Article pubs.acs.org/JPCC
Impedance Spectroscopic Study on Rotator and Disordered Phases in Trimethylammonium Chlorides Yuchen Tian,† Ji Yu,† Min Gu,*,† Yadong Lian,† Xiaoqian Ai,† and Tong B. Tang‡,§ †
National Laboratory of Solid State Microstructures and Department of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P. R. China ‡ Department of Physics, Hong Kong Baptist University, Hong Kong SAR P. R. China ABSTRACT: The intriguing dynamics of the alkyl chains in trimethylammonium chlorides remain far from being fully understood. With the help of impedance spectroscopy, we confirmed the existence of rotator phases at ambient temperature in the dodecyl, tetradecyl, and cetanecyl members of the homologous series. For each, differential scanning calorimetry identified a first-order phase transition at elevated temperature in the range of 350−370 K, with molar endothermicity increasing with alkyl chain length from about 20 to 50 kJ/mol. Variable-temperature X-ray diffractometry suggested that the transition corresponds to melting of the alkyl chains but not the ionic sublattice. In the temperature plots of both components of complex permittivity, this order−disorder transition appears as terraced jumps, which we explain by the sharp rise in conductivity due to alkylammonium cation mobility. The activation energy of ionic diffusion lies within 0.45−0.5 eV, about 0.05 eV greater than the energy barrier for alkyl dipole relaxation.
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INTRODUCTION The homologous trimethylammonium chlorides are quaternary ammonium salts, each consisting of a hydrophilic chloride, and a hydrophobic carbanion with four methyl groups attached to some alkyl chain. As cationic surfactants, they are often used to modify layered materials such as hydrotalcite and graphite oxide by way of self-assembly.1−5 They find widespread applications in biological control, micellar catalysis, and flotation.6−8 These chlorides are isomorphous with n-alkylammonium halides (n < 10), which form tetragonal crystals with space group P4/nmm around room temperature.9 Solid-state nuclear magnetic resonance (NMR) spectroscopy has shown that nalkylammonium chlorides with carbon chains longer than C3 turn, before melting, into a rotator phase accompanied by cationic two-dimensional (2D) self-diffusion in lamellar layers.9−12 In the trimethylammonium chlorides such as cetanecyltrimethylammonium chloride (CTAC), the 2D double-layered lamellar-type structure has rodlike cetanecyltrimethylammonium ions (CTA+) and anions (Cl−) stacked alternately along its C4 axis.10 It is known that CTA+ ions assume an all-trans conformation at low temperature but, as temperature rises, undertake a transition to gauche conformation followed by an order−disorder phase transition.9,11,12 Recent studies further show that the CTA+ ions undergo smallangle wobbling around their long molecular axes and, at elevated temperatures, execute fast and unrestricted rotations about symmetry axes.6 The ionic interactions in polar layers are much stronger than van der Waals forces between alkyl chains in nonpolar layers. © XXXX American Chemical Society
So when CTAC undergoes a phase transition, the nonpolar layers melt partially while the ionic layers remain practically unaffected.13,14 At the same time conformational changes occur within the alkyl chain layers, so it is an order−disorder transition.2,6,15 The present work focuses on this transition. Impedance spectroscopy has proved to be an effective probe into the molecular dynamics of anionic surfactants. This work reports our application of this technique to dodecyltrimethylammonium chloride (DTAC) and tetradecyltrimethylammonium chloride (TTAC), in addition to CTAC. The disordering of their alkyl chain parts prior to their complete melting is observed, and information is derived for their order−disorder transitions.
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EXPERIMENTAL SECTION
Materials. DTAC, TTAC, and CTAC of AR purity were sourced from Shanghai Aladdin Industrial. Before use in measurements, selected powder samples were dried for 24 h at 50 °C and 50 Pa vacuum. Characterization of Transition. Differential scanning calorimetry (DSC) proceeded in a low-temperature calorimeter of the heat-flux type, the NETZSCH DSC 200F3. Each sample weighed nearly 10 mg, and a scan went from −20 to 180 °C at 10 K/min, then back, in a dynamic atmosphere of dried Received: September 15, 2016 Revised: September 29, 2016
A
DOI: 10.1021/acs.jpcc.6b09320 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C nitrogen. This heating and cooling cycle was repeated twice for every substance, and reproducibility verified. After DSC results had located the temperature of interest, small-angle X-ray diffractometry (XRD) was performed in a variable-temperature RIGAKU D/max 2500VL/PC diffractometer using Cu Kα1 radiation (λ = 1.5406 Å), operating with conventional copper target source, at 70, 95, 100, 105, and 120 °C. The sample sat in vacuum environment during the angular scans. Dielectric Spectroscopy. A broadband dielectric spectrometer, Concept 40 from NOVOCONTROL, was used to acquire impedance spectra. Pellets 10 mm in diameter and 0.5− 0.8 mm in thickness were prepared by uniaxial compaction under 6 MPa, before drying in a dynamic vacuum of 30 Pa at 50 °C for 24 h. The chosen pellet was heated in vacuum from −150 to 150 °C at 1 K/min, while its complex permittivity was measured under a working voltage of 1.0 V applied via copper electrodes in three-terminal configuration, at 12 preselected frequencies within 111 Hz to 2000 kHz. Measurement on each sample was repeated once, and confirmed to produce essentially identical spectra, apart from some noises.
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RESULTS Figure 1 shows that, on heating, all three chlorides undergo an endothermic transition well below their melting points in excess of 200 °C. The onset temperatures are 353, 350, and 369 K, together with specific enthalpies of 22, 33, and 49 kJ/mol; these respective values for DTAC, TTAC, and CTAC are listed under the first two columns in Table 1. On cooling, the corresponding exothermic peak has delayed onset but virtually identical area. Quite clearly a longer alkyl chain leads to a greater heat of transition. The presence of both latent heat and thermal hysteresis affirms that this transition belongs to first order. Our XRD results illustrated in Figure 2 provide structural information below and above the onset. The representative diffractograms in Figures 2a and 2b include, in 2θ < 26°, a series of periodical peaks of descending intensities, which can be ascribed to diffraction from alkyl chains. But these peaks disappear in traces c, d, and e, being replaced by a diffuse band around 2θ ≈ 20°, suggesting the loss of crystallinity. As the peaks pertain originally to alkyl chains, it is those chains that melt. In contrast, peaks in the region of 2θ < 20° remain distinct at all temperatures, their changes above the onset being limited to slight shifts to slightly lower angles. In particular, the sharp diffraction peaks around 2.5° pertain to the laminated structure of the anion−ionic layers.13,14 These features indicate that the ionic sublattice stays rigid but expands slightly. Figure 3 depicts the thermal dependence of dielectric loss in the respective chloride, at low temperatures. The loss peaks all shift up as the measurement frequency f increases, indicative of their origins in some relaxation process. Accordingly, the loss is fitted with the Arrhenius relationship τ = τ0 exp(Ea/kBTf), where the relaxation time τ is reciprocal of 2πf, and Tf, the temperature at which ε″(f) peaks. Linear fits are obtained for each material, as Figure 4 shows. Calculated values of the Arrhenius parameters are included in Table 1, and it is quite clear that Ea rises with increasing length of the alkyl chain. Lastly we examine the high-temperature region, where a transition has been observed by DSC. As shown in each of Figures 5−7, both ε′ and ε″ change slowly with increasing temperature, initially, but jump abruptly at a common temperature irrespective of frequency, indicating that dielectric
Figure 1. DSC thermograms of (a) DTAC, (b) TTAC, and (c) CTAC, during heating followed by cooling.
Table 1. Materials Parameters Derived from DSC and Impedance Spectroscopy at Low and High Temperature material
onset/K
ΔH /kJ/mol
Ea/eV
log(τ0/s)
Tc/K
Eaσ/eV
DTAC TTAC CTAC
353 350 369
22 33 49
0.38 0.41 0.45
−16.0 −15.0 −17.5
357 358 371
0.50 0.45 0.51
loss originates not from dipole relaxation but probably from charge transport. Denoted by Tc, its value is included in Table 1 for each of the chlorides. Ac conductivity can be obtained from ε″ as σ = ε0ωε″, ε0 being the vacuum permittivity. Figure 8 depicts its thermal B
DOI: 10.1021/acs.jpcc.6b09320 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Figure 2. XRD patterns of CTAC at (a) 343, (b) 368, (c) 373, (d) 378, and (e) 393 K. The distance d = λ/2sin θ is derived from Bragg’s law. Figure 3. Imaginary part of complex permittivity at various frequencies, in low-temperature region, for (a) DTAC, (b) TTAC, and (c) CTAC.
dependence for CTAC as example; at the same temperature, σ increases with frequency. Its log vs 1/T plot is found to be linear for T > Tc, as is so for the other salts (inset). The selfdiffusion coefficients of mobile charge carriers can be derived from σ via the Nernst−Einstein relation, and their activation energies Eaσ are listed under the last column in Table 1. These activation energies exceed those for the relaxation peaks at lower temperatures, as it should be, because the energy barrier is expected to be greater for hopping than for wobbling.
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DISCUSSION CTAC is known to exist in a rotator phase: its alkyl chains rotate, but the CTA+ ions stay in parallel arrangement. We detect this ordered phase among all three chlorides from their low-temperature XRD patterns, and also, from their dielectric loss spectra, small-angle wobbling of the alkyl chains around their long molecular axes, that gives rise to relaxation peaks. The activation energy derived from these peaks, interpreted as energy barrier for this motion, rises with lengthening alkyl chain. As temperature increases, the rotation of the chains becomes faster. Moreover, from DSC, XRD, and dielectric spectroscopy we detect a first-order order−disorder transition, known from previous work to occur when the alkyl chains change from alltrans conformation to gauche disorder. Analogous chain melting occurs in the nanocomposite of CTAC and graphite oxide, which has alkylammonium ions intercalated between two carbon skeletons.2 Confined to two dimensions, those ions cannot undertake translational motion and diffusion is prohibited. The permittivity of this nanocomposite exhibits relaxation peaks, and, on the higher temperature side, cusps at
Figure 4. Arrhenius plots of the dielectric loss peaks in DTAC, TTAC, and CTAC.
the same position for both real and imaginary components, independent of frequency. The trimethylammonium chloride exhibits similar dielectric characteristics except that distinct steps replace cusps. After chain melting occurs, its alkylammonium cations can drift under an electric field along planes perpendicular to their axes. C
DOI: 10.1021/acs.jpcc.6b09320 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Figure 5. Complex permittivity of DTAC.
Figure 7. Complex permittivity of CTAC.
Figure 8. Conductivity σ of CTAC; inset, log σ plotted against 1/T for DTAC, TTAC, and CTAC.
deduced from X-ray diffractometry and electrical measurements. The latter order−disorder phase transition appears as an endotherm in differential scanning calorimetry and frequencyindependent step jumps in both components of the complex permittivity.
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Figure 6. Complex permittivity of TTAC.
AUTHOR INFORMATION
Corresponding Author
Such carrier mobility drastically increases conductivity and dielectric susceptibility, explaining the step jump in permittivity.
*Tel: +86 25 83593508. Fax: +86 25 83595535. E-mail: mgu@ nju.edu.cn.
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CONCLUSION The alkyl chains in various trimethylammonium chlorides execute rotation, as confirmed by relaxation peaks in dielectric loss spectra. The chains suffer melting at higher temperatures (but yet below the melting point of the respective chloride), as
Present Address §
T.B.T.: Asia Power Development (Group), Win Plaza, San Po Kong, Hong Kong SAR, P. R. China. Notes
The authors declare no competing financial interest. D
DOI: 10.1021/acs.jpcc.6b09320 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (under Grants 2016YFA0201604 and 2012CB934000) and the National NSF of China (No. 10674060).
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REFERENCES
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