- Email: [email protected]
Molecular interactions in high conductive gel electrolytes based on low molecular weight gelator
Accepted Manuscript Molecular interactions in high conductive gel electrolytes based on low molecular weight gelator Michał Bielejewski, Andrzej Łapiński, Oleg Demchuk PII: DOI: Reference:
S0021-9797(16)30936-5 http://dx.doi.org/10.1016/j.jcis.2016.11.059 YJCIS 21787
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
23 September 2016 14 November 2016 15 November 2016
Please cite this article as: M. Bielejewski, A. Łapiński, O. Demchuk, Molecular interactions in high conductive gel electrolytes based on low molecular weight gelator, Journal of Colloid and Interface Science (2016), doi: http:// dx.doi.org/10.1016/j.jcis.2016.11.059
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Molecular interactions in high conductive gel electrolytes based on low molecular weight gelator Michał Bielejewski*1, Andrzej Łapiński1 and Oleg Demchuk2
1
Institute of Molecular Physics Polish Academy of Sciences ul. M. Smoluchowskiego 17, 60-179 Poznań, Poland
2
Maria Curie-Skłodowska University, Department of Organic Chemistry ul. Gliniana 33, 20-614 Lublin, Poland
*Corresponding Author: Dr eng. Michal Bielejewski Institute of Molecular Physics, PAS 60-179 Poznań, Poland phone: +48-061 8695-186 e-mail: [email protected]
Abstract Organic ionic gel (OIG) electrolytes, also known as gel electrolytes or ionogels are one example of modern functional materials with the potential to use in wide range of electrochemical applications. The functionality of OIGs arises from the thermally reversible solidification of electrolytes or ionic liquids and their superior ionic conductivity. To understand and to predict the properties of these systems it is important to get the knowledge about the interactions on molecular level between the solid gelator matrix and the electrolyte solution. This paper reports the spectroscopic studies (FT-IR, UV-Vis and Raman) of the gel electrolyte based on low molecular weight gelator methyl-4,6-O-(p-nitrobenzylidene)-α-D-glucopyranoside and solution of quaternary ammonium salt, tetramethylammonium bromide. The solidification process was based on sol-gel technique. Below characteristic temperature, defined as gel to sol phase transition temperature, Tgs, the samples were solid-like and showed high conductivity values of the same order as observed for pure liquid electrolytes. The investigations were performed for a OIGs in a wide range of molar concentrations of the electrolyte solution.
Keywords: gel electrolytes; low molecular weight gelator; ionic interactions; electric conductivity; FT-IR, Raman and UV-Vis spectroscopy; hydrogen bonding
1. Introduction
In recent years in the field of transport and energy storage much attention has been put into projects such as fuel cells, proton conductors, ionic liquids, solid electrolytes, gel electrolytes etc., as a source of new materials for use in modern electronics [1-11]. Organic ionic gels (OIGs) possess both, adhesive properties of a solid state and diffusive properties of a liquid electrolyte which receive considerable attention as systems capable to combine high conductivity, characteristic for liquid state and mechanical properties of solid or semi-solid materials. A key to succeed in designing and producing new functional conductive materials is to understand the basic interactions underlining the process of gelation and after it. The design process starts already at the stage of choosing the type of gelator. Depending on its nature, the gel polymer electrolytes (GPE) [12-25] and low molecular weight gel electrolytes (OIG) [26-39] can be distinguish. For the first case the system with high mechanical strength and resistance to external conditions can be created, however the irreversible degradation and low conductive properties are the main disadvantages of GPEs. On the other hand we have gel electrolytes based on low molecular weight gelators (LMWG) which are thermally reversible systems, thus can be treated as renewable materials, with the conductivity properties as good as for liquid electrolyte from which they are created. However, due to the physical nature of non-covalent interactions (i.e. hydrogen bonding, electrostatic interactions, van der Waals interactions, - stacking) responsible for OIGs formation, the mechanical properties and usable temperature range are limited. Therefore, the GPEs are still the leading solution for obtaining solidified electrolytes which finds practical applications in batteries and electrochemical devices. In recent years new GPEs with enhanced conductivity properties ( ~10-5 – 10-3 Scm-1 around room temperature) have been obtained [4043], however this is still below what is observed for very simple OIGs systems [44]. Beside higher conductivity properties, the ionogels have also other advantages over gel polymer electrolytes, which makes them very promising systems for applications in the near future. Thanks to the physical nature of interactions responsible for gelation in ionogels, the big issue of GPEs, that is short cycle life, is solved by thermally reversible gelation leading to renewing of the OIGs microstructure. Thank to that all mechanical damages, inhomogeneity and material consumption can be removed. Also the process of manufacturing the complete elements and parts will be simpler as organic ionic gels can be created directly in the casing ensuring the best electrical contact with the electrodes. Latest progress in investigations of OIGs, showed that the thermal and conductive properties of such systems are very reproducible over many cycles of renewing process [45]. In this paper we present the investigation of OIG formed by saccharine based LMWG, methyl-4,6-O-(p-nitrobenzylidene)-α-D-glucopyranoside (1), and a electrolyte solution of tetramethylammonium bromide (TMABr) in wide range of molar concentrations. The gelator 1 creates in the solid state one-dimensional hydrogen-bond-based chains that are the building blocks of the fibril-like gelator network in the gel phase. It is believed that the tendency to form one-dimensional hydrogen-bonded structures is a prerequisite for a good gelator. This can be understood in case of non-polar solvents where the driving forces for gelation are intermolecular hydrogen bonds between gelator molecules, where the solvent doesn’t interfere with the gelation. However, a question arise, what happens in the case of polar solvent or additional presence of charged particles as we have in studied system. Polar solvents can usually form hydrogen bonds
with gelator molecules competing with the gelator-gelator hydrogen bond formation, furthermore the presence of charged species during the gelation stage can lead to electrostatic interactions disrupting the process of gelation. The gelator 1 has a unique properties, allowing to form gel phase in both non-polar and polar solvents, thus it is expected to allow OIGs formation with different electrolyte solutions. Such bifunctional gelators are limited in the literature [46,47]. The TMABr salt has been chosen for the preparation and investigations of the gel electrolytes due to its simple form, where both anion and cation can be treated as spherical species, and the fact of possessing unprotonated anion which would not take part in hydrogen bonding process during gelation. The TMA+ cation on the other hand has four methyl groups which protons are passive in the process of hydrogen bonding. Therefore it is expected not to incorporate into gelator network by covalent bonds and modify the gel matrix. To study the process of gelation and investigate the gelator-gelator, gelator-ion and ionion intermolecular interactions in created OIGs, different spectroscopic methods: FT-IR, UVVis, and Raman, have been applied. For interpretation of recorded spectra the quantum chemical calculations method with Gaussian’03 and GaussView software were used. Additionally to check the conductive properties of investigated ionogels, the conductometry method was used.
2. Experimental section 2.1 Materials The gelator 1 was synthetized according to the method described elsewhere [47]. From our previous studies on the physical gels based on gelator 1 made with organic solvents and water we found that mainly the hydrogen bonding interactions between gelator molecules were responsible for the molecular aggregation in the gel matrix [48-51]. As in the present work, the aqueous solution of TMABr was used to prepare the OIG with 1, we postulate that also in this case the hydrogen bonding and/or - stacking of aromatic ring, play the main role in the gelation process. TMABr was obtained commercially from the Sigma-Aldrich and was used without further purification. Figure 1 presents chemical structure of the gelator molecule and the TMABr.
FIGURE 1. Chemical structure of the gelator molecule: methyl-4,6-O-(p-nitrobenzylidene)-α-D-glucopyranoside (a) and tetramethylammonium bromide (b).
2.2 Preparation of OIG The gelator 1 and TMABr aqueous solution in molar concentrations ranging from 0.5 to 3.77 M have been taken in appropriate weight proportions and heated in flame sealed glass tube until the gelator was completely dissolved. After cooling down to room temperature, the physical gel phase was obtained. To verify the gel phase, the “bottom-up” test was performed to check for no flow. After gelation, the sample was fully transparent and stable in time. The concentration of 1.5% weight of the gelator to the weight of water used to prepare all electrolyte solutions for studied OIGs were chosen for the investigations. 2.3 Spectral investigations The vibrational spectra of the gelator, electrolyte solutions, and organic ionic gels were measured at ambient temperature using Raman scattering and absorption techniques. The Raman spectra of polycrystalline gelator, electrolyte solutions, and OIG were investigated in the range from 100 to 4000 cm-1 with a JobinYvon HORIBA LabRAM HR 800 confocal spectrometer with a liquid-N2-cooled charge couple device (CCD) and Stabilite 2017 argon ion laser (laser=514 and 488 nm). The laser power on the sample was less than 7 mW. The spectral resolution was better than 2 cm-1. The absorption spectra were recorded using a FT-IR Bruker Equinox 55 spectrometer within the spectral region from 400 to 7000 cm-1 with a resolution of 2 cm-1. The absorption electronic spectra were measured using a UV-Vis Hitachi U-2900 spectrometer in the wavelength range from 190 to 1100 nm. The electrolyte solutions and OIGs samples were measured within special measuring cell with BaF2 windows and 0.015 mm spacer. 2.4 Computational methods The quantum-chemical calculations, conducted in two-step procedure including optimization of the ground-state geometry and vibrational transitions calculations with the B3LYP theory level and 6-31++G(d,p) basis set, followed by determination of the electronic transition energies by means of TD-B3LYP/6-31++G(d,p) [52-54] where used to interpret intramolecular excitations in the electronic spectra. Calculations were performed for single molecules taking into the account interaction with surrounding potential from solvent molecules, using the Gaussian’03 software package [55]. The B3LYP theory with 6-31++G(d,p) basis set including mixture of HF exchange with DFT exchange correlation, given by Becke’s threeparameter functional [56,57] with the Lee, Yang and Parr correlation functional [58] where used to perform vibrational transitions. For elimination of known systematic errors and correcting the anharmonicity, the computed frequencies were scaled [59]. The mode description was performed by visual inspection of the individual modes using the GaussView program. Moreover, the theoretical Raman intensities for room temperature and excitation light 514 nm were calculated from scattering activities and calculated wavenumbers obtained from the Gaussian calculations [60-62]. 2.5 Electrical conductivity measurements The electric conductivity was characterized using the digital conductivity meter S230 SevenCompact with InLab 710 four electrode conductivity cell. The sample was loaded in a
closed glass tube with conductivity cell placed in. The cell constant was calibrated using 1.413 mS/cm standard (aqueous solution of KCl 0.01 M). All measurements were carried out in specially designed measurement chamber, the heating medium was nitrogen gas. The sample cooler was used to lower the temperature of the nitrogen gas to 253.15 K and the home-build temperature controller was used to change the temperature of the sample. The measurements were carried out with use of TSC method and stabilized temperature work mode with accuracy of temperature stabilization 0.1 K [45]. The uncertainty in the measurement of conductivity was estimated to be 1%. Measurements were performed at the gel phase (298 K), around gel to sol phase transition (323 K) and in the sol phase (363 K).
3. Results and discussion 3.1 Characterization of the molecular interactions in electrolytes and gel electrolytes Ion-Ion interactions The organic ionic gels are unique systems in which small number of gelator molecules (below 1% wt% of all molecules in the system) can immobilize huge number of electrolyte solution molecules. Therefore, the intermolecular interactions in solution are very important to understand what can happen during the gelation and afterwards at the surface of the geltor matrix. In studied OIG samples the electrolyte solution was composed of water and TMABr salt diluted in wide range of molar concentrations (0.5 – 3.77 M). As the gelation process involves non-covalent interactions between gelator molecules, mostly the formation of hydrogen bonds as will be shown in following section, the ion-ion and ion-gelator interactions can significantly influence on the aggregation mechanism between the gelator molecules. It is reasonable to assume that the electric potential from the ions of the electrolyte can disrupt the H-bond pattern, depending on the concentration of the ions. In case of diluted electrolyte we did not expect that the intermolecular interactions affects the intermolecular interaction between gelator 1 molecules. However, in the case of concentrated electrolyte solutions (important from the point of view of conductive properties), the ion-ion and ion-gelator interactions were expected to have some influence. It is well known that the creation of molecular aggregates can be identify by investigations of absorption electronic spectra of diluted systems. The change in the position of bands or/and the appearance of new lines with increase of the concentration of the solute (TMABr ions) suggest that the aggregations of molecules can be present in the system. On the basis of observed line shifts it is possible to determine the type of created aggregates to be either J or H type and by comparing the diluted spectra with the ones for higher molar concentrations the presence of aggregation process can be identified.
FIGURE 2. Vertical electronic transitions calculated using TD-DFT methods and the simulated electronic absorption spectrum for the case of non-interacting cation and cation-anion ion pair (a). Absorption spectra of TMABr dissolved in water for concentrations Cm=0.91 and 3.77 M (b).
The electronic spectra of TMABr salt dissolved in water are shown on Fig. 2b. They are similar to each other and show the absorption maximum at about 225 nm. In order to interpret the experimental data, the characterization of electronic transitions was further performed using the time-dependent DFT method on its optimized ground state geometry. The transitions from the ground to excited states of cation-anion ion pair and TMA ion were investigated. The energy of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were equal to -0.18148 and -0.04570 Hartree for TAMBr ions and -0.42912 and 0.01884 Hartree for TMA ion, respectively. On this basis the band at 225 nm was assigned to HOMOLUMO transition. The results of theoretical calculations are compared with experimental measurements on Fig. 2. Together with increasing the molar concentration of prepared electrolyte solutions, the probability of ion pairing was rising, as the approach distance between cation and anion was decreasing. The possibility of ion-ion pair creation has significant importance in terms of conductive properties of created electrolytes and organic ionic gels, as well as for gelation process. Therefore, a theoretical calculations were performed to check how ion pairing would affect the electronic absorption spectra. The inset of Fig. 2a shows the total energy of such ion pair depending on the cation – anion distance. The theoretical absorption
spectra obtained for non-interacting cation and cation-anion ion pair optimized for the minimum energy are presented on Fig. 2a. As can be seen in the case of ion pairs the absorption band is significantly shifted towards longer wavelengths. Figure 2b shows the experimental spectra recorded for TMABr ions diluted in water at 0.91 and 3.77 M. No evidence of molecular aggregation is observed even in the case for the highest electrolyte concentration. On this basis we conclude that the ion pairing process is not present or the life time of ion-pairs is too short to be observed with UV-Vis spectroscopy. This result is in agreement with the results obtained by others in different type of studies form TMABr salt [63]. From UV-Vis investigations it is clearly seen that no ionic aggregates (ion pairs) are created in studied electrolyte solution even for the highest molar concentrations. However, it can’t be assumed that there are no interactions between cations and anions especially in highly concentrated electrolyte solutions and at least some weak electrostatic attraction or repulsion are expected. Therefore, to investigate the intermolecular interactions between the ions, the FT-IR and Raman spectroscopies were applied. Basing on the selection rules for both methods we were looking for the evidence of cation-anion interactions manifested as the change of the dipole moment and polarization of the cation molecule. For very diluted systems the distance separation between TMA+ and Br– is so big that both of them doesn’t feel the other thus the observed spectra should be similar as calculated for isolated molecule.
FIGURE 3 Experimental FT-IR spectra of TMABr electrolyte solution for C m=0.5M together with the theoretical spectra of the isolated cation molecule calculated at the B3LYP/6311++G(d,p) level of theory (a). The parts of the FT-IR absorption spectra relevant to observed bands shifts and splitting owing to ion-ion interactions (b).
Figure 3a shows the experimental spectrum recorded for diluted TMABr in water (Cm=0.5M, upper panel) together with theoretical spectrum calculated for isolated cation of TMA +. Three absorption maxima has been observed in presented spectral range which has been assigned to CH3, CH2 bending (1489cm-1 and 1419 cm-1) and C-N stretching modes (946 cm-1). Both spectra, experimental and theoretical ones give the same result what indicates that for low concentration of the salt the ions can be treated as individual non-interacting species. Figure 3b shows the experimental spectra obtained for TMABr electrolyte solutions in function of salt molar concentration. For better clearance only selected spectral regions are presented for visualization of changes observed for CH2 bending and C-N stretching modes. As can be seen starting from the concentration of Cm=1.67M a splitting and shift of the band corresponding to C-N stretching mode are observed. Also a slight shift of the CH2 bending band towards lower wavenumbers can be noticed. Such behavior can be understand if we assume that the Br – anion approaches the TMA+ cation and interacts with it. The shift of the bands towards lower wavenumbers indicate increase of the C-N bond length, the shift towards higher wavenumbers indicate shortening of the bond. How this can be understand? If we assume that the anion approaches the cation on one side, most probably from the three methyl group side, then we would expect shortening of the C-N bond as the charge distribution would be moved to one side of the cation, and the band position would shift towards higher wavenumbers. On the other hand if we assume that the cation interacts with more than one anion, what is possible if we assume small approach distances between the ions in concentrated electrolytes, than we can imagine a situation where two or more anions approach the cation from opposite sides. Such interaction would lead to stretching of the C-N bond what would shift the corresponding band towards lower wavenumbers. In our case we can observe both scenarios therefore it is reasonable to assume that in concentrated TMABr electrolyte solutions close approach distance between cations and anions leads to different ionion intermolecular interaction (most probably of electrostatic nature), however they don’t lead to creation of stable ion aggregates (ion pairs). To confirm this hypothesis the Raman measurements were conducted to check if some change of the polarization of the cation molecule can be detected.
FIGURE 4. Experimental Raman spectra of TMABr electrolyte solution for C m=0.5M together with the theoretical spectra of the isolated cation molecule calculated at the B3LYP/6311++G(d,p) level of theory (a). The parts of the Raman spectra relevant to observed bands shifts owing to ion-ion interactions (b).
Figure 4a shows the experimental spectrum of diluted TMABr electrolyte solution (upper panel) and theoretical one (lower panel) with assigned bands. The bands at 1453 cm -1 and 1420 cm-1 were assigned to CH2 and CH3 bending modes, the band at 1172 cm-1 to N-C-H bending and at 950 cm-1 to C-N stretching. On Fig. 4b the Raman spectra for TMABr electrolyte solution in function of salt molar concentrations are shown. Analyzing the spectral range corresponding to C-H stretching modes it can be notice that starting from concentration Cm=1.67M the bands shifts to lower wavenumbers. This corresponds exactly to changes observed for FT-IR measurements. Additionally, it is worth to notice that three lower bands have comparable shifts equal to around 2-3 cm-1, whereas the band at 3060 cm-1 has shift of 8.3 cm-1. This can be understand if we look to what part of the cation these bands corresponds. The three lower bands come from the side methyl groups and the highest one from the apical methyl group. Changes in band position for all of them can indicate that the interaction with the anion occurs at both sites, what confirms our hypothesis from FT-IR studies. Reassuming, from our ion-ion interaction
studies we conclude that no ion aggregates are formed and in case of concentrated electrolyte solutions an electrostatic interactions between cation and anions leads to changes in the absorption vibrational spectra.
Ion-gelator and gelator-gelator interactions Knowing the situation in pure electrolyte solutions at different molar concentrations of the TMABr, we can now closer look at the ion-gelator and gelator-gelator interactions in the created organic ionic gels. The knowledge about these intermolecular interactions is essential from the point of view of possible applications of OIGs and understanding the conductivity properties of created gel electrolytes. The chemical composition of 1 (Fig. 1) suggests that the Hbonding of the saccharide part together with the intermolecular - interactions of the aromatic ring are the driving forces of gelators’ molecular aggregation. Such type of gelation process was also found in other representatives of LMWGs [48, 50, 64, 65]. To study the influence of the TMABr ions on the H-bonding pattern created during gelation process, we study also the hydrogel created with 1 and water as a reference system for our ionic gels. Figure 5 shows the FT-IR and Raman spectra recorded for all stages and components used for creation and referring the organic ionic gels.
FIGURE 5. The parts of the experimental FT-IR spectra of water, gelator 1, hydrogel, organic ionic gel and electrolyte solution (Cm=1.67M) presented in a spectral range from 920 cm-1 to 1600 cm-1 (a). The experimental Raman spectra of gelator 1, organic ionic gel and electrolyte solution (C m=1.67M) presented in a spectral range from 300 cm-1 to 1650 cm-1 (b).
On Fig. 5a we can see that in the spectrum of studied OIG we can find bands characteristic for electrolyte solution and for gelator 1, this gives us certainty that all components are present in the gel electrolyte. Furthermore to check the influence of the ions on the gelation process we compare the gel electrolyte spectra with the hydrogel (HG) spectra. On that basis we have found that the characteristic shifts of the bands taking part in the formation of the hydrogen bonds between gelator molecules in OIG and HG are exactly the same and equal to 31 cm-1. Unfortunately, due to saturation of the signals coming from water used as solvent in investigated samples, we couldn’t perform full analysis of bands taking part in hydrogen bonding. Figure 5b shows the Raman spectra corresponding to gelator 1, electrolyte solution at Cm=1.67M, and OIG sample, no evidence of interactions between ions and gelator were noticed. The analysis of the FT-IR spectra for OIGs in function of salts’ molar concentration is shown on Fig. 6, and also gives no evidence for influence of TMABr on the gelation process.
FIGURE 6 The experimental FT-IR absorption spectra of OIGs in function of electrolyte molar concentration presented in a spectral range from 950 cm-1 to 1600 cm-1. The stars indicate the bands related to the gelator 1.
In all investigated samples we can easily find bands characteristic for electrolyte and gelator (marked with * on Fig. 6), as can be seen (Fig. 6) none of them have change its position. Basing on all obtained data we believe that the gelation process with non-covalent driving forces is not disrupted by the presence of the ions of electrolyte solution (Fig. 5a). Such behavior suggests that at the stage of gelation, the electrolyte can be treated as inert environment. The lack of iongelator intermolecular interactions at the gelation stage and after the gel is created (Fig.6), can shed some light on the conductivity properties measured for studied gel electrolytes.
3.2 Electrical properties of organic ionic gels (OIGs) Gel electrolytes are systems which already find more and more applications, owing to its main advantage over liquid electrolytes, that is solid or semi-solid physical state. The most commonly used ones are polymer gel electrolytes (PGE). However, due to low conductivity values, a short cycle life time and none biodegradable character, new materials are searched for. The alternative solution is to use organic materials such as low molecular weight gelators. Gel electrolytes based on LMWG have two major advantages over PGEs: they create thermally reversible physical gels and are bio-compatible. Our first studies of LMWG based gel electrolyte showed enhanced conductivity properties in relation to liquid electrolyte from which it was created [44]. Based on our present study we can say about the intermolecular interactions between all components of created OIG systems and transfer this knowledge on conductive properties of investigated samples. It is assumed that electrical conduction in gel electrolytes based on LMWGs occurs primarily through the movement of charge carriers (ions) in the liquid phase of the electrolyte, immobilized in the free space within the gel matrix. As a consequence, the sample has solid-like mechanical properties, such as shape retention, stiffness, and elasticity on a macroscopic scale, but microscopically consists of large free interconnected volumes in which liquid can diffuse almost freely. This assumption has been confirmed by our spectroscopic
investigations, which allows us to say that despite the fact that the bulk flow in ionogel is stopped the ions behaves as none interacting in permanently way with the pores surface. From the close approach distance of cations and anions the physical obstacles in form of gel matrix are not seen. Therefore the conductivity in gel and in liquid phase are essentially the same. The values of the OIGs conductivity registered for 25°C, 50°C and 90°C are plotted in Fig. 7a and summarized in Table 1. Figure 7b shows the relative conductivity of the gel electrolytes to liquid electrolyte used during gelation process. As can be seen for all three temperatures the conductivity of gel electrolyte at low electrolyte concentration is higher than the corresponding one recorded for pure electrolyte. For higher electrolyte concentrations the relative conductivity seems to level away. It is worth to notice that this change of the behavior of relative conductivity is observed at the concentration of Cm=1.67M, what corresponds directly to observed bands shifts in FT-IR and Raman measurements. Therefore we propose to relate this change of conductivity behavior to closing approach distance in ion-ion interactions. This could also explain reaching a plateau value of conductivity for the highest molar concentrations of electrolyte solutions. Although no stable ion pairs are created some weak ion association process can lead to reduction of charge carriers, as a consequence the conductivity increases much slower and level away with increase of the ion concentration.
FIGURE 7 Electric conductivity of electrolyte solution and organic ionic gel in function of electrolyte solution molar concentration registered at 25°C, 50°C and 90°C (a). Relative conductivity dependence of the OIGs in function of electrolyte solution molar concentration (b). Table 1. Electric conductivity values of OIGs and electrolyte solutions in function of electrolyte solution molar concentration registered at 25°C, 50°C, and 90°C.
Cm (M) 0.5 0.91 1.3 1.67 2.87 3.77
o
25 C / mS cm-1 42 63 81 94 118 121
OIG 50 C / mS cm-1 67 102 127 153 192 200 o
o
90 C / mS cm-1 102 159 205 242 316 317
Electrolyte solution 25 C / mS 50oC / mS 90oC / mS cm-1 cm-1 cm-1 39 60 95 63 99 156 84 129 208 99 153 245 124 190 311 123 191 314 o
4. Conclusions In this paper, we have studied the ion-ion, ion-gelator and gelator-gelator intermolecular interactions in created organic ionic gels to determine the mechanisms of molecular aggregation present in the system and responsible for the gelation. We have demonstrated, that even for the highest molar concentrations of prepared TMABr electrolyte solutions, no ion-pairing can be observed. This has beneficial effect in terms of using this quaternary ammonium salt as the source of charge carriers for preparation of ionogels based on LMWG. Furthermore we have shown that the presence of TMA+ cations an Br– anions does not disrupt the self-assembly process of gelator molecules forming the gel matrix. On the basis of conducted spectroscopic studies no ion-gelator interactions have been confirmed. On the other hand the non-covalent ion-ion interaction within the electrolyte solution has been detected and attributed to close approach distance between cations and anions in concentrated electrolyte solutions. The measured conductivity properties of created organic ionic gels shows values comparable with observed in pure electrolyte solutions. In comparison with new generation of polymer gel electrolytes and proton conductors [40-43] our system shows the conductivity values from 1 to 5 orders of magnitude higher. Additionally some dependence of the relative conductivity of the gel electrolytes to electrolyte solutions has been noticed to correlate with the occurring ion-ion interactions. Summarizing we can conclude that gel electrolytes based on low molecular weight gelators can constitute a new approach in searching of alternative solid electrolytes for use in electrochemical applications [38, 39]. As on the basis of presented data we can conclude that the gel matrix in presented system is determined only by gelator-gelator interactions, its microstructure and properties will depend therefore on the gelator concentration. The change in the gelator concentration will affect the thermal properties (like Tgs and thermal stability) and electrical properties (ion - gel matrix interactions) of created ionogels. The detailed investigations of ionic conductivity and thermal properties of ionogel based on LMWG and TMABr electrolyte
were the subject of our other paper by Bielejewski M., Nowicka K., Bielejewska N. and TrittGoc J. in J. Electrochem. Soc. 2016, 163, G187 [66]. As the task for future, the non-water based electrolytes for OIGs are necessary to search for. It can be predicted that such non-water electrolyte solution or ionic liquid based OIGs should have better thermal properties, what would increase the temperature limits in its applications. Acknowledgments The partial financial support for this work was provided by the National Center for Science as Grant No. DEC-2013/11/D/ST3/02694
References [1] T. Norby, A. Magrasó, J. Power Sources, 2015, 282, 28. [2] H. Zhang, P. K. Shen, Chem. Rev., 2012, 112, 2780. [3] R. S. Daries Bella, S. Karthickprabhu, A. Maheswaran, C. Amibika, G. Hirankumar, P. Devaraj, Physica B: Condensed Matter, 2015, 458, 51. [4] A. C. Nwanya, C. I. Amaechi, A. E. Udounwa, R. U. Osuji, M. Maaza, F. I. Ezema, Appl. Phys. A-Mater. DOI 10.1007/s00339-014-8979-x Articel in press. [5] M. Yamada, Y. Moritani, Electrochim. Acta, 2014, 144, 168. [6] A. S. Rewar, H. D. Chaudhari, R. Illathvalappil, K. Sreekumar, U. K. Kharul, J. Mater. Chem., 2014, 2, 14449. [7] E. Kartini, M. Nakamura, M. Arai, Y. Inamura, K. Nakajima, T. Maksum, W. Honggowiranto, T. Y. S. P. Putra, Solid State Ionics, 2014, 262, 833. [8] M. Smiglak, J. M. Pringle, X. Lu, L. Han, S. Zhang, H. Gao, D. R. MacFarlane, R. D. Rogers, Chem. Comun., 2014, 50, 9228. [9] P. Ramaswamy, N. E. Wong, G. K. H. Shimizu, Chem. Soc. Rev., 2014, 43, 5913. [10] A. Rachocki, K. Pogorzelec-Glasser, P. Ławniczak, M. Pugaczowa-Michalska, A. Łapiński, B. Hilczer, M. Matczak, A. Pietraszko, Cryst. Growth Des., 2014, 14, 1211. [11] I. Smolarkiewicz, A. Rachocki, K. Pogorzelec-Glasser, R. Pankiewicz, P. Ławniczak, A. Łapiński, M. Jarek, J. Tritt-Goc, Electrochim. Acta, 2015, 155, 38. [12]10 J.J. Xu, H. Ye, J. Huang, Electrochem. Commun., 2005, 7, 1309. [13] D. Chen, Q. Zhang, G. Wang, H. Zhang, J. Li, Electrochem. Commun., 2007, 9, 2755. [14] Md. A.B. H. Susan, T. Kaneko, A. Noda, and M. Watanabe J. Am. Chem. Soc., 2005, 127, 4976. [15] l. Y. Chang, C. P. Lee, R. vital, J. J. Lin and K. C. Ho, J. Matter. Chem. A, 2013, 1, 3055. [16] L. Etgar, G. Schuchardt, D. Costenaro, F. Carnito, C, Bisio, S. M. Zakeeruddin, M. K. Nazeeruddin, L. Marchese and M. Gratzel, J. Matter. Chem. A, 2013, 1, 10142. [17] R. T. Carlin, J. Fuller, Chem. Commun., 1997, 15, 1345. [18] J. Fuller, A.C. Breda, R.T. Carlin, J. Electroanal. Chem., 1998, 459, 29. [19] P. Wang, S.M. Zakeeruddin, I. Exnar, M. Gratzel, Chem.Commun., 2002, 24, 2972 [20] R.P. Swatloski, S.K. Spear, J. D. Holbrey, R. D. Rogers, R. D. J. Am. Chem. Soc., 2002, 124, 4974. [21] M. A. Klingshirn, S. K. Spear, R. Subramanian, J.D. Holbrey J.G. Huddleston, R. D. Rogers, Chem. Matter., 2004, 16, 3091. [22] E. Stathatos, P. Lianos, U. Lavrencic-Stangar, B. Orel, B. Adv. Mater., 2002, 14, 354. [23] P. Wang, S. M. Zakeeruddin, P. Comte, I. Exnar, M. Gratzel, M. J. Am. Chem. Soc., 2003, 125, 1166. [24] P. Snedden, A. I. Cooper, K.Scott, N. Winterton, N. Macromolecules, 2003, 36, 4549. [25] T. Fukushima, A. Kosaka, Y. Ishimura, T. Yamamoto, T. Takigawa, N. Ishii, T. Aida, Science, 2003, 300, 2072. [26] K. Hanabusa, D. Inoue, m. Suzuki, M. Kimura, H. Shirai, Polymer Journal, 1999, 31, 1159.
[27] K. Hanabusa, K. Hiratsuka, M. Kimura, H. Shirai, Chem. Mater., 1999, 11, 649. [28] K. Hanabusa, H. Fukui, M. Suzuki, H. Shirai, Langmuir, 2005, 21, 10383. [29] A. Ikeda, K. Sonoda, M. Ayabe, S. Tamaru, T. Nakashima, N. Kimizuka, S. Shinkai, Chem. Lett., 2001, 11, 1154. [30] M. Amaike, H. Kobayashi, S. Shinkai, Bull. Chem. Soc. Jpn., 2000, 73, 2553. [31] W. Kubo, S. Kambe, S. Nakade, T. Kitamura, K. Hanabusa, Y. Wada, S. Yanagida, J. Phys. Chem. B, 2003, 107, 4374. [32] N. Mohmeyer, P. Wang, H.W. Schmidt, S. M. Zakeeruddin, M.; Gratzel, J. Mater. Chem., 2004, 14, 1905. [33] X. Dong, H. Wang, F. Fang, X. Li, Y. Yang, Electrochim. Acta, 2010, 55, 2275. [34] J. Zou, D. S. Frost, L. L. Dai, Colloids and Surfaces A: Physicochem. Eng. Aspects, 2012, 414, 477. [35] S. Sun, J. Song, Z. Shan, R. Feng, J. Electroanal. Chem., 2012, 676, 1. [36] N. Minakuchi, K. Hoe, D. Yamaki, S. Ten-no, K. nakashima, M. Goto, M. Mizuhata, T. Maruyama, Langmuir, 2012, 28, 9259. [37] L. Tan, X. Dong, H. Wang, Y. Yang, Electrochem, Commun., 2009, 11, 933. [38] S. Sun, J. Song, Z. Shan Electrochim. Acta, 2014, 130, 689. [39] Y. Ishioka, N. Minakuchi, M. , Mizuhata, T. Maruyama Soft Matter, 2014, 10, 965. [40] S. Sikkanthar, S. Karthikeyan, S. Selvasekarapandian, D. VinothPandi, S. Nithya, C. Sanjeeviraja J. Solid State Electrochem., 2015, 19, 987. [41] S. Aminah Mohd Noor, H. Yoon, M. Forsyth, D. R MacFarlane Electrochim. Acta, 2015, 169, 376. [42] N. Rajeswari, S.Selvasekarapandian, S. Karthikeyan, C. Sanjeeviraja, Y. Iwai, J. Kawamura Ionics, 2013, 19, 1105. [43] W. Li, Y, Xing, Y. Wu, J. Wang, L. Chen, G. Yang, B. Tang Electrochim. Acta, 2015, 151, 289. [44] M. Bielejewski, A. Puszkarska, J. Tritt-Goc Electrochim. Acta, 2015, 165, 122. [45] M. Bielejewski Electrochim. Acta, 2015, 174, 1141. [46] J. H. Jung, G. John, M. Masuda, K. Yoshida, S. Shinkai, T. Shimizu, Langmuir, 2001, 17, 7229. [47] O. Gronwald, S. Shinkai, J. Chem. Soc., Perkin Trans., 2001, 2, 1933. [48] M. Bielejewski, A. Łapiński, R. Luboradzki, J. Tritt-Goc, Tetrahedron, 2011, 67, 7222. [49] J. Tritt-Goc, M. Bielejewski, R. Luboradzki, Tetrahedron, 2011, 67, 8170. [50] J. Kaszyńska, A. Łapiński, M. Bielejewski, R. Luboradzki, J. Tritt-Goc, Tetrahedron, 2012, 68, 3803. [51] J. Kowlaczuk, S. Jarosz, J. Tritt-Goc, Tetrahedron, 2009, 65, 9801.
[52] Lipkowitz, K. B., Larter, R., Cundari, T. R., Eds. Reviews in Computational Chemistry, Vol. 20; John Wiley & Sons, Inc.: USA, 2004. [53] Kohanoff, J. Electronic Structure Calculations for Solids and Molecules. Theory and Computational Methods; Cambridge University Press: Cambridge, U.K., 2006. [54] Burke, K.; Werschnik, J.; Gross, E. K. U. J. Chem. Phys., 2005, 123, 062206-1. [55] Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03, revision B.03; Gaussian, Inc.: Pittsburgh, PA, 2003. [56]Becke, A. D. Phys. Rev. A, 1998, 38, 3098. [57]Becke, A. D. J. Chem. Phys., 1993, 98, 5648. [58] Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B, 1988, 37, 785. [59] Scott, A. P.; Radom, L. J. Phys. Chem., 1996, 100, 16502. [60] Sun, R.; Yao, J.; Li, S.; Gu, R. Vib. Spectrosc., 2008, 47, 38. [61] Michalska, D.; Wysokinski, R. Chem. Phys. Lett., 2005, 403, 211. [62]Polavarapu, P. L. J. Phys. Chem., 1990, 94, 8106.
[63] Giesecke M., Meriguet G., Hallberg F., Fang Y., Stilbs P., Furo I. Phys. Chem. Chem. Phys., 2015, 17, 3402. [64] Kowalczuk J., Bielejewski M., Łapiński A.,Luboradzki R., Tritt-Goc J. J. Phys. Chem. B, 2014, 118, 4005. [65] Bielejewski M., Kowalczuk J., Kaszyńska J., Łapiński A., Luboradzki R., Demchuk O., TrittGoc J. Soft Matter, 2013, 9, 7501. [66] Bielejewski M., Nowicka K., Bielejewska N. and Tritt-Goc J. J. Electrochem. Soc. 2016, 163, G187.
S0021-9797(16)30936-5 http://dx.doi.org/10.1016/j.jcis.2016.11.059 YJCIS 21787
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
23 September 2016 14 November 2016 15 November 2016
Please cite this article as: M. Bielejewski, A. Łapiński, O. Demchuk, Molecular interactions in high conductive gel electrolytes based on low molecular weight gelator, Journal of Colloid and Interface Science (2016), doi: http:// dx.doi.org/10.1016/j.jcis.2016.11.059
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Molecular interactions in high conductive gel electrolytes based on low molecular weight gelator Michał Bielejewski*1, Andrzej Łapiński1 and Oleg Demchuk2
1
Institute of Molecular Physics Polish Academy of Sciences ul. M. Smoluchowskiego 17, 60-179 Poznań, Poland
2
Maria Curie-Skłodowska University, Department of Organic Chemistry ul. Gliniana 33, 20-614 Lublin, Poland
*Corresponding Author: Dr eng. Michal Bielejewski Institute of Molecular Physics, PAS 60-179 Poznań, Poland phone: +48-061 8695-186 e-mail: [email protected]
Abstract Organic ionic gel (OIG) electrolytes, also known as gel electrolytes or ionogels are one example of modern functional materials with the potential to use in wide range of electrochemical applications. The functionality of OIGs arises from the thermally reversible solidification of electrolytes or ionic liquids and their superior ionic conductivity. To understand and to predict the properties of these systems it is important to get the knowledge about the interactions on molecular level between the solid gelator matrix and the electrolyte solution. This paper reports the spectroscopic studies (FT-IR, UV-Vis and Raman) of the gel electrolyte based on low molecular weight gelator methyl-4,6-O-(p-nitrobenzylidene)-α-D-glucopyranoside and solution of quaternary ammonium salt, tetramethylammonium bromide. The solidification process was based on sol-gel technique. Below characteristic temperature, defined as gel to sol phase transition temperature, Tgs, the samples were solid-like and showed high conductivity values of the same order as observed for pure liquid electrolytes. The investigations were performed for a OIGs in a wide range of molar concentrations of the electrolyte solution.
Keywords: gel electrolytes; low molecular weight gelator; ionic interactions; electric conductivity; FT-IR, Raman and UV-Vis spectroscopy; hydrogen bonding
1. Introduction
In recent years in the field of transport and energy storage much attention has been put into projects such as fuel cells, proton conductors, ionic liquids, solid electrolytes, gel electrolytes etc., as a source of new materials for use in modern electronics [1-11]. Organic ionic gels (OIGs) possess both, adhesive properties of a solid state and diffusive properties of a liquid electrolyte which receive considerable attention as systems capable to combine high conductivity, characteristic for liquid state and mechanical properties of solid or semi-solid materials. A key to succeed in designing and producing new functional conductive materials is to understand the basic interactions underlining the process of gelation and after it. The design process starts already at the stage of choosing the type of gelator. Depending on its nature, the gel polymer electrolytes (GPE) [12-25] and low molecular weight gel electrolytes (OIG) [26-39] can be distinguish. For the first case the system with high mechanical strength and resistance to external conditions can be created, however the irreversible degradation and low conductive properties are the main disadvantages of GPEs. On the other hand we have gel electrolytes based on low molecular weight gelators (LMWG) which are thermally reversible systems, thus can be treated as renewable materials, with the conductivity properties as good as for liquid electrolyte from which they are created. However, due to the physical nature of non-covalent interactions (i.e. hydrogen bonding, electrostatic interactions, van der Waals interactions, - stacking) responsible for OIGs formation, the mechanical properties and usable temperature range are limited. Therefore, the GPEs are still the leading solution for obtaining solidified electrolytes which finds practical applications in batteries and electrochemical devices. In recent years new GPEs with enhanced conductivity properties ( ~10-5 – 10-3 Scm-1 around room temperature) have been obtained [4043], however this is still below what is observed for very simple OIGs systems [44]. Beside higher conductivity properties, the ionogels have also other advantages over gel polymer electrolytes, which makes them very promising systems for applications in the near future. Thanks to the physical nature of interactions responsible for gelation in ionogels, the big issue of GPEs, that is short cycle life, is solved by thermally reversible gelation leading to renewing of the OIGs microstructure. Thank to that all mechanical damages, inhomogeneity and material consumption can be removed. Also the process of manufacturing the complete elements and parts will be simpler as organic ionic gels can be created directly in the casing ensuring the best electrical contact with the electrodes. Latest progress in investigations of OIGs, showed that the thermal and conductive properties of such systems are very reproducible over many cycles of renewing process [45]. In this paper we present the investigation of OIG formed by saccharine based LMWG, methyl-4,6-O-(p-nitrobenzylidene)-α-D-glucopyranoside (1), and a electrolyte solution of tetramethylammonium bromide (TMABr) in wide range of molar concentrations. The gelator 1 creates in the solid state one-dimensional hydrogen-bond-based chains that are the building blocks of the fibril-like gelator network in the gel phase. It is believed that the tendency to form one-dimensional hydrogen-bonded structures is a prerequisite for a good gelator. This can be understood in case of non-polar solvents where the driving forces for gelation are intermolecular hydrogen bonds between gelator molecules, where the solvent doesn’t interfere with the gelation. However, a question arise, what happens in the case of polar solvent or additional presence of charged particles as we have in studied system. Polar solvents can usually form hydrogen bonds
with gelator molecules competing with the gelator-gelator hydrogen bond formation, furthermore the presence of charged species during the gelation stage can lead to electrostatic interactions disrupting the process of gelation. The gelator 1 has a unique properties, allowing to form gel phase in both non-polar and polar solvents, thus it is expected to allow OIGs formation with different electrolyte solutions. Such bifunctional gelators are limited in the literature [46,47]. The TMABr salt has been chosen for the preparation and investigations of the gel electrolytes due to its simple form, where both anion and cation can be treated as spherical species, and the fact of possessing unprotonated anion which would not take part in hydrogen bonding process during gelation. The TMA+ cation on the other hand has four methyl groups which protons are passive in the process of hydrogen bonding. Therefore it is expected not to incorporate into gelator network by covalent bonds and modify the gel matrix. To study the process of gelation and investigate the gelator-gelator, gelator-ion and ionion intermolecular interactions in created OIGs, different spectroscopic methods: FT-IR, UVVis, and Raman, have been applied. For interpretation of recorded spectra the quantum chemical calculations method with Gaussian’03 and GaussView software were used. Additionally to check the conductive properties of investigated ionogels, the conductometry method was used.
2. Experimental section 2.1 Materials The gelator 1 was synthetized according to the method described elsewhere [47]. From our previous studies on the physical gels based on gelator 1 made with organic solvents and water we found that mainly the hydrogen bonding interactions between gelator molecules were responsible for the molecular aggregation in the gel matrix [48-51]. As in the present work, the aqueous solution of TMABr was used to prepare the OIG with 1, we postulate that also in this case the hydrogen bonding and/or - stacking of aromatic ring, play the main role in the gelation process. TMABr was obtained commercially from the Sigma-Aldrich and was used without further purification. Figure 1 presents chemical structure of the gelator molecule and the TMABr.
FIGURE 1. Chemical structure of the gelator molecule: methyl-4,6-O-(p-nitrobenzylidene)-α-D-glucopyranoside (a) and tetramethylammonium bromide (b).
2.2 Preparation of OIG The gelator 1 and TMABr aqueous solution in molar concentrations ranging from 0.5 to 3.77 M have been taken in appropriate weight proportions and heated in flame sealed glass tube until the gelator was completely dissolved. After cooling down to room temperature, the physical gel phase was obtained. To verify the gel phase, the “bottom-up” test was performed to check for no flow. After gelation, the sample was fully transparent and stable in time. The concentration of 1.5% weight of the gelator to the weight of water used to prepare all electrolyte solutions for studied OIGs were chosen for the investigations. 2.3 Spectral investigations The vibrational spectra of the gelator, electrolyte solutions, and organic ionic gels were measured at ambient temperature using Raman scattering and absorption techniques. The Raman spectra of polycrystalline gelator, electrolyte solutions, and OIG were investigated in the range from 100 to 4000 cm-1 with a JobinYvon HORIBA LabRAM HR 800 confocal spectrometer with a liquid-N2-cooled charge couple device (CCD) and Stabilite 2017 argon ion laser (laser=514 and 488 nm). The laser power on the sample was less than 7 mW. The spectral resolution was better than 2 cm-1. The absorption spectra were recorded using a FT-IR Bruker Equinox 55 spectrometer within the spectral region from 400 to 7000 cm-1 with a resolution of 2 cm-1. The absorption electronic spectra were measured using a UV-Vis Hitachi U-2900 spectrometer in the wavelength range from 190 to 1100 nm. The electrolyte solutions and OIGs samples were measured within special measuring cell with BaF2 windows and 0.015 mm spacer. 2.4 Computational methods The quantum-chemical calculations, conducted in two-step procedure including optimization of the ground-state geometry and vibrational transitions calculations with the B3LYP theory level and 6-31++G(d,p) basis set, followed by determination of the electronic transition energies by means of TD-B3LYP/6-31++G(d,p) [52-54] where used to interpret intramolecular excitations in the electronic spectra. Calculations were performed for single molecules taking into the account interaction with surrounding potential from solvent molecules, using the Gaussian’03 software package [55]. The B3LYP theory with 6-31++G(d,p) basis set including mixture of HF exchange with DFT exchange correlation, given by Becke’s threeparameter functional [56,57] with the Lee, Yang and Parr correlation functional [58] where used to perform vibrational transitions. For elimination of known systematic errors and correcting the anharmonicity, the computed frequencies were scaled [59]. The mode description was performed by visual inspection of the individual modes using the GaussView program. Moreover, the theoretical Raman intensities for room temperature and excitation light 514 nm were calculated from scattering activities and calculated wavenumbers obtained from the Gaussian calculations [60-62]. 2.5 Electrical conductivity measurements The electric conductivity was characterized using the digital conductivity meter S230 SevenCompact with InLab 710 four electrode conductivity cell. The sample was loaded in a
closed glass tube with conductivity cell placed in. The cell constant was calibrated using 1.413 mS/cm standard (aqueous solution of KCl 0.01 M). All measurements were carried out in specially designed measurement chamber, the heating medium was nitrogen gas. The sample cooler was used to lower the temperature of the nitrogen gas to 253.15 K and the home-build temperature controller was used to change the temperature of the sample. The measurements were carried out with use of TSC method and stabilized temperature work mode with accuracy of temperature stabilization 0.1 K [45]. The uncertainty in the measurement of conductivity was estimated to be 1%. Measurements were performed at the gel phase (298 K), around gel to sol phase transition (323 K) and in the sol phase (363 K).
3. Results and discussion 3.1 Characterization of the molecular interactions in electrolytes and gel electrolytes Ion-Ion interactions The organic ionic gels are unique systems in which small number of gelator molecules (below 1% wt% of all molecules in the system) can immobilize huge number of electrolyte solution molecules. Therefore, the intermolecular interactions in solution are very important to understand what can happen during the gelation and afterwards at the surface of the geltor matrix. In studied OIG samples the electrolyte solution was composed of water and TMABr salt diluted in wide range of molar concentrations (0.5 – 3.77 M). As the gelation process involves non-covalent interactions between gelator molecules, mostly the formation of hydrogen bonds as will be shown in following section, the ion-ion and ion-gelator interactions can significantly influence on the aggregation mechanism between the gelator molecules. It is reasonable to assume that the electric potential from the ions of the electrolyte can disrupt the H-bond pattern, depending on the concentration of the ions. In case of diluted electrolyte we did not expect that the intermolecular interactions affects the intermolecular interaction between gelator 1 molecules. However, in the case of concentrated electrolyte solutions (important from the point of view of conductive properties), the ion-ion and ion-gelator interactions were expected to have some influence. It is well known that the creation of molecular aggregates can be identify by investigations of absorption electronic spectra of diluted systems. The change in the position of bands or/and the appearance of new lines with increase of the concentration of the solute (TMABr ions) suggest that the aggregations of molecules can be present in the system. On the basis of observed line shifts it is possible to determine the type of created aggregates to be either J or H type and by comparing the diluted spectra with the ones for higher molar concentrations the presence of aggregation process can be identified.
FIGURE 2. Vertical electronic transitions calculated using TD-DFT methods and the simulated electronic absorption spectrum for the case of non-interacting cation and cation-anion ion pair (a). Absorption spectra of TMABr dissolved in water for concentrations Cm=0.91 and 3.77 M (b).
The electronic spectra of TMABr salt dissolved in water are shown on Fig. 2b. They are similar to each other and show the absorption maximum at about 225 nm. In order to interpret the experimental data, the characterization of electronic transitions was further performed using the time-dependent DFT method on its optimized ground state geometry. The transitions from the ground to excited states of cation-anion ion pair and TMA ion were investigated. The energy of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were equal to -0.18148 and -0.04570 Hartree for TAMBr ions and -0.42912 and 0.01884 Hartree for TMA ion, respectively. On this basis the band at 225 nm was assigned to HOMOLUMO transition. The results of theoretical calculations are compared with experimental measurements on Fig. 2. Together with increasing the molar concentration of prepared electrolyte solutions, the probability of ion pairing was rising, as the approach distance between cation and anion was decreasing. The possibility of ion-ion pair creation has significant importance in terms of conductive properties of created electrolytes and organic ionic gels, as well as for gelation process. Therefore, a theoretical calculations were performed to check how ion pairing would affect the electronic absorption spectra. The inset of Fig. 2a shows the total energy of such ion pair depending on the cation – anion distance. The theoretical absorption
spectra obtained for non-interacting cation and cation-anion ion pair optimized for the minimum energy are presented on Fig. 2a. As can be seen in the case of ion pairs the absorption band is significantly shifted towards longer wavelengths. Figure 2b shows the experimental spectra recorded for TMABr ions diluted in water at 0.91 and 3.77 M. No evidence of molecular aggregation is observed even in the case for the highest electrolyte concentration. On this basis we conclude that the ion pairing process is not present or the life time of ion-pairs is too short to be observed with UV-Vis spectroscopy. This result is in agreement with the results obtained by others in different type of studies form TMABr salt [63]. From UV-Vis investigations it is clearly seen that no ionic aggregates (ion pairs) are created in studied electrolyte solution even for the highest molar concentrations. However, it can’t be assumed that there are no interactions between cations and anions especially in highly concentrated electrolyte solutions and at least some weak electrostatic attraction or repulsion are expected. Therefore, to investigate the intermolecular interactions between the ions, the FT-IR and Raman spectroscopies were applied. Basing on the selection rules for both methods we were looking for the evidence of cation-anion interactions manifested as the change of the dipole moment and polarization of the cation molecule. For very diluted systems the distance separation between TMA+ and Br– is so big that both of them doesn’t feel the other thus the observed spectra should be similar as calculated for isolated molecule.
FIGURE 3 Experimental FT-IR spectra of TMABr electrolyte solution for C m=0.5M together with the theoretical spectra of the isolated cation molecule calculated at the B3LYP/6311++G(d,p) level of theory (a). The parts of the FT-IR absorption spectra relevant to observed bands shifts and splitting owing to ion-ion interactions (b).
Figure 3a shows the experimental spectrum recorded for diluted TMABr in water (Cm=0.5M, upper panel) together with theoretical spectrum calculated for isolated cation of TMA +. Three absorption maxima has been observed in presented spectral range which has been assigned to CH3, CH2 bending (1489cm-1 and 1419 cm-1) and C-N stretching modes (946 cm-1). Both spectra, experimental and theoretical ones give the same result what indicates that for low concentration of the salt the ions can be treated as individual non-interacting species. Figure 3b shows the experimental spectra obtained for TMABr electrolyte solutions in function of salt molar concentration. For better clearance only selected spectral regions are presented for visualization of changes observed for CH2 bending and C-N stretching modes. As can be seen starting from the concentration of Cm=1.67M a splitting and shift of the band corresponding to C-N stretching mode are observed. Also a slight shift of the CH2 bending band towards lower wavenumbers can be noticed. Such behavior can be understand if we assume that the Br – anion approaches the TMA+ cation and interacts with it. The shift of the bands towards lower wavenumbers indicate increase of the C-N bond length, the shift towards higher wavenumbers indicate shortening of the bond. How this can be understand? If we assume that the anion approaches the cation on one side, most probably from the three methyl group side, then we would expect shortening of the C-N bond as the charge distribution would be moved to one side of the cation, and the band position would shift towards higher wavenumbers. On the other hand if we assume that the cation interacts with more than one anion, what is possible if we assume small approach distances between the ions in concentrated electrolytes, than we can imagine a situation where two or more anions approach the cation from opposite sides. Such interaction would lead to stretching of the C-N bond what would shift the corresponding band towards lower wavenumbers. In our case we can observe both scenarios therefore it is reasonable to assume that in concentrated TMABr electrolyte solutions close approach distance between cations and anions leads to different ionion intermolecular interaction (most probably of electrostatic nature), however they don’t lead to creation of stable ion aggregates (ion pairs). To confirm this hypothesis the Raman measurements were conducted to check if some change of the polarization of the cation molecule can be detected.
FIGURE 4. Experimental Raman spectra of TMABr electrolyte solution for C m=0.5M together with the theoretical spectra of the isolated cation molecule calculated at the B3LYP/6311++G(d,p) level of theory (a). The parts of the Raman spectra relevant to observed bands shifts owing to ion-ion interactions (b).
Figure 4a shows the experimental spectrum of diluted TMABr electrolyte solution (upper panel) and theoretical one (lower panel) with assigned bands. The bands at 1453 cm -1 and 1420 cm-1 were assigned to CH2 and CH3 bending modes, the band at 1172 cm-1 to N-C-H bending and at 950 cm-1 to C-N stretching. On Fig. 4b the Raman spectra for TMABr electrolyte solution in function of salt molar concentrations are shown. Analyzing the spectral range corresponding to C-H stretching modes it can be notice that starting from concentration Cm=1.67M the bands shifts to lower wavenumbers. This corresponds exactly to changes observed for FT-IR measurements. Additionally, it is worth to notice that three lower bands have comparable shifts equal to around 2-3 cm-1, whereas the band at 3060 cm-1 has shift of 8.3 cm-1. This can be understand if we look to what part of the cation these bands corresponds. The three lower bands come from the side methyl groups and the highest one from the apical methyl group. Changes in band position for all of them can indicate that the interaction with the anion occurs at both sites, what confirms our hypothesis from FT-IR studies. Reassuming, from our ion-ion interaction
studies we conclude that no ion aggregates are formed and in case of concentrated electrolyte solutions an electrostatic interactions between cation and anions leads to changes in the absorption vibrational spectra.
Ion-gelator and gelator-gelator interactions Knowing the situation in pure electrolyte solutions at different molar concentrations of the TMABr, we can now closer look at the ion-gelator and gelator-gelator interactions in the created organic ionic gels. The knowledge about these intermolecular interactions is essential from the point of view of possible applications of OIGs and understanding the conductivity properties of created gel electrolytes. The chemical composition of 1 (Fig. 1) suggests that the Hbonding of the saccharide part together with the intermolecular - interactions of the aromatic ring are the driving forces of gelators’ molecular aggregation. Such type of gelation process was also found in other representatives of LMWGs [48, 50, 64, 65]. To study the influence of the TMABr ions on the H-bonding pattern created during gelation process, we study also the hydrogel created with 1 and water as a reference system for our ionic gels. Figure 5 shows the FT-IR and Raman spectra recorded for all stages and components used for creation and referring the organic ionic gels.
FIGURE 5. The parts of the experimental FT-IR spectra of water, gelator 1, hydrogel, organic ionic gel and electrolyte solution (Cm=1.67M) presented in a spectral range from 920 cm-1 to 1600 cm-1 (a). The experimental Raman spectra of gelator 1, organic ionic gel and electrolyte solution (C m=1.67M) presented in a spectral range from 300 cm-1 to 1650 cm-1 (b).
On Fig. 5a we can see that in the spectrum of studied OIG we can find bands characteristic for electrolyte solution and for gelator 1, this gives us certainty that all components are present in the gel electrolyte. Furthermore to check the influence of the ions on the gelation process we compare the gel electrolyte spectra with the hydrogel (HG) spectra. On that basis we have found that the characteristic shifts of the bands taking part in the formation of the hydrogen bonds between gelator molecules in OIG and HG are exactly the same and equal to 31 cm-1. Unfortunately, due to saturation of the signals coming from water used as solvent in investigated samples, we couldn’t perform full analysis of bands taking part in hydrogen bonding. Figure 5b shows the Raman spectra corresponding to gelator 1, electrolyte solution at Cm=1.67M, and OIG sample, no evidence of interactions between ions and gelator were noticed. The analysis of the FT-IR spectra for OIGs in function of salts’ molar concentration is shown on Fig. 6, and also gives no evidence for influence of TMABr on the gelation process.
FIGURE 6 The experimental FT-IR absorption spectra of OIGs in function of electrolyte molar concentration presented in a spectral range from 950 cm-1 to 1600 cm-1. The stars indicate the bands related to the gelator 1.
In all investigated samples we can easily find bands characteristic for electrolyte and gelator (marked with * on Fig. 6), as can be seen (Fig. 6) none of them have change its position. Basing on all obtained data we believe that the gelation process with non-covalent driving forces is not disrupted by the presence of the ions of electrolyte solution (Fig. 5a). Such behavior suggests that at the stage of gelation, the electrolyte can be treated as inert environment. The lack of iongelator intermolecular interactions at the gelation stage and after the gel is created (Fig.6), can shed some light on the conductivity properties measured for studied gel electrolytes.
3.2 Electrical properties of organic ionic gels (OIGs) Gel electrolytes are systems which already find more and more applications, owing to its main advantage over liquid electrolytes, that is solid or semi-solid physical state. The most commonly used ones are polymer gel electrolytes (PGE). However, due to low conductivity values, a short cycle life time and none biodegradable character, new materials are searched for. The alternative solution is to use organic materials such as low molecular weight gelators. Gel electrolytes based on LMWG have two major advantages over PGEs: they create thermally reversible physical gels and are bio-compatible. Our first studies of LMWG based gel electrolyte showed enhanced conductivity properties in relation to liquid electrolyte from which it was created [44]. Based on our present study we can say about the intermolecular interactions between all components of created OIG systems and transfer this knowledge on conductive properties of investigated samples. It is assumed that electrical conduction in gel electrolytes based on LMWGs occurs primarily through the movement of charge carriers (ions) in the liquid phase of the electrolyte, immobilized in the free space within the gel matrix. As a consequence, the sample has solid-like mechanical properties, such as shape retention, stiffness, and elasticity on a macroscopic scale, but microscopically consists of large free interconnected volumes in which liquid can diffuse almost freely. This assumption has been confirmed by our spectroscopic
investigations, which allows us to say that despite the fact that the bulk flow in ionogel is stopped the ions behaves as none interacting in permanently way with the pores surface. From the close approach distance of cations and anions the physical obstacles in form of gel matrix are not seen. Therefore the conductivity in gel and in liquid phase are essentially the same. The values of the OIGs conductivity registered for 25°C, 50°C and 90°C are plotted in Fig. 7a and summarized in Table 1. Figure 7b shows the relative conductivity of the gel electrolytes to liquid electrolyte used during gelation process. As can be seen for all three temperatures the conductivity of gel electrolyte at low electrolyte concentration is higher than the corresponding one recorded for pure electrolyte. For higher electrolyte concentrations the relative conductivity seems to level away. It is worth to notice that this change of the behavior of relative conductivity is observed at the concentration of Cm=1.67M, what corresponds directly to observed bands shifts in FT-IR and Raman measurements. Therefore we propose to relate this change of conductivity behavior to closing approach distance in ion-ion interactions. This could also explain reaching a plateau value of conductivity for the highest molar concentrations of electrolyte solutions. Although no stable ion pairs are created some weak ion association process can lead to reduction of charge carriers, as a consequence the conductivity increases much slower and level away with increase of the ion concentration.
FIGURE 7 Electric conductivity of electrolyte solution and organic ionic gel in function of electrolyte solution molar concentration registered at 25°C, 50°C and 90°C (a). Relative conductivity dependence of the OIGs in function of electrolyte solution molar concentration (b). Table 1. Electric conductivity values of OIGs and electrolyte solutions in function of electrolyte solution molar concentration registered at 25°C, 50°C, and 90°C.
Cm (M) 0.5 0.91 1.3 1.67 2.87 3.77
o
25 C / mS cm-1 42 63 81 94 118 121
OIG 50 C / mS cm-1 67 102 127 153 192 200 o
o
90 C / mS cm-1 102 159 205 242 316 317
Electrolyte solution 25 C / mS 50oC / mS 90oC / mS cm-1 cm-1 cm-1 39 60 95 63 99 156 84 129 208 99 153 245 124 190 311 123 191 314 o
4. Conclusions In this paper, we have studied the ion-ion, ion-gelator and gelator-gelator intermolecular interactions in created organic ionic gels to determine the mechanisms of molecular aggregation present in the system and responsible for the gelation. We have demonstrated, that even for the highest molar concentrations of prepared TMABr electrolyte solutions, no ion-pairing can be observed. This has beneficial effect in terms of using this quaternary ammonium salt as the source of charge carriers for preparation of ionogels based on LMWG. Furthermore we have shown that the presence of TMA+ cations an Br– anions does not disrupt the self-assembly process of gelator molecules forming the gel matrix. On the basis of conducted spectroscopic studies no ion-gelator interactions have been confirmed. On the other hand the non-covalent ion-ion interaction within the electrolyte solution has been detected and attributed to close approach distance between cations and anions in concentrated electrolyte solutions. The measured conductivity properties of created organic ionic gels shows values comparable with observed in pure electrolyte solutions. In comparison with new generation of polymer gel electrolytes and proton conductors [40-43] our system shows the conductivity values from 1 to 5 orders of magnitude higher. Additionally some dependence of the relative conductivity of the gel electrolytes to electrolyte solutions has been noticed to correlate with the occurring ion-ion interactions. Summarizing we can conclude that gel electrolytes based on low molecular weight gelators can constitute a new approach in searching of alternative solid electrolytes for use in electrochemical applications [38, 39]. As on the basis of presented data we can conclude that the gel matrix in presented system is determined only by gelator-gelator interactions, its microstructure and properties will depend therefore on the gelator concentration. The change in the gelator concentration will affect the thermal properties (like Tgs and thermal stability) and electrical properties (ion - gel matrix interactions) of created ionogels. The detailed investigations of ionic conductivity and thermal properties of ionogel based on LMWG and TMABr electrolyte
were the subject of our other paper by Bielejewski M., Nowicka K., Bielejewska N. and TrittGoc J. in J. Electrochem. Soc. 2016, 163, G187 [66]. As the task for future, the non-water based electrolytes for OIGs are necessary to search for. It can be predicted that such non-water electrolyte solution or ionic liquid based OIGs should have better thermal properties, what would increase the temperature limits in its applications. Acknowledgments The partial financial support for this work was provided by the National Center for Science as Grant No. DEC-2013/11/D/ST3/02694
References [1] T. Norby, A. Magrasó, J. Power Sources, 2015, 282, 28. [2] H. Zhang, P. K. Shen, Chem. Rev., 2012, 112, 2780. [3] R. S. Daries Bella, S. Karthickprabhu, A. Maheswaran, C. Amibika, G. Hirankumar, P. Devaraj, Physica B: Condensed Matter, 2015, 458, 51. [4] A. C. Nwanya, C. I. Amaechi, A. E. Udounwa, R. U. Osuji, M. Maaza, F. I. Ezema, Appl. Phys. A-Mater. DOI 10.1007/s00339-014-8979-x Articel in press. [5] M. Yamada, Y. Moritani, Electrochim. Acta, 2014, 144, 168. [6] A. S. Rewar, H. D. Chaudhari, R. Illathvalappil, K. Sreekumar, U. K. Kharul, J. Mater. Chem., 2014, 2, 14449. [7] E. Kartini, M. Nakamura, M. Arai, Y. Inamura, K. Nakajima, T. Maksum, W. Honggowiranto, T. Y. S. P. Putra, Solid State Ionics, 2014, 262, 833. [8] M. Smiglak, J. M. Pringle, X. Lu, L. Han, S. Zhang, H. Gao, D. R. MacFarlane, R. D. Rogers, Chem. Comun., 2014, 50, 9228. [9] P. Ramaswamy, N. E. Wong, G. K. H. Shimizu, Chem. Soc. Rev., 2014, 43, 5913. [10] A. Rachocki, K. Pogorzelec-Glasser, P. Ławniczak, M. Pugaczowa-Michalska, A. Łapiński, B. Hilczer, M. Matczak, A. Pietraszko, Cryst. Growth Des., 2014, 14, 1211. [11] I. Smolarkiewicz, A. Rachocki, K. Pogorzelec-Glasser, R. Pankiewicz, P. Ławniczak, A. Łapiński, M. Jarek, J. Tritt-Goc, Electrochim. Acta, 2015, 155, 38. [12]10 J.J. Xu, H. Ye, J. Huang, Electrochem. Commun., 2005, 7, 1309. [13] D. Chen, Q. Zhang, G. Wang, H. Zhang, J. Li, Electrochem. Commun., 2007, 9, 2755. [14] Md. A.B. H. Susan, T. Kaneko, A. Noda, and M. Watanabe J. Am. Chem. Soc., 2005, 127, 4976. [15] l. Y. Chang, C. P. Lee, R. vital, J. J. Lin and K. C. Ho, J. Matter. Chem. A, 2013, 1, 3055. [16] L. Etgar, G. Schuchardt, D. Costenaro, F. Carnito, C, Bisio, S. M. Zakeeruddin, M. K. Nazeeruddin, L. Marchese and M. Gratzel, J. Matter. Chem. A, 2013, 1, 10142. [17] R. T. Carlin, J. Fuller, Chem. Commun., 1997, 15, 1345. [18] J. Fuller, A.C. Breda, R.T. Carlin, J. Electroanal. Chem., 1998, 459, 29. [19] P. Wang, S.M. Zakeeruddin, I. Exnar, M. Gratzel, Chem.Commun., 2002, 24, 2972 [20] R.P. Swatloski, S.K. Spear, J. D. Holbrey, R. D. Rogers, R. D. J. Am. Chem. Soc., 2002, 124, 4974. [21] M. A. Klingshirn, S. K. Spear, R. Subramanian, J.D. Holbrey J.G. Huddleston, R. D. Rogers, Chem. Matter., 2004, 16, 3091. [22] E. Stathatos, P. Lianos, U. Lavrencic-Stangar, B. Orel, B. Adv. Mater., 2002, 14, 354. [23] P. Wang, S. M. Zakeeruddin, P. Comte, I. Exnar, M. Gratzel, M. J. Am. Chem. Soc., 2003, 125, 1166. [24] P. Snedden, A. I. Cooper, K.Scott, N. Winterton, N. Macromolecules, 2003, 36, 4549. [25] T. Fukushima, A. Kosaka, Y. Ishimura, T. Yamamoto, T. Takigawa, N. Ishii, T. Aida, Science, 2003, 300, 2072. [26] K. Hanabusa, D. Inoue, m. Suzuki, M. Kimura, H. Shirai, Polymer Journal, 1999, 31, 1159.
[27] K. Hanabusa, K. Hiratsuka, M. Kimura, H. Shirai, Chem. Mater., 1999, 11, 649. [28] K. Hanabusa, H. Fukui, M. Suzuki, H. Shirai, Langmuir, 2005, 21, 10383. [29] A. Ikeda, K. Sonoda, M. Ayabe, S. Tamaru, T. Nakashima, N. Kimizuka, S. Shinkai, Chem. Lett., 2001, 11, 1154. [30] M. Amaike, H. Kobayashi, S. Shinkai, Bull. Chem. Soc. Jpn., 2000, 73, 2553. [31] W. Kubo, S. Kambe, S. Nakade, T. Kitamura, K. Hanabusa, Y. Wada, S. Yanagida, J. Phys. Chem. B, 2003, 107, 4374. [32] N. Mohmeyer, P. Wang, H.W. Schmidt, S. M. Zakeeruddin, M.; Gratzel, J. Mater. Chem., 2004, 14, 1905. [33] X. Dong, H. Wang, F. Fang, X. Li, Y. Yang, Electrochim. Acta, 2010, 55, 2275. [34] J. Zou, D. S. Frost, L. L. Dai, Colloids and Surfaces A: Physicochem. Eng. Aspects, 2012, 414, 477. [35] S. Sun, J. Song, Z. Shan, R. Feng, J. Electroanal. Chem., 2012, 676, 1. [36] N. Minakuchi, K. Hoe, D. Yamaki, S. Ten-no, K. nakashima, M. Goto, M. Mizuhata, T. Maruyama, Langmuir, 2012, 28, 9259. [37] L. Tan, X. Dong, H. Wang, Y. Yang, Electrochem, Commun., 2009, 11, 933. [38] S. Sun, J. Song, Z. Shan Electrochim. Acta, 2014, 130, 689. [39] Y. Ishioka, N. Minakuchi, M. , Mizuhata, T. Maruyama Soft Matter, 2014, 10, 965. [40] S. Sikkanthar, S. Karthikeyan, S. Selvasekarapandian, D. VinothPandi, S. Nithya, C. Sanjeeviraja J. Solid State Electrochem., 2015, 19, 987. [41] S. Aminah Mohd Noor, H. Yoon, M. Forsyth, D. R MacFarlane Electrochim. Acta, 2015, 169, 376. [42] N. Rajeswari, S.Selvasekarapandian, S. Karthikeyan, C. Sanjeeviraja, Y. Iwai, J. Kawamura Ionics, 2013, 19, 1105. [43] W. Li, Y, Xing, Y. Wu, J. Wang, L. Chen, G. Yang, B. Tang Electrochim. Acta, 2015, 151, 289. [44] M. Bielejewski, A. Puszkarska, J. Tritt-Goc Electrochim. Acta, 2015, 165, 122. [45] M. Bielejewski Electrochim. Acta, 2015, 174, 1141. [46] J. H. Jung, G. John, M. Masuda, K. Yoshida, S. Shinkai, T. Shimizu, Langmuir, 2001, 17, 7229. [47] O. Gronwald, S. Shinkai, J. Chem. Soc., Perkin Trans., 2001, 2, 1933. [48] M. Bielejewski, A. Łapiński, R. Luboradzki, J. Tritt-Goc, Tetrahedron, 2011, 67, 7222. [49] J. Tritt-Goc, M. Bielejewski, R. Luboradzki, Tetrahedron, 2011, 67, 8170. [50] J. Kaszyńska, A. Łapiński, M. Bielejewski, R. Luboradzki, J. Tritt-Goc, Tetrahedron, 2012, 68, 3803. [51] J. Kowlaczuk, S. Jarosz, J. Tritt-Goc, Tetrahedron, 2009, 65, 9801.
[52] Lipkowitz, K. B., Larter, R., Cundari, T. R., Eds. Reviews in Computational Chemistry, Vol. 20; John Wiley & Sons, Inc.: USA, 2004. [53] Kohanoff, J. Electronic Structure Calculations for Solids and Molecules. Theory and Computational Methods; Cambridge University Press: Cambridge, U.K., 2006. [54] Burke, K.; Werschnik, J.; Gross, E. K. U. J. Chem. Phys., 2005, 123, 062206-1. [55] Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03, revision B.03; Gaussian, Inc.: Pittsburgh, PA, 2003. [56]Becke, A. D. Phys. Rev. A, 1998, 38, 3098. [57]Becke, A. D. J. Chem. Phys., 1993, 98, 5648. [58] Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B, 1988, 37, 785. [59] Scott, A. P.; Radom, L. J. Phys. Chem., 1996, 100, 16502. [60] Sun, R.; Yao, J.; Li, S.; Gu, R. Vib. Spectrosc., 2008, 47, 38. [61] Michalska, D.; Wysokinski, R. Chem. Phys. Lett., 2005, 403, 211. [62]Polavarapu, P. L. J. Phys. Chem., 1990, 94, 8106.
[63] Giesecke M., Meriguet G., Hallberg F., Fang Y., Stilbs P., Furo I. Phys. Chem. Chem. Phys., 2015, 17, 3402. [64] Kowalczuk J., Bielejewski M., Łapiński A.,Luboradzki R., Tritt-Goc J. J. Phys. Chem. B, 2014, 118, 4005. [65] Bielejewski M., Kowalczuk J., Kaszyńska J., Łapiński A., Luboradzki R., Demchuk O., TrittGoc J. Soft Matter, 2013, 9, 7501. [66] Bielejewski M., Nowicka K., Bielejewska N. and Tritt-Goc J. J. Electrochem. Soc. 2016, 163, G187.