Probing Molecular Mobility in Nanostructured Composites by

Article pubs.acs.org/JPCC

Probing Molecular Mobility in Nanostructured Composites by Heteronuclear Dipolar NMR Spectroscopy Boris B. Kharkov,†,‡ Vladimir I. Chizhik,‡ and Sergey V. Dvinskikh*,†,§ †

Department of Chemistry, Royal Institute of Technology KTH, SE-10044 Stockholm, Sweden Faculty of Physics, St. Petersburg State University, St. Petersburg 198504, Russia § Laboratory of Biomolecular NMR, St. Petersburg State University, St. Petersburg 199034, Russia ‡

ABSTRACT: Conformational and reorientational dynamics of surfactant molecules confined within solid inorganic frameworks of nanostructured composites are studied by solid-state NMR. Since the organic part in the nanocomposites exhibits a great variation in the local molecular mobility, a significant challenge in these materials is to develop an experimental method efficient both for nearly immobilized and for highly dynamic flexible molecules. We introduce a dipolar recoupling NMR technique to study local molecular motion via heteronuclear dipolar spin interactions in a wide range of coupling constant values. The approach is efficient in both rigid and mobile molecules. The method is applied to ordered nanostructured composites to obtain model-independent information on the local molecular dynamics. On the basis of experimentally determined local order parameters of the molecular segments, we put forward physical models for the motion of surfactant molecules in nanoconfined assemblies. In composites with lamellar structure, the hydrocarbon chains adopt well-ordered essentially all-trans conformations and, depending on sample composition, undergo either small-angle wobbling or free rotation around the long molecular axes. In contrast, surfactant molecules in hexagonal mesoporous silica exhibit highly dynamic conformationally disordered chains.

1. INTRODUCTION Organic−inorganic nanostructured composite materials are now integrated in the field of nanoscience and nanotechnology.1 These composites are used directly as advanced materials or as precursors to novel materials with potential applications in optics, mechanics, energy, catalysis, and biology. Besides practical importance, they also offer a unique opportunity to study dynamic properties of small molecules and macromolecules in nanoconfinement. It has been recognized that the organic part in many nanocomposites is inherently dynamic; many properties and processes are dependent on conformational rearrangements of the flexible dynamically disordered molecular parts; molecular assemblies at the interfaces and in confinement show complex phase behavior and many unconventional effects which remain poorly understood and characterized; new structures and aggregates are formed different from those in the bulk. Dynamic processes at the molecular level are central to understanding this complex behavior. Recent study of template-synthesized solid nanocomposites has shown that segmental mobility of organic component is much higher than previously anticipated.2 At a microscopic level, the structure of nanocomposites is verified by employing diffractional, scattering, and imaging techniques such as WAXD, SAXS, TEM, SEM, and AFM, while the dynamics and organization of the material on a molecular level are not available. Based on structural parameters, composition, and grafting densities, various models of confined assemblies of assumingly immobilized molecules have been suggested.3 Calorimetric measurements often reveal complex © XXXX American Chemical Society

phase behavior in the composites, different from that in the bulk. Distinct sharp heat flow peaks associated with the firstorder phase transitions in confined bilayer have been attributed to “chain melting” processes.4,5 Change in the glass transition’s behavior in polymer/clay nanocomposites has also been studied.6 The presence of different conformational states in the hydrocarbon chains of the confined organic molecules can be identified by IR spectroscopy.7 This approach, however, does not provide a sufficient level of chemical resolution and quantification. Computer simulation studies suggested complex molecular dynamics of the confined molecules dependent on packing density and temperature.8 NMR is an atomic and molecular specific spectroscopic tool capable of delivering detailed structural and dynamic information. NMR has been widely used to study nanocomposites.9−11 The dynamic investigations of nanostructured composite materials have been greatly advanced by solid-state NMR techniques probing heteronuclear dipolar spin interactions such as 1H−13C dipolar coupling.2,9,11−13 Since the dipolar interaction constant has a well-defined orientation and distance dependence d ∝ (3 cos2 ϑ − 1)/r3 (ϑ is angle of internuclear vector with external magnetic field and r is internuclear distance), it is often straightforward to translate experimental dipolar couplings into conformational and dynamical parameters and constraints. This model-independent information is used to put forward, verify, or rule out the Received: October 3, 2014

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dx.doi.org/10.1021/jp510016z | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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spectrometer equipped with a high-power 4 mm magic-anglespinning (MAS) probe. Carbon-13 spectra were recorded at a resonance frequency of 125 MHz and at a sample spinning speed of 8 kHz. Sample temperature was stabilized to 25 °C. Two-dimensional dipolar spectra were recorded using a heteronuclear dipolar recoupling pulse sequence of Figure 1

physical models of the molecular motion. Since the organic part in nanocomposites exhibits a great variation in the local molecular mobility, a significant challenge in these materials is to develop an experimental method efficient both for nearly immobilized and for highly dynamic flexible molecules. Anisotropic molecular motion results in scaling of the spin dipolar couplings by a local orientational order parameter 0 ≤ |S| ≤ 1. Hence, an efficient dipolar NMR spectroscopy approach in nanocomposites should achieve a high dipolar resolution in a wide range of the coupling constants. Previous NMR approaches in nanocomposites provided a dipolar resolution in rather limited range of dipolar couplings.2,12−15 Some techniques are insensitive to small couplings,16,17 while others are strongly susceptible to the effects of interfering spin interactions.18−20 Accordingly, the dipolar spectra are often recorded with low or suboptimal resolution, and two or more complementary techniques have to be applied to access a full range of the parameters of the underlying dynamical processes. In the following, we first introduce an NMR approach to obtain model-independent information on motional parameters in nanocomposites. The novel dipolar recoupling technique is efficient in rigid and soft solids and provides high dipolar resolution in a wide range of dipolar couplings in organic− inorganic nanocomposites. The method, demonstrated on model samples, is then applied to organic template/modifier confined within the ordered inorganic frameworks formed by nanostructured natural and synthetic solids. Obtained detailed information on the local molecular dynamics is discussed in terms of physical models of the molecular motion.

Figure 1. Rf pulse sequence for 2D dipolar spectroscopy under MAS. During evolution period t1, homonuclear proton interaction is suppressed by magic-echo sandwiches,25 while heteronuclear dipolar recoupling is achieved by introducing amplitude- and phase-modulated cross-polarization (APM-CP).20,23 The rf field amplitude γB1 in Schannel is changed by 2ωr, synchronously with the phase inversion (ωr is angular frequency of the sample rotation). Black rectangles represent 90° pulses. Dwell time in t1 dimension corresponds to 3τ.

at a radio frequency (rf) field strength during lock pulses of γB1/2π = 120 kHz in the 1H channel and alternated between 128 and 112 kHz in the 13C channel. The length of 90° proton pulses was 1.7 μs. During signal acquisition, the proton heteronuclear decoupling by TPPM pulse sequence21 with rf field of 80 kHz was applied. 2.3. Numerical Simulations. Numerical simulations were performed using SIMPSON platform.22 For the dipolar evolution period of the pulse sequence of Figure 1, the proton rf field during lock pulses was set to γB1/2π = 120 kHz and carbon-13 rf field was alternated between 128 and 112 kHz. The length of 90° proton pulse was 1.7 μs. Spinning speed was set to 8 kHz. For immobile spin systems, dipolar coupling constants were set to dCH = −21.5 kHz and dHH = −20 kHz. For rotating CH2 group, the coupling constants were reduced by a factor of 2. Powder averaging for CH and CH2 groups was performed respectively with 376 and 4180 pairs of (α, β) angles and 11 γ angles.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. All chemicals were purchased from Sigma-Aldrich and used as received. Nanocomposites were synthesized following published procedures2,12,13 and were characterized by X-ray diffraction, differential scanning calorimetry, and 1H, 13C, and 29Si NMR spectroscopy. CTA/Magadiite.13 Natural Californian Na-magadiite clay with composition of NaSi7O13(OH)3·xH2O was obtained from an online mineral supply store. The amount of 0.25 g of magadiite powder was added into 51 g of 0.05 M CTAB aqueous solution. After stirring for 24 h at room temperature the resultant mixture was filtered, washed with excess of water, and air-dried for 3 days at room temperature. CTAC/AlPO. 12 An appropriate amount of aluminum triisopropoxide was mixed with water. An 85% solution of H3PO4 was added. After several hours of stirring, a 9 wt % aqueous solution of cetyltrimethylammonium chloride (CTAC) was added. The mixture was stirred for 1 h, and a 25% aqueous solution of tetramethylammonium hydroxide (TMAOH) was added. The resulting mixture had the molar ratio 1.0 Al2O3:3.05 P2O5:0.5 CTAC:8.11 TMAOH:350 H2O. After additional stirring for 96 h, the resultant precipitate was filtered, washed with deionized water, and dried overnight at 50 °C. CTAB/MCM41.2 A 32% solution of aqueous ammonia and ethanol was added to an aqueous solution of CTAB. After 20 min of stirring, tetraethyl orthosilicate (TEOS) was added. The resulting mixture had the following molar composition: 1 TEOS: 0.3 CTAB: 11 NH3: 144 H2O: 58 EtOH. After stirring for 2 h, the resultant precipitate was filtered, washed with deionized water, and dried for 20 h at 40 °C. 2.2. NMR Measurements. The solid-state NMR measurements were carried out on a Bruker Avance-HD 500 MHz

3. RESULTS AND DISCUSSION 3.1. Dipolar Recoupling. A new dipolar recoupling approach was designed suitable for nanocomposites exhibiting a great variation in the local molecular mobility. The experiment is based on recent advances in the design of the recoupling sequences using amplitude- and phase-modulated cross-polarization (APM-CP) under MAS sample condition.20,23 APM-CP has been demonstrated by us in the mesostructured periodic composites with a highly mobile organic component.2 Unfortunately, this technique, which lacks active proton homonuclear decoupling, was inefficient in other nanocomposite samples characterized by more restricted molecular mobility. The off-resonance version of this method, FSLG-CP,19 while capable of suppressing homonuclear proton coupling, is strongly susceptible to interfering chemical shift interaction and frequency offsets. It has been demonstrated in nonrotating samples that onresonance magic-echo based homonuclear decoupling can be combined with CP to design robust approach for dipolar NMR spectroscopy, SAMMY, and its variant, SAMPI4.24,25 This B



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technique was shown to be very efficient in measuring heteronuclear dipolar couplings in uniformly aligned biomolecules and liquid crystals under stationary sample condition.26,27 However, when this approach is applied to a spinning sample, the fast MAS suppresses heteronuclear interaction. Dipolar recoupling is necessary to recover spin interactions averaged by MAS.28 The new recoupling sequence, which combines CPMAS and magic echo, is shown in Figure 1. The main features are (i) a magic sandwich block in I-spin channel adopted from SAMMY experiment24,25 and (ii) an amplitude alternated and Hartmann−Hahn matched irradiation of S-spin channel performed according to the APM-CP scheme.20 Note also the different scheme of the phase modulation of the S-spin lock pulses when compared to original SAMMY/SAMPI4 sequences. With this design, unwanted single-quantum terms in the lowest order dipolar Hamiltonian are canceled. The dipolar scaling factor is 2/3 and is the same for static and rotating samples. For a pair of heteronuclear dipole-coupled spins in a sample rotating at the magic angle, the recoupling sequence generates polarization transfer at the frequency (2/3)(b/2√2) sin(2β), where b = −(μ0/4π)(γIγSℏ/r3) and the polar β angle determines the orientation of the internuclear vector in the rotor frame of reference. Hence, the splitting in the powder spectrum is given by Δω = |(2/3)(b/√2)|. The additional scaling by a factor of 2/3 as compared to conventional CP or APM-CP stems from the fact that CP conditions are fulfilled for 2/3 of the sequence cycle period. Synchronization of the rf pulses sequence to rotor spinning is not required. This general property of APM-CP based sequences makes the technique flexible in choosing rf amplitudes and spinning speeds, in contrast to rotor synchronized sequences. Because of phase inversions and on-resonance rf fields in both frequency channels, the new scheme is insensitive to rf maladjustments as well as to 1H frequency offsets and chemical shift terms. The sequence is γ-encoded;29 hence, destructive signal interference due to the orientational distribution in the powdered samples is minimized. 3.2. Numerical Simulation and Experiments on Model Samples. The results of the spin-dynamics numerical simulations of two- and three-spin systems are shown in Figure 2. Simulated spectral shapes for rigid CH pair and CH2 methylene group are in agreement with expected powder spectra for γ-encoded CP-based recoupling sequence.19,30 In the case of the methylene group, including the homonuclear proton coupling (coupling constant of −20 kHz) in the simulation does not affect the spectral shape, thus verifying the efficiency of the homonuclear proton decoupling. Active homonuclear decoupling makes the sequence suitable for studies of rigid and mobile molecules with strong homonuclear proton interaction, in contrast to APM-CP. The robustness toward the chemical shifts and frequency offset terms was also verified (Figure 3). Within the practical ranges of the chemical shift of about 10 ppm (5 kHz in 11.7 T magnetic field) and 100 ppm (12.5 kHz) for protons and carbons, respectively, the offresonance effect on the dipolar splitting is small. The effect of 13 C and 1H chemical shift anisotropy was also found negligible (data not shown). Insensitivity to the offset terms is a significant advantage compared to competing schemes.18,19 The experimental performance of the technique in rigid solids with strong dipolar couplings was demonstrated on Lalanine and α-glycine amino acid powders for CH and CH2 groups, respectively. A good agreement with theoretical shapes

Figure 2. Simulated 13C−1H dipolar recoupled spectra of (a) CH and (b) methylene CH2 group, using SIMPSON22 software. Coupling constants bCH/2π = −21.5 kHz and bHH/2π = −20 kHz, spinning speed of 8 kHz, proton rf field during lock pulses of γB1/2π = 120 kHz, and proton 90° pulses of 1.7 μs. Experimental spectra of (c) CH group in L-alanine and (d) CH2 group in α-glycine are compared to respective simulated shapes. Experimental spinning speed and rf fields correspond to those in numerical simulation.

was found (Figure 2). The experimental dipolar scaling factor was calibrated to k = 0.61, which is close to the theoretical value of 2/3 in the limit of the infinitely short 90° pulses. Nonadecane C19H40 was chosen as a model sample to study effect of molecular mobility on the dipolar spectral shape. In the rotator phase of nonadecane, the molecules in extended alltrans conformation undergo fast reorientation about the symmetry axis.31 This uniaxial rotation leads to the scaling of the C−H couplings with the order parameter of SCH = −0.5, while the coupling to two equivalent protons in the CH2 group results in the increase of the splitting by a factor of √2.32 The simulated dipolar spectrum of rotating CH2 methylene group reproduces accurately the experimental spectral shape of the central CH2 methylenes (C4 to C16 carbons) in nonadecane (Figure 4). From the experimental splitting of Δω = |kSCHbCH| = 2π × 6.4 kHz and known rigid coupling constant bCH ≈ 2π × 21.5 kHz, the C−H bond order parameter is estimated to |SCH| = 0.49, thus confirming a model of axially rotating all-trans chain. 3.3. Conformational Dynamics and Molecular Reorientation in Nanocomposites. Previous highly informative studies of local dynamics in nanocomposites by measuring heteronuclear dipolar couplings between rare 13C spins and abundant protons have been reported.14,15 Brus et al. studied polymer dynamics in polymer/clay nanocomposites.14 Because of highly restricted mobility of the polymer chain, the offresonance recoupling method combined with homonuclear decoupling, FSLG-CP,19 was employed. The amplitude of the reorientational motion of CH2 groups of the intercalated amorphous polymer was estimated to be maximum 33° and was slightly higher as compared to that in the bulk. Han et al. reported on chain dynamics of surfactant in lamellar titania studied by dipolar rotational spin-echo (DRSE) 13C NMR.15 The results were interpreted assuming a wide range of motional modes. DRSE technique generates sideband spectra, and analysis in terms of motional parameters requires suitable motional model assumptions. We have recently performed dynamic studies of mesostructured composites with hexagonal and lamellar morphologies exhibiting great variations in molecular mobilities.2,12 Fast conformational dynamics in the C



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Figure 3. Numerical analysis of the influence of the frequency offset/isotropic chemical shift on the dipolar splitting of immobile C−H bond, measured in the simulated spectral shapes as indicated on the right.

second lamellar sample was synthetic composite CTAC/AlPO obtained by molecular self-assembly through template synthesis.34 Different molecular assemblies of the surfactant within AlPO lamellar framework have been previously observed exhibiting also a range of the motional modes.34 Dipolar spectra obtained in these two layered composites are compared in Figure 5. In the sample CTA/magadiite, all

Figure 4. (a) Simulated 13C−1H dipolar recoupled spectrum of fast axially rotating methylene group CH2. (b) Experimental spectrum of the central methylenes C4−16 in nonadecane. Experimental parameters are as in Figure 2.

ordered hexagonal mesostructured CTAB/silica composite, comparable to that in fluid lyotropic hexagonal phase, has been found.2 However, no translational dynamics was detected in contrast to lyotropic analogue. Rotator-type phase has been observed in CTAC/AlPO layered composite at ambient temperature phase, which at elevated temperature transformed to the phase with flexible mobile chains and head groups anchored to the inorganic surfaces.12 Since studied nanocomposites exhibited a very wide range of the local order parameters in different phases and also for different molecular segments within a phase, different dipolar recoupling approaches were used to measure motional parameters in a full range of magnitudes. In the present work, the new method was applied to study molecular mobility in nanostructured organic−inorganic ordered composites with layered and hexagonal structures of the inorganic porous frameworks. Two lamellar structures prepared by different molecular assembly strategies were examined. Natural clay magadiite was organo-modified with CTA+ (cetyltrimethylammonium) ions using block assembly via intercalation process.13 The exploration of dynamic properties of the functionalizing organic layer in clay materials provides important input into the improved design of new organic−inorganic hybrid materials. Previous studies have revealed complex phase behavior in this type of composite.13,33,34 The room temperature phase was characterized as a solid with very limited mobility of the surfactant chain. The

Figure 5. 1H−13C dipolar recoupled spectra of methylene groups of the CTA+ hydrocarbon chain in mesolamellar organic−inorganic composite CTA/magadiite (a) and CTAC/AlPO (b). Experimental parameters are as in Figure 2.

resolved methylene groups in the chain exhibit similar C−H dipolar patterns (Figure 5a), resembling that obtained for immobilized CH2 group in glycine. The dipolar splittings are, however, lower by about 10% and hence correspond to the local orientational order parameters SCH ≈ 0.9. This indicates the presence of restricted motion, which is ascribed to smallangle wobbling around the molecular symmetry axis, while the general all-trans molecular conformation is preserved.13 CTA+ ions in CTAC/AlPO composite display dipolar spectral shapes (Figure 5b) different from those in CTA/ D



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while the resolution of the smaller couplings, corresponding to highly mobile species, is significantly improved. The method has been used to investigate the local site-specific molecular dynamics of surfactant molecules in nanostructured organic− inorganic hybrids of different morphology. The results show that, by applying the new technique, accurate motional parameters of the confined surfactant can be obtained in samples with very different degrees of molecular mobility. No motional models and/or adjustable parameters were involved in the analysis; thus, site-specific dynamic parameters of the molecules in different structural environments or in different phases were directly compared. On the basis of experimental profiles of the local order parameters, we put forward and confirm motional models for the intercalated surfactant molecules in different nanoconfined assemblies. In layered materials, the hydrocarbon chains adopt well ordered essentially all-trans conformations and, depending on sample composition, undergo either small-angle wobbling or free rotation around the long molecular axes. In contrast, in hexagonal MCM41 silica, the experimental data suggest highly dynamic conformationally disordered chains with more restricted motion of the segments close to the headgroup. We anticipate that developed approach will also be useful in dynamic studies of the intercalated and confined organic molecules in other classes of advanced nanostructured composite materials such as organo-modified composites based on clays,35,36 nanofibrillated cellulose,37 carbon nanotubes,38 and graphene.35,39

magadiite in spite of similar lamellar structure of the materials. In fact, spectral shapes and dipolar splitting values in the CTAC/AlPO sample correspond to that observed for the fast axially rotating CH2 group in nonadecane (cf. spectrum in Figure 4). The estimated order parameters are all of about 0.5. Hence, CTA+ chain preserves an extended all-trans conformation and undergoes fast unrestricted axial rotation about the symmetry axis. In other words, CTA+ double layers in CTAC/AlPO composite exhibit rotator phase, analogous to the nonadecane sample. High dipolar resolution is also obtained in spectra of extremely dynamic surfactant assembly in the channels of the hexagonally ordered silica composite CTAB/MCM41. It has been shown that the segmental orientational order parameters in this solid material are low and comparable in magnitudes to those typically observed in f luid hexagonal mesophases. There exists also significant motional gradient with the order parameters decreasing toward the end of the chain.2 The local order parameters can be directly estimated from the observed spectral splittings in Figure 6. Order parameters

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph +46 8 790 8824; Fax +46 8 790 8207 (S.V.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the grant of the Saint-Petersburg State University, and by RFBR grant # 13-03-01073.

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Figure 6. 1H−13C dipolar recoupled spectra of methylene groups of the CTA+ hydrocarbon chain in hexagonal mesoporous silica CTAB/ MCM41. Experimental parameters are as in Figure 2.

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4. CONCLUSIONS We have demonstrated an NMR approach to study dynamic properties of confined molecules in the nanocomposites characterized by a wide range of the motional modes and mobility degrees. The well-resolved dipolar spectra were obtained both for rigid immobile molecules and in samples with highly dynamic flexible molecules undergoing fast conformational and reorientational transitions. The resolution of large dipolar couplings is comparable to that obtained with some other techniques employing homonuclear decoupling, E



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Probing Molecular Mobility in Nanostructured Composites by

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