J. Phys. Chem. C 2008, 112, 575-580
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Geometry and Dynamics of Intercalated Water in Na-Fluorhectorite Clay Hydrates Roˆ mulo P. Teno´ rio,†,⊥ Lars Ramstad Alme,‡ Mario Engelsberg,*,§,⊥ Jon Otto Fossum,‡ and Fernando Hallwass|,⊥ Programa de Po´ s-Graduac¸ a˜ o em Cieˆ ncia de Materiais, Department of Physics, Norwegian UniVersity of Science and Technology, N-7491 Trondheim, Norway, Departamento de Fı´sica, Departamento de Quı´mica Fundamental, UniVersidade Federal de Pernambuco, 50670-901 Recife, Pernambuco, Brazil ReceiVed: August 17, 2007; In Final Form: October 18, 2007
Proton and deuterium NMR measurements are performed in Na-fluorhectorite powdered samples. The results are compared with NMR data from other 2:1 clays, and with the recent results of molecular simulations, they yield new information about the factors governing the geometry and dynamics of intercalated water in these materials. In the one-water layer regime, two different sites were identified, permitting us to elucidate the structure of interlamellar water. The role of proton exchange appears to be more pronounced in Na-Fht than in Na-vermiculite and is not limited to the 2WL regime. It appears to be promoted by a considerable amount of interlamellar water outside the hydration sphere of the cation.
I. Introduction The geometry and dynamics of water molecules intercalated in the interlamellar space of clays have been studied intensively for many years.1-4 Among several reasons that have contributed to make such a long lasting interest justifiable, one can mention: the enormous variety of clay structures found in nature, the sensitivity of water geometry and dynamics to the structure and composition of a particular clay, and the many important chemical and geological phenomena that depend upon confined water-clay interactions.5,6 The availability of synthetically manufactured clays largely free from impurities, of improved experimental techniques, and continuous advances in molecular computer simulations have contributed to the further increase in the interest in these materials both from experimental and theoretical points of view.7-13 Nuclear magnetic resonance (NMR) spectroscopy of water protons2 and/or deuterons1 has been found to be a powerful research tool in many specific cases, especially as a complement to X-ray and neutron diffraction studies,7,14 but other spectroscopic techniques such as infrared, Raman, and electron paramagnetic resonance have also been employed successfully in the study of intercalated water in clays.15 Focusing on the so-called 2:1 clays, formed by two inverted silicate tetrahedral sheets sharing their apical oxygens with an octahedral sheet, one of the most widely studied structures from the point of view of NMR is Na-exchanged vermiculite, where crystallites can be relatively large and oriented assemblies are readily attainable.2,4 Vermiculite is also interesting for a different reason. It has a rather large negative charge (1.8 e-/unit cell) in the aluminosilicate structure, caused by an aluminum for silicon substitution in the tetrahedral sites, and therefore accepts a * To whom correspondence should be addressed. E-mail: mario@ df.ufpe.br. † Programa de Po ´ s-Graduac¸ a˜o em Cieˆncia de Materiais. ‡ Department of Physics, Norwegian University of Science and Technology. § Departamento de Fı´sica, Universidade Federal de Pernambuco. | Departamento de Quı´mica Fundamental, Universidade Federal de Pernambuco. ⊥ Universidade Federal de Pernambuco.
relatively large amount of exchangeable interlayer cations with their associated hydration shells.5 It is generally believed that the substitution of aluminum for silicon in the tetrahedral sites, closer to the interlamellar region, together with the relatively large negative charge in vermiculite, are responsible for a more-ordered interlayer structure compared to other clays.5,16 As an example, the appearance of relatively disordered interlamellar structures in laponite could be attributed to the relatively small negative charge (0.4 e-/unit cell) and to the fact that it resides in the octahedral sites.5,17 In this article, we study the geometry and dynamics of intercalated water in Na-exchanged synthetic fluorhectorite (Na-Fht). In fluorhectorite, the magnesium for lithium substitution takes place at the octahedral sites, but the negative charge of fluorhectorite is also relatively large (1.2 e-/unit cell).17 As a consequence, the interlayer structure is still considerably ordered. An interesting aspect of fluorhectorite is that the OH groups of hectorite have been replaced by fluorine, making the interpretation of proton spectra less ambiguous. One drawback exists, however, compared to vermiculite. The small size of the crystallites makes it rather difficult to obtain well oriented samples. As a consequence, the results reported in this article were obtained in powders. Proton and deuterium NMR measurements in Na-exchanged fluorhectorite as a function of relative humidity and temperature in one-water layer (1WL) and two-water layer (2WL) hydrates are presented. The results suggest that the geometry and dynamics of the Na-water complexes exhibit quite significant differences with what has been observed in Na-vermiculite.2,4 In the single layer hydrate, two different types of sites for water molecules are observed. For the most abundant type, the reorientation of the water molecules around the interlamellar axis C* was found to take place in such a way that one of the OH bonds of the water molecule points in a direction almost exactly perpendicular to the silicate layers. For the less abundant type of site, this only approximately true. Moreover, the data presented here also address a different process. The role of proton chemical exchange, via the making and breaking of
10.1021/jp0766407 CCC: $40.75 © 2008 American Chemical Society Published on Web 12/22/2007
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hydrogen bonds, as well as its evolution in the transition from a single to a double layer hydrate. II. Experimental Details Samples employed in our measurements were prepared according to the procedure described in ref 7. The starting material was synthetic fluorhectorite (Corning Inc., New York), which was suspended in deionized water and subjected to ionic exchange with NaCl followed by a dialysis process to remove excess ions. The whole process, which lasted several weeks, yielded pure Na-Fht with the chemical composition Nax(Mg6 - xLix)Si8O20F4 (x ) 1.2). Powdered Na-Fht samples, obtained by this method, were then dried for 48 h at 80 °C before placing them, for at least 48 h, in a controlled relative humidity atmosphere, prior to NMR measurements. To that end, several aqueous solutions of different salts with standard compositions were employed, and the resulting relative humidity (RH) values were checked against hygrometer readings. The probehead temperature was controlled by a standard variable temperature module (Varian) that provided a resolution of (0.1 °C. After setting the desired temperature, the samples were first equilibrated for at least 15 min before data acquisition. Deuterated samples were prepared by using D2O in the saline solutions. Samples were first placed in a box saturated with D2O vapor for 3 days. Next, the container with D2O was replaced by a solution of the chosen salt in D2O and equilibrated for three more days. Finally, the samples were placed in sealed 5 mm diameter tubes, and NMR spectra were obtained in a 7.04 T magnetic field using a Varian UNITY plus-300 spectrometer. The 1H NMR spectra were obtained using by a single π/4 pulse with a 4.7 µs width and an acquisition time of 20 ms. The time delay was 0.640 s, the spectral window was 100 kHz, and the number of transients was set to 500 scans. For the adamantane 1H spectra, all of the parameters were set as mentioned above with the exception of the time delay, which was set at 10 s. For the 2H experiment, the pulse width was 12.5 µs (π/4 pulse) with an acquisition time of 5 ms, and the number of transients was set to 150 000 scans. The other parameters were left unchanged. III. Experimental Results Figure 1 shows proton NMR spectra obtained at a temperature of 20 °C in Na-Fht powdered samples equilibrated at various values of RH. The powder patterns exhibit well-defined peaks that can only be attributed to the anisotropic motion of water proton pairs with relatively weak intermolecular dipolar couplings. In contrast, the measured proton NMR spectra of synthetic powdered laponite and natural montmorillonite samples were found to be completely structureless. However, in the case of montmorillonite, which has been studied earlier by NMR,18 the broadening effect of the paramagnetic impurities may be masking the structure. For RH between 32 and 42%, at 20 °C, in addition to the larger amplitude Pake doublet with frequency splitting of ∆f1 ) 16.5 kHz, observed in Figure 1, a smaller amplitude, but still observable doublet with a splitting of ∆f2 ) 33.65 kHz can be seen. These values remain approximately constant, changing only a little provided that the value of RH does not exceed ∼45%. For example, at RH ) 11%, the measured splitting at 20 °C increased somewhat to 17.05 and 35.22 kHz. Another feature of the proton spectra is the presence of a central line with a relative intensity that depends upon water vapor partial pressure and temperature. Quite significantly, this central line is not observed in the deuterium NMR spectra at
Figure 1. 1H NMR spectra in polycrystalline Na-Fht for different values of relative ambient humidity (RH), at a temperature of 20 °C. RH values shown are: RH1 ) 32%, RH2 ) 42%, RH3 ) 65%, and RH4 ) 75%. The signal amplitude of the central peak was normalized to unity in all of the spectra.
any value of RH, a characteristic that has been recognized earlier as a signature of the chemical exchange of water protons.1,4,18 Furthermore, as shown in Figure 1, the relative amplitude of this central line increases somewhat abruptly in the RH interval of 45 to 65%, the spectrum eventually collapsing into a single line at RH ) ∼75%. This will be shown to further confirm the interpretation that the central line arises from proton chemical exchange. In a powdered sample, there is another factor that must be taken into account. Water adsorbed on the external surface of the crystallites could, given the large surface area, become a non-negligible fraction of total water. From our 2H spectra, it is possible to conclude that the effect of surface water is relatively minor and might only have some influence at the highest values of RH. Because most of the water is believed to be interlamellar, it is useful to estimate the number of water molecules per sodium ion by comparing proton Na-Fht spectra at various RH with a reference sample. To that end, we used solid adamantane (C10H16) that permitted a good comparison without any changes in spectrometer settings. From the ratio of the areas of Na-Fht to the adamantane spectra and the known chemical composition of Na-Fht, we obtained the following number of water molecules per Na+: 2.4 (RH ) 8%), 3.2 (RH ) 40%), 4.8 (RH ) 45%), 6.2 (RH ) 59%), and 10 (RH ) 67%). As will be shown below, this excess interlamellar water, compared to the Na+ coordination numbers of 2 and 4 for 1WL and 2WL NaFht, respectively,7 appears to play an important role in promoting proton exchange. To obtain a quantitative estimate of the fraction of protons for which chemical exchange takes place at a rate, which is fast compared to the dipolar width, we used a deconvolution procedure. Given that the width of the central line is a relatively small fraction of the total width, it yields acceptable results. The procedure, which involves a short parabolic interpolation joining points in the spectrum where the central line is not expected to interfere appreciably, is illustrated in Figure 2 (right). Figure 3 shows the ratio of the areas of the central peaks to the total area for Na-Fht samples equilibrated at various values of RH and a temperature of 20 °C. It shows a relatively
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Figure 2. (Left) 1H NMR spectrum in a Na-Fht sample obtained at a temperature of 30 °C. The sample was first equilibrated at 20 °C in an RH ) 42%. (Right) Deconvolution of the spectrum into two components; a central component and two Pake doublet patterns. The sum of these two components reproduces the spectrum at the left.
Figure 3. Ratio of the area of the central peak to the total area for 1H NMR Na-Fht spectra, obtained at 20 °C, as a function of relative ambient humidity.
pronounced transition taking place in the region between RH ) 45% and RH ) 65%. X-ray diffraction measurements on the same samples, employing a more elaborate RH control,19 indicate that the swelling transition from a 1WL regime, with interlayer spacing of 12.3-12.4 Å, to a 2WL regime with interlayer spacing of 15.2-15.3 Å, takes place in the range of RH ∼55-60%. It appears from Figure 3 that the ratio of the area of the central peak to the total area is sensitive to the 1WL to 2WL transition in Na-Fht. The data of Figure 3 further reinforce the conclusion that the central peak corresponds to excess interlamellar water because, for surface water, one would not expect to find such a correlation with the interlayer spacing. Figure 4 shows proton NMR spectra in Na-Fht as a function of temperature for a sample that had been previously equilibrated at 20 °C in an atmosphere with RH ) 42% corresponding to the 1WL regime. Although the central peak to total area ratio increases smoothly with temperature in a reversible manner, the spectrum does not collapse into a single line as in Figure 1. The two doublets are always detectable, albeit with a relatively small reduction in the splitting.
Figure 4. 1H NMR spectra in Na-Fht at different temperature values for a sample that had been previously equilibrated at 20 °C in an atmosphere with RH ) 42%. Temperature values shown are: T1 ) 20 °C, T2 ) 35 °C, T3 ) 45 °C, and T4 ) 75 °C. The signal amplitude of the central peak was normalized to unity in all of the spectra.
Figure 5 shows the central peak to total area ratio as a function of inverse temperature for Na-Fht samples equilibrated at two different values of relative humidity (RH ) 32 and 42%). IV. Discussion (a) Pake Doublets. The NMR spectrum of static, isolated pairs of spin 1/2 nuclei, in a high polarizing magnetic field interacting via intramolecular magnetic dipole-dipole interactions, consists of two lines with frequency splitting:2
( )
∆f(θ) ) (3/2π)pγ 2
2 1 |3 cos (θ) - 1| 2 |b| r 3
(1)
Here, br denotes the internuclear vector, θ is the angle between band r the polarizing magnetic field B B0, and γ is the gyromagnetic factor of the nuclei. If the internuclear vector br is not static but rather it reorients about a single axis (C*), at a rate which is
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Figure 5. Ratio of the area of the central peak to the total area for 1H Na-Fht spectra as a function of inverse temperature. (∆) For a sample initially equilibrated at 20 °C with RH ) 32%. (1) For a sample initially equilibrated at 20 °C with RH ) 42%.
fast compared to ∆f(θ), the frequency splitting of the Pake doublet is given by:2,21,22
∆f(ψ,φ) )
( )
(3/2π)pγ 2
2 2 1 |3 cos (ψ) - 1| |3 cos (φ) - 1| (2) 2 2 |b| r 3
where φ denotes the fixed angle between band r the reorientation axis (C*) and ψ is the fixed angle between C* and B B0. For intercalated water molecules in the interlamellar space of clays, the C* axis defined by the normal to the silicate planes has been found to play a particularly important role. Because in a powder or disordered solid the orientation of the rotation axis C* with respect to B B0 is random, the spectrum is obtained from eq 2 by a weighted average over all of the possible orientations of this axis.20-23 The resulting powder pattern exhibits two characteristic singularities, which have been widely employed as a tool for structural studies.24-26 The frequency splitting ∆fp(φ) between the two singularities can be shown to be given by the same expressions of eqs 1 and 2 by setting the angles θ ) π/2 or ψ ) π/2, respectively.20,23 The arguments leading from eqs 1 and 2 can be iterated to include a simultaneous fast reorientation about a second axis (C2). In some clays, a fast rotation about an axis C2, which is itself reorienting about C*, has been invoked to explain the dynamics of intercalated water molecules.4 For intercalated water in clays, the C2 axis coincides with the direction of the electric dipole moment of the water molecule. If the internuclear vector br is rapidly reorienting about the C2 axis, which is itself rapidly reorienting about C*, the splitting of the Pake doublet is given by:2,21,22
( )
1 |b| r 3 |3 cos2(ψ) - 1| |3 cos2(Φ) - 1| |3 cos2(δ) - 1| (3) 2 2 2
∆f(ψ,Φ,δ) ) (3/2π)pγ 2
where Φ denotes the angle between the C2 and C* axes and δ denotes the angle between the internuclear vector br and C2.
Teno´rio et al. Although the samples employed in our measurements were nonoriented powders, as revealed by X-ray diffraction measurements, the possibility of a partial alignment of the smaller crystallites in the relatively large polarizing magnetic field B B0 ) 7.04 T should be examined. It is known that the anisotropy of the diamagnetic susceptibility of Na-Fht platelets in a nematic phase is sufficiently large to cause the platelets to orient with their C* axis perpendicular to B B0, even at moderate fields.27 However, even if this somewhat unlikely alignment process were possible in a powder, its effect on the spectrum, at least regarding the frequency splitting between peaks, would be indistinguishable from that of a true powder pattern, because it would correspond to setting ψ ) π/2 in eqs 2-3. The presence of well-defined doublets in Na-Fht with frequency splitting considerably smaller than 46 kHz suggests fast anisotropic reorientation of intercalated water molecules. The 46 kHz splitting corresponds to a rigid array of proton pairs in a powder pattern, for an internuclear distance of |rb| ) 1.58 Å. Unlike laponite or montmorillonites, where the negative charge per unit cell is smaller, the alignment of the reorientation axis appears to have relatively little dispersion in Na-Fht, leading to well-resolved peaks. The structure of the water molecules in the interlamellar space of the swelling clays has been a subject of considerable controversy. According to some authors, it is mainly controlled by the type of counterion,28 whereas other authors29 claim that the structure is probably determined by the silicate itself. NMR measurements, on the other hand, tend to point to differences in structure arising from both the type of counterion and the type of silicate network. Recent molecular simulations suggest that, in 1WL and 2WL Na-exchanged montmorillonite clay, intercalated water molecules are oriented with one of the OH groups pointing along the C* axis, perpendicular to silicate planes. Different counterions do not appear to change this picture considerably.12 However, proton NMR results in 1WL vermiculite, where the negative charge resides in the tetrahedral silicate sites, agree with this picture only for some counterions such as Li+ but not Na+. This result suggests that both the type of counterion and the silicate structure can have a substantial effect upon intercalated water. The results presented here offer an interesting variation because in Na-Fht, like in montmorillonites, the negative charge resides in the octahedral silicate sites but the size of the charge is larger. Another difference between montmorillonite and vermiculite is that, in the later case, all of the water molecules appear to form complexes with the charge compensating cations,2,4 at least in the 1WL regime. In contrast, molecular simulations in montmorillonite suggest that a sizable fraction of water molecules may be localized outside the hydration sphere of the counterion, both in the 1WL and 2WL regimes.12,13 This is also borne out by our measurements of the area under the proton spectra in Na-Fht. Figure 6 shows schematically a [Na(H2O)2]+ complex in 1WL Na-Fht with its hydration shell containing two water molecules.7 Assuming that the model of eq 3 was applicable to NaFht in the 1WL regime, the angle Φ necessary to make ∆f(π/ 2,Φ,δ) coincident with the frequency splitting for the observed doublets could be calculated. Because the H-H vector is perpendicular to C2, an angle δ ) π/2 must be assumed in eq 3. Furthermore, assuming an H-H distance |rb| ) 1.58Å for the water molecule, one obtains Φ1 ) 25.6° for the ∆f1 ) 16.55 kHz doublet. Moreover, the width ∆f2 ) 33.65 kHz of the weaker doublet is too large to be accounted for by this model for any value of Φ.
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Figure 6. Schematic representation of a Na+ cation in the interlamellar space of 1WL Na-Fht. The hydration sphere, containing two water molecules, is reorienting about the C/ axis, perpendicular to the silicate planes. Figure 8. 2H spectra in polycrystalline Na-Fht at 20 °C in the 2WL regime.
Figure 7. Orientation of the C2 axis with respect to the normal C/ of the silicate planes in 1WL Na-Fht for: (Left) the most abundant type of site (∆f1 ) 16.55 kHz) and (right) the less abundant type (∆f2 ) 33.65 kHz).
Even for the larger-intensity doublet, an angle of Φ1 ) 25.6° would be difficult to justify. Assuming an HOH angle of 104°, the OH direction would be tilted away from the C* axis by 26.4°. This angle is too large compared to what is predicted by molecular simulations in montmorillonite12 and too small compared to the angle of approximately 38° inferred from proton NMR2 in 1WL Na-exchanged vermiculite. We next examine the possibility that the fast reorientation about the C2 axis of the model of eq 3 might be strongly hindered in the 1WL regime.2 From eq 2, with ψ ) π/2, we can then calculate the angle φ between the H-H vector and C* necessary to make ∆f(π/2,φ) coincident with the frequency splitting of the observed doublets. For ∆f1 ) 16.55 kHz we obtain φ1 ) 40.8°, and for ∆f2 ) 33.65 kHz we obtain φ2 ) 25.0°. Hence, for the larger-intensity doublet, the angle between the OH direction and the C* axis, the normal of the silicate planes, would be only 2.8°, whereas for the weaker doublet it would be 13°. One can conclude that a model where fast reorientation about the C* axis takes place, but reorientation about the C2 axis is strongly hindered, yields results that are in general agreement with molecular simulations in montmorillonite.12 In contrast, there appears to be a striking difference between the dynamics of intercalated water in 1WL Na-Fht and 1WL Na-vermiculite. To emphasize the general agreement with the molecular simulations of ref 12, Figure 7 shows the angles between the direction of the electric dipole moment of the water molecules and the normal of the silicate planes for the two types of Pake doublets in Na-Fht. (b) The Central Line. Although 2H NMR spectra in NaFht-powdered samples do not furnish sufficient information for an accurate determination of the orientation of the principal axes of the electric field gradient tensor (EFG) or the quadrupole coupling constants, they permit us to draw some important qualitative conclusions about the dynamics. The central line, observed in all of the proton spectra (Figure 2 and Figure 4), is
not observed in the 2H spectra even at the highest values of RH. In the 2H spectra at 20 °C, for RH ) 87 and 97% shown in Figure 8, only a very small central signal is observed at RH ) 97%, but this narrow signal is believed to arise from surface water. The absence of a central line in the 2H spectra suggests that the ubiquitous line at the center of the 1H spectra does not originate in the fast isotropic motion of some water molecules. Such an isotropic motion of the principal axes of the EFG tensor for 2H should, arguably, also lead to a central line in the deuterium spectra. The chemical exchange of protons, on the other hand, can have a more pronounced effect upon the dipolar coupling. The intramolecular dipolar coupling between a given 1H and its partner in a water molecule is substantially affected when this partner is replaced by an incoming 1H in a different spin state.4,30 In contrast, the quadrupole coupling of a 2H nucleus with the EFG at the nucleus should be affected little by the spin state of its partner nucleus.18 A proton exchange mechanism was invoked by Hougardy et al. to explain the central line in the proton spectra of 2WL Navermiculite.4 In this case, it was assumed to be promoted by a relatively small fraction of water molecules, outside the Na+ hydration shell, which relay the exchange of protons between water molecules inside the hydration shell. The same mechanism appears to be effective in Na-Fht but, in contrast to Navermiculite, the fraction of protons participating in the exchange process and the fraction of water molecules outside the hydration sphere appear to be considerably larger. Because the presence of water molecules outside the hydration shell, and the associated proton exchange mechanism, are evidenced by the presence of a central line, one can conclude from Figure 1 and Figure 3 that such water molecules should be present in Na-Fht in the 2WL regime as well as in the 1WL regime. This is in agreement with some results of recent molecular simulations in montmorillonite12 summarized schematically in Figure 9. Notice that the OH vector on the water molecule outside the hydration shell of Na+ is assumed to also point in a direction almost perpendicular to the silicate planes, tilted by the same angle as that for water molecules inside the hydration shell. The proton exchange mechanism shown in Figure 9 can be represented by:4
H2O + [Na(H2O)2]+ h H3O+ + [Na(H2O)OH]
(4)
One is tempted to associate the value ∆E ) 9.5 ( 0.5 kJ/ mol, obtained from a fit of the data of Figure 5, with the activation energy of the process represented by eq 4. This value
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Teno´rio et al. Acknowledgment. We wish to thank E. N. de Azevedo for able assistance. This work has been supported by Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico CNPQ (Brazilian agency) and by the Research Council of Norway (RCN). References and Notes
Figure 9. Schematic representation of a [Na(H2O)2]+ complex exchanging a proton with a water molecule outside the hydration shell.
is surprisingly close to 10.0 kJ/mol, the activation energy for proton exchange in water obtained from 17O NMR measurements.31 It has been pointed out, however, that such an association is, in general, not warranted.4 The area ratio of Figure 5 represents the probability that the residence time of a proton, prior to an exchange, is much shorter than a quantity of order of the inverse dipolar width (∼10-5 sec). Although the actual activation energy of the process of eq 4 partly contributes to the value obtained from Figure 5, one should rather interpret ∆E loosely as the energy required for a proton to effectively participate in the exchange process. V. Conclusions Our proton and deuterium NMR measurements in Na-Fht powdered samples, when compared with data obtained from other 2:1 clays and from molecular simulations, yield useful new information about the factors governing the geometry and dynamics of intercalated water in these materials. Because in Na-Fht and in Na-montmorillonite the negative charge is caused by aliovalent substitution in the octahedral sites, some analogies could be expected. This is borne out by the general agreement between our results and computer simulations in Namontmorillonite.12 Two different sites for intercalated water molecules could be clearly observed permitting us to elucidate the geometry and dynamics of intercalated water. Furthermore, the role of interlamellar water, not complexed to the Na+ cation, in promoting proton exchange is clarified. Proton exchange appears to be more pronounced in Na-Fht than in Na-vermiculite and is not limited to the 2WL regime. This result should enable one to better understand this process by comparing it with theoretical simulations.
(1) Porion, P.; Michot, L. J.; Fauge`re, A. M.; Delville, A. J. Phys. Chem. C 2007, 111, 5441. (2) Sanz, J.; Herrero, C. P.; Serratosa, J. M. J. Phys. Chem. B 2006, 110, 7813. (3) Smirnov, K. S.; Bougeard, D. J. Phys. Chem. B 1999, 103, 5266. (4) Hougardy, J.; Stone, W. E. E.; Fripiat, J. J. J. Chem. Phys. 1976, 64, 3840. (5) Sposito, G.; Skipper, N. T.; Sutton, R.; Park, S. -H.; Soper, A. K.; Greathouse, J. A. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 3358. (6) Cervini-Silva, J.; Larson, R. A.; Stucki, J. W. Langmuir 2006, 22, 2961. (7) Da Silva, G. J.; Fossum, J. O.; DiMasi, E.; Måløy, K. J.; Lutnæs, S. B. Phys. ReV. E 2002, 66, 011303. (8) Fossum, J. O. Physica A 1999, 270, 270. (9) Eypert-Blaison, C.; Sauze´at, E.; Pelletier, M.; Michot, L. J.; Villie´ras, F.; Humbert, B. Chem. Mater. 2001, 13, 1480. (10) Chang, F. -R. C.; Skipper, N. T.; Sposito, G. Langmuir 1998, 14, 1201. (11) Young, D. A.; Smith, D. E. J. Phys. Chem. B 2000, 104, 9163. (12) Tambach, T. J.; Bolhuis, P. G.; Hensen, E. J. M.; Smit, B. Langmuir 2006, 22, 1223. (13) Hensen, E. J. M.; Smit, B. J Phys. Chem. B 2002, 106, 12664. (14) Powell, D. H.; Fischer, H. E.; Skipper, N. T. J. Phys. Chem. B 1998, 102, 10899. (15) Sposito, G.; Prost, R. Chem. ReV. 1982, 82, 553. (16) Chang, F. -R. C.; Skipper, N. T.; Sposito, G. Langmuir 1995, 11, 2734. (17) Kaviratna, P. D.; Pinnavaia, T. J.; Schroeder, P. A. J. Phys. Chem. Solids 1996, 57, 1897. (18) Woessner, D. E.; Snowden, B. S., Jr. J. Chem. Phys. 1969, 50, 1516. (19) Alme, R. A.; Master’s Degree Thesis, Norwegian University of Science and Technology, Trondheim, Norway, 2007. (20) Abragam, A. The Principles of Nuclear Magnetism; Clarendon Press: Oxford, U.K., 1961. (21) Pake, G. E. J. Chem. Phys. 1948, 16, 327. (22) Gutowsky, H. S.; Pake, G. E. J. Chem. Phys. 1950, 18, 2. (23) Halle, B.; Wennerstro¨m, H. J. Chem. Phys. 1981, 75, 1928. (24) Engelsberg, M.; Yannoni, C. S.; Jacintha, M. A.; Dybowski, C.; Souza, R. E. J. Phys. Chem. 1994, 98, 2397. (25) Engelsberg, M.; Yannoni, C. S.; Jacintha, M. A.; Dybowski, C. J. Am. Chem. Soc. 1992, 114, 8319. (26) Engelsberg, M.; Yannoni, C. S. J. Magn. Reson. 1990, 88, 393. (27) de Azevedo, E. N.; Engelsberg, M.; Fossum, J. O.; de Souza, R. E. Langmuir 2007, 23, 5100. (28) Michot, L. J.; Villie´ras, F.; Franc¸ ois, M.; Bihannic, L.; Pelletier, M.; Cases, J. -M. C. R. Geoscience 2002, 334, 611. (29) Del Pennino, U.; Mazzega, E.; Valeri, S.; Alietti, A.; Brigatti, M. F.; Poppi, L. J. Colloid Interface Sci. 1981, 84, 301. (30) Woessner, D. E. J. Magn. Reson. 1974, 16, 483. (31) Luz, Z.; Meiboom, S. J. Am. Chem. Soc. 1964, 86, 4768.
