6430
J. Phys. Chem. C 2008, 112, 6430-6438
H2O and Cation Structure and Dynamics in Expandable Clays: 2H and Investigations of Hectorite
39K
NMR
Geoffrey M. Bowers,*,†,§ David L. Bish,‡ and R. James Kirkpatrick†, ⊥ Department of Geology, UniVersity of Illinois Urbana-Champaign, Urbana, Illinois 61801, and Department of Geological Sciences, Indiana UniVersity, Bloomington, Indiana 47405 ReceiVed: December 19, 2007; In Final Form: February 13, 2008
Variable temperature 39K and 2H nuclear magnetic resonance (VT NMR) spectroscopy of K+-saturated hectorite, a prototypical smectite clay, provides new insight into the relationships between the structural and dynamical behavior of K+ and H2O in confinement and at surfaces. In d ) 10 Å K-exchanged hectorite, interlayer K+ is rigidly held by the silicate rings, probably in 12-coordinate inner-sphere sites as in muscovite mica. In a 1/1.5 by weight hectorite/water paste, K+ occurs on interlayer and external surface sites that are indistinguishable by 39K NMR. The K+ environments experience changes in dynamical behavior over the temperature range from -50 to 60 °C that are directly related to H2O dynamics. 39K NMR of the paste sample shows dynamic line narrowing at low temperatures due to modulation of the electric field gradient (EFG) at frequencies of the order of the static line width (≈20 kHz) and two “melting”-type dynamic transitions near -10 °C, one for surface and one for confined K+. At and above 0 °C, K+ remains closely associated with the clay surfaces and experiences motion at frequencies greater than 200 kHz and less than 10 MHz, as revealed by 39K T1 relaxation behavior, nutation behavior, and the 39K quadrupolar product. These data are consistent with rapid exchange between inner- and outer-sphere K+ sites reported previously for K-montmorillonite based on molecular dynamics simulations. Deuterium NMR shows the presence of two unique H2O environments in the system: one structurally and dynamically consistent with bulk water between particles and one attributable to H2O confined in the interlayer. Confined H2O experiences anisotropic motion between -50 and 0 °C via fast rotation (>2 MHz) about a single axis oriented 127.5 ( 0.5° from the principal axis of the 2H EFG, potentially due to C2 rotation. This motion does not affect the 39K EFG significantly. Melting of free and confined H2O occurs between -10 and 0 °C and near 0 °C, respectively, similar to the melting behavior of K+ and likely reflecting the onset of molecular diffusion. At and above 10 °C, all H2O environments experience motion near or in excess of 300 kHz through at least three NMR-indistinguishable mechanisms, including Brownian motion of free water, exchange of free and confined H2O near particle edges, and diffusive motion of H2O that remains confined on the experimental time scale. The correlation between the rates of 2H and 39K motion and the observed melting transitions for both spin populations strongly suggest that 39K melting and dynamics above the melting transition are linked to an increase in the motional freedom of H2O.
Introduction The dynamic behavior of H2O and ionic species in two- and three-dimensional confinement plays a variety of important roles in processes such as ion transport and adsorption,1-5 water storage in hostile environments,6,7 dissolution/precipitation reactions in aqueous environments,8,9 and the swelling of smectite clays (low charge 2:1 type phyllosilicates with expandable interlayers).3,10,11 Historically, the structure and dynamics of ions and water in confined spaces and at solid-fluid interfaces have been difficult to characterize on the molecular scale, but the continued evolution of molecular modeling, neutron scattering, and nuclear magnetic resonance (NMR) spectroscopy has permitted ever more detailed theoretical and experimental investigations, particularly regarding the special case of H2O in the two-dimensional, nanometer-scale interlayer * Corresponding author. E-mail:
[email protected]. † University of Illinois Urbana-Champaign. ‡ Indiana University. § Current address: Department of Chemistry, Michigan State University, East Lansing, MI 48824. ⊥ Current address: College of Natural Science, Michigan State University, East Lansing, MI 48824.
space of phyllosilicates. Differences between the structural and dynamical properties of bulk water and H2O confined in lowcharge 2:1 phyllosilicate interlayers were recognized as early as 1971 based on quasi-elastic neutron scattering studies of a Li+-vermiculite.12 Recent neutron,13-17 NMR,18-27 and molecular modeling19,28-42 studies show that H2O within one to two statistical monolayers from a phyllosilicate surface (in the interlayer or at the particle exterior) exhibits structural, diffusional, rotational, and vibrational properties that differ from bulk water. Unfortunately, much less is known regarding the structure and dynamics of charge-balancing ions in these systems and how the dynamics of confined H2O and ions are related. These are essential components for a complete understanding of ion sequestration and transport in porous materials. We present here a 2H and 39K NMR study of the structure and dynamics of H2O and K+ in the prototypical smectite clay mineral, hectorite, that has been saturated with K+. The focus of this work is to determine how H2O dynamics in the kilohertz to megahertz range (“intermediate” regime) affects the behavior of interlayer and surface K+ via line-shape analysis of 2H and 39K variable temperature (VT) NMR spectra and NMR relax-
10.1021/jp7119087 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/28/2008
2H
and
39K
NMR Investigations of Hectorite
ation rates. NMR spectroscopy is an attractive approach for molecular-level structural and dynamical studies of confined and surface-associated water and ions because it is the only experimental method capable of identifying, characterizing, and quantifying structural environments while simultaneously probing molecular-scale dynamic behavior over a wide frequency range (hertz to megahertz). However, 39K NMR of inorganic solids can be challenging because of its low gyromagnetic ratio (γ/2π ) 1.994259 MHz/T) and moderate nuclear electric quadrupole moment43 (I ) 3/2, Q ) 0.06 × 10-24 cm2; comparable to 23Na). Despite these limitations, 39K is 93% naturally abundant and can be readily observed with modern spectrometers. Including this work, there have been three published 39K NMR studies of K+ binding in clays or micas to date.44-46 Prior studies have focused principally on probing the structural properties of bound K+ under anhydrous conditions and as a function of hydration and hydration cycling. This publication represents the first instance where 39K VT NMR is used to examine the dynamic properties of K+-exchanged clay minerals and also the first use of 39K and 2H VT NMR to correlate the dynamics of K+ and H2O in confinement and at surfaces, providing important new insights into the behavior of K+ and H2O in these environments. Materials and Methods Materials. The hectorite used in this study was obtained from the Clay Minerals Society Source Clays Repository currently housed at Purdue University (sample SHCa-1). Hectorite is ideal for NMR investigations of smectite clays because the San Bernardino county sample used here contains a low concentration of paramagnetic impurities (0.28 wt % Fe and Mn).47,48 A sample of hectorite was saturated with K+ by exposure to 1 or 0.01 M aqueous KNO3 in three separate ion-exchange reactions. After the final exchange period, the clay was dried at 110 °C for 25 h. The aggregated hectorite was gently ground in an agate pestle and mortar and sieved to a particle size of 5000 MHz at 300 K, substantially larger than the range of room temperature rates revealed by our 39K NMR results. The discrepancy in jump rate is likely related to the reduced tetrahedral charge in San Bernardino hectorite, although dynamic behavior in MD calculations are model-dependent,80,81 and the behavior of simulated montmorillonite at 300 K may not quantitatively reflect the behavior of San Bernardino hectorite at the same temperature. However, the suggested hopping between inner- and outer-sphere sites in the interlayer or on the external surface would generate the nonzero time-averaged quadrupolar interaction suggested by our nutation, T1, and PQ data because the motion involves at least one asymmetric binding site. If two-site exchange is responsible for the observed 39K EFG modulation, NMR experiments at temperatures low enough to freeze out motion at frequencies greater than or equal to the approximate static peak width could result in multiple observable resonances, analogous to the results of Weiss et al. for Cs-exchanged smectites.75,76 The presence of only a single broad resonance at -50 and -30 °C indicates that if multiple sites are present, their NMR resonances are not resolvable under our experimental conditions. This may be because the two peaks overlap sufficiently to be indistinguishable. As noted earlier, however, the increase in sensitivity associated with the temperature increase from -50 to -30 °C is opposite the behavior predicted by the Curie magnetization law and suggests that dynamic line narrowing occurs via motion at a rate close to the static peak width. Thus, we cannot rule out the possibility of multiple K+ sites on the basis of NMR results from -50 °C and must obtain lower temperature NMR experiments to truly test this possibility. Unfortunately, signal-to-noise and equipment limitations prevented such experiments under static or MAS conditions at this time. MD simulations of K-hectorite are planned within our group to verify the two-site exchange model
2H
and
39K
NMR Investigations of Hectorite
J. Phys. Chem. C, Vol. 112, No. 16, 2008 6435
Figure 6. Variable τ delay 2H solid echo spectra (thick lines) and simulations (thin, red lines) using tetrahedral jump motion for the 1/1.5 wt K-hectorite/2H2O paste at -50 °C (a) and -30 °C (b). The corresponding τ delays from top to bottom are 40, 70, and 100 µs, respectively.
Figure 5. 2H VT NMR spectra from 14.1 T of the 1/1.5 wt K-hectorite/ 2 H2O paste. The thin lines represent dynamic simulations incorporating both tetrahedral jump motion for the free H2O and fast rotation for confined H2O at an angle between the principal axis of the 2H electric field gradient and the rotation axis of θ ) 127.5 ( 0.5°.
TABLE 2: 2H Jump Rates (in kHz) for the Bulk Water Phase in the Hectorite Paste Compared with Those of Wittebort et al.82 temp (°C)
K-hectorite paste
water-ice 1h82
-50 -30 -10 0
1.5 6.0 54
2 MHz is absent in the 39K NMR spectra below -10 °C. At 10, 25, and 40 °C, there are two narrow 2H resonances (Figure 9) reflecting melting of the bulk water and the interlayer melting transition observed in the 39K NMR data. At 10 °C and above, the intense and very narrow resonance arises from isotropic motion in the free water phase. This strong, narrow peak is not observed for free water at -10 °C due to motionrelated signal dephasing by fast tetrahedral jumps (see Figure 5 and ref 81) but does appear in the 2H NMR spectrum at 0 °C as a decrease in the depth of the central dip near 0 ppm. This is because melting of bulk water/ice produces intensity in a narrow band near the reference frequency. The second, somewhat broader resonance is due to interlayer H2O and possibly H2O undergoing exchange with the bulk exterior water phase. This assignment is justified because confinement introduces a residual time-averaged quadrupolar interaction that leads to line broadening relative to the free water case. We know that a fraction of H2O remains confined on the experimental time scale based on the average H2O interlayer residence time. The average H2O interlayer residence time calculated using the MDcomputed diffusion coefficient for interlayer H2O in 12.5 Å
K-montmorillonite (D ) 1.3 × 10-10 m2/s)37 and assuming that this value corresponds to predominantly lateral diffusion and that the hectorite particles are plate-like and 1 µm in diameter is greater than 6 ms. This value is longer than the observed 2H T2* values (0.8 and 4.1 ms for the broad and narrow resonances, respectively), where T2* is determined from the line width and represents an upper limit on the true T2. Thus, most of the interlayer water remains confined on the experimental time scale. This residence time also suggests that a small fraction of confined H2O near particle edges exchanges with bulk H2O, in agreement with VT 2H NMR experiments of K-hectorite equilibrated in a 100% relative humidity 2H2O environment (Figure S3). This sample contains significantly less bulk H2O and complete narrowing of the confined H2O fraction is not observed above 0 °C, suggesting that exchange between interlayer and free states is an important contributor to motional narrowing of interlayer H2O above melting. Once again, molecular dynamics simulations will be needed to identify specific mechanisms of motion for H2O between -50 and 40 °C. The increase in relative intensity of the broader 2H peak from 10 to 40 °C (Figure 9) should be interpreted with caution given the extent of left shifting and back-prediction used in processing the data (see Experimental Section and Supporting Information). It may, however, reflect an increase in the fraction of water molecules that spend part of their time in the interlayer, a loss of water from the sample, or temperature-dependent variations in T2 relaxation that affect the relative intensity. The epoxy seal remained intact, and the sample’s physical appearance did not change over the course of the NMR experiments, suggesting that water was not lost. If exchange of water molecules between the interlayer and bulk fluid is important, an increasing H2O diffusion coefficient with increasing temperature would cause a greater fraction of H2O molecules to experience confinement during the acquisition period, increasing the intensity of the broader peak. Likewise, an increase in the T2 relaxation rate
2H
and
39K
NMR Investigations of Hectorite
for the free water phase with respect to the confined fraction(s) may also affect the relative intensity in a solid-echo NMR experiment in the fashion observed in the 2H NMR spectra. A thorough evaluation of these and other potential origins for the relative intensity changes requires a substantial number of NMR and computational investigations of multiple clays and clay/ H2O ratios and is beyond the scope of this paper. Dynamic Coupling of K+ and H2O. The NMR data presented here demonstrate a close relationship between the dynamic behavior of K+ and H2O in the hectorite paste. As noted earlier, the absence of 39K dynamic averaging in the 10 Å powder at room temperature is clear evidence that dynamic averaging in the paste is associated with the presence of H2O. The melting transition observed for bulk H2O by 2H NMR between -10 and 0 °C is directly associated with the onset of 39K line narrowing for a small spin population, possibly associated with the external hectorite surface (Figures 3 and 5). Assuming the 39K signal does come from the surface spins, this relationship likely reflects increased mobility of surface K+ that depends directly on the dynamics of bulk interparticle H2O. There is also a close relationship between the dynamics of the remaining K+ population and H2O confined in the interlayer. At 0 °C, we observe 39K dynamic averaging via EFG modulations at a rate g200 kHz and narrowing of the confined H2O 2H NMR spectrum by motion at a rate that is insufficient to fully average the 2H quadrupolar interaction (rate
