J. Phys. Chem. C 2009, 113, 10623–10631
10623
Proton Dynamics in Layered Double Hydroxides: A 1H T1 Relaxation and Line Width Investigation Marc X. Reinholdt,*,† Panakkattu K. Babu,†,‡,§ and R. James Kirkpatrick†,| Department of Geology, UniVersity of Illinois at Urbana-Champaign, 1301 West Green Street, Urbana, Illinois 61801, and Department of Chemistry, UniVersity of Illinois at Urbana-Champaign, 600 South Mathews AVenue, Urbana, Illinois 61801 ReceiVed: February 4, 2009; ReVised Manuscript ReceiVed: May 1, 2009
The investigation of the dynamics of water and organic species confined in minerals or adsorbed at their surface is of significant geochemical, environmental, catalytic, biomedicine, and life’s growth interests but is poorly understood on the molecular scale. This work explores the behavior of water molecules and glutamate species adsorbed on and between the double hydroxide layers of hydrotalcite [HT; (Mg2Al)(OH)6A- · nH2O, where A- is a counteranion which may bear different charges] and compares the results to those for HT containing small inorganic anions. The relative humidity (RH) dependence of the 1H T1 relaxation rates for all samples reveals the existence of two separate spin systems with 1/T1 relaxation rates differing by a factor of approximately 2 × 103. The static 1H spectral line widths allow assigning the fast relaxing protons to the fixed “static” interlayer and adsorbed speciessi.e. bound water, bound organic species, and most of the structural hydroxyl groups (-OH)sand the slow ones to the “mobile” speciessi.e. free water and solvated organic molecules and some of the structural -OH groups. Introduction The dynamics of confined and adsorbed species on minerals and analogous synthetic oxide and hydroxide compounds is of great interest to such diverse fields as geochemical transport and reactivity, water purification, environmental chemistry, catalysis, drug delivery, biomedicine, fossil fuel characterization, and the origin of life, but it remains incompletely understood at the molecular scale.1-9 1H NMR is a versatile and effective probe of molecular scale water dynamics.2,10-17 Organic species are of comparable interest, but molecular scale study of their adsorption and confinement is more limited.1,2,18-20 Many of these organic species, including amino acids, carboxylic species, peptides, and larger proteins and molecules of natural organic matter (NOM), occur as anions at neutral to basic pH values and are thus expected to interact strongly with solids having positive structural or pH dependent charges. Many silicate minerals, such as clays, normally have negative structural charges, but layered double hydroxides (LDHs) have a large, permanent positive structural charge and are known to effectively exchange many organic species. We present here an experimental NMR study of the relative humidity (RH) dependence of the 1H spin-lattice relaxation times (T1) and 1H spectral line widths of the prototypical LDH, hydrotalcite [HT; nominally (Mg2Al)(OH)6A- · nH2O, where Ais a counteranion which may bear different charges] containing surface and interlayer glutamate (GA), and for comparison similar data for HT with small inorganic anions. To our * To whom correspondence should be addressed. E-mail: marc.reinholdt.1@ ulaval.ca. Telephone: 418-656-2131ext. 4318. Fax: 418-656-5993. Present address: De´partement de Ge´nie Chimique, Universite´ Laval, 1065 ave. de la Me´decine, Que´bec, QC G1V 0A6, Canada. † Department of Geology. ‡ Department of Chemistry. § Present address: Department of Physics, Western Illinois University, 1 University Circle, Macomb, IL 61455. | Present address: Office of the Dean, College of Natural Science, Michigan State University, East Lansing, MI 48824-1115.
knowledge, this is the first such investigation of LDHs containing an organic anion. The only previous 1H T1 study of the dynamical behavior of anions, water molecules, and -OH groups in LDH compounds is for Zn-Al hydrotalcite-like compounds with inorganic anions.21 In our previous NMR, X-ray diffraction (XRD), and thermal analysis (TGA-DTA) study of HT containing glutamate, we showed that much of the adsorbed GA is not intercalated in the HT interlayer but occurs on external particle surfaces.22 The 13 C NMR results for these samples obtained with 1H-crosspolarized (CP) and magic angle spinning (MAS) show that the relative abundance of GA-2 increases with increasing pH and suggests that at 100% relative humidity (RH) the GA on exterior surfaces occurs in a surface fluid film and is not rigidly held on the surface. The results also show that at 100% RH interlayer GA is not in simultaneous direct contact with both sides of the interlayer, whereas at 0% RH it lies parallel to the layers and at 79% RH it occurs with its long axis at a high angle to them. Here, the RH dependence of the 1H spin-lattice relaxation times (T1) shows the existence of two different spin systems with 1/T1 relaxation rates differing by a factor of approximately 2 × 103. The static spectra of all samples are composed principally of a broad peak with a narrower superimposed one. The RH dependence of the 1H line width allows assignment of the broad resonance to the fast relaxing system and the narrow resonance to the slowly relaxing one. The rapidly relaxing system includes the “static” protons of the interlayer/adsorbed species, i.e. bound water, bound organic species, and the majority of the structural -OH groups. The more slowly relaxing system includes the “mobile” protons, i.e. free water and solvated organic species, and perhaps some structural -OH groups that have only weak homonuclear dipolar coupling. Our results provide a basis for future computational studies of the structure, dynamics, and energetics of intercalated and surface organic and water molecules.23-25
10.1021/jp9010263 CCC: $40.75 2009 American Chemical Society Published on Web 05/19/2009
10624
J. Phys. Chem. C, Vol. 113, No. 24, 2009
Materials and Methods Materials Synthesis. Our Mg2Al HT samples containing GA were synthesized using a coprecipitation method involving hydrolysis of Mg2+ and Al3+ ions in the presence of glutamic acid that is similar to that of Aisawa et al.26 and which we previously described in detail.22 The samples used for the 1H NMR study were synthesized at 51 ( 1 °C and pH 9.0, 10.0, and 12.0. The Mg2Al-NO3- HT was synthesized under the same conditions as the Mg2Al-GA samples at 52 ( 1 °C and pH ) 10.1. The Mg2Al-CO32- sample was prepared using the method described by Yun and Pinnavaia,27 which consists of the coprecipitation from magnesium and aluminum nitrate solutions in the presence of sodium carbonate at a pH of 10.0 and a temperature of 70 ( 2 °C. The Mg2Al-Cl- sample was prepared using the method described by Constantino and Pinnavaia,28 which consists of coprecipitation from magnesium and aluminum chloride solutions at pH 10.0 and a temperature of 70 ( 2 °C. The LiAl2-Cl- sample was prepared using the method described by Hou and Kirkpatrick,29 which consists of a reaction between lithium chloride solution and aluminum hydroxide at 90 ( 2 °C. Relative Humidity Control. In order to control the hydration state of our samples, they were kept for about four months over phosphorus pentoxide for 0% RH and over various cationic salts for the other specific RH values.30 Before the NMR measurements, the samples were quickly transferred into zirconia rotors and then sealed. Some samples were dried at 80 °C for 48 h before MAS NMR data were collected. Thermal Analysis. Simultaneous thermogravimetric analysis (TGA) and differential thermal analysis (DTA) measurements were carried out using a Netzsch STA 409 Thermal Analyzer. The samples, equilibrated at 79% RH, were weighed and then rapidly introduced into the cold oven of the instrument. The experiment was started immediately after the balance reached equilibrium. All samples were heated from room temperature to 800 °C at a heating rate of 5 °C · min-1 under flowing air. NMR Experiments. 1H T1 measurements were carried out under static conditions using the saturation recovery pulse sequence (π/2, τ, π/2),31 at a frequency of 599.819 MHz on a Varian Infinity-Plus wide bore spectrometer using a 4 mm Triple-resonance MAS probe (Varian). The 90° pulse width was 3 µs, and each spectrum was obtained from the average of 16 transients. 1H chemical shifts are referenced to an external adamantane sample at 1.74 ppm. Magnetization recovery profiles were obtained from the spectral amplitudes recorded for 18 different τ values in the range 0.001 to 6 s. The magnetization recovery profiles were analyzed using Origin 6.1 (OriginLab Corporation) software with a double exponential function M/M0(τ) ) 1 - R exp(-τ/T1a) - (1 - R) exp(-τ/T1b), where T1a and T1b are the two components of relaxation, R is the fraction of the T1a component, and τ is the time between the two pulses in the saturation recovery sequence.31 The fit was carried out through a three-parameter minimization scheme with the weight factor constrained to be 0 e R e 1. The first run started from R ) 0.5, T1a ) T1b ) 1 with T1b fixed, yielding an approximate value of R. For the second run, R was fixed to the value determined in the first run, and both T1 values were varied starting from 1. The third run started with the results of the second and allowed all variables to be free. All final convergences had residues greater than 0.99, with the ideal value being 1. The 1H line widths were determined by simulating the fully relaxed static spectra using the NMR Utility Transform Software (NUTS, Acorn NMR software). Because of the large breadths and superposition of the peaks, the reported line widths and
Reinholdt et al. the peaks areas have uncertainties of 10-15%. However, since all the spectra are simulated in the same way, comparison of the line widths at different RH values provides useful trends. Results and Discussion 1
H T1 Relaxation Rates as a Function of Relative Humidity. The T1 magnetization recovery curves for seven samples containing organic and inorganic anions obtained at RHs from 0 to 100% are well fit by a double exponential function M/M0(τ) ) 1 - R exp(-τ/T1a) - (1 - R) exp(-τ/T1b) (e.g., Figure 1a to 1d).31 Attempts to fit several of the data sets with a single exponential (M/M0(τ) ) 1 - exp(-τ/T1)) or stretched exponential (M/M0(τ) ) 1 - exp(-τ/T1)β) functions31 resulted in much poorer fits with convergences having residues less than 0.96 (e.g., Figure 1e and 1f) compared to values more than 0.99 for the double exponential function. Thus, the relaxation data clearly indicate the presence of at least two different spin systems, a fast relaxing component (T1a) with T1 room temperature values of the order of a few tenths of a millisecond, and a more slowly relaxing component (T1b) with T1 values of the order of 500 to 1,200 ms. For the LDHs containing small inorganic anions, the more slowly relaxing T1 component is most abundant and increases in abundance with increasing RH (Figure 2a). Over most of the RH range, R is less for Mg2Al-NO3- and LiAl2-Cl- than for Mg2Al-CO32- and Mg2Al-Cl-. The Mg2Al-NO3- and LiAl2Cl- phases show greater water loss at low temperatures than the other two phases (Table 1), indicating that water molecules constitute a significant fraction of the slowly relaxing component. For the Mg2Al-GA compounds, the more slowly relaxing component is dominant at all RHs, is slightly higher at high RHs, and is essentially independent of the pH of synthesis (Figure 2b). The water contents of LDH compounds are wellknown to increase with increasing RH, and the overall decrease of R with increasing RH for all samples and the differences of the R values between the samples both suggest that the slowly relaxing component is related to water molecules. For the samples with inorganic anions, the R values averaged over all RHs are between 0.18 and 0.31, with the highest values observed for the less hydrated Mg2Al-CO32- and Mg2Al-Cl- samples (Table 1), 0.28 and 0.31, respectively. The more hydrated Mg2Al-NO3- and LiAl2-Cl- samples have smaller values of 0.18 and 0.20, respectively. These differences are significantly greater than the fitting error, which is estimated to be at most 15%, making the absolute error about 0.05. The average R values for the Mg2Al-GA samples are 0.16 -0.18, within experimental error of each other. The hydration state of these samples is not known. The values of T1a (the fast relaxing component) are almost all between 0.2 and 0.4 ms, and the variation with RH and among samples is almost entirely within analytical error (Figure 3). Thus, T1a is not affected by the hydration state or the nature of the anion and appears to be a characteristic of the fundamental HT metal hydroxide structure. The only samples for which this is not the case are LiAl2-Cl- and Mg2Al-CO32- at 100% RH, and even under these conditions, the values are less than 1 ms. In contrast, the values of T1b (the more slowly relaxing component) vary significantly among samples, with the more hydrated ones typically having smaller values (faster relaxation). The values range from about 500 to 1,200 ms (Figure 4). The observed differences among the samples are much greater than the fitting errors, which are in the 20 to 45 ms range. For the samples with inorganic anions, those that adsorb less water, Mg2Al-CO32- and Mg2Al-Cl- (Table 1), have larger T1b values.
Proton Dynamics in Layered Double Hydroxides
J. Phys. Chem. C, Vol. 113, No. 24, 2009 10625
Figure 1. Magnetization curve fittings of samples (a, e, and f) Mg2Al-GA pH ) 10.0, 79% RH; (b) Mg2Al-GA pH ) 10.0, 51% RH; (c) Mg2AlGA pH ) 10.0, 100% RH; and (d) LiAl2-Cl-, 79% RH; using double (a-d), single (e), or stretch (f) exponential functions for the fit.
For the samples that contain GA (Figure 4b), the one synthesized at pH ) 12.0 has the largest T1b values, and this sample is expected to have a lower hydration state at 79 and 100% RH than the samples synthesized at pH ) 9.0 and 10.0.27 For some of the samples, such as Mg2Al-Cl-, the T1b values do not vary much with RH, but for most, T1b decreases with increasing RH, again indicating that faster T1b relaxation is associated with higher hydration. However, although T1b is typically smaller at RHs between 50 and 90% than at lower RH values, for several samples it increases slightly at 90 or 100% RH. The rate of 1H T1 relaxation depends on the intensity of the dynamical power spectrum at the Larmor frequency (here 600 MHz), and for materials containing water and solid components, it can be influenced by such processes as relaxation to paramagnetic centers, -OH libration, H2O libration and diffusion, the motions of other species containing protons (here GA), and, in some, extend proton exchange and spin diffusion.31-34 In
general, incorporation of water molecules and dissolved species in interlayer galleries or association with solid surfaces reduces their T1 relaxation times (increases the relaxation rate) relative to the values in bulk solution.10,17,18,32 This is because the characteristic frequencies of the vibrational and rotational motions of bulk water are greater than 1012 Hz and those of diffusional motion are of the order of 1010 Hz. These frequencies are all much greater than the Larmor frequency, and association with the solid increases the density of states at the Larmor frequency and also allows for greater interaction with paramagnetic centers.1,2,15,18,35 For instance, the T1 relaxation times of bulk water are of the order of 2000 to 4000 ms at room temperature36-38 and published T1 values for water associated with solids are of the order of a few milliseconds to a few tens of milliseconds depending on the magnetic field and the substrate, e.g., zeolites,2,18 carbon powders,39 aluminum phosphates,15 coal,6 clays10,17 and other minerals.12 However, in some cases the T1 values may reach a few hundred milliseconds.11,13,14
10626
J. Phys. Chem. C, Vol. 113, No. 24, 2009
Reinholdt et al.
Figure 2. Weighing coefficient R, deduced from the T1 relaxation measurements, as a function of relative humidity for (a) Mg2Al-CO32-, LiAl2-Cl-, Mg2Al-NO3-, and Mg2Al-Cl- samples; and (b) Mg2Al-GA samples synthesized at pH ) 9.0, 10.0, and 12.0.
Figure 3. T1a parameter, deduced from the T1 relaxation measurements, as a function of relative humidity for (a) Mg2Al-CO32-, LiAl2-Cl-, Mg2Al-NO3-, and Mg2Al-Cl- samples; and (b) Mg2Al-GA samples synthesized at pH ) 9.0, 10.0, and 12.0.
TABLE 1: Thermogravimetric Analysis Data Obtained for Mg2Al-Cl-, Mg2Al-CO32-, Mg2Al-NO3-, and LiAl2-ClSamples at 79% Relative Humidity sample Mg2Al-Cl
-
Mg2Al-CO32Mg2Al-NO3LiAl2-Cl-
temp range (°C)
weight loss (%)
25-250 250-800 25-250 250-800 25-250 250-800 25-200a 200-800
12 28 13 27 18 30 18 25
a All adsorbed and intercalated water is already lost at 200 °C; beyond that, the structural water (OH-) starts to be removed.
For amino acids in bulk aqueous solution or interacting with alginate, 1H T1 values are of the order of 100-3000 ms, and for amino acid copolymers in solution, they are of the order of 400-1500 ms.40,41 The measured room temperature T1 of our glutamic acid in 10-3 M aqueous solution is about 1340 ms (data not shown). In most cases, 1H T1 relaxation of water adsorbed on substrates can be adequately described by a single exponential function, although in some cases they exhibit more complex double or multiple exponential behaviors. For instance, Gotoh et al. observed two components in the T1 relaxation of water adsorbed on internal surfaces of microporous metal phosphates.15 The fast relaxing component has values of the order of a few tens of milliseconds at room temperature and represents about 0.1 of the total water content. In contrast, the more abundant, more slowly relaxing component has room temperature T1 values of the order of a few seconds. The slowly relaxing component is assigned to water molecules adsorbed in the micropores, and the rapidly relaxing component is assigned to water molecules located near trace paramagnetic impurities. For water adsorbed on carbon powders, Yu and Lee observed single-exponent T1
Figure 4. T1b parameter, deduced from the T1 relaxation measurements, as a function of relative humidity for (a) Mg2Al-CO32-, LiAl2-Cl-, Mg2Al-NO3-, and Mg2Al-Cl- samples; and (b) Mg2Al-GA samples synthesized at pH ) 9.0, 10.0, and 12.0 (error bars are hidden by the symbols).
relaxation at temperatures below 185 K39 but double exponential behavior at higher temperatures. The high temperature T1 relaxation has a much weaker temperature dependence than the low temperature one. In this case, the two components have T1 values of the same order of magnitude, separated only by a few tens of milliseconds. The low temperature relaxation is thought
Proton Dynamics in Layered Double Hydroxides to be dominated by thermally activated intramolecular rotational motions of the adsorbed water molecules, and the high temperature relaxation by intermolecular diffusional motions with diffusion constants proportional to temperature. In an even more complicated situation, Harmer et al. used an elegant 2D 1H NMR approach to determine the spin-lattice (T1) and spin-spin (T2) relaxations of a series of air-dried bituminous coals.6,42 Their method involves a unique response surface. Correlation of the T1 and T2 values led to the resolution of five different components, three of them observable in the T1 dimension alone. The authors did not assign the various components but did speculate that the shorter T1 component (13 ms) interacts with efficient relaxation centers such as transition metal ions in the mineral matter, stable organic radicals, and paramagnetic molecular oxygen. The average of the two longer T1 values is ca. 56 ms. For our samples, there are several types of 1H sites, structural -OH in the metal hydroxide layers on particle surfaces, structural -OH on either side of interlayers, water molecules coordinated via H-bonding to interlayer or external hydroxide surfaces, and interlayer or surface water molecules not directly coordinated to the hydroxide surface (“free water molecules”). For samples with GA, there are also protons in the GA ions in interlayers and on external surfaces. Assignment of these various protons to the two observed components of the T1 relaxation data can be made based on the RH (hydration) dependence of their R and T1 values and the known behavior of 1H relaxations in hydrous materials.15,17,22,23 The invariance of the T1 value for the rapidly relaxing component (T1a) and its decreasing abundance (decreasing R) with increasing RH indicates that this component is due mostly to structural -OH groups. However, water molecules strongly held to the surface via H-bonding and potentially relaxing to paramagnetic metal centers may contribute.2,17,18,23 For the samples containing GA, some GA species may also relax to paramagnetic centers and thus contribute to the short T1 component. The RH dependence of values for the slow relaxing component (T1b) and its increasing abundance (decreasing R) with increasing RH indicate that this component is due to the “mobile” species, either located in the interlayer or of the external surfaces. These “mobile” species include free water molecules, which increase in abundance with increasing RH, and water molecules adsorbed at the LDH surface undergoing site hopping but not relaxing to a paramagnetic center.23,39 For the GA-HT samples, the contribution of the GA species to the slow relaxing component becomes more pronounced with increasing RH, because of the carboxylic acid species affinity for water solvation.22,25,43 The observed shortening of T1 values of the slow relaxing system with increasing RH is consistent with an increase of the mobility of the “mobile” species.17 The overall shorter T1b values observed for samples known to contain more watersi.e. Mg2Al-NO3- and LiAl2-Cl- (Figure 3a, Table 1) and GA-LDH samples with decreasing pH of synthesis (Figure 3b, ref 22)sare also consistent with this observation. Thus, the slower T1 system (T1b) contains mostly surface and interlayer species whose environment depends on the hydration state, including free water and organic molecules, and perhaps some adsorbed water molecules undergoing diffusional motions via a hopping process. The adsorbed GA species are unlikely to undergo such processes, mostly because of steric effects and lower diffusion rates. The faster relaxing system (T1a) contains mostly protons of the lattice hydroxyls and perhaps water molecules strongly held on the surface near paramagnetic centers. A quantitative assignment of the various species of
J. Phys. Chem. C, Vol. 113, No. 24, 2009 10627
Figure 5. 1H static NMR spectra of sample Mg2Al-GA pH ) 10.0 as a function of relative humidity (100% RH is represented separately for clarity purposes).
protons is unfortunately impossible because of the complexity of the system. What is clear is that the dipolar interaction between the two spin systems is sufficiently decoupled by molecular motion that the relaxation rates are very different, even though the many protons are in close proximity in these hydrogen-rich materials. 1 H T1 results previously published by Dupuis et al. for (Zn2Al)(OH)6Cl- · nH2O HT are somewhat different than those for our samples but provide information about the temperature dependence of the T1 values.21 They observed two T1 spin systems but used a T1 measurement technique that provided only a single global value. They interpreted their results to indicate that the T1 values of the two systems are almost equal due to a strong dipolar spin-spin interaction. All their room temperature T1 values were in the range of 100 to 400 ms, intermediate between the values of the slow and fast relaxing systems for our samples. Their observations show that at a given frequency the T1 values increase with decreasing temperature and decrease with increasing hydration. We also observe the latter phenomenon. 1 H Static NMR Spectral Line Widths as a Function of Relative Humidity. The static 1H NMR spectra of our samples (e.g., Figure 5) contain a broad peak with a full width at halfheight (fwhh) of the order of 20 kHz with a narrower, superimposed peak with fwhh’s of the order of a few kilohertz (e5 kHz). For all of our samples, the relative intensity of the broad feature is greater than 0.8 at low RHs. For the samples with inorganic anions, this value remains nearly constant or decreases slightly up to 80 or 90% RH and then decreases dramatically, for Mg2Al-CO32- to essentially 0 (Figures 6 and 7). For the samples containing GA, the decrease in relative intensity of the broad component begins near 50% RH and is more gradual. Because of the 10-15% peak area fitting accuracy (see Experimental Section), the error on the relative intensity is approximately 0.2. The widths of the broad component for the samples with inorganic anions are all between 20 and 22 kHz and for the
10628
J. Phys. Chem. C, Vol. 113, No. 24, 2009
Reinholdt et al.
Figure 6. Broad (b) and narrow (O) peak relative surface areas of the 1H static NMR spectra for the various LDH samples as a function of relative humidity.
most part do not change significantly with RH (Figure 7a). The widths of the broad component for the samples containing organic anions are all between 17.5 and 22.5 kHz at all RHs, except at 100% for the samples Mg2Al-GA pH ) 9.0 and 10.0, for which it decreases to less than 2 kHz (Figure 7b, Figure 8). Chemical and thermal analysis of similar samples synthesized under the same conditions as those for these two Mg2Al-GA samples shows that they contain more carbon (GA) than the pH ) 12.0 sample and are more hydrated at 79% RH and slightly more hydrated at 100%.22 The pH ) 10.0 sample is the most hydrated. For all the GA-HT samples, the decrease in width of the broad component at high RHs is accompanied by a parallel decrease in relative peak area (Figure 6), and both correlate with increasing water content. The 1H spectra of our samples and their RH dependence are similar to the previously published results of Dupuis et al. for a (Zn2Al)(OH)6Cl- · nH2O LDH.21 Their spectra also consist of a broad peak superimposed by a much narrow one. Although they did not give line width or peak area data, they did observe that the relative intensity of the narrow component increases with the increasing RH, as it does for our samples. Their variable temperature measurements for a sample maintained at 90% RH show that the line width and relative proportions of the two components remain the same in the temperature range from 180 to 300 K. This result shows that the structure of the compound and the types of motion contributing to the line widths do not change in this temperature range. Their 1H line width results for partially deuterated samples show considerable narrowing
Figure 7. Broad resonance line width of 1H static NMR spectra as a function of relative humidity for (a) Mg2Al-CO32-, LiAl2-Cl-, Mg2AlNO3-, and Mg2Al-Cl- samples; and (b) Mg2Al-GA samples synthesized at pH ) 9.0, 10.0, and 12.0.
Proton Dynamics in Layered Double Hydroxides
J. Phys. Chem. C, Vol. 113, No. 24, 2009 10629
Figure 9. Narrow resonance line width of 1H static NMR spectra as a function of relative humidity for (a) Mg2Al-CO32-, LiAl2-Cl-, Mg2Al-NO3-, and Mg2Al-Cl- samples; and (b) Mg2Al-GA samples synthesized at pH ) 9.0, 10.0, and 12.0. Figure 8. 1H static NMR spectra and simulations of samples Mg2AlGA pH ) 9.0 (a) and 10.0 (b) at 100% relative humidity.
of both components, demonstrating that the line shapes of the undeuterated samples are dominated by 1H-1H dipolar interactions. They also observed a nearly order of magnitude reduction in the absolute peak intensities, demonstrating that the protons of the hydroxide layers undergo site exchange during sample synthesis and storage. Since 1H NMR peak widths are dominated by 1H-1H dipolar coupling, changing line widths reflect the dynamics of processes with frequencies within an order of magnitude of the static peak width. These frequencies are typically 100 kHz or less, much lower frequencies than those that influence T1 relaxation (here, 600 MHz).31,44 Thus, for our samples, the broad peak is due to the signal for 1H spins that are not dynamically averaged or only partially averaged, and the narrow peak is due to spins averaged by essentially isotropic motion.45,46 Isotropic averaging can arise from motions such as isotropic tumbling or proton exchange among sites.46-48 For the samples with only inorganic anions at low RHs, 80% to nearly 100% of the 1H spins contribute to the broad peak and are thus not fully averaged. We attribute the broad component to -OH groups and surfacesorbed (fixed) water, and the narrow component to interlayer free water, as suggested by Gotoh et al.15 Unless the structural -OH groups undergo rapid proton exchange or dissolution reprecipitation, their librational motion would not cause isotropic narrowing.23,24,29 It is possible that the interlayer water molecules do undergo isotropic motion at the required frequencies, but the RH dependence of the results for the LiAl2-Cl phase argues that they do not (Figures 6, 7a, and 9a). Previous water sorption and RH controlled XRD results for this phase show that it undergoes a phase transition near 20% RH that results in filling of one interlayer water layer.24 If these water molecules were dynamically averaged, we would expect an increase in the narrowed peak at the phase transition. Computational molecular
dynamics modeling of this phase shows that the interlayer water molecules do undergo librational motion,49 which would not cause isotropic averaging of their 1H NMR resonances.23,49-51 The LiAl2Cl phase does not incorporate much additional water at RHs between 20% and 95%, and the lack of significant change in the breadth of the broad 1H NMR resonance is consistent with the lack of change in the interlayer structure and dynamics. Unfortunately, there are no published results correlating the RH dependence of water sorption and layer spacing for the Mg2Al phases used here. The increased intensity of the narrow peak at high RH for the samples with only inorganic anions, then, must be due to increased free water content. Rapid Mn+-OH site exchange may also contribute, as suggested by Dupuis et al.22 and others.46-48 For the Mg2Al samples containing GA, the relative abundance of the broad peak is also higher than 80% at low RHs but begins to decrease near 50% RH (Figure 6). This is the same RH range where changes occur in the T1 relaxation times and R values (Figures 2 and 4b). For these samples, the 1H signal contains intensity from not just the -OH groups and water but also the protons of the GA. The similarity between the intensity ratios at the low RHs suggests that the narrow peak for the samples containing GA can be assigned to free water, both at the external surfaces and in the interlayer,15 and probably to free GA; and the broad resonance to -OH groups, surface-sorbed (fixed) water molecules, and potential surface-sorbed (fixed) GA. Our previous NMR, XRD, and TGA-DTA results for GA-HT show that the amount of interlayer water increases with increasing RH, that at 100% RH the GA on exterior surfaces occurs in a surface fluid film, and that the interlayer GA is hydrated and no longer in simultaneous direct contact with both sides of the gallery.22 These results are consistent with the affinity of carboxylate amino acid species for solvation by water.22,25,43 Thus, at high RHs the main contributors to the broad peak are probably -OH groups and surface-sorbed (fixed) water molecules. The small
10630
J. Phys. Chem. C, Vol. 113, No. 24, 2009
line width changes for the broad component over the full RH range show that there is little effect of water content on the dynamics of the species contributing to this resonance. The widths of the narrow 1H spectral component decrease with increasing RH for all samples (Figure 9). This behavior contrasts with that of the broad component and indicates that the extent of dynamical averaging increases with increasing hydration for the sites contributing to the narrow component. For Mg2Al-Cl-, the widths are nearly constant up to 79% RH and then drop dramatically at 90 and 100% RH (Figure 9a), paralleling the increase in relative intensity of the narrow peak (Figure 6). The Mg2Al-CO32- sample shows a steady decrease in peak width up to 90% RH and then a large decrease at 100% RH (Figure 9a). The widths for the LiAl2-Cl- and Mg2Al-NO3samples are nearly constant up to 79% RH and then decrease at 90 and 100% (Figure 9a). In contrast, the widths of the narrow components for the Mg2Al-GA samples decrease continuously from 33 to 100% RH (Figure 9b), again paralleling the increasing relative intensity of this resonance (Figure 6). Thus, the dynamical behaviors of the samples containing organic matter at frequencies less than ca. 200 kHz are significantly more affected by the hydration level than the samples containing inorganic anions. For the Mg2Al-CO32- and pH ) 10.0 Mg2Al-GA samples at 100% RH, all the 1H signal is in the narrow peak. For the LDHs, dynamical averaging of all sites requires that the -OH sites participate in 1H exchange at frequencies greater than about 200 kHz.23,35,39 Since the RH values at which changes in line width and population differ among samples, there may be a minimum number of water molecules required for the initiation of proton exchange. The amount of water needed probably depends on several parameters, including the metal species in the hydroxide, their distribution and amount, the interlayer counteranion, the particle size, and the way the sample is synthesized. Overall, for all our samples, the width of the narrow peak is reduced by half, at least, with increasing RH from 0 to 100%, making it much more sensitive to hydration state than the broad peak, especially at high RH values. Thus, we ascribe the narrow component to “mobile” interlayer and surface species, i.e. free water and free solvated GA molecules. Comparison of the fractions of fast and slow relaxing components from the T1 data and the fractions of signal intensity in the broad and narrow spectral components (Figures 2 and 6) show that they are quantitatively not the same, as might be expected under the simple assumption that the slowly relaxing component and the narrow resonance represent principally water molecules in rapid isotropic motion. Quantitative comparison of the two data sets is not possible, because the frequencies probed are so different (600 MHz for the T1 relaxation and tens to hundreds of kilohertz for changes in peak width), because of the many different structural types of 1H in the samples and because of uncertainties in understanding spin diffusion and the effects of paramagnetic centers on the relaxation of the different types of 1H. Conclusion 1
H spin-lattice (T1) NMR relaxation and the 1H static and MAS NMR spectra of layered double hydroxide compounds containing glutamate ions (GA) and inorganic anions obtained over a range of controlled relative humidities (RHs) provide significant new insight into the motional dynamics of surface and interlayer water and organic molecules exchanged onto this important class of compounds. All samples have two T1 spin systems. The fast relaxing system has T1 values of the order of
Reinholdt et al. 0.2 to 0.4 ms and is assigned to structural -OH groups and protons in sorbed water molecules rapidly relaxing to paramagnetic centers. The more abundant, slower relaxing system has T1 values of 500 to 2,000 ms and is assigned to mobile species, protons in most of the water molecules, and GA ions on the surface and in interlayers. The relative abundance of the slowly relaxing system increases with increasing RH. The static 1H spectra of all samples are composed of a broad peak with a fwhh of the order of 20 kHz with a narrower superimposed peak with fwhh of the order of a few kilohertz. For most samples, the widths of the broad component do not change greatly with RH, whereas the widths of the narrow component decrease by at least 50% at high RHs. This component is assigned to the mobile interlayer and surface water and GA undergoing isotropic motion. At low RHs, the broad resonance is assigned to -OH groups, surface-sorbed (fixed) water molecules, and possibly surface-sorbed GA. At high RHs, the main contributors to the broad peak are -OH groups and surface-sorbed water molecules. At these RH values, GA is highly solvated and occurs predominantly in a fluid film on the exterior surface of the LDH.22 Unfortunately, a quantitative assignment of the various species of protons is impossible because of the complexity of the system and because of the large difference in frequency between the processes affecting T1 relaxation and the line widths. Acknowledgment. This work was supported by Grant DOEFG02-00ER15028 from the Geoscience program of the U.S. Department of Energy Division of Basic Energy Sciences. References and Notes (1) Winkler, H.; Steinberg, K.-H.; Kapphahn, G. J. Colloid Interface Sci. 1984, 98, 144. (2) Winkler, H. Catal. Today 1988, 3, 501. (3) Bank, S.; Yan, B.; Edwards, J. C.; Ofori-Okai, G. Langmuir 1994, 10, 1528. (4) Hill, A. R., Jr.; Bo¨hler, C.; Orgel, L. E. Origins Life EVol. Biospheres 1998, 28, 235. (5) Liu, R.; Orgel, L. E. Origins Life EVol. Biospheres 1998, 28, 245. (6) Harmer, J.; Callcott, T.; Maeder, M.; Smith, B. E. Fuel 2001, 80, 417. (7) Fischer, K. Water, Air, Soil Pollut. 2002, 137, 267. (8) Ding, X.; Henrichs, S. M. Mar. Chem. 2002, 77, 225. (9) Wilson, E. E.; Awonusi, A.; Morris, M. D.; Kohn, D. H.; Tecklenburg, M. M. J.; Beck, L. W. Biophys. J. 2006, 90, 3722. (10) Fripiat, J. J.; Letellier, M.; Levitz, M. Philos. Trans. R. Soc. London A 1984, 311, 287. (11) Vogeley, J. R.; Moses, C. O. Geochim. Cosmochim. Acta 1991, 56, 2947. (12) Nakashima, Y.; Nakashima, S.; Gross, D.; Weiss, K.; Yamauchi, K. Geothermics 1998, 27, 43. (13) Nakashima, Y.; Mitsumori, F.; Nakashima, S.; Takahashi, M. Appl. Clay Sci. 1999, 14, 59. (14) Nakashima, Y. Clay Clay Miner. 2000, 48, 603. (15) Gotoh, K.; Ishimaru, S.; Ikeda, R. Chem. Lett. 2001, 30, 1250. (16) Nakashima, Y. Clay Clay Miner. 2002, 50, 1. (17) Sanz, J.; Herrero, C. P.; Serratosa, J. M. J. Phys. Chem. B 2006, 110, 7813. (18) Winkler, H.; Birner, B.; Bosa´e`ek, V. Zeolites 1989, 9, 293. (19) Volpert, E.; Selb, J.; Candau, F.; Green, N.; Argillier, J. F.; Audibert, A. Langmuir 1998, 14, 1870. (20) Weir, M. R.; Facey, G. A.; Detellier, C. Stud. Surf. Sci. Catal. 2000, 129, 551. (21) Dupuis, J.; Battut, J. P.; Fawal, Z.; Hajjimohamad, H.; de Roy, A.; Besse, J.-P. Solid State Ionics 1990, 42, 251. (22) Reinholdt, M. X.; Kirkpatrick, R. J. Chem. Mater. 2006, 18, 2567. (23) Hou, X.; Kalinichev, A. G.; Kirkpatrick, R. J. Chem. Mater. 2002, 14, 2078. (24) Hou, X.; Bish, D. L.; Wang, S.-L.; Johnston, C. T.; Kirkpatrick, R. J. Am. Mineral. 2003, 88, 167. (25) Padmanabhan, P. K.; Kalinichev, A. G.; Kirkpatrick, R. J. J. Phys. Chem. B 2006, 110, 3841. (26) Aisawa, S.; Takahashi, S.; Ogasawara, W.; Umetsu, Y.; Narita, E. J. Solid State Chem. 2001, 162, 52.
Proton Dynamics in Layered Double Hydroxides (27) Yun, S. K.; Pinnavaia, T. J. Chem. Mater. 1995, 7, 348. (28) Constantino, V. R. L.; Pinnavaia, T. J. Inorg. Chem. 1995, 34, 883. (29) Hou, X.; Kirkpatrick, R. J. Inorg. Chem. 2001, 40, 6397. (30) Standard Salt Solutions for Humidity Calibration. In CRC Handbook of Chemistry and Physics, 88th ed. (Internet Version 2008); Lide, D. R., Ed.; CRC Press/Taylor and Francis: Boca Raton, FL. (31) Fukushima, E.; Roeder, S. B. W. Experimental Pulse NMR: A Nuts and Bolts Approach; Addison-Wesley Inc.: 1981. (32) Haase, J.; Park, K. D.; Guo, K.; Timken, H. K. C.; Oldfield, E. J. Phys. Chem. 1991, 95, 6996. (33) Kim, Y.; Kirkpatrick, R. J. Am. Mineral. 1998, 83, 339. (34) Yu, P.; Kirkpatrick, R. J. Cem. Concr. Res. 2001, 31, 1479. (35) Houston, R. J.; Philips, B. L.; Casey, W. H. Geochim. Cosmochim. Acta 2006, 70, 1636. (36) Hindman, J. C.; Svirmickas, A.; Wood, M. J. Chem. Phys. 1973, 59, 1517. (37) Lang, E.; Lu¨demann, H.-D. J. Chem. Phys. 1977, 67, 718. (38) Baumgartner, S.; Lahajnar, G.; Sepe, A.; Kristl, J. AAPS PharmSciTech 2002, 3, E36. (39) Yu, I.; Lee, M. J. Magn. Reson. A 1994, 109, 41. (40) Desmukh, M. V.; Vaidya, A. A.; Kulkarni, M. G.; Rajamohanan, P. R.; Ganapathy, S. Polymer 2000, 41, 7951.
J. Phys. Chem. C, Vol. 113, No. 24, 2009 10631 (41) Di Cocco, M. E.; Bianchetti, C.; Chiellini, F. J. Bioact. Compat. Polym. 2003, 18, 283. (42) Harmer, J.; Callcott, T.; Maeder, M.; Smith, B. E. Fuel 2001, 80, 1341. (43) Li, Q.; Kirkpatrick, R. J. Am. Mineral. 2007, 92, 397. (44) Levitt, M. H. Spin Dynamics: Basic Principles of NMR Spectroscopy; Wiley: Chichester, 2001. (45) Modig, K.; Halle, B. J. Am. Chem. Soc. 2002, 124, 12031. (46) Azaı¨s, T.; Tourne´-Pe´teilh, C.; Aussenac, F.; Baccile, N.; Coelho, C.; Devoisselle, J.-M.; Babonneau, F. Chem. Mater. 2006, 18, 6382. (47) Golubev, N. S.; Asfin, R. E.; Smirnov, S. N.; Tolstoi, P. M. Russ. J. Gen. Chem. 2005, 76, 915. (48) Suzuki, K.; Hayashi, S. Solid State Ionics 2006, 177, 3223. (49) Wang, J.; Kalinichev, A. G.; Amonette, J. E.; Kirkpatrick, R. J. Am. Mineral. 2003, 88, 398. (50) Kalinichev, A. G.; Kirkpatrick, R. J.; Cygan, R. T. Am. Mineral. 2000, 85, 1046. (51) Wang, J.; Kalinichev, A. G.; Kirkpatrick, R. J.; Hou, X. Chem. Mater. 2001, 13, 145.
JP9010263
