ARTICLE pubs.acs.org/JPCC
Alkali Metal and H2O Dynamics at the Smectite/Water Interface Geoffrey M. Bowers,*,†,‡ Jared W. Singer,‡ David L. Bish,§ and R. James Kirkpatrick|| †
Division of Chemistry, College of Liberal Arts and Science, Alfred University, 1 Saxon Drive, Alfred, New York 14802, United States Department of Materials Engineering, Kazuo Inamori School of Engineering, Alfred University, 1 Saxon Drive, Alfred, New York 14802, United States § Department of Geological Sciences, Indiana University, 1001 East 10th Street Bloomington, Indiana 47405, United States College of Natural Science, Michigan State University, East Lansing, Michigan 48824, United States
)
‡
bS Supporting Information ABSTRACT: Molecular-scale dynamic processes involving ions and water at smectitewater interfaces play crucial roles in issues such as contaminant transport, reactivity of geochemical systems, and carbon sequestration, yet little is known about the specific manner in which interfacial ion and water dynamics influence one another, particularly at temperatures below 0 °C. In this work, we discuss the effects of the charge-balancing cation on the structure and dynamics of alkali metals and H2O at the mineralwater interfaces of alkali metalsmectite pastes over a broad range of temperatures. New variable-temperature 2 H and 23Na NMR spectroscopic data for a Nahectorite paste presented here in combination with the results from our previous studies of Cs and Khectorite pastes reveal a common anisotropic mechanism of 2H motion for 2H2O restricted by proximity to a surface or cation between 50 and 20 °C. This motion is well modeled by combined fast-limit C2 librations about the 2H2O molecular dipole moment and fast-limit octahedral-type jumps of 2H2O molecules about the C3 symmetry axis of a slightly compressed metalH2O complex. At higher temperatures, 2H2O dynamics are dominated by diffusion and/or chemical exchange of deuterons and differ for Na- and K-exchanged samples. Comparing our collective 39K, 133Cs, and 23Na VT NMR results shows that Na+ has less affinity for the smectite surface in pastes than K+ or Cs+, that the influence of 2H2O and 2H2O content on alkali metal motion decreases from Na+ to K+ to Cs+, and that slow-to-intermediate (rate < 104105 Hz) two-site exchange is a significant dynamic process above 80 °C only for Cs+.
’ INTRODUCTION Chemical and biochemical processes that involve molecularscale events at mineralwater interfaces play central roles in many critical scientific and societal issues, including soil stabilization of carbon,13 deep geological CO2 storage,48 and long-term nuclear waste management.911 For example, sorption desorption reactions1217 and mobilization of chemical species due to weathering1821 of geological materials depend fundamentally on the behavior of water and dissolved species at the mineral surface. Thus, the molecular-scale structure, dynamics, and energetics of water and dissolved species interacting with mineral surfaces are central to addressing these issues and serve as the foundation for the multiscale understanding required to confront them. Many of these crucial interfacial processes (such as chemical exchange or diffusion through restricted environments) have correlation times on the order of tens of picoseconds to hundreds of milliseconds, and nuclear magnetic resonance (NMR) spectroscopy, neutron scattering, and computational molecular dynamics (MD) modeling are especially effective tools for investigating such behaviors.2227 In many geochemical environments, clay minerals such as smectites (the so-called swelling clays) are critical players in r 2011 American Chemical Society
chemical sequestration and transport. Thus, the behavior of H2O molecules restricted by their proximity to external surfaces or confined in interlayer galleries has been studied extensively for smectitewater systems.2860 The results show a growing consensus that water within 5 Å of an external or interlayer clay mineral surface exhibits structural order that differs from ice-1 h and bulk H2O, that there is observable dynamic behavior of this H2O well below 0 °C, and that the behavior of confined H2O and H2O restricted dynamically by proximity to a surface (termed proximity-restricted H2O in the remainder of this paper) is controlled by a combination of the smectite structure and composition, the overall water content of the system, and the hydration properties of the charge-balancing cations. However, the details of the molecular-scale H2O and cation dynamics in clays and their temperature dependence remains an issue of debate, particularly for saturated natural smectite pastes that may represent many natural conditions better than the more commonly studied dilute clay suspensions or low-water hydrates. Received: July 28, 2011 Revised: October 4, 2011 Published: October 07, 2011 23395
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The Journal of Physical Chemistry C Previously published results from neutron scattering, NMR spectroscopy, and MD calculations for natural and synthetic smectites at a variety of H2O contents have shown that roomtemperature H2O diffusion coefficients are a factor of 45 less than in bulk water,29,32,37,5462 including some data for paste samples. Early NMR studies also invoked an isotropic rotational diffusion model to describe the room-temperature motion of proximity-restricted H2O in pastes and suspensions,33,48 but more recent neutron, MD, and NMR studies have suggested that anisotropic motion occurs in both paste and low-water samples at a variety of temperatures.31,39,41,42,63 The most common model of this anisotropic motion is fast C2 reorientation (rotation/hopping) of the H2O molecules around their molecular dipole moment, often combined with some type of restricted molecular reorientation of the hydration complex. This latter reorientation is often modeled as occurring around a vector perpendicular to the smectite surface, typically coincident with the C3 symmetry axis of a partially or fully hydrated interlayer cation,31,41,64,65 though a specific mechanism of this reorientation has not been agreed upon. NMR studies have also provided evidence that proton exchange between H2O populations plays an important role in the room-temperature dynamics of smectite H2O interfaces and that the role of this exchange becomes less significant as the total water content decreases.36,41,42,48,66,67 Fewer experimental studies have examined the nanoseond to second molecular-scale dynamics of dissolved cations at smectite H2O interfaces using NMR,15,39,6878 and there has been little experimental effort to understand the relationships between cation and H2O dynamics, particularly how they influence one another over a broad range of temperatures.35,39 MD modeling of room-temperature cation diffusion in one- to three-layer smectite hydrates has shown that coefficients for continuous diffusion of Na+ are 2 orders of magnitude slower than in bulk solution and that more restrictive jump diffusion of Cs+ is up to 7 orders of magnitude slower than in bulk solution.60,79,80 NMR relaxation rates yield Na+ and Li+ diffusion rates for dilute and dense smectite suspensions35,68,71,77,8183 that are in general agreement with the MD results. We know of no modeling studies focusing on low-temperature cation dynamics, but a few variabletemperature NMR studies have provided insight at temperatures where cation dynamics are dominated by mechanisms such as direct site exchange or dynamic averaging by motion of the coordinating H2O. Laboriau et al.75 report chemical exchange between Na+ sites in smectites and suggest that Na+ site exchange at paste levels of hydration occurs between sites that are different than those observed in “dry” or dilute suspension samples. Weiss et al.72,73 observed two-site exchange of Cs+ in pastes and low-water samples of natural hectorite using variabletemperature 133Cs NMR and show that site exchange in this system becomes observable at 70 °C and approaches the fast exchange limit continuously over a ∼90 °C temperature range. To the best of our knowledge, the only experimental study focused on relating the nanoseond to millisecond H2O and ion motion in smectite pastes as a function of temperature is our previous 39K and 2H variable-temperature NMR study of Khectorite.39 These data do not show clear evidence of multisite cation exchange but do show a connection between the onset of K+ dynamic averaging and a shift to rapid diffusion of proximity-restricted H2O. In this work, we use variable-temperature 23Na and 2H NMR spectroscopy to extend and refine ideas from our previous work39,72,73 into novel insight regarding changes in H2O and
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cation dynamics of alkali metalsmectite pastes over a broad range of temperatures. The experiments use the same hectorite fraction as previous studies, and any differences between the cation or 2H2O dynamics is related to the properties of the charge-balancing alkali metal. Hectorite is a low-Fe smectite that develops a rather large negative layer charge (ca. 0.8/formula unit) by Li+ substitution for Mg2+ in the octahedral sheet, making it ideal for NMR investigations of smectitewater interfacial behavior.73 By analyzing data for the entire suite of common charge-balancing alkali metals, we find that the dynamics of ions and water depend on one another and the total system H2O content to varying degrees depending on the system temperature and propose a model for the common cation-independent mechanism of 2H2O motion we observe for alkali metalhectorite pastes between 50 and 20 °C.
’ EXPERIMENTAL SECTION Materials. The hectorite sample was obtained by isolating the clay fraction from the standard San Bernardino hectorite (SHCa-1) available from the Clay Mineral Society’s Source Clays Repository, and the chemical composition of this fraction before ion exchange was reported previously.73 This hectorite develops permanent charge almost exclusively from Li+ for Mg2+ substitution in the octahedral sheet, has very low levels of iron, and has 60% of the structural OH sites substituted by F. The chargebalancing cations of the as-received smectite are predominantly Na+ with lesser amounts of Mg2+, K+, and Sr2+. We performed a multistep Na+ ion-exchange procedure (detailed in the Supporting Information) to ensure that the hectorite is saturated with respect to Na+. Some of the fully exchanged Nahectorite was used to prepare a dry sample, a low-water hydrate sample, and 1.5:1 by mass 2H2O:smectite paste for NMR analysis. The dry sample was made by freeze drying the Na-exchanged sample and transferring it immediately to a desiccator over P2O5 to maintain a vaporphase water activity equivalent to approximately 0% relative humidity (RH). The low-water hydrate sample was made by storing a portion of the freeze-dried sample over K2CO3saturated 2H2O to maintain a vapor-phase RH between 40% and 43%. Controlled RH XRD experiments show that this sample is a mixture of 1-layer and 2-layer hydrates. Thermogravimetric analysis (TGA) of these two samples after several days of equilibration shows that the dry hectorite contained 0.6 H2O per Na+ and low-water hectorite 8 H2O per Na+. Pastes were prepared from the P2O5-equilibrated sample immediately before beginning the NMR experiments. To prevent changes in moisture content, the pastes and low-water samples were sealed in rotors with custom rubber gaskets or in cut NMR tubes with water-tight epoxy. Details of the controlled relative humidity XRD experiments and TGA results are described in the Supporting Information. 23 Na Variable-Temperature NMR. Conventional Bloch-decay 23Na magic angle spinning (MAS) NMR spectra were collected at temperatures of 80, 50, 35, 20, 10, 0, 10, 25, 40, and 55 °C for the paste and low-water Nahectorite samples using a 5 mm HX Varian T3 probe and a 9.4 T Varian Infinity-Plus spectrometer in the Max T. Rodgers NMR facility at Michigan State University. The 23Na MAS NMR spectrum of the dry sample was also obtained with this instrument at 25 °C. All 23Na NMR results are referenced with respect to a 1 M aqueous solution of NaCl. Temperature control was achieved by passing 23396
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The Journal of Physical Chemistry C liquid-nitrogen-cooled N2(g) into a standard Varian VT stack and allowing 30 min for the temperature to equilibrate after each temperature change. Actual sample temperature and a suitable equilibration period were calibrated against the VT thermocouple using Pb(NO3)2 according to the procedure of Bielecki and Burum84 and varied within (0.1 °C of the set point at each temperature. Weighing the sealed rotors before and after the NMR experiments showed minimal water loss. Bloch-decay 23Na MAS NMR experiments were also performed for the dry and low-water Nahectorites at 20 and 0 °C using a 3.2 mm BL3.2 MAS probe on the 21.1 T Bruker Avance-900 spectrometer at Michigan State University. Details of the 23Na NMR experiments are described in the Supporting Information. Data from the two fields were used to quantify the isotropic 23 Na chemical shifts and quadrupolar products of the sites in the dry hectorite using the method of Mueller et al.85 To extract the position data required to quantify these parameters, the NMR spectra were fit with a stretched Lorentzian function 0 1 ðω ω0 Þ 2 C B f whh þ Q C B IðωÞ ¼ a 3 B C @ f whh2 þ ðω ω0 Þ A where a is an amplitude scaling factor, fwhh is the full width at half height of the Lorentzian line, ω0 is the observed chemical shift, and Q is the skewness parameter that describes the extent of tailing toward lower resonance frequency. This function is a slightly modified version of the equation proposed by Prawer et al.86 for fitting Raman spectral lines, and though the skew parameter in this function lacks the direct relationship with the distribution of quadrupolar parameters like the σ parameter of the related Czjzek model for MAS NMR of 1/2 integer quadrupolar nuclei,8789 it produces excellent fits to the spectral data without having to guess at the distribution of chemical shift parameters. The skewness in this function is related to the distribution of local bonding environments but likely is some combination of the distribution in quadrupolar interactions and distribution in shielding. Iterative fitting of the data with this function was performed using the freeware program Abscissa for MacOSX written by R€udiger Br€uhl. Abscissa was also used to fit the 23Na VT data for the paste with either pure or stretched Lorentzian functions as needed. The peak areas for the paste samples were determined using the integration routine in Acorn’s NUTS software. 2 H Variable-Temperature NMR. 2H NMR experiments were performed for the paste and low-water Nahectorite samples at temperatures of 80, 50, 30, 20, 0, and 25 °C under static conditions using a Chemagnetics dedicated deuterium wide-line probe and a 7.0 T Tecmag HF-3 Apollo spectrometer at the Pennsylvania State University NMR facility. The samples were packed in cut 5 mm NMR tubes and sealed immediately with H2O-tight epoxy. Sample mass was recorded before and after the NMR experiments and showed no loss or gain of water. The 2H resonance frequencies in this paper are referenced to pure 2H2O at room temperature. 2H spectral simulations were performed using functions in Mathematica originally developed by Benesi et al.90 and modified to reflect the rates and geometries of motion modeled in this paper. These functions are based on the formalism of Vega and Luz.91 No line broadening or other attempts to model disorder were used. Details of the 2H NMR experiments are described in the Supporting Information.
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Figure 1. High-field 23Na MAS NMR spectra of the dry Nahectorite powder compared with the room-temperature spectrum of the powder from 9.4 T. Spectra have been scaled to roughly the same maximum intensity. Note the presence of two well-resolved dominant resonances in the 21.1 T results that tail toward low frequency and do not change between 20 and 0 °C. The small peak at high resonance frequency arises from a minor contaminant phase that cannot be observed at lower field.
’ RESULTS 23
23
Na spectra of the dry hectorite powder, which contains 0.6 H2O/ Na+, are dominated by resonances representing two disordered Na+ environments that are well resolved at 21.1 T but overlap at 9.4 T (Figure 1). At 21.1 T these two dominant resonances are centered near 3 and 18 ppm and are essentially featureless within the signal-to-noise limits except for tails toward lower frequencies, the classic line shape of a quadrupolar nucleus confined in a disordered phyllosilicate interlayer.89,92 The positions of these two resonances shift to higher frequency with increasing H0 field, indicating that nuclei on both sites experience a quadrupolar interaction, with the higher frequency site exhibiting a slightly larger weighted-average quadrupolar product (Pq = 3.04 ( 0.09 MHz vs Pq = 2.66 ( 0.06 MHz; Table 1). At both 9.4 and 21.1 T the two dominant resonances are well fit with stretched Lorentzian line shapes with the same skewness parameter for each component (Q = 29 ( 2 at 9.4 T vs Q = 66 ( 34 at 21.1 T), consistent with relatively similar distributions of binding environments for each site in the sample. The 21.1 T spectrum also contains a small peak near 5.6 ppm that is probably due to NaCl crystallizing in the sample following the initial experiments at 9.4 T. Our results suggest that the dominant resonances correspond to two inner-sphere Na+ sites, one of which is anhydrous and the other partially hydrated. The controlled humidity XRD data (Figure 2) shows that the clay exists predominantly as a 9.7 Å fully collapsed phase with a tail in the (001) basal peak toward a one-layer hydrate at very low relative humidities relevant to our dry Nahectorite sample. The lack of available space in the interlayer combined with the water content of 0.6 H2O/Na+ 23397
Na VT NMR of Dry NaHectorite Powder. The
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Table 1. Shifts from Two Fields and Calculated Isotropic Chemical Shifts and Quadrupolar Products for the Two Dominant Na+ Sites in Dry NaHectorite (0.6 H2O/Na+) δ9.4T ((0.2 ppm)
temperature (9.4 T/21.1 T)
δ21.1T ((0.1 ppm)
δiso (ppm)
Pq (MHz)
25/0 °C
site 1
20.0
3.2
0.90 ( 2.6
3.05 ( 0.09
25/20 °C
site 2 site 1
30.9 20.0
17.9 3.2
14.7 ( 0.6 0.90 ( 2.6
2.68 ( 0.03 3.05 ( 0.09
site 2
30.9
18.4
15.3 ( 0.6
2.63 ( 0.04
Figure 2. Controlled RH XRD patterns for Nahectorite. (Top) Similar pattern for the increasing and decreasing relative humidities with minor hysteresis and a basal spacing of ∼9.7 ( 0.2 Å for the 0% RH samples. (Bottom) 40% RH XRD pattern for increasing RH (purple) and decreasing RH (black). Both have the basal spacing of their main peaks labeled. The intensity/peak breadth in the box represents a fraction of the smectite layers with a ∼15 Å basal spacing, the so-called two-layer hydrate.
shows that all Na+ in the system must have a direct coordination to the mineral surface, in agreement with the previously reported room-temperature 23Na NMR observations of this hectorite by Aranda and Ruiz-Hitzky93 and comparable observations for a montmorillonite by Reinholdt et al.94 Since hectorite has a very low tetrahedral charge, it is unlikely that either of the two sites occurs near tetrahedral substitution sites. They are much more likely to be either anhydrous inner-sphere sites, such as two unique Na+ environments in ditrigonal cavities, or a combination of anhydrous and partially hydrated inner-sphere sites. 23Na resonances centered at 18 and 33 ppm at Ho = 11.7 T for montmorillonite, which contains no structural F, were previously assigned to partially hydrated inner-sphere Na+ and anhydrous inner-sphere Na+, respectively.94 These positions compare well with our observations at 9.4 T (Table 1) and those of Aranda and Ruiz-Hitzky93 after accounting for the changes in resonance frequency at different fields due to the isotropic second-order quadrupolar shift and the large uncertainties in the position, width, and intensity of the low-frequency peak in the Reinholdt et al. paper. Awarding two or three H2O to each partially hydrated Na+ yields partially hydrated Na+ fractions of 33% and 20%, respectively, based on the TGA results. The 2 H2O/Na+ value compares favorably with the relative intensity of
Figure 3. Variable-temperature 23Na MAS NMR results from 9.4 T for the Nahectorite paste compared with spectra for the dry sample and low-water sample at 25 °C. Spectra have been scaled to roughly the same maximum intensity to better focus on changes in line width and position with respect to temperature.
the 20 ppm site from our two-site line fits (∼40% Na+). We therefore assign the 20 ppm resonance at 9.4 T to partially hydrated inner-sphere Na+ and the resonance near 30 ppm at 9.4 T to anhydrous inner-sphere Na+. With this assignment, the partially hydrated site has the larger weighted-average quadrupolar product, suggesting either that it is more asymmetric than the anhydrous site or that the difference in the ability of O on the basal surface and in an H2O molecule to draw electrons generates a larger electric field gradient for the partially hydrated site. The absence of any differences in peak position, line width, or relative intensity of these sites at 0 and 20 °C at 21.1 T (Figure 1) indicates that there is no change in the type and/or rates of any dynamic processes affecting the line shape at these temperatures. 23398
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Table 2. Fit Results for Resonances of NaHectorite Paste at 9.4 T as a Function of Temperature site 1
site 2
δ ((0.23 ppm)
fwhh (Hz)
55
1.25
34 ( 1
40
0.95
35 ( 1
25
0.69
37 ( 1
10
0.42
39 ( 1
0
0.25
40 ( 1
10
0.08
42 ( 1
T (°C)
site 3
δ (ppm)
fwhh (Hz)
δ (ppm)
sites 2 and 3 combined fwhh (Hz)
area (107)
20
0.07
44 ( 1
25.0 ( 5
1750 ( 394
1.24 ( 0.44
35 50
0.90 1.93
155 ( 2 262 ( 3
30.0 ( 5 29.8 ( 5
962 ( 95 1107 ( 147
1.07 ( 0.10 1.50 ( 0.13
80
3.40
675 ( 39
25.0 ( 0.6
741 ( 150
23
Na VT NMR of Hectorite Paste. The
33.4 ( 1.6
763 ( 250
8.76 ( 0.68
Q
10.6 ( 5.3
23
Na VT NMR data for the 1:1.5 by mass hectorite: H2O paste is dominated by a symmetric peak whose breadth and position vary substantially with temperature, indicating that a majority of the Na+ sites in this sample are very different than in the dry sample and greatly affected by dynamical processes (Figure 3, Table 2). There is also a broad, less intense resonance centered near 25 ppm that is observable at 20 °C and below. The peak maximum of the dominant resonance occurs at significantly more positive chemical shifts and is much narrower than the 23Na peaks for the dry and low-water samples (Figures 3 and S2, Supporting Information) at all temperatures. The dominant resonance both narrows and shifts to higher resonance frequencies with increasing temperature up to 20 °C, where the resonance resembles that of Na+ in bulk solution. The peak position between 80 and 20 °C occurs between what is expected for outer-sphere Na+ sorbed on clay surfaces, which is in the 10 to 13 ppm range for other smectites,9496 and that of free Na+ in solution at 0 ppm. At 20 °C and above, the full width at half height and chemical shift of the dominant resonance continue to resemble those of Na+ in bulk aqueous solution, although the peak maximum shifts to slightly lower frequencies with increasing temperature up to 50 °C. This slight shift may be associated with an increase in the rate of 2H2O exchange between the Na+ hydration sphere and the free 2H2O population with increasing temperature. It has previously been reported that such exchange of H2O molecules initiates in smectite suspensions and gels near 20 °C,48 and this interpretation is consistent with our own 2H VT NMR results discussed below. It is difficult to characterize the rates of Na+ motion over this temperature range without knowing the line width and resonance frequency of rigidly surfacesorbed outer-sphere Na+ in the paste, which we cannot be sure is observed even at 80 °C. However, the NMR data does support assigning the dominant resonance to fully hydrated Na+ that experiences an increasing rate of diffusional exchange between surface-sorbed outer-sphere sites with increasing temperature, eventually transitioning into exchange between surface sites and the free 2H2O domain between platelets such that the residence time of Na+ on the surface is nearly zero at and above 20 °C. The broad, low resonance centered near 25 ppm that occurs below 20 °C (Figure 4) is similar to the resonances observed for the dry hectorite sample, suggesting the presence of a small population of inner-sphere Na+ in the paste. From the 9.4 T spectra alone, however, it is not clear whether this intensity 2
Figure 4. Variable-temperature 23Na MAS NMR results from 9.4 T for the Nahectorite paste scaled to highlight the characteristics of the lowfrequency resonance. Note how this resonance corresponds well to the dry hectorite powder rather than the low-water hydrate and how the signal-to-noise ratio degrades rapidly with increasing temperature, indicative of dynamic averaging. Perceptible intensity from this resonance persists to 35 °C, is perhaps barely noticeable at 20 °C, and is not visible at any higher temperature.
corresponds to a single site with a large quadrupolar interaction or two overlapping resonances. Satisfactory fitting of this spectral component at 80°C (where its S/N ratio is largest) requires use of two stretched Lorentzian functions with the 23399
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The Journal of Physical Chemistry C same skewness parameter, as for the dry smectite. There is no statistically significant difference between the relative abundances of the two components for the dry smectite and the paste at 80 °C, consistent with the interpretation that this signal represents a small fraction of inner-sphere Na+ in the paste. At all other temperatures up to 35 °C the reduced S/N ratio for this feature allows acceptable fitting with a single pure Lorentzian line shape and prevents a more detailed interpretation (Table 2). The lack of observable signal in the low-frequency region above 20 °C has two possible explanations: attenuation due to dynamic processes that do not occur in the dry or low-water samples or an inhomogeneous 2H2O distribution in the paste at the start of the experiment. Without at least one of these effects or a phase transition, the relative intensity of this signal would remain constant with increasing temperature, since the signal generating population remains the same and the number of transients is the same for each experiment. If the signal attenuation were related to inhomogeneities in the water distribution, we would expect the moisture content to become more homogeneous with time and the signal intensity to decrease with increasing time after mixing. Since the high-temperature spectra were acquired before the low-temperature spectra, our data have the opposite trend, indicating that the loss of signal is due to dynamics. The dynamic transition leading to this signal loss happens gradually up to 20 °C, a temperature that is similar to the “melting” transition described for inner-sphere K+ in our previous work with Khectorite.39 It is not clear whether this Na+ population becomes incorporated in the dominant spectral resonance or is just broadened to the point that it cannot be observed at and above 20 °C, although we note that the line widths do not change significantly between 80 and 35 °C. However, the difference in dynamic behavior with respect to the dry sample and low-water samples shows that total 2H2O content and temperature play crucial roles in activating the dynamic process(es) for the inner-sphere Na+ population as well. 2 H VT NMR of Paste and Low-Water NaHectorites. The variable-temperature 2H NMR spectra of the paste and low-water Nahectorites exhibit drastically different behavior with increasing temperature and show that the dynamics leading to changes in the 2H line shape of smectite2H2O systems depend on both 2 H2O content and temperature (Figure 5). The low-water sample yields a single, quadrupole-dominated resonance that is consistent in width (residual quadrupolar splitting = 19 ( 1 kHz) and resolution between 50 and 0 °C, with a slightly higher residual splitting observed at 80 °C (22 ( 1 kHz) and slightly lower splitting at 25 °C (17 ( 1 kHz). Because this sample is a mixture of predominantly 1 water layer hydrated hectorite with a minor 2 water layer component (Figure 2), we interpret this signal to be principally interlayer confined water, although proximity-restricted water on external surfaces may also contribute. In contrast, the paste sample yields signal for two 2H populations at low temperatures, both with quadrupole-dominated line shapes that change significantly with temperature. The broad 2H resonance with singularities near 70 and 80 kHz is due to bulk 2H2O between the smectite platelets in the ice-1 h crystal structure.39 The characteristics and dynamics of this resonance are discussed in our previous work and the references cited there.39 Briefly, the line shape and loss of intensity with increasing temperature are due to irreversible dephasing via increasingly rapid tetrahedral jump motion of the deuterons in ice-1 h due to the presence of Bjerrum defects.97 Once the ice
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Figure 5. Variable-emperature 2H NMR quad-echo experiments for the paste (left) and low-water (right) Nahectorite. The broad resonance at low temperatures in the paste sample corresponds to 2H2O with the ice1 h crystal structure in free-2H2O domains and decreases in intensity with increasing temperature as a result of irreversible dephasing due to tetrahedral jump motion until the ice melts. The narrow feature in both sets of spectra indicates anisotropic motion and corresponds to the proximity-restricted 2H2O population within 5 Å of a smectite platelet surface or cation, which includes all of the 2H2O in the low-water sample. The slight differences in line width for these narrow sites below 20 °C likely reflect some variation in the extent of Na+ hydration shell compression or extension from an ideal octahedral geometry. The narrowing of the proximity-restricted resonance in the paste sample that is not observed in the low-water sample indicates that a new type of dynamic process has activated in the paste that depends on the presence of excess 2H2O. Note that the spectra are scaled to be relatively uniform in their maximum intensity to focus on changes in the line shape with temperature.
melts, diffusive motion and/or proton exchange become the dominant mechanisms affecting the line shape and the spins contribute to the single site observed at higher temperatures. The narrow 2H resonance for the paste sample at low temperatures between 50 and 20 °C has a line shape similar to that for the low-water sample, but the resonance is narrower and has a more pronounced variation with temperature (residual quadrupolar splitting between 10 and 14 kHz). For wet samples such as ours, this type of resonance has previously been assigned to interlayer confined 2H2O,39 although we believe terming this population proximity-restricted 2H2O is more appropriate, since much of the Na+ probably interacts strongly only with the surfaces of fully dissociated smectite platelets under these conditions. The relatively small changes in the spectral width between 50 and 20 °C is evidence that the molecular-scale process(es) causing dynamic averaging in the paste is(are) essentially the same throughout this temperature range and occurs at frequencies an order of magnitude larger than the residual line width or greater 23400
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Figure 6. Zoom of the near room-temperature 2H VT NMR results for Nahectorite paste (left) and Khectorite paste (right). Note the presence of a single 2H population experiencing a uniform residual quadrupolar splitting in the Na+ smectite at 25 °C, while two resonances are observable in the K+ smectite. The sharp central feature is associated with a 2H population experiencing isotropic motion and is associated with diffusional motion of a free 2H2O population. The broader feature that increases in relative intensity with increasing temperature is associated with a fraction of 2H2O molecules that experience a residual quadrupolar splitting much like the 2H2O in the Nahectorite paste, indicating a residual proximity-restricted 2H2O fraction in Khectorite paste.
(i.e., >∼120 kHz). The lack of well-defined singularities for this resonance at 80 °C suggests that the one or more of the dynamic mechanisms responsible for the averaging observed between 50 and 20 °C slows significantly and/or deactivates between 50 and 80 °C. Above 20 °C, the paste sample yields only a single 2H resonance, and this resonance retains some residual quadrupolar splitting to at least 25 °C. Importantly, there is no evidence of a narrow component at 0 ppm that would be due to 2 H2O undergoing isotropic motion in bulk water (Figure 6). This latter observation is substantially different from our previous results for Khectorite,39 for which a bulk isotropic water resonance is observed at 10 °C and above (Figure 6). These differences are clear evidence that excess water plays a critical role in activating dynamic line narrowing for 2H2O at temperatures above 20 °C.
’ DISCUSSION Smectite Expansion in the Paste Samples. On the basis of the controlled humidity XRD data and 2H NMR observations, it is likely that the T-O-T sheets of our Nahectorite paste are fully delaminated, meaning that there are few interlayer regions in which Na+ and 2H2O are confined in two-dimensional nanoscale pore spaces. The extent of interlayer expansion and delamination is critical to the interpretation of our results, because it influences the number of sorption sites accessible at a given temperature, whether the 2H2O population is influenced by one or two basal surfaces (proximity-restricted or interlayer-confined, respectively) and thus the rate of 2H2O and alkali metal diffusion and/or site exchange. It is well known that Na+-exchanged smectites more readily delaminate than smectites bearing other charge-balancing cations.98100
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The controlled relative humidity XRD data for our Na-hectorite (Figure 2) show that at the highest RHs (up to 95%) this sample experiences expansion into the osmotic regime accompanied by a loss of reflections associated with stepwise expansion (the one and two water layer regimes). The greater amount of 2H2O in our paste makes it likely that the hectorite in the paste experiences osmotic swelling to full delamination. This interpretation is supported by the 25 °C 2H NMR spectrum of the paste, in which the residual quadrupolar splitting experienced by all 2H2O molecules shows that all 2H2O have equal access to the Na+ hydration shells and/or clay surfaces and that there is no detectable population of confined interlayer 2H2O or completely free 2H2O at this temperature. Since the pastes were prepared and mixed at room temperature, it is likely that the delaminated state is preserved in the low-temperature NMR experiments. 2 H and 23Na NMR data for the Nahectorite presented here are in many ways comparable to our previous 39K and 2H NMR results for Khectorite paste39 but suggest that the earlier physical interpretation of the results for that sample should be slightly modified. In the earlier paper we suggest that the interlayers of Khectorite paste remain at least partially collapsed, whereas complete delamination may occur in this system as well. As for the Nahectorite, the controlled RH XRD for Khectorite at room temperature shows a loss of stepwise expansion and full osmotic swelling at 95% RH. The 2H NMR results for Khectorite paste at low temperatures are also nearly identical to those of the fully delaminated Nahectorite paste. If the Khectorite paste is fully delaminated, our interpretations about the relationships between K+ and the 2H2O dynamics are still valid, but the narrow 2H resonance for Khectorite paste and its changes with temperature (Figure 6) are associated with a proximity-restricted 2H2O population on the surfaces of delaminated platelets rather than interlayer confined 2H2O. This interpretation also suggests that the most likely explanation for the melting-type dynamic transition of the inner-sphere K+ is an increase in the mobility of proximity-restricted 2H2O, leading to modulation of the 39K electric field gradient of fully hydrated surface sorbed outer-sphere K+ or partially hydrated inner-sphere K+. MD calculations and cryogenic electron microscopy experiments101,102 will help in determining the extent of delamination in both of these alkali metalhectorite pastes and are planned in the near future. Low-Temperature Structure and Dynamics of 2H2O in Hectorite Pastes. Between 50 and 20 °C the 2H spectral component of the proximity-restricted 2H2O for both the K and the Nahectorite pastes is well fit with a two-component reorientation model involving fast (.120 kHz) restricted librational motion of 2H2O about its C2 symmetry axis combined with fast (>∼120 kHz) octahedral jumps of each 2H2O molecule about the C3 symmetry axis of a slightly compressed or elongated alkali metal hydration sphere (Figures 79), consistent with several previous reports for smectites and vermiculites.31,41,64,65 This combined C2 and C3 motional model can be understood in the following way. For 2H2O, the principle component of the 2H electric field gradient (EFG) tensor is aligned along the OH bond,103 but in the fast limit, libration of 2H2O about the C2 axis causes the time-averaged principle component of the 2H electric field gradient to be aligned along the molecular dipole moment of 2 H2O, which is the bisector of the HOH angle. It is well known that in infrared and Raman spectra the H2O librational frequencies occur in the 500 cm1 region (15 GHz) at room 23401
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Figure 7. Schematic representation of the two-component fast-limit motion model for confined 2H motion between 50 and 20 °C with the fast C2-type librations (a) and fast octahedral jump motion about the C3 symmetry axis (b) separated onto separate lines. The images in the left column represent a half-hydrated alkali metal, the images in the right column a fully hydrated six-coordinate alkali metal, and the black vectors in the center column indicate the primary director of the specific type of motion with respect to the hydrated complex and the smectite surface. The different octahedral sites involved in the jump motion in b are labeled 13 and 1*3* with asterisks indicating octahedral sites in the second plane of 2H2O molecules in the octahedral hydrated complex. The possible jumps for one particular atom/molecule in each mechanism are indicated with red arrows.
temperature, a rate at least 3 orders of magnitude greater than the static 2H line width of rigid 2H2O in ice-1 h (∼230 kHz), suggesting that this motion can remain in the NMR fast limit well below room temperature when in the proximity of smectite surfaces or ions. We also note that while it is present at all temperatures, its influence on the 2H NMR line shape can be insignificant if other processes that lead to isotropic averaging, such as diffusion and proton exchange, are present or if this type of libration is significantly restricted by crystallization (as in ice-1 h). Librational motion by itself cannot adequately model the experimental spectra (Figure 8), and a second, additional anisotropic motion is needed. Because of the fast C2 librations, 2H2O molecules in a six-coordinate hydration shell around a cation have their time-averaged EFGs aligned with octahedral symmetry about the metal cation when undergoing fast C2 librations, and restricted jumps of the 2H2O molecules among the sites in the octahedron are feasible (Figures 6 9). The hydrated alkali metal ions spend most of their time near the smectite surface at low temperatures to effectively balance the layer charge, and it is likely that a Na(H2O)6+ complex near a surface will orient such that its time-averaged C3 symmetry axis is perpendicular to the smectite platelets (parallel to the surface normal). This is because such an arrangement allows the shortest distance between the cation and the surface, which should more effectively balance the negative platelet charge. In addition, this geometry theoretically allows for the densest packing of hydrated metal complexes in
smectite interlayers, and the overall shape of this low-temperature 2H resonance is quite similar for the delaminated paste and the one/two-layer hydrated smectites. For an ideal octahedron with this orientation, the angles between the NaO vectors and the jump motion-directing C3 axis are ∼55° and 125°. Calculations assuming combined fast-limit C2 2H librations that align the principle component of the 2H EFG along the metaloxygen bonds and fast-limit C3 octahedral 2H2O jumps with the ideal octahedral angles generates a narrow component at 0 ppm that differs greatly from the experimental spectra of our K and Nahectorites between 50 and 20 °C (Figure 9). However, the 2H residual quadrupolar splitting due to octahedral jump motion is exquisitely sensitive to the bond angles with respect to the director (Figure 9), in this case the surface normal/C3 axis of the hydrated metal ion. Assuming that the octahedron is slightly extended or compressed along the C3 axis such that the NaO bond vectors make angles of 51° and 129° or 59° and 121° with respect to the C3 axis, respectively, results in an excellent match between the calculated spectra and the low-temperature 2H data for the K and Nahectorite pastes. Compression of the hydration shell along the surface normal is consistent with the effects of electrostatic interactions between the ion and the surface(s), suggesting that the compressed octahedral model is more likely to be correct in the delaminated pastes. The hydration shells of metal ions in the interlayers of the low-water sample should be more compressed or stretched than those on the surface due to electrostatic attraction and hydrogen bonding to both surfaces. 23402
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Figure 8. Comparison of spectra for the proximity-restricted 2H2O population between 50 and 20 °C calculated using several feasible fast-motion limit models for restricted 2H motion and experimental values from 50 °C for low-water Nahectorite (a), Khectorite paste (b), and Nahectorite paste (c). Simulated spectrum (d) corresponds to octahedral jump motion about the C3 symmetry axis at a rate in excess of ∼120 kHz for a fully hydrated six-coordinate outer-sphere Na+ such that the octahedron is slightly compressed along the C3 axis (surface normal2H2O bisector angle of 59 ( 0.5°). The model in e is similar to d but corresponds to a half shell of three 2H2O molecules on a partially hydrated inner-sphere Na+ jumping between their respective positions in excess of ∼120 kHz. (f) Model used in our previous study of Khectorite, which includes three coordinating 2H2O molecules on a partially hydrated inner-sphere K+ jumping in excess of ∼120 kHz between any of the six positions in a plane such that each K2H2O bisector vector makes the same 59 ( 0.5° angle with respect to the surface normal. The spectrum from model g assumes two-site jump motion about a C2 director alone, which corresponds to fast librational motion of 2H2O without an accompanying C3 reorientation. The final model spectrum (h) corresponds to a fast-motion limit tetrahedral jump motion one would expect for ice-1 h adjacent to the smectite surface, indicating that proximity-restricted 2H2O is not ice like in this system between 50 and 20 °C. Note that models df give identical results.
This structural difference would lead to a larger residual quadrupolar splitting for the low-water sample than for the paste, as observed in our 2H data (Figure 5). Another possible explanation for the octahedral compression/extension might be exchange of coordinating 2H2O between neighboring charge-balancing cations or with surface sorbed 2H2O, which could conceivably manifest as a change in the time-averaged geometry of the octahedron. Molecular modeling at several temperatures and hydration states is necessary to evaluate the types and importance of fundamental processes that influence the time-averaged geometry of the hydrated metal complex. 2 H line-shape calculations involving simultaneous fast-limit C2 libration and C3 octahedral jumps yield the same calculated line shape whether the alkali metal ion has a half or full shell of coordinating 2H2O molecules (Figure 9). Thus, such models could be relevant for smectite interlayers with one water layer (e.g., inner-sphere Na+ with a partial hydration shell of H2O molecules), two or more water layers (e.g., outer-sphere Na+ with a full, six-member hydration shell), or surface-sorbed outersphere Na+ in delaminated systems. The model used to simulate
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Figure 9. Effect on the combined fast-motion limit C2/C3 jump reorientation model due to compression or extension of the hydration shell along the C3 symmetry axis compared with the experimental spectra for low-water Nahectorite (a) and Nahectorite paste at 50 °C (b). Calculated spectra (blue) are for octahedral-type jumps of 2 H2O involving C3Na2H2O bisector angles of 62°/118° (c), 60°/ 120° (d), and 57°/123° (e), which are associated with compression of the hydration shell, and 54°/126° (f), 51°/129° (g), 50°/130° (h), 47°/ 133° (i), and 45°/135°(j), which are associated with hydration shell extension. Note the symmetry in the line shapes about the ideal octahedral angles of 54.74°/125.26°, which indicates that either C3 compression or extension of the hydration shell can explain our experimental 2H NMR spectra. However, it seems unlikely that extension of the octahedron will occur adjacent to a charged smectite surface in a fully delaminated clay paste, where compression of the octahedron due to one-sided alkali metalclay surface electrostatic interactions is more probable.
the 2H spectra in our previous Khectorite paper39 is in fact a variation of the half-hydration shell C3 jump model involving C3 compression/extension that allows the 2H2O molecules to jump between six positions in one plane (rather than the three positions in an ideal half-shell octahedral jump model) such that the surface normal/2H2O bisector angle remains 51° or 59° for each site. Tetrahedral jump motion of ice-like 2H2O in the fast motion limit or at any other rate does not generate an acceptable fit of the narrow 2H resonance observed at low temperatures (Figure 8). The absence of a sharp central feature makes it clear that any type of isotropic motion is at most a small contributor to 2 H2O line shapes of the Na and Khectorite pastes at temperatures below 20 °C. The similarity of the 2H resonances for the proximity-restricted 2 H2O between 50 and 20 °C for the Nahectorite and Khectorite pastes39 suggests that water molecules in this environment in alkali metalsmectite pastes may undergo similar types of motion at low temperatures irrespective of the chargebalancing cation. One possible explanation for this is that the 2 H2O motion is governed predominantly by the structure of the 23403
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The Journal of Physical Chemistry C smectite itself rather than the cation. Studies of residual 2H quadrupolar splittings for clay suspensions suggest that the smectite structure is dominant in ordering of the surface 2H2O for phases with charge originating in the octahedral sheet, whereas ordering via cation hydration dominates in phases with tetrahedral charge development.32 This is consistent with the similarities observed in the octahedrally substituted hectorite pastes via NMR. However, published 2H spectra of low-water Nahectorite and Nasaponite (which develops its layer charge predominantly in the tetrahedral layer)33 show similar narrow resonances at moderate to low 2H2O content, consistent with cation control of the 2H structure and dynamics in the interlayer galleries when full delamination does not occur. Since the paste samples we use here represent an overall 2H2O content between dilute suspensions and stepwise hydrates, it seems that additional experiments involving pastes of smectites with tetrahedral charge sites are necessary to clearly resolve whether surface ordering of 2H2O in pastes is clay structure or cation dominated and at what level of hydration this dominance may change. Many authors have reported H2O dynamics in smectite water systems at room temperature that depend critically on the water content,33,41,104,105 and the differences in quadrupolar splitting between the low-temperature 2H NMR spectra of the K and Nahectorite pastes and the low-water Nahectorite presented here suggest that 2H2O content influences the lowtemperature dynamics of proximity-restricted 2H2O as well. However, the differences in residual quadrupole splitting between the paste and the low-water samples are relatively small, indicating that diffusive exchange between 2H2O populations and deuteron exchange between 2H2O molecules are minor players in the observable 2H2O dynamics below 20 °C. It is unlikely that deuteron exchange between 2H2O molecules and the structural OH sites located below the ditrigonal cavities of the hectorite has an influence on the 2H line shape between 50 and 20 °C. 2H NMR studies of natural and synthetic smectites that are fully hydroxylated and fully fluorinated (where deuteron exchange with structural OH is not possible) yield resonances that are similar to those reported here,29,33,41 and a computational study of 2H quadrupolar couplings in aluminosilicate clusters yield predominantly values similar to those of ice-1 h.105 The latter results suggest that exchange between 2H2O and OH would produce either a very broad resonance or a narrow central peak rather than the intermediate residual quadrupolar splitting observed experimentally. In addition, the OD distance between 2H2O sorbed in a ditrigonal cavity and the structural OH is large enough that the probability of a deuteron exchange between these sites on the time scale and at the temperatures of these NMR experiments is likely to be quite low. High-Temperature 2H Motion in Alkali MetalHectorite Pastes. In contrast to the low-temperature dynamical behavior just described, the spectra for Na- and K-exchanged hectorite pastes above 20 °C differ significantly and show that another process must affect the line shape under these conditions and that this process is influenced by the nature for the cation. For both the K and the Nahectorite pastes the residual quadrupolar splitting decreases with increasing temperature above 20 °C. This change is not observed for the low-water Nahectorite sample, suggesting that the additional process involves bulk 2H2O in the pastes. Diffusional exchange between proximity-restricted and free 2H2O has been reported to activate in the vicinity of 20 °C for dilute smectite suspensions,48 and increasingly rapid exchange with increasing temperature is
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consistent with the peak narrowing observed here and the relatively rapid diffusion of confined H2O at room temperature observed in other studies.29,37,51,55,57,59,60,104 At 0 °C and above, the differences in the 2H NMR spectra of Khectorite and Nahectorite pastes (Figure 6) show that the nature of the cation causes observable affects on the motion of 2 H2O in the system. At 10 °C and above, the spectra for the Khectorite paste contain two resolvable 2H resonances, a sharp feature near 0 ppm and a broader feature with a residual quadrupolar splitting. The relative intensity of the resonance with residual splitting increases with increasing temperature. We interpret the narrow resonance to represent free 2H2O undergoing isotropic motion and the resonance with residual quadrupolar splitting to represent 2H2O molecules that spend at least part of their time in a proximity-restricted environment (near surface or near K+) on the NMR time scale. The increase in intensity of the broad peak with increasing temperature is consistent with an increasing population of the total 2H2O that spends some time in a proximity-restricted environment on an NMR time scale, consistent with a faster rate of 2H2O diffusion with increasing temperature and our earlier proposed mechanism.39 Another possibility is that there are two distinct environments that have relatively consistent populations but substantially different rates of motion. This model can explain the observed changes in relative intensity if the near-surface 2H2O population initially experiences intermediate-regime diffusion dynamics at 10 °C that lead to irreversible dephasing and a consummate loss of signal intensity. As the temperature increases and the near-surface 2 H2O begins to enter the fast-motion regime, we would expect less signal loss due to irreversible dephasing and an increasing intensity for this 2H population. Sorting between these two mechanisms will require additional molecular-scale insight that likely will involve molecular dynamics simulations. As a final note, we find that the spectra for the Nahectorite under these conditions do not contain the narrow resonance at 0 ppm but do become narrower with increasing temperature, suggesting the presence of a single 2H population that experiences a less significant proximity restriction with increasing temperature. These conclusions are consistent with the difference in the hydration energies of K+ and Na+. K+ has a smaller hydration energy (322 vs 406 kJ/mol for Na+) and a larger ionic radius than Na+ and thus a stronger interaction with the surface. This interaction appears to limit the population of proximity-restricted 2H2O until the increased 2H2O diffusion rate at higher temperature enables more 2H2O molecules to sample a nearsurface environment on the ∼101 microsecond NMR time scale. In contrast, the lack of a narrow, solution-like resonance for the Nahectorite pastes at the same temperatures reflects the lower affinity of Na+ for the surface leading to either all or most of the 2 H2O molecules sampling the Na+ hydration shell or a surface interaction on the NMR time scale. Whether these effects on the high-temperature 2H line shape are due purely to increased 2H2O diffusion or involve some influence of cation exchange between inner- and outer-sphere sites or desorption of the cation from the surface and migration into the free 2H2O domain, the experimental results are consistent with the hypothesis that small cations with high hydration energies prohibit purely isotropic motion of 2H2O in alkali metalsmectite pastes above 20 °C. Alkali Metal Dynamics for Alkali MetalHectorite Pastes. The variable-temperature 23Na NMR results for Nahectorite pastes presented here and similar, previously published 39K and 133 Cs NMR data for K and Cshectorite pastes39,72,73 are all 23404
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The Journal of Physical Chemistry C consistent with the 2H NMR data in showing that Na+ interacts less strongly with the smectite surface than K+ and Cs+. This difference in surface interaction leads to nearly liquid-like NMR behavior for 23Na+ and multiple observable inner-sphere sites for 133 Cs+. As noted in the Introduction, Weiss et al.72,73 observed exchange of 133Cs between 9 and 12 coordinate sites in a 0.1 M Cshectorite paste at temperatures as low as 70 °C and a continuously increasing frequency of exchange between these sites with increasing temperature until reaching the fast-exchange limit at 10 °C.73 This interpretation is consistent with the low charge/radius ratio of Cs+ promoting inner-sphere surface sorption. The continuous increase in the rate of Cs+ site exchange with increasing temperature does not parallel the discontinuous temperature variation in 2H2O dynamics observed for the K or Nahectorite pastes, suggesting that the behavior of H2O in Cshectorite paste has little influence on the Cs+ dynamics. 2H NMR experiments for 2H2O Cshectorite pastes and low-water samples are necessary to test this hypothesis. In contrast, the 39K NMR spectra of Khectorite paste shows only a single, asymmetric peak at low temperatures that is similar to that of muscovite mica and experiences a rapid melting-type dynamic transition at 20 °C that appears related to 2H2O dynamics.12,92 Our combined 39K and 2H data show that the melting-type dynamic transition of K+ at 20 °C is closely associated with the transition to diffusion-dominated dynamics for 2H2O in this system and coincides with the onset temperature of water molecule exchange between the hydration shell and bulk fluid in dilute smectite suspensions reported by Fripiat et al.48 The mica-like 39K resonance for the Khectorite paste at low temperatures suggests that K+ occurs in a disordered innersphere association with the surface even in the presence of excess 2 H2O under these conditions. The relatively large residual line width of the 39K resonance at higher temperatures compared to K+ in bulk aqueous solution also suggests that K+ maintains a relatively strong association with the surface even after the dynamic transition. This strong surface interaction is most likely a result of K+ maintaining an inner-sphere association with the smectite layers and a partial hydration shell of 2H2O molecules at all temperatures. In this case, the observed melting-type dynamic averaging arises from the increase in mobility of proximity-restricted 2H2O. This conclusion is supported by the observation of the two 2H2O populations for Khectorite paste at higher temperatures described above (Figure 6). In contrast, the data presented in this paper for the Nahectorite paste (Figures 3 and 6) suggest that both Na+ and 2H2O are diffusing rapidly at temperatures greater than 20 °C, which maximizes the probability of each 2H2O molecule coming into contact with a Na+ ion or the clay surface during the NMR experiment. If K+ remains sorbed directly to the smectite surface, the probability of each 2H2O interacting with a K+ ion or the clay surface on the same time scale is reduced with respect to the Na+ case and should lead to two distinct 2H2O populations for Khectorite paste, as observed. An alternative, although less likely, explanation of the 39K dynamic averaging is that the K+ association with the smectite changes from inner sphere to outer sphere with increasing temperature, leading to K+ averaging via 2H2O-mediated desorption and diffusion or twosite exchange. In either case, it is clear that dynamic averaging of K+ is more influenced by the behavior of H2O in hectorite pastes than the dynamic behavior of Cs+. Unlike K+ and Cs+, a majority of the Na+ in Nahectorite pastes has a low affinity for the surface and exhibits significant
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solution-like character at even 80 °C (Figure 3). The Na+ in Nahectorite paste shows a dominant, relatively symmetric environment indicating weak outer-sphere sorption at low temperatures that becomes more like Na+ in bulk aqueous solution with increasing temperature. Above 20 °C, the 2H2O and 23 Na NMR data show that all 2H2O molelcules spend some time in the Na+ hydration shell or near the clay surface. The progressive narrowing of the 2H2O resonance with increasing temperature above 20 °C suggests that the rate of 2H2O exchange between the Na+ hydration shell or clay surface and the bulk water domain increases with increasing temperature with the exchange rates of hydration-shell 2H2O leading to the observed shift of the 23Na resonance toward lower frequency for the dominant Na+ resonance. As for 39K but in contrast to 133Cs there is no clear evidence of 23Na site exchange even at 80 °C. We observe only a broadening of the resonance, a shift toward the frequency range previously assigned to surface-sorbed outer-sphere Na+,94 and growth of the low-frequency feature that is most likely due to a small fraction of low-mobility inner-sphere Na+. The observed narrowing of the dominant 23Na resonance between 80 and 20 °C is not accompanied by an associated change in the 2H2O 2 H spectra, suggesting that diffusive mobility of outer-sphere Na+ is the dominant mechanism of 23Na spectral averaging for most Na+ in hectorite paste, even at low temperatures. Dynamic averaging of the minor, low-frequency 23Na resonance is more closely associated with the temperature at which diffusional mobility or proton exchange become significant for the surfaceassociated 2H2O population, suggesting either 2H2O-mediated desorption or dynamic averaging of Na+ due to exchange of 2 H2O in and out of the Na+ coordination sphere for these sites. The dramatic differences between the 23Na VT NMR results for the paste and low-water Nahectorite show that the presence of excess 2H2O significantly affects Na+ dynamic averaging in smectitewater systems at all observed temperatures.
’ CONCLUSIONS New 2H and 23Na NMR results for Nahectorite pastes presented in this study, combined with previously published 133 Cs NMR results for Cshectorite pastes72,73 and 2H and 39K NMR results for Khectorite with a range of water contents,39 shows that excess 2H2O promotes diffusive motion of alkali metals in hectorite pastes at low temperatures when the alkali metal ion is small and has a large hydration energy. As the radius of the alkali metal ion increases, we find that inner-sphere sorption is favored more strongly at low temperature, even in the water-rich conditions of the paste. The 2H2O content and dynamics become more important in determining the rate, mechanism, and temperature dependence of cation dynamic averaging as the strength of the ionsurface interaction decreases. The hydration energy and affinity of the alkali metals for the mineral surface also appear to influence the high-temperature diffusion-related dynamics of 2H2O, with the small and easily hydrated ions leading to a greater fraction of 2H2O molecules experiencing ion2H2O (and/or smectite surface2H2O) interactions on the NMR time scale. At lower temperatures, the dynamics of proximity-restricted 2H2O appear to be independent of the alkali metal and well described by a model involving fast-limit librational motion of 2H about the 2H2O molecular dipole moment and fast-limit hopping of either a half or full shell of 2H2O molecules about the hydrated metal C3 symmetry axis. An important component of this model is compression or extension of the hydration complex along the C3 axis parallel to the 23405
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The Journal of Physical Chemistry C surface normal, most likely compression in the case of our paste samples due to electrostatic interactions between the alkali metal ions and the smectite surfaces. On the basis of these results and the higher hydration energies of the alkaline earth metals, we expect a stronger influence of alkaline earth metals on high-temperature 2 H2O dynamics and increased importance of cation diffusion as a dynamic mechanism at lower temperatures in alkaline earth metal smectite pastes.
’ ASSOCIATED CONTENT
bS
Supporting Information. Details of the NMR experiments, TGA results, and supplementary figures referenced in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: 607-871-2822. Fax: 607-871-2831. E-mail: bowers@ alfred.edu.
’ ACKNOWLEDGMENT This work was supported by the United States Department of Energy, Office of Basic Energy Science through grants DE-FG0210ER16128 and DE-FG02-08ER15929. The authors thank Dr. Alan J. Benesi for access to the NMR facilities at Penn State University and for helpful consultations regarding the dynamic interpretations of the 2H NMR spectra and Mr. Kermit Johnson of Michigan State University for access to the 9.4 T solid-state NMR instrument and performing the high-field 23Na VT NMR experiments. We also thank Mr. Michael Tomik for assistance in gathering low-field 23Na VT NMR data. ’ REFERENCES (1) Moni, C.; Rumpel, C.; Virto, I.; Chabbi, A.; Chenu, C. Eur. J. Soil Sci. 2010, 61, 958–969. (2) Sanderman, J.; Amundson, R. Biogeochemistry 2009, 92, 41–59. (3) Wagai, R.; Mayer, L. M. Geochim. Cosmochim. Acta 2007, 71, 25–35. (4) Credoz, A.; Bildstein, O.; Jullien, M.; Raynal, J.; Petronin, J.-C.; Lillo, M.; Pozo, C.; Geniaut, G. Energy Procedia 2009, 1, 3445–3452. (5) Rimmele, G.; Barlet-Gouedard, V.; Renard, F. Oil Gas Sci. Technol. 2010, 65, 565–580. (6) McGrail, B. P.; Schaef, H. T.; Glezakou, V. A.; Dang, L. X.; Owen, A. T. Energy Procedia 2009, 1, 3415–3419. (7) Kaszuba, J. P.; Janecky, D. R.; Snow, M. G. Chem. Geol. 2005, 217, 277–293. (8) Shao, H.; Ray, J. R.; Jun, Y.-S. Environ. Sci. Technol. 2010, 44, 5999–6005. (9) Bodvarsson, G. S.; Boyle, W.; Patterson, R.; Williams, D. J. Contam. Hydrol. 1999, 38, 3–24. (10) Long, J. C. S.; Ewing, R. C. Annu. Rev. Earth Planet. Sci. 2004, 32, 363-401, 362 plates. (11) Marklund, L.; Simic, E.; Woerman, A. In Proceedings of the 11th International High-Level Radioactive Waste Management Conference, Las Vegas, NV, April 30May 4, 2006; Curran Associates, Inc.: Red Hook, NY, 2006; pp 369373. (12) Bowers, G. M.; Bish, D. L.; Kirkpatrick, R. J. Langmuir 2008, 24, 10240–10244. (13) Chen, C.-C.; Hayes, K. F. Geochim. Cosmochim. Acta 1999, 63, 3205–3215. (14) Galunin, E.; Alba, M. D.; Santos, M. J.; Abrao, T.; Vidal, M. Geochim. Cosmochim. Acta 2010, 74, 862–875.
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