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Arrangement and Mobility of Water in Vermiculite Hydrates Followed by 1H NMR Spectroscopy J. Sanz,* C. P. Herrero, and J. M. Serratosa Instituto de Ciencia de Materiales, C.S.I.C., Cantoblanco, 28049 Madrid, Spain ReceiVed: January 4, 2006; In Final Form: February 22, 2006
The arrangement of water molecules in one- and two-layer hydrates of high-charged vermiculites, saturated with alkaline (Li+, Na+) and alkali-earth (Mg2+, Ca2+, Ba2+) cations, has been analyzed with 1H NMR spectroscopy. Two different orientations for water molecules have been found, depending on the hydration state and the sites occupied by interlayer cations. As the amount of water increases, hydrogen bond interactions between water molecules increase at expenses of water-silicate interactions. This interaction favors water mobility in vermiculites. A comparison of the temperature dependence of relaxation times T1 and T2 for one and two-layer hydrates of Na-vermiculite shows that the rotations of water molecules around C2-axes and that of cation hydration shells around the c*-axis is favored in the two-layer hydrate. In both hydrates, the anisotropic diffusion of water takes place at room temperature, preserving the orientation of water molecules relative to the silicate layers. Information obtained by NMR spectroscopy is compatible with that deduced by infrared spectroscopy and with structural studies carried out with X-ray and neutron diffraction techniques on single-crystals of vermiculite.
Introduction Vermiculites are layer silicates found in nature as a product of mica alteration.1 Like micas, silicate layers of ideal vermiculite, (Si3Al)Mg3(OH)2O10.Mg0.5.nH2O, are formed by an octahedral and two adjacent tetrahedral sheets (phyllosilicates 2:1). During vermiculitization of micas, interlayer K+ is replaced by hydratable cations (usually Mg2+).1 The possibility of inserting different cations in the interlayer space allows one to study water-cation interactions in different homoionic phases. Depending on the partial water pressure, one- or two-layer hydrates have been detected in exchanged vermiculites.2 Thus, these natural materials are well suited to study confined water in interlayer spaces, with the advantage that the water content can be externally controlled. For these reasons, phyllosilicates have been employed for many years as model materials to study physicochemical properties of water in two-dimensional systems.3-6 Arrangement and mobility of water molecules in phyllosilicates 2:1 have been analyzed, so far, with different experimental and theoretical approaches. Among experimental studies, spectroscopic (infrared,7 Raman,8,9 electron,10 and nuclear magnetic resonances7,11,12) and diffraction (X-ray,13-18 neutron19-21) techniques have been used. In particular, diffraction studies have allowed the determination of the relative disposition of adjacent layers, as well as sites occupied by water molecules and compensating cations.15,16 On the other hand, lattice energy calculations and molecular dynamics simulations have provided interesting information on the disposition and mobility of water molecules in phyllosilicates.22,23 1H NMR (nuclear magnetic resonance) spectroscopy has been used to study the arrangement and mobility of water molecules in hydrated phyllosilicates. The study of oriented aggregates of phyllosilicates allowed the water orientation to be determined.24 * To whom correspondence should be addressed.
From the temperature dependence of spin-spin (T2) and spinlattice (T1) relaxation times, the mobility of water molecules was analyzed in a low-charged Li-hectorite25 and an high charged Na-vermiculite.24 These works emphasized the importance of water mobility on NMR spectral features. In particular, it was proposed that the rotation of water molecules around their C2 symmetry axis precedes the rotation of hydrated cation shells around the c*-axis and/or the diffusion of water in the interlamellar space of phyllosilicates.24 In this work, 1H NMR spectroscopy has been used to analyze one- and two-layer hydrates of vermiculites, where Mg2+ ions have been exchanged by monovalent (Li+ and Na+) or divalent (Ca2+ and Ba2+) cations. In particular, we have concentrated on the arrangement and mobility of water molecules in the interlayer space of vermiculites. The disposition of water molecules has been deduced from the evolution of NMR spectra with the orientation of monocrystalline samples relative to the external magnetic field. Some insight into the water mobility has also been obtained from the analysis of intra and intermolecular H-H interactions in 1H NMR spectra of different phases. In the case of one-layer Na-vermiculite, T1 and T2 relaxation times vs temperature have been used to analyze the water motion. Structural and dynamical information obtained in this work has been compared with that yielded earlier by spectroscopic and diffraction techniques. Experimental Section Samples used in this work are natural vermiculites from Llano County (Texas), with mineralogical formula (Si2.89Al1.11)(Al0.08Ti0.02Fe0.06Mg2.82)O10(OH)2Mg0.48.nH2O. To analyze disposition of water molecules in vermiculites, 4 × 4 × 0.5 mm3 platelets were separated from natural samples. The interlayer Mg2+ cations were replaced by Li+, Na+, Ca2+, and Ba2+ cations by immersion of vermiculite platelets in renewed 1N solutions of the corresponding chloride salts. To
10.1021/jp0600561 CCC: $30.25 © 2006 American Chemical Society Published on Web 03/30/2006
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TABLE 1: Shifts between Contiguous Layers Deduced from XRD in Monocrystalline Samples.15,16 Vb-Vd Shifts Correspond to One-layer, and VI-VIII Correspond to Two-layer Vermiculite Phases layer shift Vb Vc Vd VI VIII
tx ( a/6 +a/3 ∼a/5 0 0
ty ( b/3 0 ( b/8 ( b/3 0
d001 (A) 12.20 11.85 12.20 14.30 14.92
hydrate Li+‚3
H2O Na+‚2 H2O 2+ Ba ‚6 H2O Mg2+‚9 H2O Ca2+‚12 H2O
favor cation diffusion, vermiculite platelets were previously pierced along the direction perpendicular to the layer planes in points uniformly distributed. After cation exchange, samples were washed with water to eliminate residual chlorides. Cation replacement was followed by X-ray diffraction. The ion exchange process was assumed to be complete when the X-ray diffraction (XRD) pattern of the original Mg-vermiculite (d001 ) 14.30 Å) was substituted by those reported in homoionic vermiculites. In most cases, cation exchange processes were achieved after 15 days of treatment. Basal spacings corresponding to different phases, measured under a partial water pressure of P/P0 ) 0.3, are given in Table 1. In one-layer vermiculite phases, basal d001 spacings are near 12 Å, and two-layer vermiculites display d001 values near 14.5 Å. 1H NMR spectra were recorded at 60 and 90 MHz in a SXP 4/100 pulse spectrometer (Bruker) with an Aspect 2000 data system. Spectra were obtained by the Fourier transform procedure after irradiation of the sample with a π/2 pulse (3 µs). To analyze the spectra dependence on the sample orientation, crystal platelets were oriented parallel inside a square-section tube. In these experiments, a goniometer was fixed to the sample holder. 1H NMR spectra were recorded for different orientations, at 5° intervals, with respect to the external magnetic field B0. Hydration processes were studied by exposing vermiculite samples to different partial water pressures. Humidity of the flowing air was controlled by passing it through different salt solutions with known partial water pressure. For each pressure considered, the exposition time was chosen to ensure equilibrium conditions. The amount of water adsorbed in the interlayer space was determined by integration of the NMR water signal with respect to that of OH groups of the vermiculite. The monocrystalline character of these samples allowed us to neglect adsorbed water on the external surfaces of the material. Spin-lattice relaxation times (T1) were measured between 170 and 320 K by using the classical (π-τ-π/2) pulse sequence. Spin-spin relaxation times (T2) were obtained from the line width of the NMR components detected in the proton spectrum. For that, reciprocal spin-spin relaxation rates (T -1 2 ) were deduced from the full-width at half-height (fwhh) of the NMR lines. In the case of Gaussian lines, T -1 2 was calculated as 0.6π was times the fwhh; in the case of Lorenzian lines, T -1 2 calculated as π times the fwhh. Results 1H NMR spectra of exchanged vermiculites exposed to the partial water pressure P/P0 ∼ 0.3 are given in Figure 1. These spectra were obtained at room temperature with vermiculite platelets disposed perpendicular to the external magnetic field B0. In all cases, the spectra are formed by one doublet (named Pake doublet hereafter),26 that increases with the water content, and one central line that corresponds to OH groups of vermiculite layers. For quantitative purposes, the water content of vermiculites was deduced from the integration of the doublet signal. In one-layer hydrates, the number of water molecules per cation was near 2, 3, and 6 in Na, Li, and Ba-vermiculites,
Figure 1. 1H NMR spectra of vermiculite samples saturated with alkaline and alkali-earth cations. Spectra were recorded at 300 K with the magnetic field B0 disposed perpendicular to vermiculite platelets.
respectively. In two-layer hydrates of Mg- and Ca-vermiculites, the amount of water molecules per cation was near 6 and 12, respectively. In Ca vermiculite, the central line is much smaller than the doublet components, because the water loading is larger and doublet components are narrower than the central line (OH groups of the silicate layer). In 1H NMR spectra of the analyzed phases, the splitting of Pake doublets changes considerably. In one-layer Na-vermiculite, the splitting (about 21 G) is much larger than that found in Li and Ba vermiculites (7-8 G). For the natural two-layer Mg vermiculite, we find a similar splitting (7 G). This is not the case of the two-layer Ca vermiculite, for which we find a smaller splitting of 4.5 G. As discussed below, these features are related to the arrangement and mobility of water molecules in the interlayer space of vermiculites. In analyzed phases, small changes detected on chemical shift values of water molecules correspond to differences on cation-water or silicate-water interactions. The analysis of NMR spectra recorded at increasing partial water pressures, showed that the intensity of the water signal increases with the water pressure. This is shown in Figure 2, where we have displayed the water loading of the phyllosilicate as a function of P/P0 for various interlayer cations. When oneand two-layer phases are formed, plateaus are detected in water isotherms. This observation goes parallel with specific basal distances detected by XRD. In particular, for Na, Li, and Ba vermiculites, one-layer phases are observed for P/P0 between 0.2 and 0.4, reaching the two-layer hydrate at P/P0 ∼ 0.9. For Mg and Ca cations, the two-layer phase is obtained at much lower water pressures (P/P0 < 0.3). The line width of the doublet components is found to depend on the water content (Figure 1). For one-layer vermiculites exchanged with Li, and Ba cations, line width values are similar (3 G), but clearly lower than that observed in one-layer Navermiculite (4.5 G). In the case of two-layer hydrates, the line width decreases from 3 G in Mg-vermiculite (∼6 H2O per Mg2+ cation) to 0.8 G in Ca-vermiculite (12 H2O per Ca2+ cation). From the above observations, it is clear that the line width of doublet components decreases with the amount of adsorbed water, reaching consecutive plateaus when one and two water layers are filled in vermiculites (∼6 and 12 H2O/unit formula) (see Figure 3). One-Layer Na Vermiculite. 1H NMR spectra of one-layer Na-vermiculite recorded at different orientations of vermiculite platelets are given in Figure 4. In this Figure, δ indicates the angle between the perpendicular to the silicate layers (c* axis) and the external magnetic field B0. The analysis of these spectra
1H NMR Spectroscopy and Water in Vermiculite Hydrates
J. Phys. Chem. B, Vol. 110, No. 15, 2006 7815
Figure 2. Dependence of the water signal intensity on the partial water pressure P/P0 in (a) Li+, Na+, and Ba2+-vermiculites and (b) Ca2+ and Mg2+ vermiculites. Intensity values were deduced from 1H NMR spectra recorded at room temperature with the magnetic field B0 perpendicular to the silicate layers.
Figure 3. Dependence of the line width of NMR components on the water content of vermiculites. Experimental points correspond to different vermiculite phases and different water pressures P/P0.
shows that the separation ∆ between doublet components follows the strict dependence ∆ ) ∆0 (3 cos2δ -1) This means that the separation between doublet components is maximum, 2 ∆0, when the magnetic field is perpendicular to the phyllosilicate sheets, and decreases to ∆0 when it is disposed parallel to the layer platelets (B0 ⊥ c*). For δ ) 54°44′ (the so-called magic angle), the doublet collapses into a single line. The same angular dependence has been observed in all analyzed vermiculites. The Pake doublet detected in NMR spectra of one-layer Navermiculite does not change in a significant way in the temperature range 150-320 K. However, the line width of the doublet components (::T2-1) decreases considerably as the temperature increases (see Figure 5a). This observation suggests the existence of local anisotropic motions for water molecules, that reduce intermolecular (H-Al, H-H,...) interactions, preserving the disposition of water molecules relative to the silicate layers (intramolecular interactions). Moreover, a certain intensity loss was detected at 80 °C for the water signal, indicating that water desorption occurs above this temperature. To analyze the water mobility, spin-lattice T1 relaxation times have been studied as a function of the inverse temperature for the perpendicular disposition of platelets with respect to the
Figure 4. 1H NMR spectra of one-layer hydrate Na-vermiculite, when sample platelets are tilted different angles with respect to the external magnetic field B0.
external magnetic field B0 (Figure 5b). Below 170 K, T1 values are nearly constant; however, above 170 K, the water mobility increases, making that T1 decreases. In the temperature range analyzed here, T1 values decrease as temperature is raised, reaching first a shallow minimum at about 250 K, and a deeper one at 320 K. However, a better resolution of the two minima was not possible for the two frequencies, 60 and 90 MHz, employed in this work. In the same Figure, T1 values of the two-layer hydrate of the Na-vermiculite, measured at 60 MHz (open circles), are included as a reference (Hougardy et al., 1976). In this phase, two well-differenciated minima were detected. Discussion 1H
NMR spectra of analyzed vermiculites are always formed by a doublet ascribed to the adsorbed water and a central line associated with OH groups of the silicate layers. The presence of the doublet (Pake doublet) is associated with intramolecular
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Figure 5. (a) Temperature dependence of spin-spin (T2) relaxation time of water components, (b) Spin-lattice relaxation times (T1) vs inverse temperature. T1 and T2 values were obtained at 60 and 90 MHz in one-layer Na vermiculite (closed squares and circles). T1 results reported previously in the two-layer hydrate of Na-vermiculite have been included for comparison (open circles). In this figure, dotted lines are included to guide the eye.
interactions between protons (1/2 spins) of water molecules,12,24 but the line width of components results from intermolecular (water-water and water-silicate) interactions. In general, intermolecular interactions (H-Al, H-H,...) decrease considerably when water mobility increases. From this fact, the analysis of the doublet splitting will afford information about the water arrangement, and that of the line width of components on the water mobility. Interlayer Structure. The basal spacings d001 of analyzed vermiculites coincide within error bars with those previously reported (Table 1), indicating that the formation of one-layer hydrates at P/P0 ∼ 0.3 gives d001 values near 12 Å and the formation of two-layer hydrates gives d001 values near 14.5 Å. The monodimensional Fourier analysis of XRD patterns showed that compensating cations can occupy two different positions with respect to the silicate layers.2 In one-layer Na vermiculite, sodium ions are coordinated to two water molecules at the central plane of vermiculites (Figure 6a). In one-layer Li and Ba vermiculites, interlayer cations are coordinated to 3 and 6 molecules of water and are located near silicate layers, interacting in a preferential way with one silicate layer. In the latter case, Ba cations are partially located inside ditrigonal rings of the silicate (Figure 6b), affecting vibrational frequencies of OH groups of the nearby layer.27 In the case of the two-layer Na or Mg vermiculites, interlayer cations are coordinated to six water molecules and are located at the center of the basal spacing. In these vermiculites, water molecules interact only with oxygens of one silicate layer (Figure 6c). In the case of the two-layer Ca-vermiculite, the structural analysis revealed the existence of two sites partially occuped by Ca at the center of the interlayer space of vermiculite.18 Water Disposition. Structural analyses carried out on single crystals of vermiculite showed that contiguous layers are shifted to favor the adequate coordination of hydrated compensating cations15,16 (see Table 1). In one-layer Na vermiculite, an a/3 translation was observed between adjacent layers. In the case of two-layer Na or Ca hydrates, no shift was detected between silicate layers. In the case of the two-layer Mg hydrate, contiguous layers are disposed in a semi-ordered way with +b/3 or -b/3 shifts between layers (see Table 1).
Figure 6. Schematic view of interlamellar space of (a) one-layer Navermiculite, (b) one-layer Li or Ba-vermiculites, and (c) two-layer Naor Mg-vermiculites. Note that the coordination shell of Ba2+ and Mg2+ cations includes six molecules of water in respective hydrates. However, for the sake of clarity, only two and four molecules of water have been depicted in panels b and c.
In all analyzed vermiculites, the doublet splitting display the same angular dependence, given by the expression (1), where δ stands for the angle between the perpendicular to the silicate layers (c* axis) and the external magnetic field B0. In one-layer Na-vermiculite, the maximum doublet splitting, measured for the perpendicular disposition of platelets with respect to the external magnetic field, is near 21 G (Figure 4). This value is close to that expected for two H atoms of a water molecule, separated r ) 1.65 Å, when the magnetic field B0 is nearly parallel to the H-H vector.26 In this case, the angular dependence of the doublet splitting is given by the expression
∆ ) 2K(3cos2 δ-1)
(1)
where K ) 3/(8π)γr-3 and γ is the magnetogyric ratio of protons. From this analysis, it can be concluded that the internuclear H-H vector is disposed perpendicular to the silicate
1H NMR Spectroscopy and Water in Vermiculite Hydrates
Figure 7. ab-Projection of two adjacent silicate layers in one-layer Na vermiculite. In this projection, positions occupied by interlayer cations and water molecules have been indicated. Electric dipoles of water molecules are visualized by arrows parallel to the b-axis.
layers. This result differs from that reported by Hougardy et al. for one-layer Na vermiculite, in which two doublets, probably associated with one- and two-layer hydrates, were detected.28 Observed differences could result from the difficult control of the vermiculite hydration in aggregated films. In Figure 7 we show the relative disposition of contiguous silicate layers, determined by De la Calle et al. for one-layer Na-vermiculite16 (oxygens atoms of contiguous layers are indicated by open and closed small circles). In this phase, Na cations occupy the center of the interlayer space and electric dipoles of water molecules bounded to Na cation (two molecules per cation) are disposed parallel to the silicate layers (see arrows in Figure 7). Similar results were deduced from neutron diffraction experiments.19 The relative disposition of adjacent layers favors the symmetric interaction of water molecules with two adjacent layers. The location of Na cations at indicated positions favors also the local compensation of the charge deficit created by the substitution of one Si by one Al in each of the four nearby tetrahedra. In other analyzed phases, the doublet splitting was smaller, indicating that the water arrangement is different from that adopted in one-layer Na vermiculite. A static distribution of H-H vectors of water molecules around the c*-axis should produce a broad NMR spectrum at any orientation of the external magnetic field, B0. In this static configuration, intermolecular H-H interactions should increase considerably the line width of water doublet components when the amount of water increases. However, it has been observed that the line width of doublet components decreases considerably when the water content increases, indicating that water mobility increases with the amount of adsorbed water (Figures 1 and 3). Taking into account that in all analyzed vermiculites, the doublet collapses into a single component when the external field is oriented δ ) 54°44′ with respect to the normal to the silicate layers (c*), one has to assume that some kind of anisotropic motion of water is produced. Among different models favored by the silicate layer geometry, the fast rotation of cation coordination shells around the c*-axis gives the above-mentioned angular dependence for the doublet splitting.12 In this case,
∆ ) 2K (3cos2 Θ-1)) 2K[1/2 (3cos2 Φ-1) (3cos2 δ -1) + 3/2 sin2 Φ sin 2 δ cos R + 3/2 sin2 Φ sin2 δ cos 2R] (2) where Θ is the angle formed by H-H vectors with the external magnetic field, and Φ and δ are, respectively, the angles formed by the perpendicular to platelets with the H-H vectors of water molecules and with the external magnetic field B0 (see Figure 8). In this expression R is the angle formed by the H-H
J. Phys. Chem. B, Vol. 110, No. 15, 2006 7817
Figure 8. Relative disposition of different axes used to describe the orientation and mobility of water molecules in vermiculite hydrates (see the text).
proyection on the xy plane with the y-axis. The fast rotation of coordination shells around c*-axis cancels the last two terms, giving an expression similar to that deduced from the experimental results. Different angles used to describe the orientation of water molecules are illustrated in Figure 8. In the case of two-layer hydrate of Mg vermiculites, interlayer cations occupy the center of the interlamellar space, but water molecules are arranged in two different planes (three in each plane) (Figure 6c). In one-layer hydrates of Li- and Bavermiculites, with 3 and 6 water molecules per cation, the water disposition is similar to that found in the two-layer Mg vermiculite (Figure 6b), but interlayer cations occupy positions near the silicate layers. In all these phases, water molecules only interact with one silicate layer and electric dipoles of water molecules, deduced with the expression (1), are tilted by about 55° with respect to the perpendicular to the vermiculite layers (c* axis). In the case of Ba vermiculite, cations are located inside ditrigonal rings of the silicate, thus lying close to OH groups of the vermiculite. This is in line with IR results, that showed that Ba cations cause the perturbation of vibrational frequencies of OH groups.27 Water Mobility. Based on the water content dependence of T2 relaxation times, we have concluded that water mobility increases with the amount of water in one- and two-layer hydrates (Figure 3). According to this fact, mobility of water molecules averages intermolecular interactions (H-Al, H-H...) reducing the line width of water components. In previous papers, it has been proposed that the rotation of the water molecules around the C2 axis, precedes the rotation of the coordination shell around c*-axis and the diffusion of water molecules in ab-planes.24,25 In the case of one-layer hydrate Na-vermiculite, the splitting of the water doublet does not change appreciably when the line width of water components decreases. This result indicates that the H-H direction remains basically parallel to the c* direction (perpendicular to the layers) during rotation or diffusion of water molecules. This observation excludes the possibility of a free molecular rotation around the C2 symmetry axis, that would reduce considerably the NMR doublet splitting below 21 G, when the field B0 is disposed perpendicular to the layers. This result does not exclude, however, the existence of hindered 180° rotations of water molecules around their C2 axis, with large residence times in the stable detected configuration. In this phase, the rotation of linear Na-2H2O associations around the c*-axis is impeded by hydrogen bonds established between water molecules and oxygen atoms of silicate layers. In one-layer Li and Ba, and two-layer Mg phases, the maximum doublet splitting measured, 7-8 G, is compatible with the asymmetric arrangement of water molecules, described in the previous section. In this model, C2 axes are inclined with respect to the c*-axis, making that only one proton of water
7818 J. Phys. Chem. B, Vol. 110, No. 15, 2006 molecules interacts with oxygen atoms of the adjacent layer. In these phases, the fast rotation of water molecules around C2 axes tilted ∼ 54o (the magic angle) would reduce considerably the doublet spliting measured when platelets are disposed perpendicular to the magnetic field. If we assume the fast rotation of water molecules around the C2 and cation coordination spheres around c* axes, the NMR doublet splitting should be given by eq 2:
D ) K/2 (3 cos2 γ - 1) (3 cos2 Ψ - 1) (3 cos2 δ - 1) (3) where γ is the angle between H-H vectors and the C2 axis, Ψ is the angle between the C2 axis and the perpendicular to the layers (c*-axis), and δ is the angle between c*-axis and the external magnetic field (see Figure 8). If observed splittings are introduced in expression (3), an angle Ψ ) 27-29o between C2 and c* is deduced when γ ) 90°. This value is difficult to reconcile with the stable hydrogen disposition deduced previously for water molecules in vermiculites (Ψ ∼ 54°). From this fact, we have to conclude that water molecules do not rotate freely around C2 axis in one-layer vermiculites.24 An alternative explanation, which is compatible with spectral features deduced in these phases, is that water molecules can hop between equivalent sites, preserving the water disposition relative to the silicate layers. This model predicts similar doublet splittings to those calculated with the expression (2) for the rotation of the coordination shells around the c*-axis. Introducing the observed doublet splittings for Li and Ba vermiculites, we obtain an angle Φ between H-H vectors and the c*-axis near 40°, very close to Φ ) 38° expected for water molecules with an OH axis normal to the layers. From the above considerations, we must conclude that the anisotropic diffusion of water molecules in the plane-ab or the rotation of coordination shells around the c*-axis give similar results and cannot be differenciated on the basis of the angular dependence of the doublet splitting. In the case of two-layer Na and Ca vermiculites, where the amount of water molecules is near 12 per unit cell (Figure 2), the same disposition of water molecules has been proposed.24 In these vermiculites, one of the OH groups of water molecules interacts with the silicate layer and the other OH interacts with neighboring water molecules. Smaller splitting values detected in these phases have been explained previously by assuming the existence of the simultaneous rotation of water molecules around C2 axis, that reduces considerably the doublet spliting of water molecules, and the rotation of the coordination shell around the c*-axis, that produces the (3 cos2 δ - 1) dependence.24 In these phases, water-silicate and water-water interactions would be smaller than water-cation interactions, what favors the rotation of water molecules and coordination shells. Water Mobility in One-Layer Na Vermiculite Hydrate. Further insight into the mobility of water molecules can be obtained by measuring the relaxation times T1 and T2 as a function of temperature. Relaxation times have been measured at 60 and 90 MHz over the temperature range 170-320 K in one-layer Na-vermiculite (closed circles and squares). Results for T1 are compared with those reported for the two-layer-water Na-vermiculite (open circles) in Figure 5b.24 Similar to that previously reported, one observes a decrease in T1 values when water mobility increases. A detailed analysis of T1 plots shows two broad minima at 103/T ∼3 and 4.2 K-1 for the two NMR frequencies employed, 60 and 90 MHz. However, the intensity of the low temperature minimum detected
Sanz et al. in one-layer specimens is considerably smaller than that reported in the two-layer phase. In a previous work, Hougardy et al. analyzed the water mobility in the two-layer Na-vermiculite.24 In this case, the two maxima detected at 103/T∼ 4.3 and 3 were ascribed to the rotation of water molecules around the C2 axis and the rotation of the hydration shell around the c*-axis and/or the water diffusion along ab-planes. In one-layer Na-vermiculite, the rotation of water molecules around the C2 axis and the rotation of hydration shells around the c*-axis are hindered by interaction of water molecules with the silicate layers. According to this, we have assigned the main minimum observed at 103/T ∼ 3.2 to the hopping/diffusion of water molecules between equivalent sites and that at 103/T ∼ 4.3 to residual rotations of water molecules around the C2 axis. This assignment explain the water desorption detected above 80 °C in the one-layer Na hydrate. Taking into account the presence of two possible relaxation mechanisms in the temperature window analyzed, a determination of the activation energy for water motion is not recommended. On the other hand, strong deviations from the Debye behavior, detected in neutron scattering experiments, difficults relaxation analyses in these vermiculites.29-31 From the comparison of data obtained in the two hydrates of Na-vermiculite, it can be concluded that the rotation of water molecules around the C2-axis is only favored in the two-layer Na-vermiculite. This conclusion is reasonable if one takes into account the different dispositions of water molecules in both phases. In one-layer Na-vermiculite, each molecule forms H bonds with two neighboring silicate layers, but in two-layer Na hydrate, each molecule interacts only with one silicate layer. From this fact, the rotation of water molecules is more probable in two-layer hydrates. Similar conclusions were deduced in oneand two-layer hydrates of TaS2 and NbS2 chalcogenides.32 Conclusions 1H
NMR spectrscopy has been used to study one- and twolayer hydrates of vermiculites saturated with alkaline and alkaliearth cations. In this work, it has been shown the existence of two orientations for H-H vectors that are compatible with two different arrangements deduced for water molecules in vermiculite hydrates. In one-layer-hydrate Na-vermiculite, two water molecules coordinated to Na ions are symmetrically disposed with respect to the vermiculite layers. In this phase, H-H vectors of water molecules are found to be perpendicular to the silicate layers (parallel to the c*-axis). This arrangement favors the H bond interaction of water molecules with oxygen atoms of the two adjacent layers. These results fit well to the location of compensating cations and water molecules deduced from XRD and ND analyses. In the case of one-layer hydrates of Li- and Ba-vermiculites, for which the amount of water molecules per cation is 3 and 6, respectively, water molecules are arranged in an asymmetric way, forming only one H bond with one of the adjacent layers. In this case, the angle between the H-H vector and the c*-axis is near 38°. This disposition is particularly well adapted to the location of compensating cations near the silicate layers. A similar arrangement of water molecules with respect to the phyllosilicate is detected in two-layer hydrates. However, in these phases compensating cations occupy the central plane of the interlayer space, and water molecules are arranged in two adjacent planes. In analyzed hydrates, mobility of water increases with the amount of water adsorbed in vermiculites. In the two-layer Na-
1H NMR Spectroscopy and Water in Vermiculite Hydrates vermiculite, the rotation of water molecules around C2 axes precedes the rotation of hydration shells around c* axes and/or anisotropic diffusion of water in the interlamellar space. In onelayer Na-vermiculite, only the anisotropic diffusion of water molecules was detected. In fully hydrated one-layer phases (Li‚3H2O; Ba‚6H2O), the rotation of hydration shells around c* axes could also be favored in parallel with the water diffusion. Acknowledgment. We thank Prof. W. E. E. Stone for technical assistance and helpful discussions. References and Notes (1) Rausell-Colom, J. A.; Sweatman, T. R.; Wells, C. B.; Norrish, K. Experimental Pedology; Butterwoths: London, 1965; p 40. (2) Le Renard, J.; Mamy, J. Bull. Gr. Franc. Argiles 1971, 23, 119. (3) Sposito, G.; Prost, R. Chem. ReV. 1982, 82, 553. (4) Boek, E. S.; Covenay P. V.; Skipper, N. T. J. Am. Chem. Soc. 1995, 117, 12608. (5) Skipper, N. T.; Refson, K.; McConnell, J. D. C. J. Chem. Phys. 1994, 94 7434. (6) Skipper, N. T.; Sposito, G.; Chang, F. R. C. Clays Clay Miner. 1995, 43, 294. (7) Hougardy, J.; Serratosa, J. M.; Stone, W.; van Olphen, H. Spec. Discuss. Faraday Soc. 1970, 1, 187. (8) Wada, N.; Kamitakahara, W. A. Phys. ReV. B 1991, 43, 2391. (9) Suzuki, M.; Wada, N.; Hines, D. R.; Whittingham, M. S. Phys. ReV. B 1987, 36, 2844. (10) Clementz, D. M.; Pinnavaia, T. J.; Mortland, M. M. J. Phys. Chem. 1973, 77, 196. (11) Woessner, D. E.; Snowden, B. S. J. Colloid Interface Sci. 1969, 30, 51. (12) Hecht, A. M.; Geissler, E. J. Colloid Interface Sci. 1970, 34, 32. (13) Shirozu, H.; Bailey S. W. Am. Mineral 1966, 51, 1124. (14) Telleria, M. I.; Slade, P. G.; Radoslovich, E. W. Clays Clay Miner. 1977, 25, 119.
J. Phys. Chem. B, Vol. 110, No. 15, 2006 7819 (15) de la Calle, C.; Suquet, H.; Dubernat J.; Pezerat, H. Clay Miner. 1978, 13, 275. (16) de la Calle, C.; Planc¸ on, A., Pons, C. H.; Dubernat J.; Suquet, H.; Pezerat, H. Clay Miner. 1984, 19, 563. (17) Wada, N.; Hines, D. R. Ahrenkiel, S. P. Phys. ReV. B 1990, 41, 12895. (18) Slade, P. G.; Stone, P. A.; Radoslovich, E. W. Clays Clay Miner. 1985, 33, 51. (19) Skipper, N. T.; Soper, A. K.; McConnell, J. D. C. J. Phys. Chem. 1991, 94 5751. (20) Skipper, N. T.; Soper, A. K.; Smalley, M. V. J. Phys. Chem. 1994, 98, 942. (21) Skipper, N. T.; Smalley, M. V.; Williams, G. D.; Soper, A. K.; Thompson, C. H. J. Phys. Chem. 1995, 99, 14201. (22) Hensen, E. J. M.; Smit, B. J. Phys. Chem. B 2002, 106, 12664. (23) Hensen E. J. M.; Tambach, T. J.; Bliek, A.; Smit B. J. Chem. Phys. 2001, 115, 3322. (24) Hougardy, J.; Stone, W. E. E.; Fripiat, J. J. J. Chem. Phys. 1976, 64, 3840. (25) Fripiat, J. J.; Kadi-Hanifi, M.; Conard J.; Stone, W. E. E. Magnetic resonance in Colloid and Interface Science; Fraissard, J. J., Resing M. A. Eds.; Reidel: Boston, 1980; p 519. (26) Pake, G, E. J. Chem. Phys. 1948, 16, 327. (27) Rausell-Colom, J. A.; Fernandez, M.; Serratosa, J. M.; Alcover, J. F.; Gatineau L. Clay Miner. 1980, 15, 37. (28) Hougardy, J.; Stone, W. E. E.; Fripiat, J. J. J. Magn. Res. 1977, 25, 563. (29) Swenson, J.; Bergman, R.; Howells, W. S. J. Chem. Phys. 2000, 113, 2873. (30) Swenson J.; Bergman, R.; Bowron, D. T.; Longeville S. Philos. Mag. B 2002, 82, 497. (31) Swenson, J.; Bergman, R.; Longeville, S. J. Chem. Phys. 2001, 115, 11299. (32) Ro¨der, U.; Mu¨ller-Warmuth, W.; Scho¨llhorn, R. J. Chem. Phys. 1979, 70, 2864.
