FTIR spectroscopic study of the complex formation between H+ and DMSO in Nafion

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 178 (2017) 94–104

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FTIR spectroscopic study of the complex formation between H+ and DMSO in Nafion A.I. Karelin ⁎, R.R. Kayumov, E.A. Sanginov, Yu.A. Dobrovolsky Institute of Problems of Chemical Physics of RAS, 142432, 1 Academician Semenov avenue, Chernogolovka, Moscow region, Russia

a r t i c l e

i n f o

Article history: Received 21 October 2016 Received in revised form 26 January 2017 Accepted 27 January 2017 Available online 31 January 2017 Keywords: Nafion membrane DMSO plasticization ATR - FTIR spectroscopy Complexes H+(DMSO)2 and H+(DMSO-d6)2 Strong hydrogen bond OH+ O Symmetry decrease + Ion pair\ \SO− 3 H (DMSO)2 Isotope substitution H+\ \D+ Dipole-dipole interactions Protonic conductivity

a b s t r a c t Nafion membranes plasticized with dimethyl sulfoxide (DMSO) have been examined at room temperature using the vacuum ATR - FTIR spectroscopic technique in the range 50–4000 cm−1. The amount of the plasticizer corresponds to the molecular ratio n = DMSO/H+ = 1.2, 2.3, 4.8, 7.0, 9.7 and 13.3. The medium intensity band with two maxima at 780 and 853 cm−1 have been assigned to the ν(SO) stretching vibrations of the H+(DMSO)2 complex. The possible reason of ν(SO) splitting is symmetry decrease of hydrogen bond under the influence of the anion group\\SO− 3 electric field. Whereas the mutual association of free DMSO molecules in Nafion leads to appearance of weak band at 86 cm−1 assigned to the dipole-dipole interactions. © 2017 Published by Elsevier B.V.

1. Introduction Due to the modern rapid development of electrochemical power sources, the increasingly greater attention is focused on the new materials with high ionic conductivity. The most prominent examples of electrochemical devices are fuel cells, where proton exchange membrane is one of the main components. Nafion membranes (DuPont) currently the most commonly used which are the copolymer of tetrafluorethylene and sulfo-containing monomer [1,2] (scheme 1). Nafion is a unique ionic conductor with high mechanical and chemical properties and the electrochemical stability window [3,4]. However it has some disadvantages such as a high dependence of conductivity on humidity and temperature. It's ineffective to use it in low-temperature fuel cells at low humidity due to the conductivity drop [5]. There are different ways to affect the physical and electrochemical properties of Nafion membranes. The chemical approach refers to synthesis of Nafion copolymers with organic materials. Recently published data [6] emphasize the existence of the wide platform for designing new Nafion derivatives with controllable chemical structures. The more traditional physical approach refers to the incorporation of organic or inorganic materials into Nafion. It is well known that modification of a ⁎ Corresponding author. E-mail addresses: [email protected] (A.I. Karelin), [email protected] (R.R. Kayumov), [email protected] (E.A. Sanginov), [email protected] (Y.A. Dobrovolsky).

http://dx.doi.org/10.1016/j.saa.2017.01.062 1386-1425/© 2017 Published by Elsevier B.V.

perfluorinated ionomer by an aprotonic solvent reduces dependence on the moisture content of a membrane, while maintaining high conductivity. The advantage of DMSO – coated (or plasticized) Nafion membranes is that they can conduct ionic current to a significant extent without side reactions [7–10]. A number of data have been published regarding influence of the solvent type and cation substitution on the ionic conductivity of Nafion [7]. It was shown, in particularly, that H+ conductivity is reduced for 25 times at the uptake of DMSO, whereas Li+ conductivity is reduced only for 10 times under the same conditions. On the other hand, plasticization of membranes by DMSO leads to the fact that proton conductivity reaches 10−3–10−2 S/cm, which is commensurate with the Nafion membrane conductance at humidity 65– 85% RH. Vibrational spectroscopy techniques are enough effective for elucidation of the ion - molecular interactions within an ionomer. For this reason, in the present work, we report the first IR spectroscopic study of Nafion membrane in acid form plasticized by DMSO. The results shed light on the role of interactions between proton and DMSO molecules in the process of Nafion conductivity lowering. 2. Materials and Methods Chemicals used in this study are Nafion-115 and DMSO. Before use, membranes of Nafion-115 were treated with 3% hydrogen peroxide solution at 60–80 °C for 2 h, washed with water, immersed in 1 M Н2SO4 at 100 °C for 1 h, after which they were again washed with water. The wet

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Scheme 1. The primary chemical structure of Nafion.

samples were dried at 130 °C for one hour and then kept in an exsiccator over P2O5 for one week. DMSO was distilled over drying agents and stored over activated molecular sieves. Dried membrane samples were kept in DMSO in the presence of activated molecular sieves at room temperature (Table 1). The exposure time was varied over a wide range - from several minutes to two days. To remove liquid droplets DMSO from the surface it was used a filter paper. After that membranes were placed for two days in a closed vessel for a uniform distribution of solvent throughout the volume. With this method, samples with different contents of DMSO were prepared. The content of DMSO was determined by a gravimetric method. Deuterated plasticized samples were obtained under vacuum using liquid D2O with 99.9 at.% D (Deutero GmbH, Kastellaun, Germany). Some experiments were performed using DMSO - d6 with 99.9 at.% D (Sigma Aldrich). The Nafion membranes thickness was 125 μm before plasticization (Table 1). For a comparison purposes, the membranes also with a thickness of 25 μm were used in some experiments. The thickness of the membrane was measured with a micrometer. The ATR IR and IR absorption (transmission) spectra were recorded accordingly in ATR and Absorbance regimes at room temperature under vacuum (b1 hPa) in the range 50–4000 cm−1 (from 50 to 100 scans, resolution 4 cm−1) using FTIR spectrometer Bruker model Vertex 70 V (Germany). The first wide range beamsplitter (mid and far IR spectral ranges covering from 6000 cm− 1 to 10 cm− 1 [11,12]) and the first room temperature ultra-wide range DLaTGS detector (covers the near, mid and far IR spectral ranges from 12,000 cm−1 to 20 cm− 1 [13]) were used. Design a wide spectral range beamsplitter and ultra-wide range DLaTGS detector as well as a standard internal source makes the complete mid and the far infrared spectral ranges accessible. The ATR measurements were performed using Platinum – ATR (Bruker) accessory equipped with a pure diamond crystal (transparent in the near, mid and far IR spectral ranges from 50,000 cm−1 to 50 cm−1). The usual range limitations are removed in this case, due to the purity of the diamond crystal and the use of appropriate mirrors (all reflective, gold coated optics, no composite material construction). Platinum diamond unit is designed for the use in a wide spectral range up to 50 cm−1 or even to 10 cm−1 [11,14]. The depth of incident IR radiation penetration dp probed by ATR significantly increases at low frequencies. Accordingly, the absorbance (AB) Table 1 Amount of the plasticizer n, solvent uptake and membrane thicknesses of plasticized Nafion. n, mol DMSO/mol H+

Solvent uptake, % wt

Membrane thickness, μm

0 1.2 2.3 4.8 7.0 9.7 13.3

0 8.9 16.4 34.4 49.5 69.2 92.8

125 150 165 170 175 185 190

increases significantly. In our study, variability of absorbance with wavenumber is eliminated using the standard (Bruker) algorithm for its normalization on the dp value. The normalized spectra are presented in the ATR units scale which is connected with the absorbance scale through the formula: ATR = AB·X/1000, where X is a wavenumber. 3. Results and Discussion 3.1. Spectral Characterization of the Nafion Membranes before Plasticization The degree of dehydration of the initial reagents was monitored by the ATR - FTIR and FTIR absorption (transmission) spectra. The monitoring showed that the DMSO solvent is free of water while Nafion contains residual water in an amount H2O/H+ = 1. Fig. 1 show the ATR - FTIR spectra of initial Nafion sample dried for a week over P2O5. A very broad band of 2763 ± 20 cm−1 and a moderately broad band of 1682 ± 19 cm−1 assigned to internal vibrations of the H3O+ ion – the first one refers to the stretching vibration ν(OH) and the second one refers to the anti – symmetric deformation vibration δas(OH3). Thus we used in our spectroscopic experiments really not anhydrous Nafion but its monohydrate (NM). The H3O+ ion forms rather strong hydrogen bonds (HB) with the \\SO3- anion in Nafion membranes resulting in significant decrease of ν(OH) frequency. There are several reasons for the ν(ОН) band broadening. The bandwidth highly depends on the structural disorder as well as on the dynamic and electric coupling [15]. In our opinion, the large width of ν(OH) bands is due mainly to the structural disorder and decrease of the symmetry of the C3V ion of H3O+. Otherwise, we would observe two discrete bands, νs(OH3) and νas(OH3) rather than a broad one. A weak band at ~2200 cm−1 refers to the overtone of 2δs(OH3) or more likely to the combination vibration of δas(OH3) + τ(OH3), where τ(OH3) is a libration vibration of H3O+. The fundamental frequencies of H3O+ given in Table 2 satisfactorily correspond to literature values [11–17]. However authors [11] assigned the weak band at 2210 cm−1 to νs(OH3) which obviously is not correct. The intense broad band at 2821 cm−1 appearing in the spectrum of partially dehydrated Nafion was tentatively attributed [18] to one of vibrations of the H7O+ 3 ion, which we also cannot accept. We assigned a weak broad band at 235 cm−1 to external (translation, T) H3O+ vibration. Whereas libration bands of H3O+ which would have to be three times higher frequency are not observed because they are diffuse. Previously the T H3O+ mode band has been observed at ~ 140 cm− 1 for many proton conductors especially by neutron scattering, see e.g. [22]. This information gives some reasons to doubt in that our assignment of the band at 235 cm−1 is correct. However it is well known that the frequency of translation mode depends on the hydrogen bond strength. The hydrogen bond strengthening should increase the T H3O+ frequency. According to our IR data the strength of the hydrogen bond between H3O+ and C–SO− 3 is rather strong (ν(OH) = 2763 cm−1). Whereas the strength of the hydrogen is essentially more weak bond between H3O+ and ClO− 4

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Fig. 1. The survey ATR - FTIR spectra of Nafion membrane dried over P2O5: water content corresponds to the monohydrate (MH).

(ν(OH) ~ 3000 cm−1). Meanwhile, authors [23] assigned the T H3O+ frequencies in the following manner:

+

ClO− 4 ,

H3O P21/n

Tz (H3O+)

Tx (H3O+)

Ty (H3O+)

125 (IR) 125 (INS)

192/205 (IR) 200 (INS)

265 (IR) 240 (INS)

T (H3O+) H3O+ ClO− 4 , Pnma

3.2. Spectral Characterization of the Nafion Membranes after Plasticization Plasticizing NM with DMSO molecules leads to complete disappearing of the H 3O + ion band at 2763 cm− 1 . Instead of it a Table 2 The fundamental vibrational frequencies of Н3О+ ion (cm−1).

Н3О+ in Nafion (this work) Н3О+ in Nafion [17] Н3О+ in Nafion [16] (Н3О+)2[C6H3OH(SO3)2]2−, cryst [18] Н3О+ Н3О+)+ Н3О+ CH3C6H4SO− 3 , cryst [19] 2− [Н3О+(ClO− (Bu4N+)2, cryst [20] 4 )3] + + − [Н3О (ТBPh)3] ClO4 , solut. in ТBPha [21] ТBPh is (C4H9)3P = O.

H3 Oþ ↔Hþ þ H2 O

ð1Þ

H3 Oþ þ H2 O↔H5 O2 þ

ð2Þ

200 (INS)

In our opinion, these data strongly support the feasibility of our assignment of the 235 cm−1 frequency to H3O+ translation mode. For the \\SO− 3 group, the medium intensity band of νs(SO3) is observed at 1060 cm− 1, while a very intensive band of νas(SO3) at 1198 cm−1 is masked with the intensive band of νas(CF2). Bands of – SO2OH are not observed in the studied spectra. The papers [16,24–26] testify that Nafion is completely dehydrated only under high vacuum and heating. Accordingly, the analytical bands of C–SO2OH ν(S\\O) at 910 cm−1 and νas(SO2) at 1414 cm−1 were reported in all these papers.

a

weak band at 3278 cm− 1 appears which was assigned to the stretching vibration ν(OH) of ion H 5 O + 2 (Fig. 2a, b). It follows the H3O+ ions undergo dissociation in the presence of DMSO molecules according to scheme

νs,аs(ОН3)

δаs(ОН3)

2763 ± 20 2750 2700 2700 2600, 2700 2747 ± 4 2530

1682 ± 20 1750 1665 1680,1840 1665, 1850 1730, 1820 1700, 1890

Another feature of the ATR - FTIR spectrum depicted in Fig. 2 is the medium intensity band with two broad maxima at ≤ 780 and 853 cm−1. Along with these two maxima in this region there is also a sharp peak at 801–803 cm−1 assigned to the fluorocarbon chain vibration. Its origin is confirmed by the presence of a weak sharp peak at 801 cm− 1 in the spectrum of non-plasticized Nafion, as shown in Fig. 3a. It should be noted that neither liquid DMSO nor NM have no their medium intensity bands in this spectral region. As well as the mo+ lecular ions H3O+, H5O+ 2 and H7O3 have no the distinct absorption bands here, as evidenced by the results of studies [20,27]. Detected band with two maxima becomes the greatest intensity at the ratio n = DMSO/H+ = 2.3. The dependence of the intensity on n is shown from spectra in Fig. 3b – d. When changing from n = 2.3 to 4.8 or to n = 1.2, it is successively decreasing. We found using ATR technique that an exposure of plasticized membranes under vacuum for 3 h at room temperature results in the removal of all the molecules DMSO except the two associated most strongly. Tightly bonded molecules are not removing in vacuum (b1 hPa) for several days. Accordingly, the band with two maxima at 780 and 853 cm−1 does not disappear. This band disappears completely only in conditions of high humidity but it appears again after prolonged exposure of the sample under vacuum at room temperature. It should be noted the long time vacuum evacuation leads to complete dehydration of the plasticized samples. In particular, the characteristic band of impurities H5O+ 2 disappears completely at 3278 cm−1. The most possible mechanism is the shift of the equilibrium in the system according to the next scheme Hþ þ DMSO↔Hþ DMSO

ð3Þ

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Fig. 2. The survey ATR - FTIR spectra of Nafion membranes plasticized with DMSO (n = 2.3) before (a) and after (b) dehydration under vacuum at room temperature. Time (tvs) of the sample b vacuum storage in the ATR accessory is 18 h.

Hþ DMSO þ DMSO↔Hþ ðDMSOÞ2

ð4Þ

3.3. Spectral Determination of Complex H+(DMSO)2 in the Plasticized Nafion Membranes

Unlike plasticized samples, initial MN samples do not dehydrate under these conditions. Thus, DMSO molecules decrease the stability of the H3O+ ions in MN.

Taking into account the above considered data it can be concluded, the appearance of band with maxima at 780 and 853 cm−1 is indicating the complex H+(DMSO)2 formation. Fig. 4 shows the spectra of

Fig. 3. Contours of the analytical band ν(SO) of the H+(DMSO)2 complex (shaded area 1), translational band of the H3O+ ion (shaded area 2) and of molecular dipole-dipole coupling band (shaded area 3) in the ATR - FTIR spectra of plasticized Nafion with n = 0 (a), 2.3 (b), 4.8 (c) and 9.7 (d).

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Fig. 4. ATR - FTIR spectra of the D+(DMSO)2 (a) and H+(DMSO)2 (b) complexes between 50 and 1100 cm−1. Samples with n ~ 2 were obtained through vacuum storage in the ATR accessory (tvs = 24 h) of the DMSO - enriched membrane (initial value n = 13.3).

anhydrous deuterated and undeuterated samples of plasticized membranes. The first of them was prepared by using the heavy water, and the second with the use of the light water. Deuteration ratio exceeds 90 at.%. It can be seen that this band is not shifted to lower frequencies by substituting H+ on D+. Its intensity only decreases significantly. In addition, there is a slight displacement of its two maxima to 721 and 870 cm− 1. Isotope substitution H–D does not result in its shift to lower frequencies. The νas(OHO) and νas(ODO) stretching bands are diffuse in ATR and in IR absorption spectra. The edge of weak and very broad νas(OHO) band can hardly be detected in a general background at ~ 2500 cm− 1. While the maximum locates likely in the region of ~1300 cm−1 where it is masked by the nearby spaced strong bands of Nafion. If our assumption is correct, then we can estimate approximately the distance between oxygen atoms for hydrogen bond. The well known relationship between OH stretching frequency and R(O… O) distance [28] gives the value of 2.57 Å in this case. A rather narrow and weak band at 2323 cm−1 on a diffuse contour of νas(OHO) refers to a combination frequency 1142 + 1204 cm−1 of fluorocarbon bonds. The bending vibrations of H+ result in moderately broad weak bands at 1637 and 1530 cm−1. The first assigned to the in - plane vibration δ(OHO), and the second one to the out-of-plane vibration ω(OHO). There is a certain correlation between sets of frequencies 780–853, 1530–1637 and 2500 cm−1 of the proton complex H+(DMSO)2 in a membrane and ~ 700, 1590 and 2450 cm−1 [29] of the related complex Bu4N+ ClO− 4 ·HClO4 in a melt.

membrane with n = 1.2 (or 2.3) to IR ATR spectrum of the liquid phase DMSO multiplied by an arbitrary factor (Fig. 6). Noteworthy, the ν(SO) band intensity dependence on n differs significantly from that in the ATR spectra of plasticized Li-Nafion. In the latter case, the band is easily detected at 1020 cm− 1 for the all lowest values of n up to 1.5 in spite of the intensity significant drop [31]. Thus, its disappearance in the plasticized Nafion spectrum when n ≤ 2.3 should be explained by the contour broadening and shift to lower frequencies under the influence of strong hydrogen bond OH+ O. From this point of view, the band with maxima at 780 and 853 cm− 1 likely belongs to the ν(SO) vibrations of the H+(DMSO)2 complex. A reason for ν(SO) splitting is discussed in Section 3.8. 3.5. Influence of the Complex H+(DMSO)2 Formation on the νs(SO3) Frequency of Nafion The narrow moderate intensity band at 1060 cm−1 corresponds to the stretching symmetric vibration νs(SO3) in the NM spectrum. Plasticization of NM in the extent n = 1.2 leads to a sharp decrease of frequency from 1060 to 1050 cm−1. This effect is due to polarization weakening of three S _O bonds of the C\\SO− 3 group with the positive ion when H3O+ is substituted by H+(DMSO)2. The further increase only slightly changes the νs(SO3) frequency (Table 3). Similar effect was observed earlier for the membranes of Nafion with alkaline metals ions. For example the νs(SO3) frequency sharp decrease from 1073 to 1060 cm−1 when the dehydrated membrane of Li-Nafion plasticizing with 1.5 mol of DMSO [31].

3.4. Influence of the Complex H+(DMSO)2 Formation on the ν(SO) Frequency of DMSO

3.6. Data about Free Molecule DMSO Interactions in the Plasticized Nafion

The band ν(SO) of the DMSO molecule stretching vibration is not observed at n = 1.2 in the spectrum of plasticized membrane. Only shoulder at 1019 cm−1 appears at n = 2.3 on the contour of the band νs(SO3). Maximum at 1029 cm−1 appears if n N 2.3 (Fig. 5). The ν(SO) contour takes the form similar to contour of the liquid phase DMSO for large values of n. In the liquid DMSO, it is complicated by the associative molecular interactions [30]. In a rough approximation the envelope of ν(SO) band can be reproduced by adding IR ATR spectrum of the

The weak broad band recorded at 84 cm−1 using ATR- FTIR and FTIR absorption methods in long-wave region can be assigned to the dipoledipole coupling of free DMSO molecules. It is observed distinctly when n ≥ 2.3. The established using ATR technique details are shown in Fig. 3. Recently this band was found at 90 ± 10 cm− 1 in the longwave ATR - FTIR spectra of a liquid DMSO phase as well as of Li-Nafion plasticized membranes [31]. The intensity of the dipole-dipole coupling band increases when n increases. The intensity of an adjacent band at

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99

Fig. 5. Contours of the molecular DMSO ν(SO) band (shaded area): n = 0 (a), 2.3 (b), 4.8 (c), 7.0 (d) and 9.7 (e).

235 cm−1 assigned to translational vibrations of the H3O+ ion, on the contrary, decreases – first in the big extent then less distinctly. The decrease of its intensity can be explained by H3O+ dissociation under the influence of DMSO molecules. 3.7. Vibrational Frequencies of the DMSO Molecules and of the Nafion Polymer Chains A set of DMSO and Nafion fundamental vibrational frequencies established for all studied n values is presented in Tables 4 and 5 respectively. Assignment is based on the spectroscopic data for liquid DMSO

[32], lithium perchlorate solutions in DMSO [30] as well as for Nafion [30]. It is very approximate, because it is empirical and does not take into account for the possibility of mixing modes. The tabular data show a continuous dependence of frequency values on n for the DMSO molecule and Nafion respectively. When n increases, the ν(CH) frequency decreases (Table 4), and the ν(CF) increases (Table 5). This dependence occurs in a wide range of n values, i.e. it does not directly relate to the complex formation process. It is caused obviously by the gradual substitution of one type of molecular contacts with another type. Two effects form its basis, in particular, the extension of molecular DMSO chains in the channels of a polymer membrane and the

Fig. 6. Contours of the molecular DMSO ν(SO) band (shaded area) have been calculated by summation the membrane spectrum (n = 1.2) and the spectrum of a liquid DMSO phase.

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Table 3 The νs(SO3) frequency dependence on the value of n. n

0

1.2

2.3

4.8

7.0

9.7

13.3

νs(SO3), cm−1

1060

1050

1050

1049

1048

1047

1047

reorientation of polymer units which according to [33] have a helical zigzag conformation. 3.8. Some Spectral Peculiarities of Interaction between H+ and DMSO-d6 in Nafion The ATR FTIR spectra in the range of 50–4000 cm−1 were examined preliminarily for the liquid phase DMSO-d6 to clarify the origin of some peaks for the DMSO-d6 plasticized Nafion. In particularly the well resolved triplet splitting of ν(SO) band was found at 1007, 1026 and 1053 cm−1. Notably, the values 1009, 1030 and 1059 cm−1 were communicated [28] in the IR absorption study of liquid phase of DMSO-d6. Triplet arises from the overlap of bands ν(SO) and δas(CD3). The similar three sharp peaks are observed at 1007, 1026 and 1051 cm−1 for the values of n ≤ 13.3 at the ATR spectra recording of Nafion plasticized with DMSO-d6 (Fig. 7). The similarity of ν(SO) triplet splitting can be considered as a further evidence of the liquid – like molecular association in the channels of Nafion. An excess of DMSO-d6 is easily removed from Nafion under vacuum at the room temperature similar to excess of DMSO. At the very beginning of the free DMSO-d6 molecules process removing the complex formation is detected due to the weak bands appearing at 733 and 845 cm−1. Their intensity increases successively as the content of free molecules in a sample is getting smaller Fig. 7ac. This process almost stops after free molecules removal whereas the last two molecules of DMSO-d6 are tightly bonded with H+. ATR spectrum of Nafion, where the DMSO-d6 content corresponds to n = 2.3 is shown in Fig. 8. A similar spectrum was recorded after all free molecules removing from the solvent – rich (n = 13.3) membrane (Fig. 7c). It is easy to notice the identity of the both spectra. Here it is necessary to pay attention on the weak maximum observed at

Table 4 Fundamental vibrational frequencies of DMSO molecules in the plasticized Nafion membranes (cm−1). n

DMSO

Approximate

1.2

2.3

4.8

7.0

9.7

13.3

liquid

assignment (*)

3022

3015

3008

3004

3002

3001

2996

νas(CH3)

a,a

2928

2925

2921

2919

2917

2917

2912

νs(CH3)

a,a

1435

1436

1437

1437

1437

1437

1437

1419

1419

1419

–

–

–

–

δas(CH3)

a,a

–

–

–

1408

1408

1408

1408 δs(CH3)

a

1310

a a

–

–

–

1311

1311

1311

–

–

–

–

–

–

~ 1296

–

–

–

–

–

–

1060 sh.

–

1019 ?

1030

1031

1029

–

1043

–

–

–

–

–

1019 sh.

1020 sh.

–

–

–

953

953

953

952

–

–

–

–

928 sh.

929 sh.

930

–

–

–

–

–

–

896

–

–

–

700

699

699

697

–

–

–

663

664

664

668

νs(CS)

a

–

–

382

382

382

382

381

δ(CSO)

a

327

334

333

333

333

333

331

δ(CSO)

a

–

–

–

311sh.

310 sh.

310 sh.

310

δ(CSC)

a

–

–

84

84

88

88

83

ν intermol.

δs(CH3) ? a ν (S=O)

a

ρ(CH3)

a

νas(CS)

a

a a

(*) The assignment of the liquid DMSO vibrations on the symmetry types was made as in [30].

Table 5 Fundamental vibrational frequencies of fluorocarbon chains and of sulfonate group in the unplasticized (n = 0) and plasticized (n ≠ 0) Nafion membranes (cm−1). n 1.2

2.3

4.8

7.0

9.7

13.3

1.2

Approximate assignment

1304 1197 1144 1062 982 967 805 718 647 624 553 512 463 443 203

– 1204 1143 1051 981 967 803 – 649 624 554 513 463 443 204

– 1209 1144 1050 980 966 803 – 648 624 553 519 463 443 204

– 1212 1147 1049 980 964 803 – 647 626 554 521 462 442 204

– 1214 1149 1048 981 – 803 – 647 625 554 520 462 442 204

– 1218 1151 1047 981 – 803 – 647 627 554 521 463 441 204

– 1218 1151 1047 981 – 803 – 647 627 555 522 463 442 204

ν(СС)? νas(СF2), νas(SO3) νs(СF2) νs(SO3) νas(СOC) νs(СOC) δ(ССF), ν(СS) δ(ССF) ω(CF2) ω(CF2), ν(СS) δ(ССF), δ(СOC), δas(SO3) ρ(CF2), ω(CF2), δs(SO3) ρ(SO3) ρ(SO3) χ(СF2)

1024 cm−1 when n = 2.3. This maximum belongs to a splitted band δas(CD3). Another maximum of this band is shielded by a mean intensity νs(SO3) band of Nafion at 1051 cm− 1. While the ν(SO) band of free DMSO-d6 molecules at ~1010 cm−1 disappears almost completely. The main spectral feature of H+(DMSO-d6)2 complex is a big splitting of its analytical ν(SO) band. There is a remarkable difference between H+(DMSO)2 and H+(DMSO-d6)2 spectra. Only one broad band with two unresolved maxima is observed at ~ 780 and 853 cm− 1 in the first case (see Sections 3.3 and 3.4), whereas two well resolved separate bands are observed at 733 ± 2 cm−1 and 844 ± 2 cm−1 in the latter case (Fig. 7c, 8). Dependence of ATR - FTIR spectra on the vacuum storage time of Nafion membrane (initial value n = 13.3) that was treated with excess of DMSO-d6 + liquid D2O mixture is shown in Fig. 9a – c. At the end of this experiment, the D+(DMSO-d6)2 complex is formed (Fig. 9c). The H+\\D+ substitution significantly reduces intensity of the analytical splitted ν(SO) band. Besides, it results in the shift of the second band to 873 ± 2 cm−1 (Fig. 9c, 10). The ν(SO) band splitting resolution in the H+(DMSO-d6)2 spectrum is a result of the superposition of two narrow bands ρ(CD3) at 763 ± 3 cm−1 and 824 ± 1 cm−1. The superposition makes possible the resonant interaction ν(SO) ↔ ρ(CD3). It should be noted that the rocking vibrations ρ(CD3) of DMSO-d6 molecule per se are mixed with ν(CS). The narrow bands of mixed ρ(CD3) ↔ ν(CS) vibrations are observed in the ATR spectrum of the liquid phase DMSO-d6 at 758 and 820 cm−1. There is a little doubt in that the both bands at 733 cm−1 and at 844 cm−1 belong to the complex H+(DMSO-d6)2. However, there may be different explanations of their origin. We believe that each of these two bands is attributed to ν(SO). Their presence in the spectrum is likely an indication of nonequivalence of the DMSO-d6 molecules. Consequently, proton is shifted from the center of strong hydrogen bond OH+ O toward one of the two molecules. Reduction of symmetry occurs under the influence of an electric field of anionic group at the ion pair + \\SO− 3 · H (DMSO-d6)2 formation. A similar phenomenon has been observed previously at the melting of crystalline tetrabutylammonium biperchlorate. Namely, it was found that a weak electric field of Bu4N+ − cation reduces the symmetry of complex anion H+(ClO− 4 )2 → ClO4 ·HClO4 due to the formation of ion pair [29]. The theory of easily polarizable strong hydrogen bonds is presented in [34]. The symmetric stretching vibration of hydrogen bond σ(OHO) cannot be IR active provided the C2h point group of complex cation. The low symmetry (Cs, for example) lifts the prohibition, but the corresponding band is not detected because of its small intensity. In principle a resonance between closely spaced vibrational levels σ(OHO) and ν(SO) could be another possible reason for doublet splitting ν(SO) in the H+(DMSO-d6)2 and H+(DMSO2) complexes spectra. However, this version, which is an alternative to the previous one (about existence

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Fig. 7. Dependence of ATR - FTIR spectra between 50 and 1100 cm−1 on the vacuum storage time of Nafion membranes plasticized with DMSO-d6 (initial value n = 13.3): tvs = 3 min (a), 1 h (b) and 28 h (c).

of the two stretching SO bands), seems to us less likely, since it allows too high frequency for σ(OHO). In any case, there is a high probability for σ(OHO) ↔ ν(SO) vibrations mixing. 3.9. The Membrane Thickness and Its Influence on Infrared Spectra The most used samples of the plasticized Nafion membranes are characterized by a large thickness (from 125 to 190 μm, see Table 1). The IR absorption spectra obtained for such samples are given in the Electronic Supplementary Material. Their transmittance is about 2– 50% in the range of 490–50 cm−1, 0% in the range of 600–1350 cm−1

and 1–2% in the range of 1450–3200 cm−1. ATR method allows to record spectra of thick samples without any restrictions within the range from 50 to 4000 cm−1, while the operating range of IR absorption (transmission) spectroscopy is limited to 50–490 and 1450–3200 cm−1. Taking into account the above mentioned limitations, we used for transmission experiment additionally the samples, having a thickness 25 μm. Then, the transmittance increases from 20 to 60% in the range of 490– 50 cm−1 and from 20 to 80% in the range of 1400–3500 cm−1. An offscale reading was observed only in the range of 1100–1240 cm−1. According to [35], depth of the IR beam penetration dp in water is 1.39 μm at 1100 cm−1; the value of 3 x dp for bulk water, ca. 4 μm at

Fig. 8. Mid and Far ATR - FTIR (50–1100 cm−1) spectrum of H+(DMSO-d6)2 complex. (initial value n = 2.3, tvs = 21 h).

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Fig. 9. Dependence of ATR - FTIR spectra between 50 and 1100 cm−1 on the vacuum storage time of Nafion membrane (initial value n = 13.3) which have been treated with excess of DMSO-d6 + liquid D2O mixture: tvs = 2 min (a), 11 min (b) and 24 min (c). Peak at 982 cm−1 observed in the spectrum (c) belongs to the ν(SO) vibration of the free DMSO molecules diluted in D2O.

1100 cm−1, may be considered as the upper value for the thickness of a deposited film of TiO2, to ensure the sampling of the whole layer at the sorption phenomena. The mean value of dp in our samples presumably has an equal order of magnitude. Naturally this value increases with increasing of the wavelength, but in any case, the measured thickness of Nafion membranes is much bigger, approximately by two orders of magnitude. It is well known that the structure of a surface layer may differ significantly from the bulk structure. However, we have not found the essential differences comparing the ATR and IR absorption data. For example, the ν(CH) frequency shift does not exceed 4 cm−1. In most other cases,

it does not exceed 2 cm−1. Bandwidth and contours of bands also little depend on the choice of a spectral method. The vacuum IR absorption spectra of the samples having a thickness ≥ 125 μm, were studied initially to confirm the ATR determination of a weak band dipole – dipole interactions in the long-wavelength region. Indeed, this band was found at 80–100 cm−1. Being very weak, it is observed in this region even at n ~ 2, when the free DMSO molecules should be absent. Its appearance at the small values of n can be explained by one of the two reasons. Either there are associative interactions between the H+(DMSO)2 ions, or they are subjected to the partial dissociation. Some arguments in support of the latest version are discussed below.

Fig. 10. Mid and Far ATR - FTIR (50–1100 cm−1) spectrum of D+(DMSO-d6)2 complex. Sample with n ~ 2 was obtained in the ATR accessory through vacuum storage (tvs = 21 h) of Nafion membrane (initial value n = 13.3) which have been treated with excess of DMSO-d6 + liquid D2O mixture.

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Favorable conditions for the detection of H+(DMSO)2 analytical band occur with decreasing of the membrane thickness up to 25 μm. The IR absorption spectrum of this sample is shown for n ~ 2 in the Appendices. Note that it is completely similar to the ATR spectrum. In this case, the analytical band ν(SO) of complex has maxima at 782 and 866 cm−1. Shift one of the two maxima of splitted ν(SO) band to higher frequencies on 13 cm−1 is explained by some composition variability, but no bulk effect. Comparing the ATR and IR absorption data, we have established that a choice of a method have a certain effect on the results, provided that a plasticized membrane was subjected to the long-term exposure under vacuum. If a membrane is pressed tightly to the diamond crystal within the ATR accessory, the loss of excess DMSO molecules under vacuum (1 hPa) stops with time (from 1 to 2 days) in the stage of n ~ 2 (as it was already mentioned in the Section 3.2). But if it is in a conventional sample holder for recording the absorption spectra, the free molecules removing is getting much faster. Now the process of solvent loss stops at the stage n ≤ 1, passing the stage n ~ 2. This conclusion follows from many observations of H+(DMSO)2 absorbance ν(SO) decrease by about half. The observed effect can be explained by a shift of existing equilibrium H+(DMSO)2 → H+ DMSO + DMSO. Whereas the shift of \SO3H + 2 DMSO would cause equilibrium H+(DMSO)2 + − SO− 3 →\ the band appearance at 910 cm−1, characteristic for the undissociated sulfonate group. However this band is not observed in the IR absorption spectrum of a membrane with a thickness of 25 μm subjected to the long-term exposure under vacuum. A very weak sharp band presents in this region at 926 cm− 1 but it also presents in the spectrum of DMSO (Table 4). Two features give some evidence in favor of the version about the equilibrium shift toward the H+ DMSO. First, the intensity of the maximum at 872 cm− 1 significantly drops due to the DMSO loss but its resolution unexpectedly increases. While the maximum at ≤ 781 cm− 1 is not detected since it is masked by the closely spaced (720 cm−1) weak band of Nafion when n ≤ 1. Second, the long-term exposure of membrane with a thickness ≥ 125 μm under vacuum gives rise to a moderately broad and weak band at 2435 cm−1 which shifts on deuteration H+\\D+ to ~ 2000 cm−1, so it possibly belongs to the + ν(OH) vibration of the ion pair\\SO− 3 ·H DMSO. It was revealed also a very weak narrow satellite at 2858 cm−1. Its position does not depend on the exchange H+\\D+, so it may be attributed to the stretching vibrations of CH bonds, which available in Nafion. 4. Conclusions 1. The DMSO plasticization of monohydrated Nafion leads to the disso+ ciation of ions H3O+ thereby forming ions H5O+ 2 and H (DMSO)2. 2. High stability of complex H+(DMSO)2 ions is due to formation of the strong hydrogen bond OH+ O. 3. The symmetry of the hydrogen bond OH+ O presumably is reduced under the influence of an electrostatic field of the\\C\\SO− 3 anionic group. 4. There is a reason to believe that the formation of the complex H+(DMSO)2 leads to a decrease in the proton conductivity of Nafion at a plasticizing. 5. The free DMSO molecules (which are not included in the complex) form associates in Nafion. The analytical feature of association is a weak band of dipole-dipole coupling of molecules located at 84 cm−1. Acknowledgments This work was supported by Russian Scientific Foundation [Contract No. 14-23-00218]. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.saa.2017.01.062.

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