84 53.2 0.21

240 0.16 120 0.84 53.2 0.20

a Errors in estimating the parameters are as follows: for Q1, (5 kHz; for η1, (0.05; for θ, (0.5°; for rD · · · Cl, (0.01 nm. b Data from this work. c Taken from X-ray and neutron diffraction data of refs 10–13.

The 2H NMR spectrum of the water molecules rapidly flipping by 180° around the bisecting angle can be simulated using the formalism elaborated by Spiess20 and Wittebort.21 In the fast exchange limit, the resulting motionally averaged

12856 J. Phys. Chem. C, Vol. 112, No. 33, 2008

Kolokolov et al.

spectrum is defined by both the initial unaveraged parameters Q1 and η1 and the bisecting angle D-O-D ) 2θ.22 Simulations of spectral line shapes show a perfect agreement with the experimental data for all hydrates. Figure 2A shows the simulation results for bulk CaCl2 · 6D2O hydrate, and Figure 2B demonstrates that the simulated spectra of Figure 2A are a sum of two line shapes from static and motionally averaged by 180° flips of water molecules. The 2H NMR simulating parameters of the spectra are given in Table 1 for all studied hydrates. While parameter θ provides us structural information on the hydrogen-bonded water molecules for bulk and dispersed hydrates, the quadrupole constant Q1 for static water can also yield information on the length of the O-D · · · Cl hydrogen bond. An estimation of hydrogen bond length can be made based on semiempirical dependence between the quadrupole constant and the length of the hydrogen bond, found by Chiba et al.:9

Q ) 310 - 3

190.6 r3

Here Q is the observed quadrupole constant in kilohertz and r is the length of hydrogen bond in nanometers. The comparison of the simulated parameters θ and rD · · · Cl (Table 1) for bulk and dispersed hydrates shows that they have very close values. Thus, the local structures of bulk and dispersed hydrates are almost identical in the low-temperature region. Within the intermediate-temperature region, the intensity of the liquidlike isotropic signal at the center of the anisotropic spectrum increases rapidly with temperature for the bulk hydrate. It is clear that the liquidlike signal represents water in a mobile state, characterized by fast molecular reorientation with τC , τNMR, which completely averages the quadrupolar solid-state spectral features. It is reasonable to assign the liquidlike signal to the melted hydrate. The intermediate-temperature region ends at temperature TNMR when the solid-state signal completely disappears and the spectrum is represented by the single liquidlike signal. It follows from our 2H NMR data that TNMR is the temperature at which the hydrate completely melts. The temperature behavior of the NMR line shape for the bulk and dispersed hydrates is similar. However, the dispersed hydrate is characterized by a broader intermediate-temperature region in which the solid and melted hydrates coexist, and by an essentially lower value of TNMR at which the hydrate melts. The temperature dependence of the 2H NMR spectrum line shape described above is similar for all samples, except for the bulk sample of CaCl2 · 2D2O hydrate, which shows a more peculiar behavior. As can be seen from Figure 3A, the spectrum consists of two signals from rigid and flipping water molecules up to ∼350 K. However, the spectrum line shape drastically changes above 353 K: it consists of a superposition of three different Pake powder patterns at T g 353 K (see Figure 3B). Simulation shows (Figure 3B) that the spectrum is perfectly fitted by the typical deuterium solidlike spectra with Q1 ) 180 kHz, Q2 ) 120 kHz, and Q3 ) 60 kHz, with asymmetry parameters η1,2,3 ) 0. Relative intensities of the three contributing signals almost do not change with further temperature increase. Such a dramatic change in the total line shape of the spectrum indicates a structural change (phase transition) in the hydrate. Our further X-ray diffraction analysis of the bulk CaCl2 · 2D2O hydrate (data not presented) revealed the change of the diffraction pattern above 350 K, and thus the structural phase transition was confirmed. The observation of a structural phase transition with 2H NMR spectroscopy was earlier reported.23 Although the detailed study

Figure 3. (A) Temperature dependence of the 2H NMR spectrum of bulk CaCl2 · 2D2O. (B) Simulation of 2H NMR spectrum of bulk CaCl2 · 2D2O hydrate at 353 K as a sum of three Pake powder patterns.

of CaCl2 · 2D2O hydrate structure above 350 K is beyond the scope of our work, 2H NMR spectral line shape analysis allows us to make several suggestions on the peculiarity of water motion in a newly formed phase of the polycrystalline hydrate CaCl2 · 2D2O. The averaging of the asymmetry parameters η1,2,3 to 0 points to the fast anisotropic rotation of water molecules in the hydrate.20 One can reasonably assume that there are three different positions of water molecules freely rotating around some rotation axes in the newly formed crystalline hydrate structure. 3.2. 2H NMR T1 and T2 Relaxation Times Analysis. Figure 4 shows the temperature dependences of 2H NMR spin-lattice (T1) and spin-spin (T2) relaxation times for the liquidlike signal of both bulk and dispersed in silica CaCl2 · nD2O (n ) 2, 4, 6) hydrates in the intermediate- and high-temperature regions. The solid curves drawn through the data points are theoretical fits according to the models developed for water molecule motion (vide infra). It is clear from data in Figure 4 that at least two different types of motion are present in both bulk and dispersed hydrates. A fast motion (τC ∼ ω0-1 ) 2.6 × 10-9 s, ω0 ) 61.43 MHz) with higher activation barrier governs the T1 temperature dependence. A slower one (τC ∼ 10-6-10-7 s) with a smaller activation barrier defines the T2 temperature dependence. The qualitative difference between bulk and dispersed hydrates lies in a narrow temperature interval of rapid T2 growth (leap of T2 at TC) observed in two of the dispersed hydrates (marked by a vertical dashed line in Figure 4E,F). The slopes of T2 dependences before and after this leap are visually identical. It should be noted that there is no noticeable leap in the T2 temperature dependence for the dispersed CaCl2 · 2D2O hydrate. In order to make theoretical fits to the T1 and T2 temperature dependences, a physically reasonable picture of water molecule motion in the melted CaCl2 hydrates is needed. It is natural to assume that the fast motion, i.e., the fast 180° flips about the D-O-D bisector, which has already been observed in the lowtemperature region persists in the melted state. This motion can govern the T1 relaxation time of the liquidlike signal of the melted hydrates. Isotropic liquidlike shape can be a consequence of a relatively fast (τC < τNMR ∼ 4 × 10-6 s) isotropic reorientation of the molecule as a whole or of fast intramolecular

Water Dynamics in Bulk and Dispersed in CaCl2 · nH2O

J. Phys. Chem. C, Vol. 112, No. 33, 2008 12857

Figure 5. Schematic representation of water molecule jumping between two positions in dispersed CaCl2 · 6D2O hydrate.10–13

Figure 4. Temperature dependences of T1 (0) and T2 (4) for bulk (A-C) and dispersed in silica (D-F) CaCl2 · 2D2O, CaCl2 · 4D2O, and CaCl2 · 6D2O hydrates, correspondingly.

motion with a high symmetry, such as for tetrahedral jumps in solid ice.8 However, the values of the activation barriers and characteristic times deduced below from the experimental data indicate that the isotropic rotational diffusion is the only option. The correlation function for isotropic diffusion is wellknown,24 and the case of 2-fold jumps is also classic25 (it can be evaluated using the formalism described in ref 26), with the geometry of the jumps being defined in the previous section. Correlation times τC are assumed to obey the Arrhenius law τC ) τC0 exp(-E/kBΤ), where τC0 is the preexponential factor and E is the activation energy. Assuming that these relaxation processes are independent, the general correlation function can be obtained.20,26 However, such a simple model does not fit both T1 and T2. For all samples, the simulations of the T1 and T2 temperature dependences yield quadrupole constants Q lower by a factor of 3-10 than the values obtained from spectral line shape simulation. This suggests the presence of a second fast anisotropic motion, which additionally influences the relaxation process. The other possible internal motions of water, vibrations and librations, are present but they cannot influence the relaxation in such a drastic way. To take into account this motion and fit correctly the experimental data, we have examined possible models of water molecule motion. It is worth noting that for the analyzed models the 180° flip geometry (the θ angle) was fixed and its parameters were obtained from the low-

temperature-region spectral line shape analysis. Two elaborated models were probed to fit the experimental data. Relaxation data were very well fitted with a jump-reorientation model for the dispersed hydrates. In order to elucidate the nature of the second anisotropic motion, besides the 180° flipping motion, we have assumed that the short-range order of water molecules in the melted hydrates is kept and corresponds to the crystalline structure of hydrates.10–13 In this case, there are at least two nearby positions for water molecules, spaced by a distance of 0.2-0.3 nm for all hydrates. We have supposed that the water molecules could perform an anisotropic diffusion by jumping between nearest-neighbor sites. It is certain that such a motion is suppressed in the crystal phase; however, such jumps could be possible in the melted state. Such a motion affects the water molecule orientation in the following way: first, the molecule C2 symmetry axis is tilted by the polar angle β (Figure 5); then the molecular plane is rotated by the polar angle φ about the new symmetry axis C2′. The polar angle β is the angle between the directions of the axes C2 and C2′ of the molecule symmetry axis. It can be directly taken from the available crystallographic data.10–13 The polar angle φ defines the relative rotation angle of the water molecule plane about the C2 symmetry axis at each site. It is less defined in the literature and is regarded as a free parameter for our modeling. As can be seen in Figure 4D-F, such a model gives a good fit of the experimental data. The parameters for the molecular motion are given in Table 2. The proposed model does not describe the T2 leap for dispersed samples (see Figure 4E,F). This phenomenon, which affects T2 relaxation time, acts on relatively large correlation times and influences only isotropic diffusion. A possible explanation for this phenomenon will be discussed (vide infra). During the simulation only the regions before and after the T2 leap were taken into account for the dispersed hydrates with n ) 4 and 6; the results for both regions are given in Table 2. The temperature at which this rapid growth of T2 ends is marked as TC. In order to fit the relaxation data for bulk hydrates, another model of molecular reorientation motion had to be used. In this model, in addition to the persisting isotropic rotation and internal 2-fold flips, the water molecule is involved in a free anisotropic rotation about an axis given by the directing vector nj (Figure j 2) ) Cos(β′). Its physical 6). The angle β′ is defined as (nj, C rationalization is discussed below. The analytical evaluation is

12858 J. Phys. Chem. C, Vol. 112, No. 33, 2008

Kolokolov et al.

TABLE 2: Parameters of the Molecular Motion of Water in Bulk and Dispersed in Silica CaCl2 · nD2O Melted Hydratesa,b bulk hydrates

dispersed hydrates

n)2 n)4 n)6 Q1, kHz E1, kJ mol-1 τ10 × 1013, s θ, deg E2, kJ mol-1 τ20 × 1013, s β, deg φ, deg β′, deg ER, kJ mol-1 τR0 × 107, s

TABLE 3: Characteristic Temperatures for Bulk and Dispersed CaCl2 · nD2O Hydratesa

250 18 0.13 53.2 25 16.6

250 16 0.13 53.2 23 4

250 18 0.12 53.2 21 0.33

3 10 5

4.1 10 3.3

10 2.9 13.7

bulk hydrate

n)2

n)4

n)6

255 18 1.8 53.2 25 8.4 50 10

240 18 0.23 53.2 19 4.3 60 75

240 18 0.29 53.2 21 0.77 60 68

13 0.05

2.7 2.8 (3.4-1.7)c (3.6-1)c

a E1 and τ10 are the activation energy and preexponential factor for the intramolecular 2-fold jumps. E2 and τ20 are the activation energy and preexponential factor for the second anisotropic motion: it is the free rotation about a given axis for bulk hydrates; it is the anisotropic jump diffusion between neighboring sites for dispersed hydrates. ER and τR0 are the activation energy and preexponential factor for isotropic rotational motion. b Errors in estimating the parameters are as follows: for Q1, (5 kHz; for θ, (0.5°; for β, β′, and φ, ∼10%. Errors for activation barriersE1 and E2, ∼10%; for ER, ∼10%; for all preexponential factors, 30%. c Simulation parameters (before - after) the leap in T2 temperature dependences.

Figure 6. Schematic representation of water molecule motion in bulk hydrates. The motion is assumed to be a small-angle β′ precession of the molecule around its C2 symmetry axis or a libration of the water C2 axis in a cone with β′.

based on the general formalism proposed in ref 26. The fits in Figure 4A-C show that this model gives a good agreement with the experimental data. Its parameters are also given in Table 2. Within both models we have also tried to replace the intramolecular 2-fold jumps by a more general motion: the free rotation about the water molecules’ symmetry axis. However, with such a modification of motional models, we were unable to get any acceptable fits. This confirms that the 2-fold jumps are the only possible internal motions of water molecules. 4. Discussion The analysis of 2H NMR data shows resemblances between bulk and dispersed hydrates. They have similar environments in the solid state as follows from close values of the lengths of hydrogen O-D · · · Cl bonds between water molecule hydrogen

TNMR, K TC, K Tmelt,b K

dispersed hydrate

n)2

n)4

n)6

n)2

n)4

n)6

453

318

303

323 370

263 296

233 285

450

313

301

a Errors in estimating the parameters are as follows: for TNMR, (2 K; for TC, (3 K; for Tmelt, (1.5 K. b Defined by differential scanning calorimetry (DSC).

atoms and chlorine atoms. Water molecules are either immobile on the NMR time scale or exhibit fast 2-site jumps around the C2 axis in the solid state. Only the bulk solid CaCl2 · 2D2O hydrate exhibits a secondorder phase transition at 353 K, which changes both the structure of the crystalline hydrate and the modes of the water molecular motion. At T g 353 K the water molecules become involved in three types of anisotropic rotation. A transformation of the solidlike spectrum to the liquidlike one corresponds to the melting of the hydrates at Tmelt. The temperature of the entire transformation of the solid to liquidlike signal TNMR is in good correspondence with the melting points Tmelt for the bulk hydrates, TNMR ≈ Tmelt (see Table 3). In contrast to the bulk hydrates, the transformation of the solid to liquidlike signal occurs in a relatively broad temperature range for the dispersed hydrates. Further, the temperature TNMR is 55-130 K lower than that for the bulk hydrates. This means that water molecules exhibit isotropic reorientation at a temperature 55-130 K lower than in the bulk hydrates. Therefore, water molecules are more mobile in the dispersed hydrates. It is well-known that the melting temperature of materials can be decreased in porous media, due to the Gibbs-Thompson effect: small samples of material melt at lower temperatures than the bulk substance, due to the combination of surface curvature and surface energy.27 For our case of hydrates dispersed in silica pores, it is natural to assume that the surface curvature of hydrate grains is characterized by a radius equal to the average pore size: dj ) 10-30 nm.The melting temperature Τ′ in the silica pores can be calculated from the standard Gibbs-Thompson equation:

Tmelt - T′ )

γslTmelt 2 Fsqm r

Here γsl is the interfacial free energy between the liquid and the solid, Fs is the solid hydrate density, and qm is the melting heat; Tmelt is the melting temperature of the bulk sample. Numerical estimations show that for all dispersed studied hydrates Τ′ corresponds approximately to TNMR. Therefore, the decrease of the melting point for the dispersed hydrates is partially explained by the Gibbs-Thompson effect. Analysis of the relaxation data for the melted hydrates provides us information on the modes of water motion, their rates, and energetics. Two-fold jumps have been found in both melted bulk and dispersed hydrates, and their motional parameters are similar. The 2-fold jumps are very fast and have an average value of the activation barrier of E1∼ 18 kJ mol-1, which is similar for all hydrate samples. Such a value is typical for hydrates with hydrogen bonds of weak or moderate strength, if crossing the rotational barrier is associated with breaking the hydrogen bonds.6,7,28,29 Indeed, if we assume that each water deuteron is linked with a chlorine atom by a hydrogen bond,

Water Dynamics in Bulk and Dispersed in CaCl2 · nH2O

Figure 7. Dependence of activation barrier ER or isotropic reorientation on water content in the bulk (4) and dispersed in silica (0) CaCl2 · nD2O hydrates.

then 180° flip motion involves simultaneous breaking of two hydrogen bonds.6 Similar activation energies for 2-fold jumps presume that the intramolecular motion of water depends only on the local environment for all hydrates, which is mainly governed by hydrogen bonding of water molecules in the melted hydrates. The second anisotropic motion looks more peculiar. In the dispersed hydrates, this motion can be defined as fast anisotropic jumps between neighboring sites. It is interesting to compare the angle β between C2 symmetry axes of a two-sites jump, found in our relaxation times simulation, with available crystallographic data.10–13 In fact, for all hydrate species this angle is close to 60°, which is in a good coincidence with our results (see Table 2). Thus for the melted dispersed hydrates the second anisotropic motion is indeed a fast interchange of water between neighboring sites with a geometry which correlates well with the hydrate unit cell structures. This implies that the short-range order in the melted hydrates is kept for the dispersed hydrates. In bulk hydrates the second fast motion is completely different. Instead of a large angular jump displacement, the relaxation data are better described by a continuous anisotropic free rotation. The tilting angle is small: β′ < 10° for all hydrate species. That means that this motion is a small-angle precession of the water molecules about the C2 symmetry axis (or libration in a cone). Such a strong difference looks even more interesting if one decides to compare the activation energies and rates of these motions in bulk and dispersed states. As can be seen from Table 2, in both bulk and dispersed hydrates the second fast motion is characterized by similar activation energies and preexponential factors. As a common rule, the preexponential factor increases with decreasing the amount of water in the hydrate and thus the motion becomes slower, which intuitionally is an expected effect. For all samples, the activation energies are close and the average value is E2 ∼ 23 kJ mol-1. The value of E2 indicates that this motion is also of hydrogen bonding origin. As far as the isotropic rotation activation barrier ER is concerned, it strongly depends on the water content in the hydrates (n ) 2, 4, 6) (Figure 7). Both bulk and dispersed in silica CaCl2 · 2D2O hydrates show a relatively high activation barrier ER ∼ 10 kJ mol-1 and ER∼13 kJ mol-1, respectively, whereas it is about ER ∼ 3 kJ mol-1 for CaCl2 · 6D2O hydrate. An activation energy of 3 kJ mol-1 is a typical value for potentials created by dispersion forces,30 whereas 10 kJ mol-1 is exceeding their possible limits and may have a different nature. The hydrates with n ) 2 and 6 show no strong difference in ER between bulk and dispersed states. On the other hand, there is a profound difference in ER between these states for n ) 4 hydrates. ER is 10 kJ mol-1 for n ) 4 bulk hydrate, i.e., similar to ER in bulk n ) 2 hydrate, whereas ER ∼ 3 kJ mol-1

J. Phys. Chem. C, Vol. 112, No. 33, 2008 12859 in the dispersed state, as in bulk and dispersed hydrates with n ) 6. Thus it is clear that isotropic reorientation of water molecules in CaCl2 hydrates is defined by two sorts of interactions. Weak van der Waals forces govern the isotropic reorientation of both bulk and dispersed hydrates with n ) 6 and dispersed hydrate with n ) 4. The second interaction, which is characterized by a stronger potential, defines the isotropic reorientation for both hydrates with n ) 2 and for the bulk hydrate with n ) 4. A further detailed study of intermolecular interactions in the melted water hydrates of CaCl2 is needed to clarify the reasons for the observed difference in activation energies for isotropic reorientation. An additional remark should be made regarding the correlation time of the isotropic reorientation. It is found to be in the range of 10-6-10-7 s for all samples; i.e., water molecules are involved in a slow isotropic reorientation in melted hydrates. A characteristic time for the rotational diffusion of small molecules such as water in liquids is usually in the range of 10-10-10-12 s.31,32 We believe that this rather slow isotropic motion should be related to some collective motion, which involves the large clusters of water molecules, rather than the individual molecules. The T2 temperature dependence exhibits a rapid growth region for the dispersed hydrates (Figure 2). We marked its bordering temperature as TC. The rapid growth in T2 was earlier reported in a number of works on the studies of phase transition phenomena.33–35 Thus the leap in T2 temperature dependence could possibly indicate some phase transition in the melted dispersed hydrates. Due to this phase transition, one could expect an appearance of new modes of molecular motion. However, T1 varies smoothly with the temperature and does not show any leap. This indicates that there is no any noticeable change in fast internal and local motions of water at TC. The change in temperature dependence of T2 at TC with similar slope before and after the leap implies that only the preexponential factor of τR is affected by 2-4 times, which has a very weak impact on the general molecular motion picture. This means that the phase transition observed at TC in the dispersed hydrate does not notably influence the modes and the rates of water molecular motion in the dispersed hydrates. The absence of the leap in the T2 temperature dependence of the dispersed CaCl2 · 2D2O hydrate indicates that this phenomenon is either absent or stretched over a wide temperature range. 5. Conclusion The analysis of the temperature dependence of the 2H NMR spectral line shapes and of the T1 and T2 relaxation times for water in deuterated analogues of CaCl2 · nH2O (n ) 2, 4, 6) hydrates allowed us to make the following conclusions on the water motional behavior in both the bulk and dispersed in silica hydrates in the solid and melted states. Both the bulk and dispersed hydrates exhibit similar 2H NMR powder patterns in the solid state, corresponding to two types of water molecules: immobile and rapidly reorienting by 180° flips around the molecular C2 axis. The structures of the bulk and dispersed hydrates are similar, according to the close values of the lengths of the O-D · · · Cl hydrogen bonds. The motional behaviors of the melted hydrates are also similar. Both the bulk and the dispersed hydrates exhibit three types of molecular motion. Two of them represent fast motions: internal 180° flips with characteristic times of 10-10-10-11 s and local motions by jumps between two neighbor water positions or precession of the water molecule around some arbitrary axis, which are also performed within a time of 10-10-10-11 s. The third slow

12860 J. Phys. Chem. C, Vol. 112, No. 33, 2008 isotropic reorientation is performed on the time of 10-6-10-7 s for both the bulk and dispersed hydrates. In the dispersed hydrates, the water molecules reorient isotropically 1 order of magnitude faster in the temperature range 230-490 K; that is, water is more mobile in the dispersed hydrates. The slow isotropic reorientation of water molecules is influenced by both the quantity of water in the hydrate and the dispersibility of the hydrates. In the case of the hydrate with n ) 4, the activation energy of this motion decreases by ca. 3 times when the hydrate becomes dispersed in the silica pores. The dispersed hydrates become involved in the slow isotropic motion at a temperature 50-130 K lower than the corresponding bulk hydrates. The temperature TNMR at which the dispersed hydrates become involved in isotropic motion corresponds to the melting point of the hydrates located in the silica pores. Decrease of the melting point for the dispersed hydrates is in good accordance with the Gibbs-Thompson effect for dispersed materials. Acknowledgment. This work was performed in a framework of the French-Russian Laboratory of Catalysis. The work was supported by the Russian Foundation for Basic Research (Grant 05-03-34762). D.I.K. acknowledges Prof. V. V. Lebedev and Dr. A. R. Muratov for stimulating discussions. References and Notes (1) Plazanet, M.; Glaznev, I. S.; Stepanov, A. G.; Aristov, Y. I.; Jobic, H. Chem. Phys. Lett. 2006, 419, 111. (2) Aristov, Y. I. J. Chem. Eng. Jpn. 2007, 40, 1241. (3) Aristov, Y. I.; Tokarev, M. M.; Cacciola, G.; Restuccia, G. React. Kinet. Catal. Lett. 1996, 59, 325. (4) Simonova, I. A.; Aristov, Y. I. Russ. J. Phys. Chem. 2005, 79, 1307. (5) Benesi, A. J.; Grutzeck, M. W.; O’Hare, B.; Phair, J. W. J. Phys. Chem. B 2004, 108, 17783. (6) Long, J. R.; Ebelhaeuser, R.; Griffin, R. G. J. Phys. Chem. A 1997, 101, 988. (7) Stepanov, A. G.; Shegai, T. O.; Luzgin, M. V.; Essayem, N.; Jobic, H. J. Phys. Chem. B 2003, 107, 12438. (8) Wittebort, R. J.; Usha, M. G.; Ruben, D. J.; Wemmer, D. E.; Pines, A. J. Am. Chem. Soc. 1988, 110, 5668.

Kolokolov et al. (9) Soda, G.; Chiba, T. J. Chem. Phys. 1969, 50, 439. (10) Leclaire, A.; Borel, M. Acta Crystallogr. 1977, B33, 1608. (11) Thewalt, U.; Bugg, C. E. Acta Crystallogr. 1973, B29, 615. (12) Leclaire, A.; Borel, M.-M. Acta Crystallogr. 1977, B33, 2938. (13) Agron, P. A.; Busing, W. R. Acta Crystallogr. 1986, C42, 141. (14) Powles, J. G.; Strange, J. H. Proc. Phys. Soc. 1963, 82, 6. (15) Davis, J. H.; Jeffery, K. R.; Bloom, M.; Valic, M. I.; Higgs, T. P. Chem. Phys. Lett. 1976, 42, 390. (16) Henrichs, P. M.; Hewitt, J. M.; Linder, M. J. Magn. Reson. 1984, 60, 280. (17) Farrar, T. C.; Becker, E. D. Pulse and Fourier Transform NMR. Introduction to Theory and Methods; Academic Press: New York and London, 1971. (18) High-Resolution NMR Techniques in Organic Chemistry. In Tetrahedron Organic Chemistry Series; Claridge, T. D. W., Baldwin, J. E., Williams, F. R. M., Eds.; Pergamon: Amsterdam, 1999; Vol. 19, p 31. (19) Abragam, A. The Principles of Nuclear Magnetism; Oxford University Press: Oxford, 1961. (20) Spiess, H. W. Rotation of Molecules and Nuclear Spin Relaxation. In NMR Basic Principles and Progress; Diehl, P., Fluck, E., Kosfeld, R., Eds.; Springer-Verlag: New York, 1978; Vol. 15; p 55. (21) Wittebort, R. J.; Olejniczak, E. T.; Griffin, R. G. J. Chem. Phys. 1987, 86, 5411. (22) Macho, V.; Brombacher, L.; Spiess, H. W. Appl. Magn. Reson. 2001, 20, 405. (23) Beck, B.; Villanueva-Garibay, J. A.; Muller, K.; Roduner, E. Chem. Mater. 2003, 15, 1739. (24) Wallach, D. J. J. Chem. Phys. 1967, 47, 5258. (25) Spiess, H. W.; Sillescu, H. J. Magn. Reson. 1981, 42, 381. (26) Wittebort, R. J.; Szabo, A. J. Chem. Phys. 1978, 69, 1722. (27) Dash, J. G.; Rempel, A. W.; Wettlaufer, J. S. ReV. Mod. Phys. 2006, 78, 695. (28) Holcomb, D. F.; Pedersen, B. J. Chem. Phys. 1962, 36, 3270. (29) Silvidi, A. A. J. Chem. Phys. 1966, 45, 3892. (30) Morrison, R. T.; Boyd, R. N. Organic Chemistry; Allyn & Bacon, Inc.: Boston, 1970. (31) Teixeira, M.; Bellissent-Funel, M.-C.; Chen, S. H.; Dianoux, A. J. Phys. ReV. A 1985, 31, 1913. (32) Buchhauser, J.; Grob, T.; Karger, N.; Ludemann, H.-D. J. Chem. Phys. 1999, 110, 3037. (33) Schmider, J.; Muller, K. J. Phys. Chem. A 1998, 102, 1181. (34) Lim, A. R.; Jeong, S. Y. J. Phys.: Condens. Matter 2006, 18, 6759. (35) Tang, X. P.; Wang, J. C.; Cary, L. W.; Kleinhammes, A.; Wu, Y. J. Am. Chem. Soc. 2005, 127, 9255.

JP801223C