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Room-Temperature Icelike Water in Kanemite Detected by 2H NMR T1 Relaxation Alan J. Benesi,*,† Michael W. Grutzeck,‡ Bernie O’Hare,† and John W. Phair§ Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802, and Office of Infrastructure Research and Development, Federal Highway Administration, McLean, Virginia 22101 Received July 7, 2004. In Final Form: November 17, 2004 2H NMR was used to study the nature of deuterated water in kanemite. Evidence is presented that shows that the water changes state from liquid to solid at room temperature during the hydration reaction that forms kanemite. The deuterium nuclei in the water experience rapid tetrahedral jumps in a hydrogenbonded lattice like those observed in 2H2O ice.
Results from molecular dynamics simulations, scanning polarization force microscopy (SPFM), and sum frequency generation spectroscopy have shown the formation of room-temperature “icelike bilayers” on the surface of muscovite mica,1 a hydrophilic aluminosilicate that can be used to “seed” clouds. Room-temperature solid-state water is also commonly found in crystalline hydrates.2 At temperatures slightly above the freezing point of pure water, solid-state water is found as well in clathrate hydrates.3 In most hydrated solids at room temperature, however, the water is thought to be in the liquid state except for the water immediately adjacent to the solid surface, the nature of which has been heretofore unknown. 2 H NMR is particularly advantageous for the study of the motion of the hydrogen nucleus because the 2H quadrupolar interaction is so large that other interactions can usually be neglected. The “rigid” solid-state 2H powder spectrum for 2H2O ice is characterized by a quadrupole coupling constant (qcc ) e2qQ/h) of 213-216 kHz and asymmetry parameter η ∼ 0.1.2,4 For ice Ih at temperatures near the freezing point, motions occur at rates comparable to the qcc and directly affect the 2H NMR powder spectrum and 2H NMR T1 and T2 relaxation times.5,6 Analysis of 2 H2O ice Ih powder spectra at temperatures near the freezing point show that the hydrogen nuclei experience tetrahedral jumps around the oxygen atoms in the crystal lattice.4 At lower temperatures, slower tetrahedral jumps † Department of Chemistry, The Pennsylvania State University. ‡ Materials Research Laboratory, The Pennsylvania State University. § Office of Infrastructure Research and Development, Federal Highway Administration.
(1) (a) Hu, J.; Xiao, X.-d.; Ogletree, D. F.; Salmeron, M. Surf. Sci. 1995, 344, 221-236. (b) Hu, J.; Xiao, X.-d.; Ogletree, D. F.; Salmeron, M. Science 1995, 268, 267-269. (c) Odelius, M.; Bernasconi, M.; Parrinello, M. Phys. Rev. Lett. 1997, 78, 2855-2858. (d) Salmeron, M.; Bluhm, H. Surf. Rev. Lett. 1999, 6, 1275-1281. (2) (a) Weiss, A.; Weiden, N. In Advances in Nuclear Quadrupole Resonance; Smith, J. A. S., Ed.; Heyden: London, 1980; Vol. 4, pp 149248. (b) Reeves, L. W. In Progress in NMR Spectroscopy; Emsley, J. W., Feeney, J., Sutcliffe, L. H., Eds.; Pergamon: Oxford, 1969; Vol. 4, pp 193-234. (3) (a) Bach-Verges, M.; Kitchin, S. J.; Harris, K. D. M.; Zugic, M.; Koh, C. A. J. Phys. Chem. B 2001, 105, 2699-2706. (b) Kirschgen, T. M.; Zeidler, M. D.; Geil, B.; Fujara, F. Phys. Chem. Chem. Phys. 2003, 5, 5247-5252. (4) Wittebort, R. J.; Usha, M. G.; Ruben, D. J.; Wemmer, D. E.; Pines, A. J. Am. Chem. Soc. 1988, 110, 5668-5671. (5) Wittebort, R. J.; Olejniczak, E. T.; Griffin, R. G. J. Chem. Phys. 1987, 86, 5411-5420. (6) Torchia, D. A.; Szabo, A. J. Magn. Reson. 1982, 49, 107-121.
have been observed using the 2H spin alignment technique.7 The tetrahedral jumps of an H nucleus around a given oxygen atom leave the water molecule and its covalent bonds intact and are thought to be caused by diffusion of Bjerrum defects in the ice lattice.4,7 The translational jump of a H nucleus from one oxygen atom to another requires covalent bond breakage and is thought to be caused by diffusing ionic defects (e.g., OH- or H3O+).3b,7 When both types of defect are present, H nuclei can jump between adjacent tetrahedral sites throughout the icelike lattice.3b In this study, we used 2H NMR to elucidate the dynamics of water occurring in restricted geometries in kanemite (NaHSi2O5‚3H2O), a simple phyllosilicate (Figure 1). Kanemite consists of alternating single layer sheets of [Si2O4OH]nn- and hydrated sodium ions.8 The silicon in the silicate layers is known from solid-state 29Si NMR to be exclusively Q3. The fourth bond for each silicon atom is to -OH or to -O-. Interlayer Na+ ions in kanemite are each coordinated by up to six water molecules but may also coordinate to the silicate layers. The hydrated Na+ ions are fixed in the crystal lattice as shown by both X-ray diffraction and solid-state 23Na NMR. Reasonable positions for many of the water molecules in kanemite could be determined from residual electron densities in the X-ray structure (Figure 1), but additional water is known to be located within the puckered region of the silicate sheets.8 Kanemite was synthesized by mixing a stoichiometric preblended powder consisting of δ-Na2Si2O5 and silica gel with a slight excess of deuterated water to allow for inevitable evaporation during mixing. The hydrated powders were then stored in sealed vials at 21 ( 1 °C until used in NMR experiments. 2H NMR experiments were carried out at five magnetic fields: 6.98 T (45.65 MHz), 7.04 T (46.03 MHz), 9.39 T (61.42 MHz), 11.74 T (76.77 MHz), and 14.09 T (92.13 MHz) at 21 ( 1 °C. The quadrupole echo pulse sequence was used to obtain 2H spectra at 45.65 MHz. T1 values of the central aqueous peak were determined with the inversion recovery pulse sequence at all frequencies. Theoretical 2H NMR powder line shapes5 and relaxation times6,10 were calculated with Mathematica programs developed by the authors (see the (7) Fujara, F.; Wefing, S.; Kuhs, W. F. J. Chem. Phys. 1988, 88, 68016809. (8) (a) Garvie, L. A. J.; Devouard, B.; Groy, T. L.; Ca´mara, F.; Buseck, P. R. Am. Mineral. 1999, 84, 1170-1175. (b) Almond, G. G.; Harris, R. K.; Franklin, K. R. J. Mater. Chem. 1997, 7, 681-687. (c) Hayashi, S. J. Mater. Chem. 1997, 7, 1043-1048.
10.1021/la048302s CCC: $30.25 © 2005 American Chemical Society Published on Web 12/14/2004
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Figure 1. A cross section of the crystal structure of kanemite (refs 8 and 9): O, red; H, white; Si, gray; and Na, purple. Water molecules are octahedrally coordinated to sodium ions, as shown by the broken blue lines. The structure does not show missing water molecules that are expected in the puckered regions of the silicate sheets.
Figure 2. 2H quadrupole echo spectra of kanemite prepared with 2H2O: (a) at room temperature 10 days after mixing reactants; (b) vertical expansion of spectrum a, with a simulation (coarse dashed line) of silanol (Si-O-H) groups experiencing rapid rotational diffusion or rapid 120° jumps about the Si-O bond axis for an Si-O-H bond angle of 143°, qcc ) 240 kHz, η ) 0; (c) at -120 °C, with a simulation (fine dashed line) for rigid 2H with qcc ) 205 kHz and η ) 0, a simulation (coarse dashed line) of silanol motion as in spectrum b, and a third simulation (fine solid line) of fast C2 symmetry jumps with an HOH bond angle of 130° (ref 2).
Supporting Information). In addition, calorimetric data were obtained for kanemite on a TA Instruments 2920 Modulated DSC using Thermal Solutions software to monitor isothermal heat evolution at 25 °C. The room-temperature (21 ( 1 °C) 2H quadrupole echo spectrum obtained at 45.65 MHz for kanemite prepared with 2H2O is shown in Figure 2a,b. A large, relatively sharp isotropic peak as well as a much weaker, axially symmetric 2H powder pattern are present in the spectrum. The sharp peak is assigned to
Letters
Figure 3. Expanded view of the plot of the theoretical dependence of 2H T1 on the tetrahedral jump rate, ν, and on the inverse of the rotational correlation time, 1/τc, assuming qcc ) 193 kHz, η ) 0. T1 is plotted against ν or 1/τc. The solid and fine-dashed curves show the values calculated for tetrahedral jumps and isotropic rotational diffusion, respectively, at the different magnetic fields (6.98, 7.04, 9.39, 11.74, and 14.09 T corresponding to 2H frequencies of 45.65, 46.03, 61.42, 76.77, and 92.13 MHz). The positions of the curves vary monotonically with the magnetic field, with the lowest curve for either model corresponding to the lowest field and the highest curve to the highest field. The curve shown for the theoretical data calculated at 45.65 and 46.03 MHz used a compromise 2H frequency of 45.84 MHz. The stars show the experimental values obtained from the NMR data. The tetrahedral jump model easily reproduces the experimental T1 values for a jump rate of about 2.25 × 108 s-1. The isotropic rotational diffusion model cannot match the experimental T1 values at all magnetic fields for any value of 1/τc.
the interlayer water and is the focus of the work reported here. The weak axially symmetric powder pattern is assigned to silanol -OH groups experiencing either rapid three-site jumps or rapid rotational diffusion of the O-H bond about the Si-O torsion angle. Figure 2c shows the 2H quadrupole echo spectrum for kanemite at -120 °C. The sharp central aqueous peak has been replaced by a superposition of different 2H NMR powder patterns as would be expected for the different environments of hydrogen in crystalline kanemite.8 The outer horn maxima correspond to qcc ) 193 kHz and η ∼ 0, although qcc values between 180 and 216 kHz and η values less than 0.2 contribute to the spectrum. This falls in the expected range of qcc and η values for hydrates.2 The central feature between the outer horns may contain contributions from rapid low-temperature three-site jumps or rotational diffusion of the OH bond in silanol groups about the Si-O bond (Figure 2c, coarse dashed line) but with wider variability of the Si-O-H bond angles than observed at room temperature. It may also contain contributions from rapid C2 symmetry jumps of H in the hydrate water molecules2 of Na(H2O)n+ (n e 6, Figure 2c, thin solid line) but with wider variability of the H-O-H bonds as would be expected for kanemite.8 2H NMR T relaxation times were determined at 21 °C 1 for the single sharp water peak of the room-temperature kanemite spectrum at five magnetic fields. The T1 values stabilized after several days at 4.2 ( 0.6 ms (45.65 MHz), 4.3 ( 0.6 ms (46.03 MHz), 5.7 ( 0.6 ms (61.42 MHz), 6.2 ( 0.6 ms (76.77 MHz), and 7.4 ( 0.6 ms (92.13 MHz). The uncertainties represent the full range of experimentally determined T1 values at all magnetic fields. Inversion recovery T1 data sets for kanemite were well matched (9) Bruno, I. J.; Cole, J. C.; Edgington, P. R.; Kessler, M. K.; Macrae, C. F.; McCabe, P.; Pearson, J.; Taylor, R. Acta Crystallogr. 2002, B58, 389-397. (10) Sudmeier, J. L.; Anderson, S. E.; Frye, J. S. Concepts Magn. Reson. 1990, 2, 197-212.
Letters
with single exponentials. For comparison, bulk 2H2O at 21 °C has an experimental inversion recovery T1 value of 0.41 ( 0.01 s at 45.65 MHz and 0.38 ( 0.01 s at 76.77 MHz. The experimental T1 values at both magnetic fields for bulk 2H2O correspond closely to values calculated with the isotropic rotational diffusion model10 if qcc ) 216 kHz, η ) 0.1, and the rotational correlation time τc is 3.5 ( 0.3 × 10-12 s. The theoretical T1 dependence of kanemite was calculated using two different models: an isotropic liquid-state rotational diffusion model10 and a solid-state tetrahedral jump model assuming an average qcc ) 193 kHz and η ) 0 (Figure 3).6 Comparison of the experimental T1 values with the calculated values in Figure 3 shows that the field dependence of the T1 values is consistent only with the solidstate tetrahedral jump model. It is possible to choose a correlation time for isotropic liquid-state rotational diffusion that matches the experimental T1 value at one magnetic field, but it is impossible to find a correlation time that matches the T1 values within the experimental error at all magnetic fields. Conversely, the simulation of the T1 values with the tetrahedral jump model is robust and works within experimental error for a range of average qcc values from 183 to 216 kHz, η ) 0-0.2, for jump rates ranging from 1.9 × 108 to 3.5 × 108 s-1 (see the theoretical T1 calculations in the Supporting Information). The average qcc of 193 kHz (with η ) 0) used to calculate the T1 plots in Figure 3 is reasonable because average qcc and η values are expected for a deuterium nucleus jumping rapidly from tetrahedral site to tetrahedral site, with each site having a slightly different qcc and η value. The extended icelike tetrahedral lattice for the jumps is provided by hydrogen bonding of water octahedrally coordinated to Na+ with the “missing” water in the puckered regions of the silicate sheets. Thus, even though only two of the tetrahedral positions for a H nucleus are available on a single water molecule coordinated to Na+, the extended tetrahedral lattice allows a H nucleus starting on a coordinated water molecule to sample a large number of different tetrahedral sites. Diffusion of ionic defects (e.g., OH- in basic kanemite) enables H nuclei to jump between adjacent oxygen atoms and in combination with diffusion of Bjerrum defects promotes tetrahedral
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jumps throughout the icelike lattice. The agreement between the tetrahedral jump model and the experimental results provides compelling evidence for solid-state water in kanemite. Isothermal calorimetry for the kanemite reaction showed that heat evolution lasted for several hundred minutes after addition of 1H2O to the dry reactants. The average of two runs yielded ∆H ) -1585 J/g (reactants) or -4753 J/g (H2O). By comparison, ∆Hfusion ) -333.6 J/g for 1H2O and -317.1 J/g for 2H2O.11 Although the heat evolved during kanemite formation includes contributions from several processes such as adsorption, solvation of sodium ions, Si-O bond breaking (for SiO2), and Si-O bond making (for δ-Na2Si2O5), more than enough heat is evolved to include a contribution from the proposed change in state of the interlayer water from liquid to solid. In conclusion, theoretical analysis of 2H NMR T1 relaxation data shows that hydration water in kanemite is in the solid state at room temperature. Calorimetric data are consistent with this hypothesis. The H nuclei of the hydrogen-bonded water molecules experience rapid tetrahedral jumps similar to those observed in ice Ih. Recent work in our laboratories on other hydrated materials shows that the existence of room-temperature icelike water at interfaces is not limited to kanemite.12 The significance of room-temperature icelike water to the properties of various hydrated materials will be explored in future investigations. Acknowledgment. We thank the Pennsylvania State University for general support. Supporting Information Available: Crystallographic data for kanemite (CIF), NMR spectra for 2H2O ice, spectral simulations of silanol motion, C2 symmetry jumps (PDF), and programs used in spectral simulations and theoretical T1 calculations (Mathematica). This material is available free of charge via the Internet at http://pubs.acs.org. LA048302S (11) Weast, R. C. Handbook of Chemistry and Physics, 53rd ed.; CRC Press: Cleveland, 1972; p B-241. (12) Benesi, A. J.; Grutzeck, M. W.; O’Hare, B.; Phair, J. W. J. Phys. Chem. B 2004, 108, 17783-17790.
