Calix(4)arene sulfonic acid complexes with halogenated acetic acids

Calix(4)arene sulfonic acid complexes with halogenated acetic acids

SOSI-14165; No of Pages 5 Solid State Ionics xxx (2016) xxx–xxx Contents lists available at ScienceDirect Solid State Ionics journal homepage: www.e...

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SOSI-14165; No of Pages 5 Solid State Ionics xxx (2016) xxx–xxx

Contents lists available at ScienceDirect

Solid State Ionics journal homepage: www.elsevier.com/locate/ssi

Calix(4)arene sulfonic acid complexes with halogenated acetic acids☆ Lyubov V. Shmygleva ⁎, Ruslan R. Kayumov, Yury A. Dobrovolsky Institute of Problems of Chemical Physics of RAS, 1 Academician Semenov Avenue, Chernogolovka, Moscow Region 142432, Russia

a r t i c l e

i n f o

Article history: Received 29 July 2016 Received in revised form 22 December 2016 Accepted 23 December 2016 Available online xxxx

a b s t r a c t Effective proton conductors based on calix(4)arene sulfonic acid and CX3COOH (where X = H, F, Cl) with molar ratio 1:1 were obtained in crystalline form. It was demonstrated that the original calixarene sulfonic acid and its complex with CCl3COOH, and complexes with CH3COOH and CF3COOH have pairwise similar structures and similar proton transport patterns. © 2016 Elsevier B.V. All rights reserved.

Keywords: Calixarenes Halogenated acetic acids Proton conductivity

1. Introduction Water-soluble calixarenes are the promising class of compounds in terms of their use in different application areas. Also, these compounds are interesting due to their layered structure. If calixarene contains an acid group (\\PO3H,\\SO3H) on the upper rim, then it becomes hydrophilic and the structure is divided into two sub-lattices with alternating layers. One bilayer is composed of organic anions that target each other by hydroxyl groups [1]. Another layer contains protons of the acid groups coordinated by water molecules. The water molecules are linked together through hydrogen bonds, which creates preconditions for the effective proton transport. And, indeed, we have recently detected high proton conductivity (up to 10−1 S/cm) of such compounds [2–4], which enables their use in electrochemical devices such as lowtemperature solid polymer fuel cells, potentiometric gas sensors [5], and supercapacitors. Study of proton transfer in calix(4)arene sulfonic acid showed that due to the strong bonded of the hydrate layer a significant change in the amount of water in this layer (which results from the change in environmental humidity) does not lead to significant changes in the parameters of proton transfer. Besides, the significant mobility of these water molecules allows to hope that the part of water molecules can be replaced with acid molecules in the same manner as it was done in polymer membrane Nafion® [6]. This can lead to improvement in the electric transport characteristics.

☆ This work was presented during the 12th International Symposium on Systems with Fast Ionic Transport, Kaunas, Lithuania 03–07.07.2016. ⁎ Corresponding author. E-mail addresses: [email protected] (L.V. Shmygleva), [email protected] (R.R. Kayumov), [email protected] (Y.A. Dobrovolsky).

Moreover, due to their structural aspects calixarenes belong to supramolecular compounds with variable cavity capable of forming “host-guest” complexes with small molecules, including lowmolecular acids [7]. Therefore, such compounds may be promising in the various fields of science, from biology [8,9] to hydrogen energy [10]. The authors of [10] propose to use calix(4)arene for the recovery hydrogen from gas mixtures, since the calixarene cavity retains gases such as O2, N2 and CO2, passing hydrogen. Consequently, calixarenes in their pure form cannot be used as a proton exchange membrane in fuel cells due to the hydrogen crossover. However, another type of interaction between guest acids and calixarenes is possible: they can enter inside the calixarene cavity, forming a weak connection with hydroxyl groups. We chose calix(4)arene sulfonic acid as a main component and halogenated acetic acids (CX3COOH, where X = H, F, Cl) as guests for our research. The purpose of this study was to investigate the influence of halogenated acetic acids on the proton transfer processes in compounds calix(4)arene sulfonic acid:CX3COOH = 1:1.

2. Experimental 2.1. Materials and synthesis 5,11,17,23-tetrasulfo-25,26,27,28-tetrahydroxycalix(4)arene (SC4) was synthesized by sulfonation [11] of the starting calix(4)arene, which was purchased from abcr GmbH, Germany. Halogenated acetic acids CH3COOH (H), CF3COOH (F) and CCl3COOH (Cl), all used chemicals and solvents for synthesis were purchased. Complexes SC4-X was prepared by mixing SC4 with excess of appropriate acetic acid in the presence of water until dissolved. The resulting

http://dx.doi.org/10.1016/j.ssi.2016.12.026 0167-2738/© 2016 Elsevier B.V. All rights reserved.

Please cite this article as: L.V. Shmygleva, et al., Calix(4)arene sulfonic acid complexes with halogenated acetic acids, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.026

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solutions were frozen in liquid nitrogen and freeze-dried for 21 h until the saturated vapor pressure of 0.04 mbar.

3. Results and discussion 3.1. Thermal stability

2.2.1. NMR spectra SC4-H 1Н (DMSO, ppm): 1.96 (s 3H, CH3), 3.98 (s 8H, CH2 from SC4), 7.42 (s, 8H, ArH) (due to exchange with the solvent \\SO3H и \\OH groups proton signals give a common signal at 4.57 ppm); 13С (DMSO, ppm): 21.57 (CH3), 30.98 (CH2 from SC4), 126.95 (ArH), 127.71 (ArH), 139.14 (Ar-SO3H), 151.77 (ArOH), 172.59 (COOH). SC4-F 1Н (D2О, ppm): 4.00 (s 8H, CH2 from SC4), 7.55 (s, 8H, ArH); 13 С (D2О, ppm): 30.77 (CH2 from SC4), 126.62 (ArH), 128.25 (ArH), 135.81 (Ar-SO3H), 151.79 (ArOH); 93F (D2О, ppm): −75.62. SC4-Cl 1Н (D2О, ppm): 3.99 (с 8H, CH2 from SC4), 7.54 (s, 8H, ArH); 13 С (D2О, ppm): 30.76 (CH2 from SC4), 126.62 (ArH), 128.24 (ArH), 135.80 (Ar-SO3H), 151.79 (ArOH). 2.3. Thermal stability The thermal stability of the samples was determined by simultaneous thermal analysis (STA) along with mass spectrometric analysis of the decomposition products using a Netzsch STA 409 PC Luxx® instrument and a QMS 403 C Aëolos instrument. The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves were recorded in a temperature range of 25–300 °C under an argon atmosphere at a heating rate of 10 °C/min. 2.4. IR spectroscopy The prepared samples were analysed using IR absorption spectroscopy, IR spectroscopy with a resolution of 4 cm−1 (vacuum FTIR spectrometer Bruker Vertex 70/70 V with diamond ATR and RAMII module (λB = 1064 nm), Bruker). 2.5. X-ray powder diffraction (XRD) XRD patterns were registered on an X-ray diffraction meter ADP-201 (Сu Кα-radiation, Ni filter) using X-RAY software for automation of obtaining, processing and analysing the data obtained from X-ray diffraction from a DRON series. The approach increment of the detector block was 0.050, and the exposure time as 2 s.

3.2. X-ray powder diffraction (XRD) The presence of new peaks on XRD spectra of samples stored at 32% RH also confirms formation of the complexes of SC4 with halogenated acetic acids (Fig. 2). All three obtained complexes with CX3COOH are characterized by the emergence of two peaks 2Θ = 18.33°–18.60° and

DSC (mW/mg)

All experiments were performed at room temperature in D2O or DMSO. 1H NMR spectra were acquired in an AVANCE III 500 MHz Bruker BioSpin spectrometer operating at 500 и 125 MHz.

According to STA all three samples, which were stored at 32% RH, are stable at room and elevated temperatures (there is only water loss up to 146 °C). Disintegration of the complexes occurs at temperature range of 146–160 °C (Fig. 1) and is accompanied by the formation of halogenated acetic acid degradation products, such as CO2, CH3, CF3. A further increase in temperature over 250 °С leads to destruction of the sulfonic groups from the complex. The dehydration process occurs in several steps (as in case of the original SC4). The definition of absolute water content in the samples is impossible due to beginning of the complex decay. However, it is possible to assess the quantity of adsorbed weakly bonded water according to the first peak of dehydration. The most hygroscopic sample is the one with trichloroacetic acid – 5.5 water molecules per complex at 32% RH. This value is considerably lower than 3.5–3.7 molecules for the other two samples. Adding of the halogenated acetic acid causes endothermic peak broadening and, consequently, increase of the area of dehydration of the bound water, increase of the heat of water evaporation and the shift of maximum towards high temperatures in the row SC4H → SC4-F → SC4-Cl compared to the original SC4.

exo

SC4

SC4-H SC4-F

SC4-Cl

0.5 mW/mg

2.2. NMR

50

100

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200

T (°C) 10

100

-10

90

TG (%)

The proton transfer parameters of the powder samples were determined by impedance spectroscopy in a temperature range of + 70 to − 50°С at a relative humidity (RH) 10–75%. For measurements, we used symmetric C/sample/C cells (C is carbon paper) 5 mm diameter pressed at a pressure of 5 MPa (estimated density of all the samples is equal to 1.37 ± 0.12 g/cm3). Equilibrium with the environmental RH was established over the course of at least one week and was monitored by stabilization of the cell resistance. During the conductivity measurement typical times required to fully equilibrate the samples were 1–5 h depending on temperature value (the lower the temperature, the longer time of stabilization). The frequency dependence of the sample impedance was obtained on Z-3000 impedance meter (Elins LLC, Russia) in a frequency range 1 Hz–3 MHz with a measurement signal amplitude of 10–100 mV.

m(H 2O) 10

SC4-H SC4-F SC4-Cl

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70 m(CH ) 3

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m(CO2) 60

-12

m(SO2 )

m(CF3 ) 50

-11

Ion current (A)

2.6. Impedance spectroscopy

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200

250

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T (°C) Fig. 1. DSC and TG curves of the complexes SC4-X (32% RH).

Please cite this article as: L.V. Shmygleva, et al., Calix(4)arene sulfonic acid complexes with halogenated acetic acids, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.026

L.V. Shmygleva et al. / Solid State Ionics xxx (2016) xxx–xxx

SC4-H and SC4-F are apparently isostructural and have the second type structure. Such structural similarity should have an influence on the proton conducting properties of the obtained materials.

Intensity (a.u.)

SC4

3.3. IR spectroscopy

I type SC4-Cl SC4-H

II type 10

15

20

25

30

3

SC4-F 35

40

2Θ (deg) Fig. 2. XRD spectra of the complexes.

New absorption peaks appear at 849 and 1785 cm−1, which belong to trichloroacetic trifluoroacetic acids, respectively. Curvature of the absorption bands H5O+ \SO− 2 , sulfonic group \ 3 and organic residue undergoes minor changes under the influence of added halogenated acetic acids. On the other hand, oscillation frequency of ν(ОН) of the hydroxyl group decreases by 60–75 cm−1 under the influence of SC4 saturation with the acids. Its value decreases in the row H → F → Cl and equals to 3141 ± 5; 3136 ± 6 and 3126 ± 6 cm− 1, respectively. Bands at 1343 and 1377 cm−1 in the IR spectra of SC4 and the complexes, respectively, refer to the COH and CCH bending vibrations. It is known that the band of bending vibrations δ(OH) shifts to the higher frequencies during the H-bond formation. 3.4. Proton conductivity

Fig. 3. Equivalent circuit for the impedance spectra: Rs – electrolyte resistance, Cg – geometrical capacity, Zel – electrode impedance.

18.83°–19.0°. The powder patterns indicate two types of structures. Overlapping of the most intensive peaks 2Θ = 11.75° and 70° indicates the original SC4 and similar SC4-Cl possessing the first type of structure. -100

(a)

10% RH

The measured impedance spectra are well described by the equivalent circuit (Fig. 3). Rs is the resistance of the electrolyte. Due to their porosity electrodes are modelled by a difficult circuit including the capacitance of the electric double layer at the electrode/proton conductor interface. In calculations, we also took into account the geometrical capacitance of the measuring cell and connecting leads (Cg), which is noticeable when large resistances are measured. The experimental data were processed using the Z-view program (Scribner Association, GB). The typical impedance spectra in the example of SC4-Cl are shown in Fig. 4. The impedance plot represents a part of semicircle that transforms into a spur (Fig. 4a, c); as the humidity increases, the semicircle

-1000

(b) -800

-60

Z'' (Ω)

Z'' (kΩ)

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30 kHz 300 kHz

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-600 30 Hz

300 Hz

-400 350 Hz

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0

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20 °C 0 °C -20 °C

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Z'(Ω)

Z'(kΩ)

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20% RH 43% RH 54% RH 75% RH

-4

30 kHz 30 Hz

-2

30 kHz

30 kHz

0 0

2

4

6

Z' (kΩ) Fig. 4. Nyquist plots of SC4-Cl at 30 °C (a, b) and 32% RH (c). Green lines show the fitting by the equivalent circuit. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Please cite this article as: L.V. Shmygleva, et al., Calix(4)arene sulfonic acid complexes with halogenated acetic acids, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.026

L.V. Shmygleva et al. / Solid State Ionics xxx (2016) xxx–xxx

T (°C) 80

40

0

-40

-80

(a) -5

ln σ [S/cm]

disappears to leave only the spur (Fig. 4b, c). At 32% RH and low temperatures the value of geometrical capacitance of the measuring cell Cg is equal to (2.0 ± 0.5)·10−11 F. In this case the estimated dielectric constants ε are equal to 52, 58 and 50 for SC4-H, SC4-F and SC4-Cl, respectively (estimated ε of SC4 is 108). Form of the impedance spectra at higher RH and temperature makes it impossible to determine Cg and, therefore, ε. Fig. 5 contains the research results for proton conductivity σ of the obtained complexes. Generally, the dependences of σ from RH have percolation character similar to original SC4 [2,3]. At humidity below 20% proton conductivity started to decrease much faster, which is connected with an insufficient number of water molecules to form a full hydrogen bonds network. It was expected that the increase in number of protons will increase the conductivity, and bonding of the excess water with acids will not allow water to freeze, as it was observed for the polymer membrane Nafion doped with acids [9]. However, adding of the halogenated acetic acids leads to a decrease in proton conductivity in the obtained samples. Thus, the increase in environmental humidity causes SC4-Cl complex to demonstrate a proton conductivity dependence similar to the original SC4, although five times lower (from 6 · 10−4 to 10−2 S/cm in at RH = 20–65%). Conductivity dependences of SC4 and SC4-F with the second type structure are identical and have a weaker dependence from humidity (σ increases from 0.002 to 0.01 S/cm in the RH range of 20–65%). The decrease in the conductivity could be associated with the replacement of same water molecule by halogenated acid, which apparently leads to decreasing in the charge carrier number. After humidification up to 75% RH, the specific proton conductivity values of all investigated complexes become identical with the original SC4 and reach 0.05 S/cm. Temperature dependencies of proton conductivity of the obtained complexes (Fig. 6) are linear in Arrhenius coordinates in the temperature range from +70 to −20 °C. The transition to low temperature region leads to a vitrification of the samples. Cooling of samples below − 20 °C results in the bend on temperature dependences at both low (32%) and high (75%) RH unlike original SC4. The existence of such bend is associated with the second-order phase transition. The phase transition for SC4-H and SC4-F occurs at −30 to −40 °C; this transition for SC4-Cl is observed at −20 to −50 °C. Activation energies calculated from temperature dependencies in the positive temperature range (before the phase transition) are presented in Fig. 7. Dependencies of the activation energy from environmental humidity are non-linear. Shape of dependencies for sample pairs SC4-Cl with SC4 and SC4-F with SC4-H confirms the splitting into two types of structures that affects transport properties of the complexes. For the sample SC4-Cl one can observe a stepped dependence of

-10 SC4 SC4-H SC4-F SC4-Cl

-15 3.0

3.5

4.0

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40

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-40

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ln σ [S/cm]

4

(b)

-4

-6 SC4 SC4-H SC4-F SC4-Cl

-8 3.0

3.5

4.0

4.5

5.0

1/T (1000/K) Fig. 6. Temperature dependences of proton conductivity of the samples at 32% RH (a) and 75% RH (b).

activation energy. The first step with the energy 0.27 eV is similar to that for the original SC4, but is observed in a wider humidity range of 20–54% (Fig. 7). Further humidification leads to a steplike drop of the activation energy to 0.15 eV. The activation energy dependence of the samples SC4-F and SC4-H sharply stands out due to its humidification-induced linear typical decrease from 0.48–0.49 eV at 10% RH to 0.10 and 0.14 eV at 54% RH, respectively.

-1

10

0.5

SC4 SC4-H SC4-F SC4-Cl

-2

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Ea (eV)

σ (S/cm)

0.4 -3

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SC4 SC4-H SC4-F SC4-Cl

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RH (%) Fig. 5. Proton conductivity dependences on the relative humidity in the complexes at 25 °C.

20

40

60

80

RH (%) Fig. 7. The dependences of activation energy on relative humidity of the complexes.

Please cite this article as: L.V. Shmygleva, et al., Calix(4)arene sulfonic acid complexes with halogenated acetic acids, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.026

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One can observe that the original SC4 sample is characterized by atypical increase in activation energy of conductivity at humidification over 45% RH. Introduction of halogenated acetic acids into the structure leads to the shift of this unusual area in the direction of higher humidities (55–60%). 4. Conclusions Crystalline compounds in the system “calixarene sulfonic acid– halogenated acetic acids” were synthesized and their structural features and parameters of proton transfer were studied for the first time. It was demonstrated that the introduction of SC4 into the crystalline lattice leads to a decrease of proton conductivity of the material. Complexes SC4 and SC4-F possess similar structures, while structure of the complex SC4-Cl is close to the structure of the original SC4. Introduction of the acids into the crystalline structure leads to the phase transition beginning at temperatures of about −20 °C, resulting in a steplike increase in activation energy of conductivity of the low temperature phase, while this phenomenon is absent for SC4. It was found that the activation energy of conductivity is 0.10–0.15 eV for all investigated compounds in the high environmental humidity region. This region demonstrates a unique phenomenon of increase in activation energy along with increasing humidity. Acknowledgements This work was supported by Russian Scientific Foundation (Contract No. 14-23-00218). References

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Please cite this article as: L.V. Shmygleva, et al., Calix(4)arene sulfonic acid complexes with halogenated acetic acids, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.026