Structural Characterization of Pristine and Modified Fluoromica Using

ARTICLE pubs.acs.org/JPCC

Structural Characterization of Pristine and Modified Fluoromica Using Multinuclear Solid-State NMR Alice Silvia Cattaneo,†,‡,§ Silvia Bracco,‡ Angiolina Comotti,‡ Maurizio Galimberti,§ Piero Sozzani,‡ and Hellmut Eckert*,† †

Institut f€ur Physikalische Chemie, Westf€alische Wilhelms-Universit€at M€unster, Corrensstrasse 30, D-48149 M€unster, Germany Dipartimento di Scienza dei Materiali, Universita degli Studi di Milano-Bicocca, Via R. Cozzi 53, I-20125 Milano, Italy § Consorzio Ricerca Materiali Avanzati (CORIMAV), Via R. Cozzi 53, I-20125 Milano, Italy ‡

bS Supporting Information ABSTRACT: The structure of a synthetic fluoromica has been investigated and elucidated by complementary 29Si, 1H, 19F, 23Na, and 27Al single and double resonance experiments. The phyllosilicate possesses a charge heterogeneity arising from the presence of different charged sites in the octahedral sheets of the T-O-T layers. Part of the magnesium atoms in the octahedral sheets is missing, creating vacancies. Charge balancing proceeds mostly by incorporation of sodium ions into the interlayer space and substitution of Mg by sodium in the lattice. The latter process produces a nonexchangeable fraction of sodium ions (∼20% of the total inventory) and creates multiple sites for the fluoride ions, as evidenced from 19F NMR spectroscopy. Detailed quantitative information about the sodiumfluoride distance geometries were obtained from 19 23 F/ Na double rotational echo double resonance (REDOR) experiments; furthermore, different types of fluoride sites could be discriminated on the basis of 19F/23Na double resonance experiments. The chemical reactivity of this system was modified by ball milling, cation exchange with octadecylammonium ions, and grafting with an organosilane (γ-aminopropyltriethoxysilane), and the corresponding structural consequences were probed by multinuclear single and double resonance NMR techniques.

’ INTRODUCTION Layered silicates have been widely investigated because they can be used in a variety of applications in ceramics, catalysts, health care products, and advanced composite materials. For example, montmorillonite, saponite, and hectorite are extensively used in the formulation of various pharmaceuticals and cosmetics because of their high specific surface areas, suitable rheological properties, and high adsorption capacity. They are also exploited as fillers in polymer/clay nanocomposites applications because of their high aspect ratio and the tunability of the chemical interactions involved.19 For most of these applications, in order to obtain a fine-tuning of the selected chemical and physical properties, structural modifications of the pristine phyllosilicate are needed. For example, dispersion of the layered silicate in a polymer at a nanoscopic level is generally not possible using the neat phyllosilicate, but requires some degree of compatibilization using organic components. The latter can be accomplished by cation exchange reactions with either alkylammonium or alkylphosphonium ions.610 Other methods for modifying the properties of an ordered silicate are grafting with organosilanes,1115 as already employed for improving the chemical reactivity of silica particles and glass beads,1619 and mechanical grinding (nanosizing), which could increase the catalytic activity of clays.20,21 A fundamental understanding of the structural modifications induced r 2011 American Chemical Society

by these transformations, and an accurate description of the hybrid organicinorganic interface, are key prerequisites for designing innovative engineered materials. The present work is focused on a synthetic fluoromica, whose trade name is Somasif ME 100, which is widely used in both industrial and academic contexts,4,10,12,2225 although its structure has never been investigated in depth. Similar to fluorohectorite, this phyllosilicate possesses a large amount of structural fluorine atoms and a high cation exchange capacity, due to strongly negative charged layers. While it is known that the negative charge arises from the presence of some magnesium vacancies in the octahedral sheets and proper charge balancing is effected by intercalation of Naþ ions into the interlayer space,26,27 a comprehensive description of this fluoromica is not yet available. Moreover, the coexistence of layers with distinct charge loadings related to moisture adsorption and ammonium intercalation previously suggested by XRD studies remains an open problem.2729 Likewise, the present contribution addresses structural effects caused by chemical modifications, such as (i) cation exchange with octadecylammonium ions (ODA), (ii) grafting with γ-aminopropyltriethoxysilane (APS), and (iii) nanosizing via ball milling. Received: March 3, 2011 Revised: May 16, 2011 Published: May 18, 2011 12517

dx.doi.org/10.1021/jp2020676 | J. Phys. Chem. C 2011, 115, 12517–12529

The Journal of Physical Chemistry C Solid state nuclear magnetic resonance (NMR) has proven to be a very useful tool for the structural analysis of complex inorganicorganic hybrid materials and it proves particularly useful when these systems are amorphous or compositionally heterogeneous.3035 By providing structural insight at the local level and on the subnanometer scale, NMR is complementary to X-ray diffraction and TEM analysis, which are commonly employed for studying modified phyllosilicates and hybrid layered materials. Besides monitoring the structural evolution of each the inorganic and the organic components in a separate fashion, NMR can also help to characterize the specific interactions at the inorganicorganic interface, using special coherence transfer and spectral editing techniques. In the present contribution, we developed a comprehensive solid-state NMR strategy to elucidate the structure of Somasif in its neat state and after physical and chemical transformation. The characterization of the inorganic framework by 29Si, 27Al, and 19F NMR is complemented by studies of the sodium ion distribution as detected by high-resolution 23Na single and 23Na/19F double resonance NMR. Furthermore, 29Si{1H} HETCOR techniques are used for probing the organicinorganic interfaces formed in the hybrid materials.

’ EXPERIMENTAL SECTION Sample Preparation and Characterization. The synthetic fluoromica, named Somasif ME 100 (CO-OP Chemical CO., Tokyo), was used as received (FM), or after partial removal of water (FM-TT), obtained with an overnight thermal treatment at 160 °C under vacuum. The chemical composition (in mol %) is SiO2 (58.2%), Al2O3 (0.3%), Fe2O3 (0.06%), MgO (26.4%), CaO (0.10%), Na2O (5.0%), K2O (0.01%), and F (10.0%), equivalent to the approximate formula Na0.66Mg2.68(Si3.98Al0.02)O10.02F1.96.28,29,36 Somasif is obtained by a thermal treatment of talc in air at 850 °C in the presence of Na2SiF6. During the reaction the structural OH groups of talc are mostly substituted by fluorine ions. At the same time, there is a loss of Mg2þ from octahedral sites, and incorporation of Naþ ions into the structure for charge balancing. Thus, the transformation from talc to fluoromica takes place without complete disruption of the original atomic arrangement.27 The cation exchange treatment used for the incorporation of octadecylammonium was conducted in a similar way as in ref 10: 2.40 mmol of octadecylamine and 2.40 mmol of HCl were dissolved in 40 mL of water. The solution was stirred 30 min at 25 °C and then 60 min at 80 °C. Two grams of Somasif was added followed by stirring for 1 h at 80 °C. The mixture was then filtered and washed with 250 mL of H2O (FM-ODA). The grafting reaction was realized using the method described in ref 12: 0.6 g of liquid γ-aminopropyltriethoxysilane (APS) was introduced into 200 mL of a water/ethanol (25/75 V/V) mixture. The solution was heated at 80 °C under stirring, and 2 g of fluoromica was added, followed by stirring and refluxing for 5 h at 80 °C. No controlled atmosphere was employed. The reaction product (FM-APS) was filtered and washed using 500 mL of mixture water/ethanol (25/75 by volume). Several samples of ground fluoromica were prepared using a centrifugal ball mill (S 100, Retsch GmbH) at 450 rpm. A 250 mL grinding jar and seven balls of 20 mm diameter made of zirconium oxide were employed. The grinding time was varied from 30 min to 6 h in order to obtain several samples with different amounts of structural disorder.

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X-ray Powder Diffraction Analyses. XRD patterns were recorded using a Bruker D8 Avance diffractometer (Cu KR radiation) in the 2θ range to 2°60° with Δ(2θ) = 0.02° and 4 s per step as counting time. Thermogravimetric Analyses. TGA measurements were performed using a Mettler-Toledo TGA850 instrument. About 10 mg of sample was heated from 30 to 850 °C at a heating rate of 20 °C/min under nitrogen atmosphere. Solid-State NMR. Spectra were obtained on Bruker Avance 300, Bruker DSX 400, and Bruker DSX 500 spectrometers. All measurements were conducted at room temperature. Some of the spectra obtained were fitted to Gauss/Lorentz curves, using the DMFIT simulation routine.37 Quantitative 29Si MAS NMR spectra were acquired at 79.5 MHz in a 4 mm MAS probe operating at a spinning speed of 5 kHz. A 90° pulse of 2.4 μs length and a recycle delay of 400 s were used. Other 29Si MAS spectra were acquired at 59.6 MHz in a 4 mm MAS probe operating at a spinning speed of 9 kHz, using a 90° pulse of 4.0 μs length and a recycle delay of 40 s for single-pulse experiments, and using a 90° pulse of 2.9 μs length, a contact time of 3 ms, a recycle delay of 6 s, and TPPM decoupling for 29Si{1H} crosspolarization (CP) experiments. The CP dynamics on FM-ODA were studied varying the contact time, using a 7 mm MAS probe operated with a 90° pulse of 2.9 μs length, a recycle delay of 10 s, and CW-mode decoupling. Phase modulated LeeGoldburg 29 Si{1H} HETCOR experiments were acquired using RAMP-CP and TPPM decoupling.38,39 A contact time of 8 ms and a relaxation delay of 5 s were used. The 29Si{19F} CPMAS spectrum was recorded at 79.5 MHz in a 4 mm MAS probe operating at a spinning speed of 5 kHz. A 90° pulse of 4.5 μs, a contact time of 4 ms, and a recycle delay of 5 s were used. Decoupling was not needed. Chemical shifts are reported relative to TMS, using a Q8M8 sample as a secondary standard, with δ = 11.5 ppm for the trimethylsilyl groups. A 13C single-pulse MAS NMR spectrum of FM-ODA was acquired at 75.5 MHz in a 4 mm MAS probe operating at a spinning speed of 12 kHz, using a 90° pulse of 4.0 μs length, a recycle delay of 75 s, and TPPM decoupling. 13C{1H} CPMASNMR experiments were recorded using similar conditions, using a 90° pulse of 2.9 μs length, a contact time of 2.5 ms, and a recycle delay of 6 s. Crystalline polyethylene was taken as an external reference at 32.8 ppm from TMS. Single-pulse 23Na MAS NMR spectra were acquired at 132.2 MHz in a 2.5 mm MAS probe operating at a spinning speed of 22 kHz, using a 90° pulse length of 1.31.6 μs length (optimized on the sample), and a recycle delay of 10 s. 23Na{19F} rotational echo double resonance (REDOR) data were acquired at 105.8 MHz in a 4 mm MAS probe operating at a spinning speed of 15 kHz, using 180° pulses of 4.6 and 9.2 μs for 23Na and 19 F, respectively, and a relaxation delay of 8 s. The compensated REDOR sequence40,41 was used. For FM a 23Na{19F} CTREDOR experiment42 was also recorded, using a spinning speed of 14 286 Hz, 180° pulses of 3.40 μs for 23Na and 7.40 μs for 19F. 23 Na{19F} CPMAS-NMR spectra were acquired at a spinning speed of 10 kHz, using a 90° pulse of 4.4 μs length and a recycle delay of 5 s, varying the contact time. 23Na{19F} HETCOR experiments were acquired with similar conditions using two different contact times of 0.75 and 5 ms. In order to find the right HartmannHahn matching conditions, systematic spin locking experiments were performed on both nuclei in order to work in the “sudden” regime under low power conditions for the quadrupolar nuclei.4345 23Na triple-quantum MAS46 (TQ-MAS) 12518

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The Journal of Physical Chemistry C

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Figure 2. TGA curves obtained for several samples: pristine FM and modified samples. Figure 1. X-ray powder diffraction patterns obtained for (a) starting fluoromica (FM); (b) dehydrated sample (FM-TT); (c) cation-exchanged sample (FM-ODA); (d) APS-grafted sample (FM-APS); (e) 6 h ball-milled sample (FM-6h).

Table 1. (hkl) Indexing for XRD Pattern Reported in Figure 1a d(hkl) spacings (Å, (0.1)

a

(hkl)

FM

I

FM-TT

(001)

12.4

(001)II

9.6

9.6

9.7

(110)

4.8

4.8

4.6

4.5

4.6

(020)

4.6

4.6

4.4

4.2

(004)I

3.3

3.2

3.3

(004)II

2.7

3.1

3.1

(200)

2.6

2.7

2.6

2.7

(130)

2.1

2.6

2.5

2.6

FM-ODA

FM-APS

23.7/19.0/16.4

19.1/12.4 9.5

FM-6 h 9.8

Assignments are made on the basis of refs 50 and 74.

spectra were acquired at a spinning speed of 12 kHz by using the three pulse (z-filtering) sequence,47 employing similar conditions as reported previously,48 using strong preparation and mixing pulses (3.8, 1.6 μs) and soft detection pulses (10 μs). 2D acquisition was carried out in 232 steps (288 scans, recycle delay 1.5 s), using a t1 increment of 1.2 ms. Chemical shifts are reported relative to a 1 M solution of sodium chloride. 19 F MAS NMR spectra on FM, FM-ODA, and FM-APS were acquired on a 9.4 T magnet (at 376.3 MHz), in a 4 mm MAS probe operating at a spinning frequency of 10 kHz, using a single 90° pulse of 3.2 μs length and a relaxation delay of 45 s. This relaxation delay was found to yield quantitative results. Additional 19F MAS NMR spectra on FM and FM-6h were obtained on a 11.7 T magnet (observation frequency 470.4 MHz), in a 2.5 mm probe operating at a spinning frequency of 20 kHz, using a single 90° pulse of 3.2 μs length and a relaxation delay of 50 s. Chemical shifts are reported relative to CFCl3, using solid AlF3 (δ = 172 ppm) as a secondary standard. 19F{23Na} rotational echo double resonance (REDOR) data were acquired at 470.4 MHz, in a 4 mm MAS probe operating at a spinning frequency of 12 kHz,

Figure 3. 29Si single-pulse MAS NMR spectrum obtained for neat fluoromica (FM). The colored lines are line-shape deconvolution components.

using 180° pulses of 9.0 and 4.4 μs for 19F and 23Na, respectively, and a relaxation delay of 40 s. Simulations of the REDOR curves were carried out with the SIMPSON simulation program.49

’ RESULTS AND DISCUSSION X-ray Powder Diffraction. The X-ray diffraction pattern recorded for the pristine fluoromica FM (Figure 1a) is characterized by two main d(001) spacings at 12.4 Å and 9.6 Å, indicating the existence of two distinct interlayer distances in agreement with previous literature.27 The spacing d(001) equal to 12.4 Å is due to interlayer spacings where some water is present in the layers, while the spacing d(001) equal to 9.6 Å is associated with the sole presence of unhydrated sodium exchangeable ions. An overnight thermal treatment (160 °C, in vacuum) was able to remove most of the water in the interlayer space, leading to an XRD pattern with a single d(001) spacing at 9.6 Å (Figure 1b). The X-ray diffraction pattern for the fluoromica exchanged with octadecylammonium ions (FM-ODA) is reported in Figure 1c. The organic chains are intercalated into the layered structure, resulting in a shift toward larger d(001) spacings. The major reflection is associated with a layer repetition distance of 19.0 Å, suggesting that both components of the pristine material 12519



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Figure 4. 29Si MAS NMR spectra obtained for several samples ball milled for different amounts of time: (a) single-pulse experiments; (b) 29Si{1H} CPMAS NMR experiments recorded with 3 ms as contact time.

(with and without water in the interlayer space) are affected by the modification. For the fluoromica grafted with APS (FM-APS), the X-ray diffraction pattern is reported in Figure 1d. The d(001) spacing at 9.5 Å is barely visible, and a new d(001) spacing at 19.1 Å is detectable, while all the other reflections remain unchanged by the chemical reaction. Thus, the spacing at 12.4 Å, assigned to layers with incorporated water, remains, whereas the spacing at 9.5 Å, assigned to dehydrated layers, vanishes. This finding indicates that the grafting reaction must at least partially occur in the interlayer space, leading to a significant expansion reflected in the new d(001) spacing equal to 19.1 Å. In order to induce progressive structural changes on the starting fluoromica, a mechanochemical treatment by ball milling was performed for various durations (see Figure S1 in Supporting Information). Figure 1e shows the XRD pattern obtained following the most severe treatment (FM-6h). All peaks are significantly broadened and show a decreased intensity; however, some order is still retained both along the c-direction (d(001)) and in the ab-plane (observation of the (060) reflection). Thermogravimetric Analysis. The starting material is thermally stable up to 850 °C. Its thermogram (Figure 2) presents only a 1% weight loss due to surface-adsorbed water. FM-ODA presents a small loss of water around 110 °C, but is stable until 210 °C. A 30% weight loss occurs in the temperature range 210450 °C, reflecting the degradation of the organic material. Compared to the pure amine (Td = 140 °C), the ammonium ion intercalated into the clay possesses significantly higher thermal stability. The TGA curve of FM-APS shows different weight loss regimes, comprising the release of surface-adsorbed water (70100 °C), the loss of interlayer water (200350 °C), and the degradation of the organic component (350700 °C); the latter regimes possibly show some overlap. The total amount of organic phase is estimated as 5.6 wt %. Finally, the TGA results on FM-6h are similar to those observed for ball-milled nonfluorinated structures, such as talc or pyrophyllite.50,51 Such curves are usually explained with the loss of molecular water and hydroxyl groups, which are formed on the surface and on particle edges, due to the grinding process. 29 Si MAS NMR. The quantitative 29Si single-pulse spectrum recorded for FM (Figure 3) shows the presence of two distinct types of Q3 units, corresponding to a major signal at 95.3 ppm,

Figure 5. 29Si(19F) CPMAS NMR spectrum of FM recorded with 4 ms as contact time.

Figure 6. (a) Single-pulse 29Si and (b) 29Si{1H} CPMAS NMR spectra recorded for FM-APS (3 ms contact time).

and a minor signal (8% of the total units) at 98.0 ppm. As hectorite and talc present single 29Si signals, at 95 and 98 ppm respectively,5254 we can assign the peak at 95 ppm to a structure having a markedly negative charge, and the peak at 98 ppm to a structure with low or zero negative charge. While {1H}29Si cross polarization was unsuccessful for the starting fluoromica, presumably because of a very low proton concentration, the efficiency of 1H f 29Si magnetization transfer 12520



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Table 2. Values of TSiH and T1F(1H) for the Three Different Sites in FM-ODA Obtained from the 29Si{1H} Contact TimeDependent CPMAS Intensity Measurements Reported in Figure 7b site

Figure 7. (a) 29Si{1H} CPMAS NMR spectrum recorded for FM-ODA using a 3 ms contact time. (b) 29Si{1H} CPMAS signal amplitudes measured as a function of contact time for the three different sites in FMODA.

via CP was found to be significantly increased in the ball-milled samples, the strongest response resulting for the sample FM-6h (Figure 4a). This result can be attributed to the significant increase in the concentration of surface silanol groups (Q2 units) at the layer edges, caused by the grinding treatment. Indeed, single-pulse 29Si MAS NMR spectra (Figure 4b) indicate a marked increase of the number of Q2 sites after 3 h grinding. Finally, Figure 5 documents successful 19F f 29Si magnetization transfer via cross polarization in the pristine FM sample, indicating a homogeneous distribution of the fluorine atoms, which are close both types of Q3 units. The 29Si{1H} CPMAS-NMR spectrum of FM-APS (Figure 6b) reveals efficient magnetization transfer from the organic moieties grafted on the inorganic material to Q3 sites at 95 ppm. The complex line shape from 58 to 75 ppm comes from the Tn units of the organosilane grafted material. There is no signal intensity near 45 ppm, where the signals of T0 units attributed to unreacted or fully hydrolyzed APS would be expected.19,55,56 The observed line shape encompasses T2 (NH2CH2CH2CH2 Si(OSi)2OR0 units, R0 = OH or OCH2CH3), and T3 (NH2CH2CH2CH2Si(OSi)3) units. While these resonances show that the grafting reaction did occur, there is no information about the location of the grafting. In fact, Si OSi bonds can be formed both by grafting of hydrolyzed APS molecules on inorganic layers, and by condensation of two or more hydrolyzed molecules to form oligomers. Nevertheless, the strong intensity of the signal at 95 ppm observed under CPMAS condition suggests that the organic residues are in close proximity to the Q3 silicon atoms pointing toward the interlayer space. This evidence is in agreement with the change in the d(001) spacing, indicating that organosilane chains are enclosed into the interlayer space of FM. The 29Si single-pulse MAS experiment on FM-APS (Figure 6a) also indicates the disappearance of the shoulder at 98 ppm,

TSiH (ms)

T1F(1H) (ms)

Q2

1.6 ( 0.2

9.4 ( 0.9

Q3 (I)

2.4 ( 0.1

12.6 ( 0.5

Q3 (II)

3.6 ( 0.5

17 ( 1

suggesting that the talc-like sites have reacted completely with the grafting reagent. Figure 7a shows the CPMAS spectrum of FM-ODA. The intercalation of ODA into the interlayer space creates an abundant proton reservoir, facilitating efficient magnetization transfer, and both the Q3 sites can be observed in the CPMASNMR spectrum. Figure 7b summarizes the signal intensities as a function of the contact time. The dependence of the magnetization M(t) on the contact time can be described with the following equation, differentiating between various silicon sites with respect to their 1H29Si dipolar coupling strengths:31,5759 ( ! ) M0 t t exp MðtÞ ¼ exp TSiH T1F ð1 HÞ TSiH 1þ T1F ð1 HÞ In this expression T1F(1H) denotes the spinlattice relaxation times of the protons in the rotating frame. These values are influenced by the intrinsic motion of the protons in the aliphatic chains from which Si atoms receive magnetization. The crosspolarization time constant (TSiH) is governed by the magnetic dipolar interactions between the aliphatic chain protons and the Si atoms. Interestingly, the two different Q3 units in fluoromica show markedly different CP dynamics (see Table 2). The significantly shorter TSiH observed for the Q3 sites at 95 ppm suggests enhanced 1H29Si dipolar interactions, consistent with shorter Si/H distances. This behavior is consistent with a stronger interaction of the ODA cation with the negative charge of the hectoritelike environment, while weaker interaction is shown for the talc-like structures. The surface Q2 units, which are located at layer edges, show the shortest T1F and TSiH values. The CP dynamics of these units are due to the contribution of both geminal silanol groups and aliphatic chains. 29 Si{1H} PMLG-HETCOR. In order to explore the organic inorganic interfaces further, 2D heteronuclear correlation experiments (HETCOR) on FM-ODA and FM-APS were carried out. Using the phase modulated LeeGoldburg (PMLG) HETCOR pulse sequence, it is possible to obtain information about the spatial proximities between Si and the various proton species within distance e1 nm.3234 Figure 8a shows the results obtained on FM-ODA. While the Q3 units resonating at 98 ppm are seen to be correlated with the protons belonging to the aliphatic chains of the octadecylammonium ions (δH = 1.2 ppm), the Q3 units resonating at 95 ppm also show strong correlations with protons located on the polar heads groups (δH = 6.9 ppm). This result may indicate that the hectorite-like structure element interacts more strongly with the polar heads of the surfactants than the talc-like structure element. Nevertheless, both layer types are in close proximity to the organic ions, demonstrating that the cation exchange reaction has occurred within both structure elements. 12521



Figure 8.

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29

Si{1H} PLMG-HETCOR MAS NMR spectra recorded, using a contact time of 8 ms, for (a) FM-ODA and (b) FM-APS.

Table 3. Assignments of the 13C MAS-NMR Spectrum of FMODA (Figure 9) site

Figure 9. 13C MAS NMR spectrum acquired for FM-ODA using 75 s as recycle delay. Solid colored lines are line-shape components of the simulated spectrum.

Figure 8b shows the analogous results on FM-APS. Both the Q- and the T-types of silicon units reveal strong correlation with protons, but the maxima of the 2D cross peaks observed in the 1H dimension are different. The cross peak related to the Tn units presents a maximum intensity at 0.9 ppm in the 1H dimension, corresponding to the protons of the CH2 group directly bonded to Si of the silane tails. In contrast, the Q3 units appear to show the strongest correlation with the protons in R position to the amino group (δH = 2.6 ppm). Q2 units resonating at 88 ppm also interact with the aminosilane. They show strong correlations with both protons of the CH2 directly bonded to Si resonating at 0.9 ppm, and to protons of the polar head resonating at about 3.5 ppm. 13 C MAS NMR. Figure 9 shows the single-pulse 13C MAS NMR spectrum of FM-ODA. The poor resolution observed in this spectrum suggests reduced chain mobility. As the ammonium group strongly interacts with the polar surfaces of fluoromica, its mobility is greatly hindered, as is also suggested by the broadening of the C1 signal (near 42 ppm). The strongest signal assigned to the carbon atoms C4C15 of the aliphatic chain can be deconvoluted into three distinct components, corresponding to different chain conformations: a transtrans (T-T) conformation (signal at 33.7 ppm), and two transgauche conformations, having different degrees of mobility. The peak at 32.6 ppm is assigned to a more rigid component T-G(r), while the broader peak at 30.4 ppm is assigned to a more mobile component T-G(m), in agreement with previous literature on similar systems.31,57,60

chemical shift (ppm, ( 0.1)

C1

41.4

C2

40.1

C3

25.6; 28.0

C4C15

30.4; 32.6; 33.7

C16

34.8; 35.6

C17

21.9; 24.0

C18

15.1

Line-shape simulation of Figure 9 yields a T-T:T-G(r):T-G(m) ratio of 42:26:32. Peak splittings are also observed for the minor signals assigned to C3, C16, and C17 (see Table 3). 13C{1H} CPMAS studies (not shown) show a very similar line shape, suggesting that the organic chains are in a relatively rigid environment. Figure 10 shows the 13C{1H} CPMAS spectrum of FM-APS. The signals at 9.9 and 12.6 ppm can be assigned to methylene groups in γ position to the amino group. The signals at 22.7 and 25.8 ppm can be assigned to CH2 in β position, while the peak at 44.5 ppm arises from the methylene units directly bonded to the amino group. The peak splitting observed for the β and γ methylene groups and the minor signal at 165 ppm were observed also by Weeding et al.19 for APS-grafted glass microspheres. The peak splitting is attributed due to the coexistence of neutral and protonated aminosilane chains, while the signal at 165 ppm is assigned to a bicarbonate species formed during the grafting by reaction of the amine with CO2 from the atmosphere. The spectra give no indication of residual ethoxy groups, indicating complete reaction of the organosilane with the clay surface and total removal of ethanol during the washing procedure. 23 Na MAS NMR. The single-pulse 23Na MAS spectrum recorded for FM is reported in Figure 11a. All the signal components are strongly affected by quadrupolar broadening; however, three distinct contributions can be recognized. The signal observed at the resonance shift of 37 ppm (isotropic chemical shift near 40 ppm) can be assigned to nonexchangeable ions, while the others arise from exchangeable sodium ions.28 This assignment is confirmed by the spectrum of the ion-exchanged FM-ODA sample (Figure 11c), which shows only the peak near 37 ppm. Using peak integration on the signal in Figure 11a, we estimate the ratio of exchangeable to nonexchangeable sodium ions in FM to be near 80:20. 12522



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Table 4. Isotropic Chemical Shift δCS and Second-Order Quadrupolar Effect SOQE for Different Sodium Species Obtained from the Analysis of the 23Na 3Q-MAS-NMR Spectra of FM, FM-APS, and FM-6h (Figure 11) δCS (ppm, ( 0.2) FM

FM-APS

FM-6h

nonexchangeable Na

41.7

40.4

40.9

exchangeable Na (I)

2.3

2.4

3.3

exchangeable Na (II)

19.8

19.7

SOQE (MHz, ( 0.1)

Figure 10. 13C{1H} CPMAS NMR spectrum recorded for FM-APS using a contact time of 2.5 ms.

FM

FM-APS

FM-6h

nonexchangeable Na

1.6

1.6

1.6

exchangeable Na (I)

2.9

2.9

exchangeable Na (II)

1.5

1.7 1.2

Figure 12. Experimental 23Na{19F} REDOR curves obtained from both nonexchangeable and exchangeable sodium ions, illustrating the large difference in NaF distances between the two types of sodium species. Data collected at 15 kHz as spinning speeds are reported in red, while the data collected at 10 kHz are reported in black.

Figure 11. 23Na single-pulse MAS NMR spectra obtained for (a) FM, (b) FM-TT, (c) FM-ODA, (d) FM-APS, and (e) FM-6h. 23Na 3Q-MAS NMR spectra recorded for (f) FM, (g) FM-APS, and (h) FM-6h.

The 23Na 2D-TQMAS NMR spectrum of FM (Figure 11f) shows significantly increased resolution allowing the isotropic chemical shifts (δCS) and the second-order quadrupole effect SOQE = CQ(1 þ η2/3)1/2 to be extracted (see Table 4).61 The exchangeable Naþ ions present a dominant component with δCS = 19.8 ppm, attributed to nonhydrated sodium ions in the interlayer space. The weak signal with δCS = 2.3 ppm is assigned to hydrated exchangeable Naþ ions that are coordinated with water molecules trapped into the interlayer space, as previously observed in sodium silicate hydrates.62,63 Dehydration by heating the sample at 160 °C for 16 h leads to the disappearance of this signal (Figure 11b). The peak also shows a significantly shorter spinlattice relaxation time compared to other Naþ ions, and it disappears when the sample is cooled down to 73 °C (data not shown). All these results suggest that the signal observed at δCS = 2.3 ppm represents a highly mobile hydrated Naþ species, whose dynamics are frozen at lower temperatures.

The MAS NMR spectrum recorded for FM-APS is reported in Figure 11d. The signal at about 14 ppm is significantly broadened and decreased in intensity. Furthermore, peak integration reveals that about 30% of the exchangeable Naþ was exchanged, suggesting that a corresponding number of protonated amino groups have been intercalated into the interlayer space. In the TQMAS experiment (Figure 11g) the broad peak is resolved in only one peak in the isotropic spectrum, suggesting that all of these ions are in the same chemical environment. The absence of the 23Na NMR signal near 20 ppm in this sample agrees well with the absence of the d(001) spacing related to nonhydrated interlayer space observed in the XRD pattern of FM-APS. Figure 11, e and h, shows the MAS- and TQ-MAS NMR spectra of FM-6h. For the sodium in the interlayer space, a highly distorted environment is observed, which is characterized by strong second-order quadrupolar broadening effects and resembles that of the hydrated Naþ ions in the interlayer space as found in FM-APS. Systematic investigation of the spectra as a function of sample milling time reveals progressive line-broadening effects, reflecting an increasing fraction of sodium in the hydrated environment (see Figure S2 in Supporting Information). 23 Na{19F} REDOR. The two types of Naþ species observed in FM can be discriminated further on the basis of their proximity to the fluoride ions as determined from 23Na{19F} REDOR studies. Indeed, Figure 12 reveals a dramatic difference in 23Na19F 12523



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Figure 13. (a) Schematic picture of the fluoromica structure. The local environment of the Naþ ions (green) with the fluoride species (red) is reported in (b) for the nonexchangeable and in (c) for the exchangeable ions.

dipolar coupling strengths. For the nonexchangeable sodium ions, the extremely steep REDOR curves reveal that the fluorine atoms are within the first coordination sphere of these ions, as would be expected if the ions are located inside the T-O-T layer. In contrast, for the exchangeable sodium ions, the dipolar interactions with 19F dipolar couplings are substantially weaker, consistent with the location of these species in the interlayer space. In principle, these REDOR curves contain quantitative information on internuclear Na 3 3 3 F distances and (in systems comprising more than two interacting spins) on the relative orientations of the corresponding internuclear distance vectors. For well-defined spin geometries, such information can often be extracted by comparing the experimental data with appropriate simulations, using the SIMPSON code.49 In the present samples, guidance for the interpretation of these REDOR curves can come from the published crystal structures of a potassiummica, having the compositional formula (KMg2.7Li0.2Na0.1Si3.3Fe0.7O12F2) and a Cs-exchanged hectorite (Cs0.6Mg2.4Li0.6Si4O10F2).64,65 In these crystal structures, each Mg2þ resides at the center of a distorted octahedron and has two fluoride ions in the first coordination sphere. In principle, these fluoride ions could be arranged either in a cis configuration (Mg(I) site, FNaF angle 90°) or in a trans configuration (Mg(II) site, F NaF angle 180°). In both structures, the FNaF angle for the cis configuration is close to 83° and the Mg(I)/Mg(II) ratio is 2:1. Substitution of Na for Mg (nonexchangeable sodium ions) in Somasif can thus result in two distinct three-spin systems, corresponding to (a) an orthogonal or (b) a linear 19F23Na19F arrangement. As the REDOR curves are quite sensitive to the angles between the two internuclear vectors,66,67 these two scenarios can be differentiated by this experiment. In the case of very strong dipolar couplings, as observed here for the nonexchangeable sodium sites, the most accurate results are generally available using the constant time (CT) version of the REDOR experiment.42 Figure 14 compares such 23Na{19F} CT-REDOR data with simulated curves by assuming that each Naþ ion interacts with two 19F nuclei, at different NaF distances between 1.95 and 2.00 Å (second neighbor fluorine atoms are at least 3.7 Å away). Figure 14a indicates excellent agreement of the experimental data, assuming NaF distances of 1.95 Å and the FNaF angles of 83°. However, simulation assuming an angle of 90° is satisfactory as well. In contrast, Figure 14b indicates that the angle of 180° would result in a substantially weaker REDOR effect than observed experimentally. Finally, Figure 14c simulates a statistical substitution scenario, corresponding to a superposition of Figure 14a,b in a 2:1 ratio. Clearly our results indicate that the nonexchangeable sodium ions exclusively substitute in the Mg(I) sites.

Figure 14. 23Na{19F} CT-REDOR curves obtained for the nonexchangeable Naþ ions. Experimental data are compared with simulations for FNaF three-spin systems as discussed in the text (assuming three different NaF distance values). (a) FNaF angle 83° (Mg(I) site), (b) FNaF angle 180° (Mg(II) site), and (c) weighted superposition of (a) and (b) in a 2:1 ratio, corresponding to a statistical substitution scenario.

Figure 15. 23Na{19F} REDOR curves obtained for the exchangeable Naþ ions. Experimental data are compared with simulations for F NaF three-spin systems as discussed in the text, assuming FNaF angles of 170° and 180° and three distinct displacements of the Na ions from the center interlayer space. Also shown is the simulated scenario with the Naþ ions occupying the center. In (a) and (b) data set recorded at 10 kHz, in (c) and (d) data set recorded at 15 kHz. 12524



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Figure 16. 19F MAS NMR spectra acquired for (a) FM and (b) FM-6h (11.7 T magnet with a spinning speed of 20 kHz); (c) FM; (d) FM-ODA; and (e) FM-APS (9.4 T magnet with a spinning speed of 10 kHz). Solid colored curves are line-shape deconvolution components summarized in Table 5.

Table 5. Quantification of the Different Fluorine Species from 19F MAS NMR fluorine site quantification (%, (0.2) δCS (ppm, (0.1)

a

a

FM

FMb

FM-6 ha

FM-ODAb

FM-APSb

183.1

3.3

4.4

1.9

1.1

0.6

182.2

8.9

7.8

4.0

5.7

4.7

181.2 180.4

8.8 8.0

8.8 7.1

5.5 7.9

12.3 8.9

12.9 7.6

179.1

10.5

12.9

7.6

12.9

13.2

178.4

8.7

6.6

17.9

6.6

7.0

177.3

15.1

15.6

17.5

15.6

13.8

176.3

13.2

13.9

15.6

13.5

14.7

175.4

17.8

15.4

18.8

15.8

16.8

174.2

4.9

6.5

9.4

6.8

7.5

173.2

0.7

0.9

3.7

1.0

1.1

With a 11.7 T magnet. b With a 9.4 T magnet.

Similar quantitative considerations are possible for the exchangeable sodium species. As Figure 12 indicates, these sodium ions experience much weaker 23Na19F dipolar interactions, suggesting distances around 4 Å, as expected from the mica structure.64 In the fluoromica, the tetrahedral sheets (T) containing Si are constituted by hexagonal rings of tetrahedra linked by sheared oxygens, so that the apical oxygens form hexagonal cavities. These cavities are directly communicating with the interlayer space. The fluorine atoms inside the octahedral sheet (O), which is between the two T-sheets, are located at the center of each ring in the internal part of the hexagonal cavities, and their local environment can be influenced by the exchangeable sodium cations in the interlayer space. Figure 13 shows that the closest spatial sodiumfluorine relationship constitutes a nearly linear

Figure 17. 19F{23Na} REDOR experiments: (a) spectra obtained on FM for several selected dipolar evolution times; (b) deconvolution of the REDOR spectrum recorded for FM, and REDOR curves obtained on FM and FM-ODA for individual spectral components indicated.

F23Na19F three-spin system, with a fluorinefluorine distance of about 8.0 Å, in agreement with the proposed XRD structure. More detailed information on the exact spin geometry can be extracted by the comparison of 23Na{19F} REDOR curves with simulations (see Figure 15). In these simulations, both the 19

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Figure 18. (a) 23Na{19F} CP experiments obtained for sample FM varying the contact time (stars indicating spinning sidebands); (b) CP dynamics curves obtained for both sodium signals. Solid curves are guides to the eye.

angles between the two dipolar vectors and the dislocation of the sodium ions from the interlayer center have been systematically varied. Comparison of two distinct experimental data sets at MAS spinning speeds of 10 and 15 kHz with these simulations leads to optimal agreement with a FNaF angle of 170 ( 10° and two NaF distances of 3.4 ( 0.1/4.6 ( 0.1 Å for the REDOR curve measured at 10 kHz and 3.5 ( 0.1/4.5 ( 0.1 Å for the REDOR curve measured at 15 kHz. The data clearly rule out FNaF angles below 170° and a symmetric position of the sodium ions in the interlayer space, which would result in a considerably weaker REDOR effect than seen in the present study. 19 F MAS NMR. Figure 16 shows the 19F spectra of the pristine and the modified FM samples. Similar spectra are observed for FM, FM-APS, and FM-ODA, indicating that the corresponding modifications proceed with preservation of the bulk intralayer structure. In contrast, ball milling results in a progressive loss of resolution due to a broadening of all signals (see Figure 16b and Figure S3 in the Supporting Information), assigned to the concomitant structural disorder produced. Furthermore, a careful quantification of the fluorine amount in FM and FM-6h was obtained by quantitative 19F single-pulse experiments (not shown) of mixtures of each sample with a reference material as NaF (materials were mixed in order to get a 1:2 molar ratio). About 10% of the initial fluorine content is missing after 6 h of ball milling. The complex line shape observed for FM can be simulated with a manifold comprising at least 11 peaks. A tentative deconvolution is suggested in Figure 5. This set of 11 resonances (with identical respective line widths in sample-tosample comparisons) is the simplest one that can be found to reproduce the experimental spectra of all the modified samples, using the relative peak amplitudes as the only adjustable parameters. Comparing our results with the scarce 19F NMR data on phyllosilicates available in the literature,6872 the large number of spectroscopically distinct fluorine species appears to be a unique feature of the present system. In natural fluorohectorite, onethird of Mg2þ ions in the octahedral layers are substituted by Liþ ions, due to isomorphic substitution. As a consequence, F atoms located in the octahedral layers can experience substantially two different environments: F atoms surrounded by MgMgMg octahedra (about 176 ppm), and F atoms surrounded by MgMgLi octahedra (about 183 ppm). Furthermore, the peak at 176 ppm was reported to be 4-fold-split, due to

different arrangements of F and OH groups relative to the Mg atoms in these octahedra.68 By analogy, we assign the 19F NMR signals in the region 180 to 184 ppm observed in the present material to F surrounded by MgMgNa octahedra. This idea will be tested by 19F{23Na} double resonance experiments as detailed below. 19 23 F{ Na} REDOR. Figure 17a summarizes 19F{23Na} REDOR data of FM for selected dipolar evolution times, clearly revealing the presence of significant 23Na dipolar fields at the fluorine sites, which also appear to be correlated with the 19F chemical shifts. Based on the stronger reduction of the signals ranging from 180 to 183 ppm at shorter evolution times, we can state that the fluorine atoms resonating in this range have sodium cations in their first coordination spheres, whereas the other types of fluoride ions do not. At longer evolution times (∼1 ms), the effect of dipolar couplings with 23Na spins are also perceptible in the REDOR results obtained for the other fluorine species, although they are significantly weaker. Comparing the REDOR curves calculated for peaks resonating at 182 and 180 ppm for the FM and the ion-exchanged FM-ODA samples (see Figure 17b), one can further observe that in the evolution time range from 0 to 1.5 ms the data are similar, whereas at longer evolution times pronounced differences are observed. This result confirms our interpretation that the dephasing at short evolution times is dominated by the nonexchangeable Naþ sites, whereas the gradual REDOR dephasing at longer evolution times arises from the interaction of fluorine ions with the exchangeable Naþ ions in the interlayer spaces. As expected, the effect of the latter dephasing contribution is absent in FM-ODA (see Figure 17), where the exchangeable Naþ ions are missing. 23 Na{19F} CP Dynamics and HETCOR. The proximity of sodium ions to fluorine sites can also be characterized by the 23 Na{19F} cross-polarization dynamics, as probed by variable contact time experiments. As Figure 18 illustrates, the magnetization transfer from 19F to nonexchangeable sodium ions is already effective with a contact time equal to 0.15 ms, whereas the signal buildup of the exchangeable Naþ ions is much slower. The weak signal with δCS = 2.3 ppm assigned to hydrated exchangeable sodium ions is not observable here, presumably owing to a short rotating frame relaxation time caused by atomic dynamics. Figures 19 and 20 show 23Na(19F) HETCOR experiments recorded respectively with contact times of 0.75 and 5 ms. For 12526



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Figure 19. 23Na {19F} HETCOR recorded on FM with 0.75 ms as contact time: (a) 2D experiment with the projections; (b) slices obtained at the cross section corresponding to the 23Na resonance of the nonexchangeable Na ions.

Figure 20. 23Na {19F} HETCOR obtained for FM, using a contact time of 5 ms. The 2D spectrum is plotted (a) as a contour plot with projections on both 19F and 23Na dimension and (b) as a 3D picture, in which the z-axis denotes the intensity of the 2D peaks. In (c) the slices obtained in both dimensions for some maxima of the two 2D peaks are reported. Each pair of slices is labeled with a number, which indicates the location of the cross peak in the 2D spectrum.

the exchangeable sodium ions in the interlayer space, we observe the expected correlation with all the fluorine species independently of the contact time used. In contrast, marked differences appear for the nonexchangeable sodium ions: at short contact times they correlate only with the fluorine species observed in the range from 180 to 184 ppm, consistent with the 19F(23Na) REDOR results, whereas at long contact times (5 ms) the nonexchangeable sodium ions are able to feel the proximity of all types of fluorine. Even in this data the stronger interaction with fluorine atoms having a chemical shift ranging from 180 to 184 ppm is evident. Weaker interactions are shown by the other fluoride sites, which fall into two different subgroups: one giving rise to signals from 174 to 177 ppm and the other giving rise to signals from 178 to 180 ppm. To proceed with an assignment, it is important to consider that the compositional formula of this fluoromica, Na0.67Mg2.68(Si3.98Al0.02)O10.02F1.96,28,29,36 means that

in 0.33 octahedral sites (“empty” octahedral sites) per unit cell the magnesium atom is missing, in such a way that the negative charge is compensated by 0.67 sodium cations. From the integration of the 23Na MAS NMR spectrum, 80% of these sodium ions are in the interlayer space, and 20%, i.e., 0.13 sodium ions per unit cell, are occupying Mg sites within the layers. If only 0.13 “empty” octahedral sites are filled with sodium ions, generating MgMgNa octahedra, the other 0.20 sites per unit cell must be described as MgMgvacancy octahedral environment for the fluorine atoms. Based on the 19F{23Na} REDOR results and the variable contact time 23Na{19F} HETCOR experiments, the 19F resonances observed in Figure 16 can be grouped into three categories, corresponding to different fluorine octahedral environments: (1) peaks appearing in the range from 180 to 184 ppm come from fluorine atoms surrounded by MgMgNa configurations; (2) 12527


The Journal of Physical Chemistry C peaks from 174 to 177 ppm arise from F surrounded by MgMgMg configurations as also reported in the previous literature for fluorohectorite;6872 and (3) peaks from 178 to 180 ppm must be assigned to F surrounded by MgMgV octahedra. Finally, the presence of a small amount of aluminum in the Si position in the T-sheets (see 27 Al NMR spectrum in Figure S5 in the Supporting Information), and a small number of OH groups not substituted by F in the O-sheets are expected to generate additional local environment variations. Unfortunately, this idea could not be tested by 19F/27Al double resonance experiments, since the Al content of this fluoromica is too low.

’ CONCLUSIONS Combining the results obtained, we are able to draw a detailed picture of the structure of the fluoromica. The sample presents a complex charge distribution, because about 8% of the layers in the sample have less negative charge than expected. However, these layers do not constitute a segregated phase in the inorganic material but are randomly distributed in it, and homogeneous cation exchange can be performed, as proved by TGA analysis, 29 Si{1H}CP dynamics, and 2D-heteronuclear correlation experiments performed on FM-ODA. The 23Na NMR spectra demonstrate the presence of three different kinds of sodium ions, one of which cannot be exchanged during the reaction with alkylammonium ions. The exchangeable ions respond to the coordination with water molecules trapped in the interlayer space. If there is much more water trapped into the galleries, as it happens in the grafted sample (FM-APS), the 23 Na NMR signal is significantly broadened. The absence of the sharper signal near 20 ppm in this sample suggests that all the exchangeable Na ions are strongly affected by water coordination. Detailed information regarding the local coordination of the sodium ions (NaF distances and FNaF angles) can be obtained by 23Na{19F} REDOR experiments. The data suggest that the nonexchangeable Naþ ions are found exclusively in the Mg(I) sites (no substitution in the Mg(II) sites). For the exchangeable Naþ ions the data suggest that their position in the interlayer space is displaced by ∼0.5 to 0.6 Å from the center, resulting in one shorter and one longer NaF distance. The latter result is in excellent agreement with the molecular dynamics simulations that supported the presence of dislocations. Through the hexagonal cavities in the tetrahedral sheets, the exchangeable sodium ions are selectively attracted by the magnesium vacancies of the O-sheet of the T-O-T layers.73 The fluorine atoms in the octahedral sheets are very sensitive to the different chemical environments they explore, resulting in multiple 19F resonances that can be partially assigned to fluoride ions in charged MgMgNa, charged MgMgvacancy, as well as neutral MgMgMg octahedral environments, using 19 F{23Na} REDOR and 23 Na{19F} HETCOR experiments. Solid-state NMR is also very useful for characterizing the various sample modifications undertaken on the pristine sample. Ball-milling enhances the Q2 site concentrations, and results in fluorine loss and a significant increase in structural disorder. Cation exchange of the removable Naþ ions with alkylammonium ions increases the interlayer spacing but otherwise leaves the T-O-T layer structure unmodified.

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’ ASSOCIATED CONTENT

bS

Supporting Information. Additional XRD patterns and MAS NMR spectra are reported. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors are grateful to Dr. Marco Arimondi (Pirelli Labs S.p.A.) for the TGA measurements and to Dr. Fabio Negroni (Pirelli Tyres S.p.A.) for technical help in the preparation of milled samples. A.S.C. thanks CORIMAV for a Ph.D. scholarship and Dr. Henning Trill for useful discussions.

’ REFERENCES (1) Aguzzi, C.; Cerezo, P.; Viseras, C.; Caramella, C. Appl. Clay Sci. 2007, 36, 22. (2) L opez-Galindo, A.; Viseras, C.; Cerezo, P. Appl. Clay Sci. 2007, 36, 51. (3) Choy, J. H.; Choi, S. J.; Oh, J. M.; Park, T. Appl. Clay Sci. 2007, 36, 122. (4) Dizman, B.; Badger, J. C.; Elasri, M. O.; Mathias, L. J. Appl. Clay Sci. 2007, 38, 57. (5) Centi, G.; Perathoner, S. Microporous Mesoporous Mater. 2008, 107, 3. (6) de Paiva, L. B.; Morales, A. R.; Valenzuela Díaz, F. R. Appl. Clay Sci. 2008, 42 (12), 8. (7) Alexandre, M.; Dubois, P. Mater. Sci. Eng. 2000, 28, 1. (8) Sinha Ray, S.; Okamoto, M. Prog. Polym. Sci. 2003, 28, 1539. (9) Paul, D. R.; Robeson, L. M. Polymer 2008, 49, 3187. (10) Kaempfer, D.; Thomann, R.; M€ulhaupt, R. Polymer 2002, 43, 2909. (11) Herrera, N. N.; Letoffe, J. M.; Putaux, J. L.; David, L.; Bourgeat-Lami, E. Langmuir 2004, 20, 1564. (12) He, H.; Duchet, J.; Galy, J.; Gerard, J. F. J. Colloid Interface Sci. 2005, 288, 171. (13) Tonle, I. K.; Diaco, T.; Ngameni, E.; Detellier, C. Chem. Mater. 2007, 19, 6629. (14) Park, K. W.; Jeong, S. Y.; Kwon, O. Y. Appl. Clay Sci. 2004, 27, 21. (15) Yuan, P.; Southon, P. D.; Liu, Z.; Green, M. E. R.; Hook, J. M.; Antill, S. J.; Kepert, C. J. J. Phys. Chem. C 2008, 112, 15742. (16) Caravajal, G. S.; Leyden, D. E.; Quinting, G. R.; Maciel, G. E. Anal. Chem. 1988, 60, 1776. (17) Ek, S.; Iiskola, E. I.; Niinist€o, L.; Vaittinen, J.; Pakkanen, T. T.; Root, A. J. Phys. Chem. B 2004, 108, 11454. (18) Kovalchuk, T.; Sfihi, H.; Kostenko, L.; Zaitsev, V.; Fraissard, J. J. Colloid Interface Sci. 2006, 302, 214. (19) Weeding, T. L.; Veeman, W. S.; Jenneskens, L. W.; Angad Gaur, H.; Schuurs, H. E. C.; Huysmans, W. G. B. Macromolecules 1989, 22, 706. (20) Yariv, S.; Lapides, I. J. Mater. Synth. Process. 2000, 8 (34), 223. (21) Christidis, G. E.; Dellisanti, F.; Valdre, G.; Makri, P. Clay Miner. 2005, 40, 511. (22) Varghese, S.; Karger-Kocsis, J. Polymer 2003, 44, 4921. (23) Finnigan, B.; Jack, K.; Campbell, K.; Halley, P.; Truss, R.; Casey, P.; Cookson, D.; King, S.; Martin, D. Macromolecules 2005, 38, 7386. (24) Miwa, Y.; Drews, A. R.; Schlick, S. Macromolecules 2006, 39, 7386. (25) Panek, G.; Schleidt, S.; Mao, Q.; Wolkenhauer, M.; Spiess, H. W.; Jeschke, G. Macromolecules 2006, 39, 2191. 12528


The Journal of Physical Chemistry C (26) Tateyama, H.; Tsunematsu, K.; Kimura, K.; Hirosue, H.; Jinnai, K. EP0326019B1. (27) Tateyama, H.; Nishimura, S.; Tsunematsu, K.; Jinnai, K.; Adachi, Y.; Kimura, M. Clays Clay Miner. 1992, 40 (2), 180. (28) Tateyama, H.; Noma, H.; Nishimura, S.; Adachi, Y.; Ooi, M.; Urabe, K. Clays Clay Miner. 1998, 46 (3), 245. (29) Yang, J. H.; Han, Y. S.; Choy, J. H.; Tateyama, H. J. Mater. Chem. 2001, 11, 1305. (30) Hou, S. S.; Beyer, F. L.; Schmidt-Rohr, K. Solid State Nucl. Magn. Reson. 2002, 22, 110. (31) Simonutti, R.; Comotti, A.; Bracco, S.; Sozzani, P. Chem. Mater. 2001, 13, 771. (32) Sozzani, P.; Bracco, S.; Comotti, A.; Mauri, M.; Simonutti, R.; Valsesia, P. Chem. Commun. 2006, 1921. (33) Bracco, S.; Comotti, A.; Valsesia, P.; Chmelka, B. F.; Sozzani, P. Chem. Commun. 2008, 4798. (34) Paul, G.; Steuernagel, S.; Koller, H. Chem. Commun. 2007, 5194. (35) Duer, M. J. Introduction to Solid-State NMR Spectroscopy; Blackwell Publishing Ltd.: Hoboken, NJ, 2004. (36) Tateyama, H.; Scales, P. J.; Ooi, M.; Nishimura, S.; Rees, K.; Healy, T. W. Langmuir 1997, 13, 2440. (37) Massiot, D.; Fayon, F.; Capron, M.; King, I.; Le Calve, S.; Alonso, B.; Durand, J. O.; Bujoli, B.; Gan, Z.; Hoatson, G. Magn. Reson. Chem. 2002, 40, 70. (38) Vinogradov, E.; Madhu, P. K.; Vega, S. Chem. Phys. Lett. 1999, 314, 443. (39) Van Rossum, B. J.; F€orster, H.; De Groot, H. J. M. J. Magn. Reson. 1997, 124, 516. (40) Eckert, H.; Elbers, S.; Epping, J. D.; Janssen, M.; Kalwei, M.; Strojek, W.; Voigt, U. Top. Curr. Chem. 2004, 246, 195. (41) Chan, J. C. C.; Eckert, H. J. Magn. Reson. 2000, 147, 170. (42) Echelmeyer, T.; van W€ullen, L.; Wegner, S. Solid State Nucl. Magn. Reson. 2008, 34, 14. (43) Vega, A. J. Solid State Nucl. Magn. Reson. 1992, 1, 17. (44) Eastman, M. A. J. Magn. Reson. 1999, 139, 98. (45) Puls, S. P.; Eckert, H. J. Phys. Chem. B 2006, 110, 14253. (46) Medek, A.; Harwood, J. S.; Frydman, L. J. Am. Chem. Soc. 1995, 117, 12779. (47) Amoureux, J. P.; Fernandez, C.; Steuernagel, S. J. Magn. Reson. A 1996, 123, 116. (48) Z€uchner, L.; Chan, J. C. C.; M€uller-Warmuth, W.; Eckert, H. J. Phys. Chem. B 1998, 102, 4495. (49) Bak, M.; Rasmussen, J. T.; Nielsen, N. C. J. Magn. Reson. 2000, 147, 296. (50) Sanchez-Soto, P. J.; Wiewiora, A.; Aviles, M. A.; Justo, A.; Perez-Maqueda, L. A.; Perez-Rodríguez, J. L.; Bylina, P. Appl. Clay Sci. 1997, 12, 297. (51) Sanchez-Soto, P. J.; Perez-Rodríguez, J. L.; Sobrados, I.; Sanz, J. Chem. Mater. 1997, 9, 677. (52) Kinsey, R. A.; Kirkpatrick, R. J.; Hower, J.; Smith, K. A.; Oldfield, E. Am. Mineral. 1985, 70, 537. (53) Weiss, C. A.; Altaner, S. P.; Kirkpatrick, R. J. Am. Mineral. 1987, 72, 935. (54) Sanz, J.; Robert, J. L. Phys. Chem. Miner. 1992, 19, 39. (55) Brochier Salon, M. C.; Abdelmouleh, M.; Boufi, S.; Belgacem, M. N.; Gandini, A. J. Colloid Interface Sci. 2005, 289, 249. (56) Brochier Salon, M. C.; Gerbaud, G.; Abdelmouleh, M.; Bruzzese, C.; Boufi, S.; Belgacem, M. N. Magn. Reson. Chem. 2007, 45, 473. (57) Kubies, D.; Jerôme, R.; Grandjean, J. Langmuir 2002, 18, 6159. (58) Simonutti, R.; Comotti, A.; Negroni, F.; Sozzani, P. Chem. Mater. 1999, 11, 822. (59) Kolodziejski, W.; Klinowski, J. Chem. Rev. 2002, 102, 613. (60) Wang, L. Q.; Liu, J.; Exarhos, G. J.; Flanigan, K. Y.; Bordia, R. J. Phys. Chem. B 2000, 104, 2810. (61) Engelhardt, G.; Kentgens, A. P. M.; Koller, H.; Samoson, A. Solid State Nucl. Magn. Reson. 1999, 15, 171. (62) Koller, H.; Engelhardt, G.; Kentgens, A. P. M.; Sauer, J. J. Phys. Chem. 1994, 98, 1544.

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(63) Hanaya, M.; Harris, R. K. J. Mater. Chem. 1998, 8 (4), 1073. (64) Hazen, R. M.; Finger, L. W.; Velde, D. Am. Mineral. 1981, 66, 586. (65) Breu, J.; Seidl, W.; Stoll, A. Z. Anorg. Allg. Chem. 2003, 629, 503. (66) Naito, A.; Nishimura, K.; Tuzi, S; Saito, H. Chem. Phys. Lett. 1994, 229, 506. (67) Bertmer, M.; Eckert, H. Solid State Nucl. Magn. Reson. 1999, 15, 139. (68) Huve, L.; Delmotte, L.; Martin, P.; Le Dred, R.; Baron, J.; Saehr, D. Clays Clay Miner. 1992, 40 (2), 186. (69) Kaviratna, H.; Pinnavaia, T. J. Clays Clay Miner 1994, 42 (6), 717. (70) Labouriau, A.; Kim, Y. W.; Chipera, S.; Bish, D. L.; Earl, W. L. Clays Clay Miner. 1995, 43 (6), 697. (71) Bowers, G. M.; Davis, M. C.; Ravella, R.; Komarneni, S.; Mueller, K. T. Appl. Magn. Reson. 2007, 32, 595. (72) Jaber, M.; Miehe-Brendle, J. Microporous Mesoporous Mater. 2008, 107 (12), 121. (73) Inoue, K.; Tateyama, H.; Noma, H.; Nishimura, S. Clay Sci. 2001, 11 (4), 391. (74) Carrado, K. A.; Csencsits, R.; Thiyagarajan, P.; Seifert, S.; Macha, S. M.; Harwood, J. S. J. Mater. Chem. 2002, 12, 3228.

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Structural Characterization of Pristine and Modified Fluoromica Using

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