Phase Transitions and Chain Dynamics of Surfactants Intercalated into

Jun 12, 2014 - St. Petersburg State University, Laboratory of Biomolecular NMR and Institute of Physics, St. Petersburg, 199034 ... •S Supporting In...
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Phase Transitions and Chain Dynamics of Surfactants Intercalated into the Galleries of Naturally Occurring Clay Mineral Magadiite Boris B. Kharkov,†,‡ Robert W. Corkery,†,§ and Sergey V. Dvinskikh*,†,‡ †

Royal Institute of Technology KTH, Department of Chemistry, Teknikringen 36, SE-10044 Stockholm, Sweden St. Petersburg State University, Laboratory of Biomolecular NMR and Institute of Physics, St. Petersburg, 199034 Russia § Australian National University, Research School of Physical Sciences, Canberra ACT 0200, Australia ‡

S Supporting Information *

ABSTRACT: We investigate conformational dynamics and phase transitions of surfactant molecules confined in the layered galleries of the organo-modified, natural polysilicate clay, magadiite. We have shown that our approach to studying this class of materials is capable of delivering detailed information on the molecular mobility of the confined molecules. From the analysis of the measured heteronuclear dipolar couplings, the orientational order parameters of the C−H bonds along the hydrocarbon chain have been determined. Three phases have been observed in the nanocomposite, characterized by distinct dynamical states of the surfactant. At room temperature, restricted mobility of the molecules led to the adoption of an essentially all-trans conformation by the chains. This behavior can be described by a model incorporating small-angle wobbling around the long molecular axes of the chains. Upon heating, dynamic transformation takes place, resulting in a rotator type solid phase where molecules in extended all-trans conformations undergo fast and unrestricted rotation about their respective symmetry axes. The second phase transition is associated with chain melting and the onset of translational dynamics and results in an essentially liquid-crystalline-like state of the organic component. The mobility of the surfactant is one of the key factors facilitating the efficient penetration of macromolecules in the process of preparing of polymer/organoclay nanocomposites. The exploration of dynamic properties of the functionalizing organic layer should provide important input into the improved design of new organic−inorganic hybrid materials.

1. INTRODUCTION The intercalation of ionic surfactants into inorganic layered solids has attracted much attention from a wide range of research interests.1−4 Upon the assembly of an organic component in 2D interlayer nanospace between solid inorganic walls, new kinds of complex molecular aggregates and phases form. These nanosized aggregates are of broad interest for fundamental studies of the confinement effects on molecular conformational structure and dynamics.5 Organic−inorganic hybrids also possess remarkable technological properties exploited in diverse industrial and environmental applications.4 Furthermore, they are used as precursors for new advanced materials.3,4,6 Clay surface functionalization and interlayer space enlargement by surfactant/clay assembly are widely used to facilitate the further intercalation of macromolecules.7 The arrangement of the surfactant molecules of intercalated materials is controlled by Coulombic interactions with the internal surface of the galleries and by cooperative intermolecular interactions of the surfactant molecules. A variety of structures, including monolayers, bilayers, and paraffin-like assemblies, can be formed by adjusting the grafting density and synthesis conditions.5 These materials are characterized by complex phase behavior as a function of temperature. With © 2014 American Chemical Society

respect to the alkyl chains, transitions between highly ordered states to conformationally and partially orientationally disordered states similar to that in liquid crystals have been reported.8−12 A motivation of the present study was to understand better, on a molecular level, the finite size effects of confinement on the dynamic molecular processes underlying the phase transitions. A deep understanding of the relationship between macroscopic characteristics and the structure and dynamics on the molecular scale is important for the further development of meso- and nanostructured composites with enhanced properties. Various experimental and computational techniques have been applied in the studies of structural and dynamic parameters of organic components within meso- and nanocomposite systems. Analytical methods such as small-angle Xray scattering (SAXS) and transmission electron and atomic force microscopy (TEM and AFM) provide information on the microstructure of these materials; however, these techniques are not suitable for elucidating details of the dynamics and Received: May 16, 2014 Revised: June 12, 2014 Published: June 12, 2014 7859

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mixture was filtered, washed with excess of water to eliminate bulk surfactant, and air dried for 3 days at room temperature. With this procedure, the content of the adsorbed surfactant reached a plateau level of about 1.0−1.1 mmol/g of the sorption isotherm,8 as was also confirmed by measuring the basal spacing (see below). 13C NMR was used to verify that no trace of the surfactant outside of the clay framework was present in the final product. The weight loss of 2.5 ± 1% at about 100 °C observed in the gravimetric measurement could be ascribed to the residual water content. On the other hand, no signal which could be associated with the mobile water molecules was observed in the proton MAS spectra (acquired at ambient and at elevated temperatures and at a spinning speed of 15 kHz, spectra not shown). A lyotropic lamellar mesophase of composition CTAB/D2O (90 wt % of CTAB) was prepared according to an established phase diagram by mixing CTAB powder and D2O.33 The sample was homogenized by repeated extrusion of the mixture at 75 °C through a syringe needle. 2.2. Methods. X-ray diffraction patterns were recorded on the I911-4 SAXS beamline at Maxlab, Lund University. Powdered samples were mounted in kapton tape sandwiches, and scattering patterns were collected for 30 s. Background scattering was determined from empty kapton cells. The SAXS camera is a transmission-mode Kratky-type camera with a bent Si(111) monochromator. This was operated at a sample−detector distance of 1.97 m and a wavelength of 0.91 Å with a spot size of 300 × 200 μm2. The sample temperature was 20 °C. SAXS patterns were detected using a Pilatus CCD detector having 172 × 172 μm2 pixels. SAXS profiles were obtained by radially averaging 2D scattering data, with appropriate weighting of individual pixel sensitivity, weighting of each scattering pattern (sample and empty cell) by the incoming beam intensity, and background subtraction. The differential scanning calorimetry (DSC) experiment was conducted on a PerkinElmer DSC 7 calorimeter under a N2 atmosphere at a heating/cooling rate of 2 K/min. The solid-state NMR measurements were carried out on a Bruker Avance-II 300 MHz spectrometer equipped with a high-power doublechannel 4 mm MAS probe. Proton-decoupled carbon-13 crosspolarization (CP) spectra were recorded at a spinning speed of 8 kHz. Two-dimensional R-PDLF (R-recoupled proton detected/encoded local field)34 spectra were recorded using a heteronuclear dipolar recoupling35 pulse sequence of R1871 at a radio frequency (rf) field strength of γB1/2π = 72 kHz, a spinning speed of 8 kHz, and a contact time of 40 μs. To record the 2D APM-CP (amplitude- and phasemodulated CP)36,37 local field spectra, the proton rf field during the recoupling period was set to γB1/2π = 62.5 kHz. Time diagrams of RPDLF and APM-CP protocols are included in the Supporting Information, SI. Spectral assignment was based on the literature data of similar samples.26,38−40 Ambiguity in the assignment of some resonances at different temperatures (in particular, of C2 and C15 carbons) was resolved by acquiring 1H−13C heteronuclear correlation spectra (HETCOR,41 included in SI).

organization on a molecular level in such systems. In contrast, NMR spectral shapes and relaxation rates are sensitive to the local environment and molecular dynamics and thus can be used to study local molecular motion. Numerous NMR measurements of relaxation rates in organoclay composites have been conducted to elucidate their molecular dynamics.9,13−15 The main drawback of this approach is that the analysis of relaxation data is model-dependent, ambiguous, and involves assumptions and adjustable parameters. The measurement of 13C isotropic chemical shifts has been exploited to monitor changes in the molecular conformation.9,13−15 While the chemical shift can report on the relative populations of different conformers of the central segments in the surfactant alkyl chains, it does not provide quantitative information on the molecular dynamics. NMR techniques based on measuring residual anisotropic spin interactions have been developed for the quantitative characterization of molecular mobility in terms of internuclear bond order parameters.16,17 Due to its well-defined orientational and distance dependence, heteronuclear dipolar couplings have been proven to be efficient reporters of the conformational changes and overall mobility of flexible molecules. Heteronuclear dipolar spectroscopy, also referred to as separated local field spectroscopy (SLF), probes dipolar interactions of rare spins, such as carbon-13, with abundant protons.16 The SLF technique has been used for studies of a wide range of bulk solids,18 liquid crystals,17,19−21 and biomembranes.22−25 Recently, this approach has been also demonstrated by us in mesostructured organic−inorganic composites with hexagonal and lamellar structures of the inorganic framework.26,27 In those studies, composites were obtained by molecular self-assembly through template synthesis, in contrast to block assembly via intercalation applied in the current study.1 Here we investigate conformational dynamics and phase transitions of surfactant molecules in organo-modified natural clay magadiite. The intercalation chemistry of surfactants in magadiite has been summarized by Lagaly.28,29 Relatively low paramagnetic species content makes magadiite-based composites particularly suitable for studies by high-resolution solidstate NMR spectroscopy.30 We apply 2D 13C SLF NMR spectroscopy under magic angle spinning (MAS) conditions to quantitatively characterize the molecular motion of the intercalated cetyltrimethylammonium (CTA+) ions which are widely used for clay surface modification. We show that our approach to studying this class of materials is capable of delivering much more detailed information on molecular mobility as compared to prior measurements. On the basis of experimental profiles of the order parameters obtained here, we put forward models for the motion of intercalated surfactant molecules in different phases of the CTA/magadiite composite.

3. RESULTS AND DISCUSSION 3.1. Sample Characterization. The microscopic structures of unmodified and surfactant-intercalated magadiite were characterized by X-ray scattering (Figure 1). Untreated magadiite exhibited a basal spacing of 15.6 Å, corresponding well to literature data.31 Upon surfactant intercalation, the basal spacing expanded to 31.1 Å in agreement with previous reports.8,42 Two observed diffraction peaks are associated with the first- and second-order reflections. Taking the length of the fully extended CTA cation to be 22 Å43 and the thickness of the magadiite layer to be 11.2 Å,44 bilayer structure with tilted and partially interdigitated aliphatic chains can be suggested45 with an estimated tilt angle of 65°.8 Published temperature dependent measurements in similar composites indicated no significant variation of the basal spacing in the temperature range up to 100 °C.8

2. EXPERIMENTAL SECTION 2.1. Materials. Natural Californian Na-magadiite clay with an approximate composition of NaSi7O13(OH)3·xH2O31 was obtained from an online mineral supply store. Raw, as-received magadiite was characterized by XRD and found to be a single-phase mineral consistent with the standard crystal structure. Pieces were selected to be free of other mineral grains. Cetyltrimethylammonium bromide (CTAB) was purchased from Sigma-Aldrich and used as received. Samples of CTAB intercalated into magadiite were prepared following a previously developed procedure.32 The amount of 0.25 g of magadiite powder was added to 51 g of 0.05 M CTAB aqueous solution. After stirring for 24 h at room temperature, the resultant 7860

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inorganic framework in natural magadiite is evidenced by the Si NMR spectrum, exhibiting two partially resolved Q3 and three well-resolved Q4 sites (spectrum shown in SI). The carbon-13 isotropic chemical shift is a sensitive probe of the alkyl chain conformation.46,47 For the central methylene carbons of the surfactant chain, the chemical shift value in the range of 33 to 34 ppm is generally ascribed to the dominant populations of the trans conformers of the chains. For a dynamically disordered chain with a large population of gauche bonds, the chemical shift is lower by about 3 ppm. Powder bulk CTAB with all-trans chains exhibits a 33 ppm chemical shift for the central methylenes (Figure 3a), which changes to 30 ppm in the lyotropic lamellar phase (Figure 3e) where the chains are dynamically disordered and undergo fast trans−gauche isomerization. 3.2.1. Phase I. In the CTA/magadiite composite at room temperature, the corresponding chemical shift for the central carbons is about 1 ppm higher than in the bulk crystalline CTAB. There is also a similar difference of the chemical shift for the carbons in and close to the headgroup (resonances CN and C1−C3). This can be partially attributed to the change in the surfactant packing density as compared to that of crystalline CTAB, a different counterion, and an effect of the confinement. Even if the packing density of the intercalated surfactant is lower than in the bulk crystal, the loose structure of all-trans chains can still be stable due to less regular orientations of the chain planes.5 Since a chemical shift does not change by decreasing the temperature to −40 °C (spectrum not shown), we conclude that at ambient temperature the dynamic effects on the chemical shift are negligible. 3.2.2. Phase II. As the temperature rises from 30 to 70 °C, carbon-13 spectra display a gradual shape transformation (spectra shown in SI). The spectral intensity at 33.2 ppm, associated with the inner carbons in the chain, grows at the expense of the peak at 34.3 ppm. Moreover, new peaks originating from C2, C3, C15, and C16 resonances become apparent. The process is reversible, and the room-temperature line shape is recovered upon cooling. However, no heat-flow peak in this range was detected in the calorimetric study (Figure 2) probably because the molecular dynamics changes over the broad temperature interval. Remarkable differences in chemical shift values and the spectral shapes recorded at 22 and 75 °C (Figure 3b,c) suggest different dynamical states of the surfactant. However, the shift of 1.1 ppm for the central peaks is significantly smaller than expected for the full-scale conformational dynamics. Such peculiar behavior in other intercalated systems9,14 has been explained by the formation of kink defects5 (gauche+−trans−gauche− isomers) migrating along the chain.10 Since any particular bond adopts a gauche state for a fraction of time, the observed chemical shift reflects a weighted average of the trans and gauche states. On the other hand, ppm range variation of the chemical shift of internal carbons in the alkyl chain is commonly observed at solid−solid transitions due to local changes in the intermolecular environment. For example, in n-alkanes, on transitioning from a crystalline phase to a rotator phase, the chemical shift changes due to the alteration of the molecular interactions between the hydrocarbon chains, when the regular orientation of the chains is lost.47,48 3.2.3. Phase III. The spectral shape does not change within the interval of 70 to 80°C, while above that temperature range the second spectral transformation takes place. This transition corresponds to the strong heat flow peak observed on the DSC trace in Figure 2. The transition is accompanied by further line 29

Figure 1. SAXS diffraction patterns of magadiite (A) and the CTA/ magadiite composite (B).

DSC data obtained for the mesolamellar CTA/magadiite composite is shown in Figure 2. While NMR analysis,

Figure 2. DSC diagram of intercalated magadiite, obtained by heating (upper curve) and subsequent cooling (lower curve) at 2 K/min.

presented below, suggested that a dynamic transition takes place at around 30−70 °C, no indication of the phase transformation in this range was found in the DSC data. Upon further sample heating, a distinct peak indicating a firstorder phase transition was observed. This transition is in agreement with the second phase transformation observed by NMR spectroscopy and is associated with bilayer melting, as will be discussed in detail in the next section. The corresponding endothermic peak on the cooling DSC curve is shifted by ca. 14 °C. 3.2. Chain Conformation. Carbon-13 CP MAS spectra of CTA+ ions intercalated into magadiite galleries exhibit wellresolved narrow resonances with line widths comparable to that in the bulk crystalline material. The effect of paramagnetic centers, which often results in severe line broadening in natural clay-based composites, is negligible owing to the low content of paramagnetic iron in magadiite.30 Thus, details of molecular conformations and dynamics can be studied with high chemical resolution and sensitivity for a large number of individual segments of the flexible surfactant molecules. The carbon-13 NMR spectrum acquired from the composite at room temperature is similar to literature spectra for analogous materials based on synthetic magadiite.8 Significantly higher resolution in our spectra is attributed to a more homogeneous structure of the natural clay compared to that in synthetic magadiite. Indeed, the highly homogeneous structure of the 7861

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vector in a molecular or molecular aggregate frame and ranges from 0 for an isotropically reorienting bond to 1 for an immobilized bond. In a 13C−1H SLF experiment, order parameters are estimated from the values of dipolar splitting Δν = |kSCHDCH|, where DCH = −21.5 kHz is the dipolar coupling constant for a rigid C−H bond and k is a dipolar scaling factor. Under the MAS condition, the dipolar recoupling technique must be employed to preserve heteronuclear spin couplings while other anisotropic spin interactions are suppressed by the sample spinning or rf decoupling. In the present work, we apply R-PDLF22,34 and APM-CP36,37 approaches. These two techniques are complementary in terms of the magnitudes of the accessible dipolar couplings. Heteronuclear recoupling pulse sequence R1871 used in the RPDLF technique has an advantage of active homonuclear decoupling of abundant protons and thus is efficient in samples with strong dipolar interactions.35 It is, however, less suitable for samples with small couplings due to the relatively low value of the scaling factor, k = 0.315. 3.3.1. Phase I. Representative 1H−13C cross sections from the 2D R-PDLF spectrum of the CTA/magadiite sample at 22 °C are displayed in Figure 4a. At room temperature, all

Figure 3. 13C CP MAS spectra of (a) bulk CTAB at room temperature, CTA/magadiite at (b) 22, (c) 75, and (d) 110 °C, and (e) 13C MAS spectrum in the lyotropic lamellar mesophase of CTAB/ D2O (90 wt %) at 70 °C. The peak assignment is made according to previous data for CTAB and similar molecules38−40,49 and was verified by HETCOR spectra.

Figure 4. Representative dipolar cross sections from 2D R-PDLF spectra in CTA/magadiite obtained at (a) 22 °C and (b) 75 °C and from the APM-CP experiment at 110 °C (c). Frequency scales in (a) and (b) are corrected for scaling factor k = 0.315 of the dipolar recoupling sequence R1871. Dashed lines indicate expected splittings for C−H bonds with a dipolar coupling constant of −21.5 kHz and SCH = 1 (a) and SCH = 0.5 (b).

narrowing in the 13C NMR spectra and by a significant decrease of 2 to 3 ppm in the chemical shift of the chain carbon resonances. Above 90 °C, the spectral shape and chemical shifts closely resemble those observed in the spectrum of the fluid lyotropic lamellar phase of CTAB/D2O. The chemical shift of 30 ppm corresponding to the central carbons is indicative of fast conformational dynamics in the chains, with a significant population of gauche conformers. 3.3. Dipolar Spectra and Chain Order Parameter Profiles. Dipolar couplings are sensitive to changes in the orientation of the internuclear bond vector and thus are employed for studies of bond reorientational motion in terms of bond order parameter SCH. The value of SCH is determined by the angular amplitude of the local reorientation of the bond

methylene groups exhibit similar dipolar splittings of about 18 to 19 kHz, which is less than the value of about 21 kHz expected for immobilized bond. Note that in the bulk CTAB under the same experimental conditions, splittings of 21 kHz were found.26 The estimated C−H bond order parameters for different methylene groups in the composite displayed in Figure 5a are all ca. 0.9. Thereby, in addition to the lack of trans−gauche conformational dynamics, no large angle reorientations of the whole molecule are present in CTA/ magadiite at room temperature. Assuming the motional model of restricted small-angle reorientations with the order parameter approximated as S ≈ 1 − (3/2)⟨ϑ2⟩, one obtains the average angular amplitude of ∼15°. The motional correlation time is below the 10−4−10−3 s range defined by 7862

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lead to SCH varying along chains. Order parameter values estimated for resolved carbons are included in Figure 5a. The rotator phase has been found by us previously in synthetic lamellar composite CTAC/AlPO.27 3.3.3. Phase III. The phase transformation observed in the DSC experiment above 80 °C (Figure 2) is associated with a chain-melting transition analogous to that studied in lipid bilayers.5,50 This process is described as a transition from a state with ordered all-trans chains (referred to as Lβ phase of lipid membranes) to a disordered fluidlike state with a dynamic distribution of trans and gauche conformers (liquid-crystalline Lα phase).50 Dipolar spectra (Figure 4c) and order parameters in the high-temperature phase of the intercalated magadiite were measured using the APM-CP recoupling technique. In contrast to R-PDLF, APM-CP has the highest possible scaling factor (k = 1 for the methylene group) and is preferable in highly mobile systems with small dipolar couplings. Since no active homonuclear decoupling is implemented in this method, it has to be applied to the systems where homonuclear spin interactions are on the order or less than the sample spinning frequency. This condition is fulfilled in the high-temperature phase of CTA/magadiite. Note that for some of the carbons unresolved in the 1D spectrum such as C4 and C12, dipolar splittings were resolved in the 2D SLF spectrum due to significantly different dipolar coupling magnitudes. The order parameter profile for the lamellar CTA/magadiite composite at 110 °C is included in Figure 5a. It is also compared to that in the lamellar CTAB/D2O liquid crystal (Figure 5b). Both curves demonstrate the motional gradient toward the chain end. A notable feature is that order parameters for all alkyl carbons in the composite are smaller than those in the fluid liquid-crystalline (LC) sample. The conformational mobility of flexible CTA+ cations depends on the packing density in the composite.5 In the LC phase, CTA+ forms densely packed aggregates and experiences higher constraints from the neighboring molecules, while in the intercalated magadiite the packing density, restricted by the surface charge density, is lower. Another notable feature of the order parameter profile in the composite is the large variation of the SCH value for the first four alkyl carbons. This can be due to some subtle effects of the chain conformation under confinement and the interaction of head groups and nearby chain units with inorganic walls. It can possibly be associated with the oscillating probability of gauche bond formation for the first few segments in the chain analogous to that demonstrated for phospholipid bilayers51 and in thermotropic mesophases.52 Previously, 2H NMR has been applied to study the mobility of CTA+ intercalated in synthetic magadiite at a low grafting density.53 Quadrupolar-broadened spectral line widths for selectively deuterated methylene groups C1 and C2 indicated fast conformational dynamics at 80 °C. The obtained ratios of the line width to that in crystalline CTAB were about 0.1 and 0.12 for C1 and C2 methylenes, respectively, thus supporting our results of SCH(C1) < SCH(C2) in Figure 5. Moreover, these ratios are in the range of the SCH values obtained from our dipolar spectra. Note that model-dependent 2H spectral shape analysis would be required to estimate the order parameter of the CD bond from the 2H quadrupolar echo spectra.53 The conformational dynamics of CTA+ was recently investigated by us in a synthetic layered composite, CTAC/ AlPO.27 Interestingly, the high-temperature phase of the CTAC/AlPO sample, when compared to CTA/magadiite,

Figure 5. (a) Order parameter SCH profiles for CTA/magadiite sample at 22, 75, and 110 °C (circles, triangles, and diamonds, respectively). Data obtained in bulk CTAB at room temperature are also included (squares). (b) Data in the CTA/magadiite composite at 110 °C (diamonds) and in the CTAB/D2O 90 wt % lamellar lyotropic phase at 70 °C (circles) are compared.

the magnitude of the dipolar couplings. Such low-amplitude reorientations can be induced by packing irregularities in the confined environment when the lowest-energy perfect all-trans chain conformation cannot be achieved. This motion may correspond to independent small-angle fluctuations of the individual methylene groups or to a wobbling of whole molecules. As the angular amplitude of the fluctuations increases at relatively elevated temperatures, these two motional modes lead to quite different molecular dynamics. The first mechanism will result in trans−gauche conformational dynamics increasingly enhanced toward the chain end and thus accompanied by a mobility gradient along the chain. Due to the second mechanism, a rotator-type molecular motion will emerge without significant variation of the order parameter for different segments. In the rotator phase, the all-trans zigzag chains are rotating rapidly around the principal molecular axis. Dipolar spectra recorded in phase II at 75 °C (Figure 4b) support the second alternative. 3.3.2. Phase II. Indeed, order parameters of about 0.5 were found for all resolved methylene carbons at 75 °C. Since a C− H vector points at an angle of 90° with respect to the molecular axis, fast rotation of the all-trans molecule about the molecular axis leads to the scaling of all C−H couplings by a factor of −1/2. The molecular rotation is not accompanied by an increase in the flexibility of alkyl chains, which otherwise should 7863

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While nanosized organic−inorganic composites are of broad interest for fundamental studies of the confinement effects on molecular conformational structure and dynamics, they also possess advanced material properties exploited in diverse technological applications. In particular, the surface functionalization of hydrophilic silicate clays with surfactants is widely used to assist the further intercalation of macromolecules. The mobility of the surfactant could be one of the key factors facilitating better penetration of polymer molecules in the process of melt intercalation. An exploration of dynamic properties of the functionalizing organic layers should provide an important input for the improved design of new materials.

showed very different order parameter profiles. While inner methylene groups and the tail demonstrated high mobility and a correspondingly low order parameter SCH ≤ 0.1, similar to that in the CTA/magadiite complex, the motion of the methylenes next to the headgroup exhibiting SCH ≈ 0.4 was significantly restricted in contrast to that of the magadiite composite. It has been concluded that dynamically disordered molten chains in CTA/AlPO are maintained in place by the ionic binding to inorganic walls and the translational dynamics is absent. However, in the CTA/magadiite sample, because of much lower SCH values for all chain segments, translational dynamics cannot be excluded on the same basis. In fact, the 1D carbon-13 spectrum recorded without proton decoupling provides us with another indirect indication of the translational mobility in the high-temperature phase. Translational diffusion, if present, leads to averaging of the intermolecular proton homonuclear interactions. The remaining intramolecular homonuclear dipolar interactions in axially rotating molecules exhibit properties of an inhomogeneously broadened system and is averaged by MAS. 54 Since heteronuclear spin interactions are averaged as well, the 13C spectrum acquired without proton decoupling should display sharp peaks with resolved J-coupled multiplets. This was indeed observed at 110 °C (spectrum shown in SI) and thus suggests that translational diffusion is sufficiently fast to average out the proton intermolecular dipolar couplings. The lower limit of the diffusion coefficient can be estimated to be 10−14 m2/s (see SI). Translational dynamics in 2D layers of orientationally aligned molecules is characteristic of LC smectic phases.55,56 Hence, if positional order is destroyed by the diffusion, then one can speak of LC smectic phase formed within the layers of the composite material. Since in layered structures the molecular translational displacement does not lead to molecular reorientations, it does not affect the SLF spectra.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Pulse schemes for separated local field spectroscopy under the magic angle spinning condition. Series of temperature-dependent 13C CP MAS NMR spectra of the CTA/magadiite composite. Carbon-13 spectrum at 110 °C recorded without proton decoupling. Silicon-29 MAS NMR spectra in unmodified magadiite and CTA/magadiite composite. This material is available free of charge via the Internet at http:// pubs.acs.org.

Corresponding Author

*E-mail: [email protected]. Phone: +46 8 790 8824. Fax +46 8 790 8207. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the Saint-Petersburg State University and by RFBR grant no. 13-03-01073.

4. CONCLUSIONS In the present work, the changes in the molecular dynamics associated with the phase transformations in the ordered mesolamellar composite CTA/magadiite were studied by means of 1D 13C and 2D 1H−13C dipolar NMR spectroscopy. This is the first direct measurement of order parameter profiles in the layered composites obtained by block assembly via intercalation. We demonstrated that solid-state dipolar NMR spectroscopy can be employed as a fine tool that is sensitive to changes in the conformation and dynamics of the organic layers in ordered organic−inorganic composite materials. Three phases in the nanocomposite were observed, characterized by distinct dynamical states of the surfactants. No large-amplitude molecular motion on a time scale faster than 10−3 s was found at room temperature. The highly restricted mobility of molecules adopting essentially an all-trans conformation can be described by a model of small-angle wobbling around the long molecular axis. Upon heating, a dynamic transformation takes place, resulting in a rotator-type molecular motion. Order parameter values of 0.5 for all methylene groups indicate that rodlike surfactant molecules undergo fast and unrestricted rotation about the molecular axis. The second phase transition is associated with the chain melting and the onset of translational dynamics. Low order parameters, mobility gradients along the chains, and the high population of gauche conformers suggest a liquid-crystallinelike state of the organic layer in the composite, characterized by fast rotational and conformational dynamics.

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