Dynamics of cetyltrimethylammonium bromide head groups in bulk by

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Dynamics of cetyltrimethylammonium bromide head groups in bulk by solid-state deuterium NMR spectroscopy Madhubhashini Maddumaarachchi, Yohan L. N. Mathota Arachchige, Tan Zhang, and Frank D. Blum Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02193 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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Dynamics of cetyltrimethylammonium bromide head groups in bulk by solidstate deuterium NMR spectroscopy Madhubhashini Maddumaarachchi,1,2 Yohan L. N. Mathota Arachchige,1,3 Tan Zhang,1,4 and Frank D Blum1,* 1

Department of Chemistry, Oklahoma State University, Stillwater, OK 74078, United States

Current Addresses: 2

Department of Chemistry, University of Sri Jayewardenepura, Nugegoda, Sri Lanka

3

Department of Chemistry, University of Kelaniya, Kelaniya, Sri Lanka

4

Department of Materials Science and Engineering, Texas A&M University, College Station, TX 77843

*Corresponding Author ([email protected])

ABSTRACT Variable temperature, solid-state deuterium (2H) NMR spectroscopy has been used to probe the rather complex head-group dynamics of the surfactant cetyltrimethylammonium bromide-d9 (CTAB-d9) in bulk. Heating and cooling runs were made as the surfactant underwent supercooling. 2H NMR line shape simulations were used to identify the hierarchy of the molecular motions of CTAB as a function of temperature. Fast continuous methyl rotations about the N-Cmethyl axes and three-fold jumps about the main chain C-N axis were present at all the temperatures from -40 to 120 °C. With heating, the spectra were consistent with CTAB molecules starting 180° flips about the hydrocarbon chain molecular axis around 0 °C which continued to flip with increasing flip rates up to 80 °C. At 90 °C, the flips changed to rotation of the CTAB molecules about the hydrocarbon chain axis and that rotation continued to 120 °C.

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Comparison of spectra of bulk CTAB at different temperatures from heating and cooling runs revealed that the rotation about the long axis of the hydrocarbon chains started at around 90 °C on heating, however, it does not freeze out until between 70 and 80 °C because of supercooling.

Introduction Cetyltrimethylammonium bromide (CTAB) is one of the most widely used cationic surfactants in fields such as medicine, detergency, and nanoparticle synthesis. In most uses, CTAB is combined with solvents, often water, and these systems can have rather extensive micellar and liquid crystalline phases.1-4 In contrast, the structures of bulk CTAB seem more modest. Crystalline CTAB has a monoclinic crystal structure with CTAB molecules forming bilayers, with the cationic moieties are arranged in an antiparallel configuration with the alkyl chains tilted and somewhat overlapping.5 However, the structural and dynamics changes in the bulk surfactant are far from clear. Differential scanning calorimetry (DSC),6-9 X-ray diffraction,5,6 FTIR spectroscopy,7-9 gravimetry,8 optical microscopy,6 and dielectric spectroscopy8 have been used to probe the thermal behavior of bulk CTAB. As evident by DSC, solid CTAB is known to show a first order solid–solid phase transition on heating at around 103 °C, which is attributed to the melting of CTAB tails.6 The crystallization of the CTAB tails occurs well below the melting temperature indicating a super-cooling phenomenon.6 X-ray diffraction studies on CTAB revealed the presence of frozen hydrocarbon chains below around 20 °C. As the temperature increased, there was an expansion of the lattice spacing resulting in an amorphous phase with molten hydrocarbon chains.6 Although the temperature dependent phase behavior of solid CTAB has

Maddumaarachchi et al.

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been known for a long time, there are very few studies in the literature which report the dynamics of bulk CTAB molecules as a function of temperature. Molecular dynamics of solid substances play an important role in determining their properties and those properties determine their functions and applications. Therefore, studying molecular dynamics of solid materials is very important to expand the scope of their applications. A very early study of dielectric measurements on the long chain alkyl bromides in the solid-state revealed that those molecules can rotate about their long molecular axes between their freezing and melting points.6,10 An infra-red study along with theoretical calculations showed that a phase transition in the CTAB can be described by conformational changes, hindered rotation, and free rotation.7 Further, another study using thermal, dielectric, and spectroscopic techniques proposed a model for the thermal motions in CTAB crystals. According to that study, topological solitons are produced at 55 °C, leading to the rotation of CTAB molecules by 180°.8 As mentioned above, although the dynamics of CTAB molecules in the solid-state have been probed to some extent, a systematic study of the molecular motions of CTAB as a function of temperature is not available. NMR spectroscopic techniques, in general, and deuterium (2H) NMR spectroscopy in particular, are very powerful techniques to study the dynamics of surfactant systems3,11-19 Deuterium NMR line shapes are dominated by the quadrupole interaction. Therefore, molecular motions which result in changes in the quadrupolar coupling affect the line shapes of the spectrum in ways which can be often straightforward to interpret in terms of the type and the amplitude of the molecular motions.11 In this paper, we report the use of solid-state 2H NMR spectroscopy to probe the molecular dynamics of solid CTAB with g-methyl deuterated surfactant. Labelled CTAB has often been used to study the behavior of this important surfactant Maddumaarachchi et al.

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on surfaces,14 association with polyelectrolytes,3,20,21 in surfactant mixtures,19,22 magnetic alignment.18 However, the behavior of the head group in the solid surfactant has not been systematically studied. Given the importance of this surfactant and its complex dynamics, a systematic characterization of these motions is warranted. Experimental Materials - Cetylamine from MP Biomedicals (Santa Ana, CA, USA), methyl-d3 iodide (D, 99.5%) from Cambridge Isotope Laboratory Inc. (Tewksbury, MA, USA), methanol, chloroform, and dichloromethane from Pharmco-aaper (Brookfield, CT, USA), sodium hydroxide from EMD Chemical Inc. (Gibbstown, NJ, USA), dowex1-X8 (20-50 Mesh)-Cl- form and sodium bromide from J. T. Baker Chemicals (Center Valley, PA, USA) and celite from Acros Organics (New Jersey, USA) were used as received. N-methyl deuterated cetyltrimethylammonium-d9 bromide (CTAB-d9) was synthesized according to the literature.22-24 Cetylamine (0.215 g) was dissolved in methanol (11.25 ml) containing sodium hydroxide (0.356 g). Methyl-d3 iodide (5.16 g) was added to the above mixture which was at 0 °C. The mixture was stirred for about 12 h in tightly closed flask at 0 °C at room temperature for four days. Methanol was evaporated under vacuum and any traces of methanol were removed by the repeated addition of chloroform. The solution was filtered through a celite plug to remove sodium iodide. The filtrate was evaporated under vacuum to obtain N-methyl deuterated cetyltrimethylammonium-d9 iodide. Preparation of ion-exchange resin – A column packed with Dowex 1-X8 (20-50 Mesh)-Clform was transformed into the bromide form using an aqueous solution of 5% sodium bromide.


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The column was washed with water until no bromide ions were detected and the column was gradually changed to methanol and stored in methanol for 2 days. The synthesized N-methyl deuterated cetyltrimethylammonium-d9 iodide was exchanged to the bromide form using the ion-exchange column mentioned above using methanol: dichloromethane 1:1 as the eluent. The solvent in the eluent was evaporated under vacuum to obtain crude CTAB-d9. The product was recrystallized from acetone. The purity of the synthesized CTAB-d9 was checked by 1H NMR and the bromine exchange was confirmed by cyclic voltammetry using silver working electrode, platinum counter electrode, and saturated calomel electrode as reference electrode. The 2H-NMR spectra of bulk CTAB-d9 were obtained using a Tecmag Discovery 400 WB spectrometer equipped with a Doty 8 mm 2H wideline probe (DSI-1432) and a Doty Scientific temperature controller, version 5.0, (Doty Scientific Inc., Columbia, SC, USA). The quadrupoleecho pulse sequence (delay-90# -τ-90$ -τ-acquisition) with a 2H frequency of 61.48 MHz, a 90o pulse width of 3.2 µs, and an echo time of 30 µs was used. The second τ was typically set to around 28 µs and each echo was left shifted as appropriate to the echo maximum. The number of acquisitions used was 256 consisting of 4096 points with a sweep width of 500 kHz and a 5 s interval after each scan. The temperature controller was calibrated on a separate temperature run using a thermocouple in the sample holder. The samples were equilibrated for approximately 20 min which was deemed adequate based on the controller calibration run. The spectra were processed using MestReNova (Mnova) software (Santiago de Compostela, Spain). The experimental 2H-NMR spectra were simulated using the EXPRESS program.25 Series of simulated line shapes with different jump rates for several different possible motions were generated using the EXPRESS program. A MATLAB (The Mathworks, Inc., Natick, MA) Maddumaarachchi et al.

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program was then used to fit the experimental line shapes with the simulated line shapes by minimizing the sum of the squares of the residuals of experimental and sum of the simulated spectra. Also, the weight fractions of each simulated spectrum used to generate the fits were obtained by the MATLAB fitting. In the figures with the summaries of jump rates, the simulated spectra were grouped by different orders of magnitude, for simplicity.

Results and Discussion The experimental 2H-NMR spectra of solid CTAB-d9 in the temperature range of -40 to 120 °C are shown in Figure 1. The spectra are shown in two separate parts for the clarity. Figure 1A shows the spectra ranging from -40 to 20 °C, whereas Figures 1B shows the spectra from 20 to 120 °C. The spectra were collected by heating the sample to desired temperature from -40 °C. As shown in the Figure 1, the spectrum at -40 °C had a characteristic Pake doublet line shape with a central resonance. The quadrupolar splitting at the horns was about 40 kHz. As the temperature increased, the intensity under the horns decreased and central resonance developed. At 0 °C, there were no distinguishable horns and the spectrum was significantly narrowed. With a temperature change from 20 to 30 °C, the spectrum again changed to a characteristic Pake doublet line shape with a quadrupolar splitting of about 10 kHz at the horns. The intensity under the shoulders and the middle part gradually decreased as temperature increased up to 80 °C. There was a notable change in the spectrum when moving from 80 °C to 90 °C with a further reduction in the quadrupolar splitting to about 8 kHz at the horns. Finally, the spectra narrowed to characteristic Pake doublet line shape at 100 °C and above with a splitting of 8 kHz.


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A - heating

B - heating

Figure 1. 2H-NMR quadrupole echo spectra for solid CTAB-d9 as a function of temperature. The spectra were collected by heating the sample from -40 °C. Figure 1A shows the spectra ranging from of -40 to 20 °C, whereas Figure 1B shows spectra from 20 to 120 °C. Note the x-axes in Fig. 1A and 1B are on different scales.

The 2H-NMR spectra of solid CTAB-d9 in the temperature range of 20 to 120 °C obtained after cooling the hot sample from 120 °C are shown in Figure 2. At temperatures of 80 °C and above, the spectra showed characteristic powder patterns with about 8 kHz splitting at the horns. When cooling the sample to 70 from 80 °C, there was a significant change in the spectrum leading to a broader powder pattern with a quadrupolar splitting of about 10 kHz at the horns.


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With further decreases in the temperature, the powder pattern broadened and the middle part increasingly filled in. At 20 °C the spectrum had a completely filled middle (flat top).

cooling

Figure 2. 2H-NMR quadrupole echo spectra for solid CTAB-d9 as a function of temperature. The spectra were collected by cooling down the sample from 120 °C. The spectrum at 80 °C was indicative of supercooling.

For the 2H-NMR spectra of solid CTAB-d9 at -40 °C, a Pake powder pattern was observed with a central resonance and a quadrupolar splitting at the horns of about 40 kHz, as shown in Figure 1A. With a nuclear spin of 1, deuterium NMR line shapes are determined by the quadrupolar interaction. In the absence of molecular motions, for a deuteron in a single crystal, Maddumaarachchi et al.

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the deuterium NMR spectrum consists of a doublet where the splitting between the two lines, Dνq is given by,26 *

∆𝜈' = (+)( where

𝑒 . 𝑞𝑄2 . . . ℎ)(3𝑐𝑜𝑠 𝜃(𝑡) − 1 − h 𝑠𝑖𝑛 𝜃(𝑡)𝑐𝑜𝑠 F (𝑡))

(1)

𝑒 . 𝑞𝑄2 ℎ is known as the quadrupolar coupling constant (QCC). q and F are the polar

angles which specify the orientation of the C-D bond vector in the magnetic field with the principal axis system of the electric field gradient (EFG) tensor andh is the asymmetry parameter. h is very close to zero for axially symmetric EFG around the C-D bond. In a situation where there is an axial symmetry of the EFG, equation 1 can simplify to, *

∆𝜈' = (+)(

𝑒 . 𝑞𝑄2 . ℎ)(3𝑐𝑜𝑠 𝜃(𝑡) − 1)

(2)

and 3𝑐𝑜𝑠 . 𝜃(𝑡) − 1 term can be expanded to, A

< 3𝑐𝑜𝑠 . 𝜃(𝑡) − 1 > = @.B < 3𝑐𝑜𝑠 . 𝛽(𝑡) − 1 > (3𝑐𝑜𝑠 . 𝜒 − 1)

(3)

where 𝛽(𝑡) is the angle between symmetry axis (rotation axis) and the applied magnetic field and 𝜒 is the angle between the symmetry axis and the C-D bond. The molecular motions cause an averaging of the quadrupole coupling and as a result, there were changes in the line shapes. The quadrupolar coupling constant of C-D bonds in static methyl groups is typically about 170 kHz27-29 and hence the static quadrupolar splitting is about 128 kHz, which is about 3/4 of the quadrupolar coupling constant. For methyl groups undergoing rapid continuous rotation around its symmetric axis, the quadrupolar splitting will be reduced to value of 40 kHz, a factor of 1/3 due to the geometry of the C-D bond with respect to the axis of


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the rotation. Therefore, for solid CTAB-d9 at temperatures of -40 and -30 °C, there were some quaternary methyl groups, which showed rapid rotation around their symmetry axis, resulting in a quadrupolar splitting at the horns about 40 kHz. The presence of the central resonance even at 40 and -30 °C indicated that some additional motions were also present affecting the quaternary methyl groups. Analysis of the spectra in Figures 1 and 2 clearly showed that the methyl groups in the head group of solid CTAB undergo different types of motions with increasing the temperature from -40 to 120 °C. As shown in Figure 1, upon heating, there was a prominent transition from 80 to 100 °C. It is well known that crystalline CTAB shows a thermal transition from 95 to 110 °C in differential scanning calorimetry due to the melting of hydrocarbon CTAB tails.7,9,30,31 In the literature, chain melting has been characterized as a conformational order-disorder transition.9,32 On the other hand, there was a prominent transition when cooling the CTAB from 80 to 70 °C as shown in Figure 2. This difference is due to supercooling. Supercooling behavior in bulk CTAB has been shown from calorimetry,9 with the melting of CTAB tails occurring at 104 °C and their crystallization at 88 °C.9 We believe that the thermal transition seen in the deuterium NMR spectra of solid CTAB and its supercooling behavior is intimately related to the CTAB chain melting. The changes in headgroup mobility with the melting/freezing of the tails does not seem to have been reported, though in hindsight seems quite reasonable. Supercooling of the melting/cooling of alkyl chains or side groups has also been well known for some time for phospholipids,33 and has important implications for drug delivery.34 It has also been observed for long alkyl sidechains on synthetic polymers.35 Investigating possible thermal motions of methyl groups in the CTAB-d9 head groups


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Solid-state 2H-NMR line shapes are dependent on the orientations of C-D bonds and the nature of molecular motions experienced by the segments where the deuterons are attached. The nature of the molecular motions includes both the types of the motion, as well as the rate at which the motion occurs. Therefore, simulating an NMR line shape requires an understanding of possible orientations of C-D bonds and the type and the rate of the motions of the molecular segments experienced. A schematic representation of CTAB indicating deuterated head methyl groups is shown in Figure 3. For methyl groups in the head group of CTAB, rapid continuous rotation around N-Cg is the lowest temperature non-vibrational motion observed by NMR in terms of the sequence of the motions that the methyl groups can experience; i.e. methyl rotation (Hereafter, this motion is referred to as motion A). Motion A occurs at all temperatures studied. Since there is a tetrahedral symmetry around the Cg, χ (the angle between the C-D bond and the rotation axis) equals to 70.5° results in a reduction of quadrupolar splitting by a factor of 1/3 of its value in static state if the rotation is fast. Therefore, a reduced QCC of 54 kHz was used in the line shape simulations to account for the fast motion A, which was always present.

CγD3 β

B

N α

+

C

D

CγD3 CγD3 A

Figure 3. A schematic representation of CTAB showing deuterated methyl head groups with the curved arrows showing different rotations with their corresponding motion labels.


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Upon heating, the next most logical motions that can be experienced by the methyl groups were either the three-fold jumps or the continuous rotation of head groups around N (about the N-Ca axis) referred to as motion B. Since there is a tetrahedral symmetry around the N, χ (the angle between the C-D bond and the rotation axis) again equals 70.5° results in a further reduction of quadrupolar splitting by a factor of 1/3 of its value in the fast motion regime. Comparison of the line shapes of simulated spectra for the three-fold jumps and the continuous rotation with the experimental line shape showed that the three-fold jumps fit better than the continuous rotation. More information on the fits can be found in the Supporting Information. Figure 4 shows the fits and the experimental spectra obtained using motion B for the head methyl groups. A series of jump rates (k), from 10 Hz to 1x109 Hz, were used to fit the experimental line shapes. In Figure 4, these two motions fit the experimental spectra quite well at -40, -30, and -20 °C, but they did not fit well above -20 °C. The distribution of jump rates used in the fits of -40 to -20 °C can be found in the Supporting Information. The fits contained distribution of jump rates rather than a single jump rate. Further, the most intense contributions of jump rates for the fits moved to larger jump rates as the temperature increased. The poor fitting in Figure 4 at 0 and 10 °C suggested that another type of motion was present at temperatures above -20 °C. We note that for a given set of motions, the simulated spectra are best fits for those motions with different rates. However, visual observation of spectra clearly showed that "Bad" fits exhibit significant deviations between the experimental and simulated spectra, i.e., significant portions of the spectra just don't fit.


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Bad

Bad

Figure 4. Experimental (black, solid) and simulated (red, dashed) 2H-NMR quadrupole echo spectra for solid CTAB from -40 to 10 °C. The simulated spectra were obtained using motion A (fast) and motion B at different rates. Flipping of CTAB molecules around their long molecular axis (180° jump) was another possible motion that the CTAB molecules experience with increasing temperature referred to as motion C. According to the crystal structure of CTAB, angle between the CTAB long molecular axis and the N-Ca bond is less than 35.25° (which is half 70.5o) due to the off alignment of Ca from the mean plane of the molecule.5 Therefore, motion C was simulated with χ equal to 30° as a function of jump rate while the jump rate of motion B was kept at a constant rate (400 kHz). The constant three-fold jump rate was selected based on the largest contribution for the fit at -20 °C. Figure 5 shows the fits obtained from combined motions of fast motion A, motion B and motion C along with the experimental spectra from -40 to 40 °C. A series of jump rates (k), from


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10 Hz to 1x109 Hz were used to fit the experimental line shapes. As shown in the Figure 5, fits for the experimental spectra from -40 to -20 °C were improved with the addition of the third motion and the above motions were able to fit the experimental spectra up to 30 °C very well. For temperatures above 30 °C, the above motions failed to give good fits as shown in the Figure 5. Fractions of simulated spectra at different jump rates involved in the fittings can be found in the Supporting Information. From -40 to -20 °C major contributions for the fits were from motion B. With increasing temperature, contributions from motion B decreased and contributions from motion C increased. From 0 to 30 °C major contributions for the fits were from motion C. Furthermore, the most intense contributions of jump rates for the fits moved to higher jump rates as the temperature increased for both motion types. Inability of obtaining a better fit for the experimental spectrum at 40 °C suggested that another additional molecular motion should be taken into account.


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Bad

Figure 5. Experimental (black, solid) and simulated (red, dashed) 2H-NMR quadrupole echo spectra for solid CTAB from -40 to 40 °C. Simulated spectra were obtained from fast methylgroup rotation (motion A), a distribution of jump rates for three-fold jumps of methyl groups around the N-Ca axis (motion B) and flipping of CTAB molecule around its long molecular axis (180° jump) (motion C). The spectra which whose fits are shown in Figure 5 were simulated with motion C plus a fixed motion B with a rate of 400 kHz. Since these motions would be expected to get faster as temperature increased, motion C with a larger motion B rate than 400 kHz was tried to account for the small mismatch in the experimental spectrum at 40 °C (shown in Figure 5). As shown in the Figure 6, motion C with a fixed motion B with jump rate of 1 MHz were able to give an even


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better fit for 40 °C spectrum and also gave satisfactory fits for the temperatures up to 80 °C. Nevertheless, these motions were not able to provide acceptable fit for the 90 °C spectrum where there were significant changes in the line shape. The distributions of jump rates used in the fits of experimental spectra from 40 to 80 °C using motion B and motion C (with fixed motion B at 1 MHz) can be found in the Supporting Information. Significant contributions from motion B alone for the fits were not observable at any of these temperatures and the fits were merely based on the motion C (with fixed motion B at 1 MHz) of the molecules.

Bad

Figure 6. Experimental (solid) and simulated (dashed) 2H-NMR quadrupole echo spectra for solid CTAB from 40 to 90 °C. The simulated spectra were obtained from the series of jump rates with fast motion A, motion B and motion C (with fixed motion B at 1 MHz) with a distribution. To account for the significant experimental and simulated spectral differences in the 90 °C spectrum, another motion, which was the rotation of CTAB molecules around long molecular axis (motion D), was added with a fixed motion B. Figure 7 shows the fits of spectra from 90 to Maddumaarachchi et al.

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120 °C. The experimental spectrum at 90 °C was fit with fast motion A, a distribution motion B and motion D (with fixed motion B at 1 MHz), whereas satisfactory fits for 100 to 120 °C required an increased fixed motion B rate (100 MHz) on which the rotation of CTAB molecules around long molecular axis was performed. Distributions of jump rates used to fit the experimental spectra from 90 to 120 °C using motion B and motion D (with fixed motion B at 1 MHz for 90 °C and 100 MHz for 100 to 120 °C) are shown in the Supporting Information. The fits contained only simulated spectra obtained from motion D with a fixed motion B. Motion C was not required as the continuous rotation dominated the flips at these higher temperatures. This revealed that at 90 °C and above the CTAB molecules undergo relatively rapid motions of A, B, and D, but these were clearly not isotropic motions.

Figure 7. Experimental (solid) and simulated (dashed) 2H-NMR quadrupole echo spectra for solid CTAB from 90 to 120 °C. Simulated spectra were obtained from series of jump rates with Maddumaarachchi et al.

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fast motion A, motion B and motion D (with fixed B at 1 MHz for 90 °C and 100 MHz for 100 to 120 °C) with a distribution. We believe that based on the 2H-NMR simulations, the temperature dependent molecular motions of solid CTAB are well described. Table 1 shows an overview of the simulated motions of CTAB-d9 used to fit the experimental spectra of the bulk surfactant. At all the temperatures mentioned here, the methyl groups in the head groups of CTAB undergo fast continuous rotation around their axis N-Cg axis (methyl rotation, Table 1A) (motion A). In addition, they undergo three-fold jumps about the N-Ca axis (Table 1B) even at -40 °C (motion B). These jumps may become continuous rotations with increased temperature, but these 3-fold jumps vs rotation are difficult to discern at fast rates for this system without further experimentation.36 With increasing temperature, 180° flips of the CTAB molecules around long molecular axis of the hydrocarbon chain (Table 1C) (motion C) results and finally above 90 °C, these flips change to continuous rotation of the CTAB molecules around this long molecular axis (Table 1D) (motion D). Further, the progression of molecular motions as a function of temperature obtained from 2H-NMR seems to correlate with the previous studies on solid-state bulk8 and solution dynamics of CTAB.3


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Table 1. A schematic representation of the motions of CTAB used to simulate the spectra as a function of temperature. A) methyl group rotation around the N-Cg axis (motion A, always fast), B) head group rotation around the N-Ca axis (motion B), C) flip (180° flips) of the CTAB molecules around long molecular axis (motion C), and D) rotation of the CTAB molecules around the long molecular axis (motion D).

Description

Schematic

Temperature Range*

D D D D

Motion A

D

Continuous methyl group rotation about N-Cg axis

D

N + D

D

-40 to 120 °C

D

Motion B

CD3 N +

CD3 CD3

Three-fold jumps of the head groups around the N-Ca axis (Corresponding to a 70.5° reorientation of the methyl group)

CD3

Motion C 180° flips around the long chain axis

N +

CD3 CD3

CD3

Motion D

N +

CD3 CD3

-40 to 120 °C

Rotation around the long chain axis

0 to 80 °C

90 to 120 °C

*The temperature range when the addition of this motion was required to be added to simulate the experimental spectra.

CONCLUSIONS


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Temperature dependent motions of CTAB molecules were probed through the dynamics of the head methyl groups of CTAB by solid-state 2H-NMR spectroscopy. 2H-NMR line shape simulations showed the presence of fast continuous rotation of CTAB head methyl groups around its symmetry axis and three-fold jumps of CTAB head methyl groups around N-Ca axis at all the temperatures employed in this work (-40 to 120 °C). With increasing temperature to around 0 °C, CTAB molecules tend to make 180° flips around long molecular axis and rate at which increased with increasing temperature up to 80 °C. Further increases in temperature changed the flips to rotation of CTAB molecules around long molecular axis and rotation continued to 120 °C, the highest temperature studied. Deuterium NMR has been shown to be a very effective probe of surfactant motion for this important system, which in spite of its importance, has limited motional characterization.

Supporting Information Additional Information on the fitting of the 2H NMR spectra. Simulation results for threefold jumps and continuous rotation about the N-Ca axis.

Corresponding Author E-mail: [email protected] ORCID Frank D. Blum, 0000-0002-7884-3134 Tan Zhang, 0000-0002-8739-2769


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Dynamics of CTAB


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Dynamics of cetyltrimethylammonium bromide head groups in bulk by

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