Article pubs.acs.org/JPCC
Conformational Dynamics of Surfactant in a Mesolamellar Composite Studied by Local Field NMR Spectroscopy Boris B. Kharkov†,‡ and Sergey V. Dvinskikh*,†,‡ †
Department of Chemistry, Royal Institute of Technology KTH, Teknikringen 36, SE-10044 Stockholm, Sweden Institute of Physics, St. Petersburg State University, Ulyanovskaya 1, St. Petersburg, 198504 Russia
‡
S Supporting Information *
ABSTRACT: Ordered mesostructured materials possess unique surface, structural, and bulk properties that lead to important practical applications. Mesostructured organic−inorganic composites are also of broad interest for fundamental studies of confinement effects and surface interaction on structural and dynamic properties of organic molecules. In the present study, solid state dipolar 13C−1H NMR spectroscopy is applied to quantitatively characterize the conformational dynamics of a surfactant in a mesolamellar composite. By applying dipolar recoupling and separated local field spectroscopy techniques, the motion of surfactant molecules was studied in a wide range of mobilities from an essentially immobilized rigid state to a highly flexible and anistropically tumbling state. From the analysis of the measured heteronuclear dipolar couplings, the orientational order parameters of C−H bonds along the surfactant chain were determined. The study shows that in surfactant bilayers in AlPO layered structure at room temperature the highly ordered chains in all-trans conformation undergo fast rotation about the molecular axis. In a higher temperature phase, the order parameter is gradually decreasing toward the chain end due to conformational transitions; however, the dynamics of the segment in the vicinity of the headgroup is only slightly affected. The conformational dynamics in the surfactant bilayers confined between solid inorganic sheets is also compared to that in fluid bilayers in an aqueous lamellar phase. compared to those in the bulk.8−14 NMR spectral shapes and relaxation rates are sensitive to the local environment and, in addition to delivering the structural information on the atomic and molecular level, can be used to study local molecular motion.9−17 Carbon-13 chemical shift spectra reported on the relative populations of conformation states of the interior methylene carbons in the alkyl chain.9−11,15 NMR relaxation rates have been measured to elucidate the molecular dynamics.9,11,15 Relaxation techniques, however, do not provide direct information on the motional parameters, require model analysis and involve assumptions and adjustable parameters. Anisotropic spin interactions, such as dipolar or quadrupolar couplings, are very sensitive to the details of the molecular motion. Since the dipolar interaction has well-defined orientation and distance dependence, the experimental dipolar couplings are often exploited to reveal the details of the conformational motion in flexible molecules.18 Previously, homonuclear dipolar interactions of protons in mesostructured composites have been studied by employing 2D wide-line separation spectroscopy (WISE).9,11,17,19 Due to multiple short- and long-range spin interactions in the complex network of the abundant protons, leading to unresolved broad peaks, these spectra are difficult to analyze in terms of the motional
1. INTRODUCTION A wide variety of ordered mesostructured composite materials have been recently designed making use of co-condensation processes in the presence of structure-directing templates.1−7 Mesostructured composites possess unique surface, structural, and bulk properties which lead to potential applications in diverse areas such as optically active and semiconducting materials, adsorption of organic pollutants, and in molecular recognition. The functional properties of these heterogeneous materials are largely determined by the morphology and connectivity of the phases and interfacial interactions. Organic− inorganic mesocomposites with the organic component periodically arranged by noncovalent interactions within the crystalline or amorphous inorganic framework are also of broad scientific interest for fundamental studies of the confinement effects and surface interaction on structural and dynamic properties of organic molecules. Surfactants, as structuredirecting components, have been widely utilized in the synthesis of different types of mesostructured materials since the first introduction of mesoporous silicas.1 In many materials developed by surfactant-templating synthetic strategies, the composite structure resembles that of the lyotropic phases of surfactants in an aqueous solution where lamellar, hexagonal, or cubic aggregates can form. Experimental and computational studies of hydrocarbons confined in, intercalated between or adsorbed to inorganic surfaces report significant differences in structure and dynamics © 2013 American Chemical Society
Received: September 22, 2013 Revised: October 26, 2013 Published: October 28, 2013 24511
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H2O. After additional stirring for 96 h, the resultant precipitate was filtered, washed several times with deionized water to remove TMAOH and any excess surfactant, and dried overnight at 50 °C. Residual water content, including water molecules incorporated in inorganic framework, was about 5 wt % according to previous TGA analysis.32 The ordered layered structure of the synthesized composite was confirmed by X-ray diffraction pattern recorded with the PANalytical X′pert PRO diffractometer using Cu Kα irradiation at 0.15406 nm. By comparison of X-ray data and 13C, 1H, 27Al, and 31P NMR spectra to literature data9,32 it was verified that the synthesized material corresponds to lamellar phase labeled in work 9 as L1. The XRD pattern and 1H, 27Al, and 31P NMR spectra are included in the Supporting Information. The lyotropic lamellar mesophase was prepared according to the established phase diagram by mixing an appropriate amount of CTAC crystalline powder with 15 wt % of deuterated water.33 The mixture was homogenized by extrusion through a syringe needle at 75 °C, repeated several times. NMR Experiments. Magic angle spinning (MAS) NMR experiments were performed at a magnetic field of 7.05 T on Bruker Avance II 300 MHz spectrometer at resonance frequencies of 300 and 75 MHz for 1H and 13C, respectively. A 4 mm double-resonance CP-MAS probe was used. Onedimensional 13C spectra were acquired using cross-polarization (CP) enhancement and proton heteronuclear decoupling by two-pulse-phase-modulated (TPPM) sequence.34 A typical 1H 90° pulse length was 4 πs. 2D R-PDLF (proton detected/ encoded local field) spectra were recorded using the heteronuclear dipolar recoupling pulse sequence R1817 at radio frequency (rf) field strength of γB1/2π = 72 kHz, spinning speed of 8 kHz and contact time of 40 μs. For recording of the 2D APM-CP (amplitude- and phase-modulated CP) local field spectra, proton rf field during recoupling period was set to γB1/ 2π = 62.5 kHz (corresponding to rf cycle time of 32 μs). Time diagrams of R-PDLF and APM-CP protocols are included in the Supporting Information.
and orientational order parameters. In contrast, heteronuclear dipolar spectroscopy, which probes spin interactions of the rare carbon-13 spins with the abundant protons, can provide highly resolved, informative, and easy to interpret spectra. This class of experiments, referred to as separated local field (SLF) spectroscopy,20 has been used in studies of a wide range of bulk solids, liquid crystals, and biomembranes.18,21−23 Recently, it was also successfully applied to study the chain dynamics of the highly mobile template molecules in a hexagonal mesoporous silica.24 It is anticipated, however, that many mesostructured composite materials can be challenging to study by this approach due to generally poorer spectral resolution and sensitivity, unfavorable motional correlations times, and a heterogeneous molecular environment. Motivation of the present study was to further exploit the opportunities offered by the SLF spectroscopy developments and to explore the sensitivity of dipolar 13C−1H NMR toward the characterization of mesostructured composites exhibiting a wide range of molecular mobility. An ultimate goal is to obtain detailed, quantitative, and model-independent information on the conformational dynamics of the organic component and thus to contribute to the understanding of the fundamental processes of molecular dynamics in confinement. Efficient SLF pulse schemes for measuring dipolar coupling and order parameters in liquid crystalline and solid systems have been recently introduced.25−27 C−H bond order parameters profiles along the hydrocarbon chain are obtained simply by measuring the splittings in the 2D heteronuclear dipolar spectra. The technique is based on the state-of-the-art dipolar recoupling and SLF protocols, and, when used with insight, is able to deliver resolved dipolar spectra in a variety of challenging systems.28−31 In the present study, the solid state dipolar 13 C−1H NMR spectroscopy is applied to investigate the conformational dynamics of a surfactant template in a mesolamellar material whose main inorganic framework is composed of aluminophosphate (AlPO). The double-layer aggregate of surfactant cations is confined in a two-dimensional space between solid inorganic sheets. A variety of synthesized aluminophosphate-based composites have been characterized on the microscopic scale by SAXS and TEM.6 However, these imaging and diffraction techniques do not provide direct information on the local dynamics of the organic part. The dynamic and structural data obtained in the present work from the 1H−13C dipolar spectra is compared with the previous results from 13C chemical shift and 1H line-width measurement. Conformational dynamics in the bilayer in the solid mesocomposite is also compared to that in the fluid surfactant bilayer formed in a concentrated aqueous solution.
3. RESULTS AND DISCUSSION Figure 1 compares the conventional proton-decoupled 13C CPMAS spectra of CTAC crystalline powder, lamellar mesocomposite CTAC/AlPO, and lyotropic lamellar phase of CTAC/D2O mixture. In the crystalline CTAC (Figure 1a), the presence of magnetically in-equivalent molecular positions leads to multiple signals from each resolved carbon site in the molecule. A similar spectral structure, albeit broader lines, is observed in the CTAC/AlPO composite in Figure 1b. In both samples, the chemical shifts of the interior carbons C5−C13 are in the range of 33−34 ppm corresponding to the chain in the trans conformation.35 The increase of the sample temperature is accompanied by a gradual line narrowing and growth of new signals at about 30 ppm at the expense of the peaks in the range 33−34 ppm. When the transition is completed at about 70 °C, the spectral shape of Figure 1c is resulted. The 3 ppm lower chemical shift of the central methylene carbons is the result of the conformational mobility of the hydrocarbon chain with a considerable population of the gauche conformers in contrast to the all-trans chain conformation of the rigid chain. Upon cooling back to room temperature, the line shape of Figure 1b is recovered. The spectral transformation in the CTAC/AlPO sample upon heating has been previously analyzed in terms of the relative populations of trans- and gauche conformers.9 At a temperature of 70 °C, the molecular structural nonequivalence
2. EXPERIMENTAL SECTION Sample Preparation and Characterization. A representative material of mesolamellar aluminophosphate was synthesized according to the previously described procedure.32 Chemicals were purchased from Sigma-Aldrich and used as received. An appropriate amount of aluminum triisopropoxide was mixed with water. An 85% solution of H3PO4 was added dropwise. After several hours of stirring at an ambient temperature, when the mixture became clear, a 9 wt % aqueous solution of cetyltrimethylammonium chloride (CTAC) was added. The mixture was stirred again for 1 h, and a 25% aqueous solution of tetramethylammonium hydroxide (TMAOH) was added. The resulting mixture had the overall molar ratio 1.0 Al2O3:3.05 P2O5:0.5 CTAC:8.11 TMAOH:350 24512
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required.39 Furthermore, the resonance assignment is difficult and spectral resolution is poor in 2H-multiple-labeled molecules. A general alternative is to measure carbon−proton direct dipolar coupling using SLF spectroscopy. Heteronuclear dipolar couplings between directly bonded nuclei, such as 13 C−1H in methylene groups are particularly convenient observables since their magnitude in the absence of the motion is well-known. Under MAS, the dipolar recoupling technique must be used which preserves the heteronuclear coupling term while other spin interactions are suppressed by sample spinning and/or radio frequency pulses. In the present study we apply RPDLF and APM-CP approaches.25−27 R-PDLF has an advantage of active suppression of the homonuclear dipolar interactions and hence can be used for samples with strong homonuclear proton couplings, such as rigid solids. The spectral splitting obtained in the indirect dimension of the experiment is given by Δν = |κSCHDCH|, where DCH = −21.5 kHz is the one-bond rigid lattice dipolar coupling and κ = 0.315 is the scaling factor of the dipolar recoupling sequence.25 RPDLF, having a relatively low dipolar scaling factor,25 does not achieve sufficient resolution of weak interactions (typically, in the kHz range and below). In contrast, the APM-CP sequence, with the highest possible scaling factor (κ = 1.0 for CH2 methylene groups) is especially sensitive to the small dipolar interaction.26,27 Its application is, however, limited to the samples where the magnitude of the homonuclear proton coupling is of the order of or below the sample spinning speed. The heteronuclear dipolar couplings in rigid solid CTAC and in CTAC/AlPO composite at room temperature were measured using R-PDLF technique. For rigid immobile CTA+ cations in crystalline samples, the dipolar spectra for all resolved methyl groups exhibited splitting corresponding to C−H coupling of about 20 kHz and, hence, |SCH| ≈ 1 (spectra are shown in the Supporting Information). In the R-PDLF spectrum recorded in CTAC/AlPO sample at room temperature, the dipolar couplings between directly bonded 13C and 1 H are also well resolved and can be extracted from recoupled line shapes by direct inspections of the peak-to-peak splittings (Figure 2). Dipolar couplings for the different methylene carbon sites are all in the range of 9−10 kHz. The resulted order parameters of about 0.5 are virtually independent of the carbon position within the experimental error bars. This behavior is distinctly different from that found in hexagonal mesoporous silica sample, where motional gradient along the chain with significantly lower order parameters was found.24 The value of the order parameters of 0.5 for all methylene groups in the chain is indeed expected for the all-trans chain freely rotating or experiencing large amplitude motions about the molecular axis, similar to the rotator phase of n-alkanes.35 Since the C−H vector points at an angle of 90° to the molecular axis, in the presence of the fast axial rotation the dipolar couplings are scaled by a factor of −1/2. Any significant contribution of the gauche conformations would result in a variation of the order parameters along the chain. In spite of this regularly ordered chain structure, the resonances in 1D spectrum are rather broad (Figure 1b) when compared to the crystalline powder or to the higher temperature phase. This can be explained by a less regular orientations of the molecular axis and of the planes of the carbons skeleton of the all-trans chains. In the higher temperature phase of CTAC/AlPO sample, C− H dipolar couplings are significantly reduced, as compared to the room temperature phase, and decrease toward the chain end. To record dipolar splittings with sufficient resolution, the
Figure 1. Carbon-13 CP-MAS spectra of (a) CTAC crystalline powder, (b and c) lamellar mesocomposite CTAC/AlPO at 25 and 80 °C, respectively, (d) CTAC/D2O 85 wt % lamellar phase at 40 °C. The peak assignment is according to previous data for CTAC and similar molecules.9,13,15,36−38
vanishes as evidenced by the disappearance of the line splittings and broadening. Structural rearrangement may become possible due to an increased mobility of the chains, allowing for the relaxing of packing constraints. No further change of the carbon spectrum was observed up to a maximum applied temperature of 90 °C. It is noted that the chemical shifts for the respective carbon positions essentially coincide with those observed in the conventional aqueous lamellar phase (spectrum Figure 1d). Overall, the inspection of the 1D carbon spectra suggests the presence of a significant change of the chain conformational dynamics at an elevated temperature compared to the room temperature phase. However, such spectra alone cannot deliver quantitative information on the flexibility of the alkyl chain. Below, we explore 2D dipolar spectroscopy as a tool capable to provide detailed characterization and comparison of the chain dynamics in different phases in the terms of bond order parameters. The bond order parameter SCH characterizes the amplitude of the local anisotropic reorientation of the bond vector in a molecular or molecular aggregate frame and ranges from 1 for a rigid bond to 0 for a freely reorienting bond. By analyzing the motionally averaged NMR spectral shapes, the local dynamics with the time scale faster than 10−5 s as determined by the magnitude of the involved spin coupling in the static limit, can be characterized. A popular approach to measure the C−H bond order parameter is 2H NMR, which has, however, the important disadvantage that site-specific isotopic labeling is 24513
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from C5 to C12 sites are heavily overlapped in the chemical shift dimension (Figure 1c), the presence of several distinct powder patterns for these resonances was apparent in the 2D spectrum (shown in the Supporting Information). Hence, the dipolar splittings for the majority of the CH2 groups in the molecule were resolved. The calculated order parameters are plotted in Figure 4, where also order parameters obtained at
Figure 2. R-PDLF dipolar 13C−1H spectra in CTAC/AlPO mesolamellar alumonophosphate at 25 °C. The frequency scale is corrected for the scaling factor 0.315 of the recoupling sequence R1817. The dashed lines indicate the theoretical splitting of 10.7 kHz corresponding to the bond order parameter SCH = 0.5.
Figure 4. Order parameter profiles SCH in CTAC/AlPO mesolamellar alumonophosphate at 25 (solid circles) and 80 °C (squares) and in CTAC/D2O 85 wt % lamellar lyotropic phase at 40 °C (open circles). Lines are guides for eye. Rotating all-trans ordered chains in room temperature phase and partly disordered chains at 80 °C are schematically illustrated.
APM-CP technique was applied. Selected 1H−13C dipolar cross sections from 2D APM-CP spectrum of CTAC/AlPO sample at 80 °C are shown in Figure 3. Notably, while carbon signals
room temperature are included. To the best of our knowledge, the results of Figure 4 represent the first report of the order parameter profiles of organic components in mesolamellar composites. It is evident that the chains become much less ordered in the high temperature phase. However, C1-methylene (and to some less extent also C2) exhibits only a slight decrease of the order parameter. The implication of a much higher ordering of the first segments will be discussed below. The motion gradient, as reflected by the decreasing the SCH values toward the chain end implies a significant segmental motion besides the tumbling of the molecule as a whole. Such a “chain-melting” is related to the formation of gauche bonds and “kinks” in the alkyl chain that exist in a dynamic equilibrium.40 As mentioned previously, 30 ppm chemical shift for the interior carbons of the chain also points to the presence of gauche conformers. The structure of the composite at the microscopic scale, however, is not affected by the heating up to 100 °C, as has been shown previously by XRD.41 Our results on the chain conformational dynamics in the mesocomposite are, generally, in agreement with the previous available data in related mesolamellar compounds. Carbon-13 chemical shifts and relaxation rates, and proton line-width have provided valuable information on the motionally averaged spin interaction.9,11,17,19 Based on the analysis of the carbon-13 1D spectral shapes in mesolamellar aluminophosphate, predominantly trans-conformation at room temperature and trans− gauche transitions at ca. 70 °C have been suggested for the interior carbons in the chain.9 In a similar lamellar composite, but prepared with a lower surfactant content, a significant
Figure 3. APM-CP dipolar 13C−1H spectra in CTAC/AlPO mesolamellar alumonophosphate at 80 °C. 24514
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reveals significant differences in the chain dynamics of the two materials. First, much higher order parameters for the carbons next to the headgroup in CTAC/AlPO suggest significantly restricted conformational mobility in the mesostructure with solid walls as compared to the aqueous phase. The main process affecting the orientational order for the first segment (C1 carbon) is the overall molecular tumbling about its long axis, similarly to that in room temperature sample. Further decrease of SCH to about 0.40 can be attributed to the additional wobbling of the long axis of the molecule, whose chain, in its major part, is in mixed trans−gauche conformations in dynamic equilibrium. Second, in the central part and toward the chain end, the order parameters are lower in the mesocomposite, which can be ascribed to the difference in the surfactant packing density per surface area as well as to the temperature effect.
population of the gauche conformers was found already at ambient temperature.9 These findings were also consistent with the results of the 2D WISE experiment of the same work. Variable contact time CP-MAS and 2D WISE NMR studies of surfactant bilayer intercalated in layered CdPS3 have suggested a significant motional gradient along the surfactant chain.11 Similarly, 2D WISE spectra have indicated conformational heterogeneity of the surfactant in the intercalated montmorillonite clay19 and in the synthetic layered surfactant-silicate mesocomposite.17 However, complex molecular structure and dynamics, a large number of coupled spins and involved spin interactions, make it difficult to extract the motional parameters from the relaxation rates and proton line width. In contrast, isolated pairwise heteronuclear dipolar couplings, provided by SLF spectra, give direct access to the bond orientational order parameters. Order parameters can be also obtained from MD simulations thus allowing the direct comparison between computational and experimental data.42 To gain further insight in the surfactant dynamics in composite materials and to put the obtained results in a wider context, the order parameter profiles are compared to those obtained in aqueous lamellar lyotropic phases. In previous studies of lamellar mesophases, the bond order parameters of about 0.2 and below were observed, including those for the carbon sites next to the headgroup.43−45 One can speculate that in mesolamellar composites, due to strong electrostatic interactions of the headgroup with the solid interface, the motion of the first segments in the chain is more restricted as compared to that in aqueous analogues. Thus, to directly compare the surfactant dynamics in lamellar aggregates separated by an aqueous layer in the lyotropic phase and divided by solid walls in the mesolamellar composite we also measured APM-CP dipolar spectra in CTAC/D2O lamellar phase. The results are displayed in Figure 5. A gradual decrease of the dipolar splitting toward the chain end is evident, similar to that in the mesocomposite at high temperature. However, a comparison of the bond order parameter profiles in Figure 4
4. CONCLUSIONS In the present work, 13C−1H separated local field spectroscopy has been applied to organic−inorganic mesolamellar composite material to obtain detailed and model-independent information on the motional parameters of the organic component. Our study of the mesostructured CTAC/AlPO, in addition to confirming the previous findings on the highly dynamic state of the surfactant molecules confined in the mesolamellar structure, provided new results on the chain dynamics in the surfactant bilayer. The conclusion of the study of the CTAC/AlPO material may be summarized as follows: (i) In the room temperature phase, highly ordered chains in extended all-trans conformation undergo axial rotation or large amplitude motion about the symmetry axis. This rotation is fast on the time scale of the order 10−5 s determined by the strength of the static dipolar spin interactions. The orientational order parameters of CH bonds are close to 0.5 for all methylene groups in the chains. (ii) Upon heating to 70 °C, the molecular structural inequivalence vanishes as suggested by the disappearance of the line broadening and splittings observed at low temperatures. This structural rearrangement may become possible due to an increased mobility of the chains, allowing for the relaxing of some packing constraints. (iii) In the high temperature phase, a large part of the chain undergoes fast conformational transitions characterized by a low value of the C−H bond order parameter
