——————————— *Corresponding author. E-mail: [email protected] Received August 16, 2013; accepted November 13, 2013. Supported by the Romanian Ministry of Education, Research and Youth and the National Centre for Programme Manage-
Synthesis, Characterization and Thermophysical Properties of Three Neoteric Solvents-Ionic
Liquids Based on Choline Chloride
POPESCU Ana-Maria and CONSTANTIN Virgil* Laboratory of Molten Salts, Ilie Murgulescu Institute of Physical Chemistry of the Romanian Academy,
Bucharest 006021, Romania
Abstract The authors prepared, characterized and preliminary studied the properties of some neoteric solvents-ionic liquids based on choline chloride, i.e., three ionic liquids based on the eutectics of choline chloride(2-hydroxy- N,N,N-trimethylethanaminium chloride) with, respectively, urea, malonic acid and citric acid. The obtained mixtures were clear and colorless ionic liquids at room temperature. The thermophysical properties, namely, density, viscosity, and electrical conductivity of these mixtures were investigated as a function of temperature within a range of 298—353 K. Correlations for the temperature dependence of the measured properties were generated and discussed in terms of Arrhenius theory. Finally the electrochemical windows of the eutectic liquids were determined. Keywords Neoteric solvent; Ionic liquid; Choline chloride; Malonic acid; Citric acid; Density; Viscosity; Conductivity
1 Introduction
Neoteric solvents comprise a new family of green solvents and cover ionic liquids, supercritical fluids, and perfluorohy-drocarbons. Ionic liquids(ILs) are salts that are liquid within a wide temperature range. ILs have received much attention re-cently due to their unique properties: wide liquid range, good conductivity, wide electrochemical window, non-volatility and non-flammability. Therefore, they have been studied by many researchers as a promising alternative to the conventional or-ganic solvents or organic electrolytes[1—11].
Room temperature ionic liquids(RTILs) consist solely of ions that have been named “low temperature molten salts” in the last few years. However, unlike conventional molten salts, these materials often melt below 373 K. ILs seem to be a revolution in green chemistry for less pollution and a greener environment. There also exist mixtures of substances which have low melting points, called “deep eutectic solvent” (DES), which has many similarities with ionic liquids. A DES is a type of ionic solvent with special properties composed of a mixture which forms a eutectic with a melting point much low-er than either of the individual components. Compared to ionic liquids with which share many characteristics of the ionic compounds, DESs are cheaper to make, much less toxic and sometimes biodegradable. Such a DES is the mixture between choline chloride and malonic acid. It should be noted that eutectic mixtures of salts have been utilized for a long time to decrease the temperature for molten salts(MS) application. As ionic liquids are low or room temperature molten salts(LMS or
RTMS), one can say that similar to molten salts, eutectic mix-tures of ILs are very interesting as electrolytes in electrodeposi-tion application. RTILs are a relatively new class of compounds that have received increased attention in recent years as green designer solvents and electrolytes for electrodeposition processes. From this point of view the physical properties (density, viscosity and electrical conductivity) are of great importance.
It is already known that the mixture of choline chloride with urea produces eutectics that are liquid at ambient temperature and have unusual solvent properties[12]. The phase diagram of choline chloride-urea shows a eutectic at a urea to choline chloride molar ratio of 2 and the freezing point of this eutectic mixture is 285 K, which is considerably lower than that of either of the constituents and allows the mixture to be used as an ambient temperature solvent. This significant depression of the freezing point must arise from an interaction between urea molecule and the chloride ion[13,14]. The same phenomena take place in the case of some mixtures of choline chloride with acids.
This paper is a preliminary study on the physico-chemical properties(density, viscosity and electrical conductivity) of three choline chloride based ionic liquids. The three chosen mixtures were prepared in a molar ratio of 1 to 2 from choline chloride with urea, malonic and citric acids, respectively. The main goal of this study is to determine which of those ionic liquids are most suitable for being employed as electrolytes for electrochemical uses.
120 Chem. Res. Chin. Univ. Vol.30
2 Experimental
2.1 Chemicals
2-Hydroxy-N,N,N-trimethylethanaminium chloride(choline chloride, ChCl) is an organic compound and a quaternary ammonium salt with the molecular formula C5H14ONCl and appearance of white or deliquescent crystals. Choline chloride is a complex vitamin B4 that is added as an important nutrient in animal feeds. Diaminomethanal(urea) is an organic com-pound with the chemical formula CH4N2O, and with the ap-pearance of a white odorless solid. Propanedioic acid(malonic acid) is a dicarboxylic acid which has the molecular formula C3H4O4. 2-Hydroxypropane-1,2,3-tricarboxylic acid(citric acid) is a weak organic acid named with the molecular formula C6H8O7.
Structural formulas of the above four chemicals are shown in Fig.1.
Fig.1 Structural formulas of compounds choline chloride(A), urea(B), malonic acid(C) and citric acid(D)
In order to obtain data to be used in a future technological process, choline chloride(ChCl, 99%, Sigma Aldrich), malonic acid(>98%, Fluka), citric acid(>98%, Sigma Aldrich) and urea(99%, Sigma Aldrich) were used as purchased without further purification. Another motif because of which we were not interested in the water content of the studied mixtures was that in the electrodeposition process from IL’s water does not disturb the process. And in many cases low concentration (volume fraction) of water(even ca. 20%) is even advanta-geous.
2.2 Preparation of Ionic Liquids
Theoretically eutectic liquids are obtained at <353 K, by a simple method which means by putting together the two salts. The binary mixtures were prepared by mixing known masses of pure solid substances. All the measurements of mass were per-formed on a Shimadzu AX-200 analytical balance with a preci-sion of ±0.1 mg. The uncertainty in the composition was esti-mated to be within ±1×10–3(molar ratio). The eutectic mixtures were formed by stirring the two components ChCl+A(A=urea, malonic acid or citric acid) at 353 K until homogenous liquids were formed. For the mixture of it with citric acid, it was ne-cessary to rise the temperature to 373 K in order to obtain a clear liquid. The based eutectic liquids were then cooled slowly. From the 1:2 molar ratio compositions of ChCl+urea and ChCl+malonic acid we obtained clearly colourless liquids and from that of ChCl+citric acid we obtained a yellow viscous liquid. This ChCl+citric acid mixture is a very viscous yellow
liquid at 393 K and becomes a dense gel at 303 K. The eutectic liquids ChCl+A(A=urea and malonic acid) had a very good behavior with time and the same aspect after 2 month and longer. As for the eutectic liquid ChCl+citric acid this one be-came a very dense gel, almost a solid in 7 d. Choline chloride is a quaternary ammonium salt which has been already demon-strated to form deep eutectic solvents with urea and carboxylic acids by complexing the chloride ion from ChCl with urea/or carboxylic acid molecules. Urea is neither acidic nor alkaline and it’s one simple hydrogen bond donor. The malonic acid is a weak acid having pKa1=1.19 and pKa2=5.7. The first pKa of malonic acid is significantly lower than that of a typical mono-carboxylic acid, indicating that this compound forms a mono-anion more readily. The second pKa of this acid is sig-nificantly higher than that of a typical mono carboxylic acid. This is because that “second” acid proton is held more tightly by the mono ionized complex via intramolecular hydrogen bonding. So malonic acid is a two-hydrogen donor. As the citric acid has three acidic hydrogen atoms and has pKa1=7.5×10–4, pKa2=1.7×10–5 and pKa3=4.0×10–7, it’s important to mention that citric acid, when dissolved in water(and we have some in our mixtures), becomes a triprotic acid capable of donating 3 protons(see the numbered hydrogen atoms in the structure). Even if the eutectic composition of DES changes with the na-ture of the molecule provided the hydrogen bond, it has been already demonstrated that all the 3 compounds(urea, malonic acid and citric acid), which are in principle capable of donating a hydrogen atom to a hydrogen bond, are capable of forming ionic liquids with choline chloride in the 1 to 2 molar ratio composition, by the method presented above.
2.3 Apparatus and Procedure
The density(ρ), viscosity(η) and electrical conductivity(κ) were measured for these deep eutectic liquids at temperatures between 298 K to 353 K.
Density determinations were performed on an Archi-medes’s principle laboratory set up. The measurements were performed by the classical platinum sinker method with the equipment constructed by ourselves which was calibrated and tested in molten salt measurements for many years in our la-boratory. Details on this technique were presented elsewhere[15]. The uncertainty limits of the Archimedean technique is within ±0.002 kg/m3[16]. This equipment was modified for ionic liquids measurements by coupling it to a temperature controller which maintained the temperature to ±0.01 K[17].
Viscosities were measured with an Ubbelohde viscometer (Jenaer Glaswerk Schott & Gren, Mainz-Germany) with A=1.026, fixed in a special thermostated bath which limited the temperature fluctuation to ±0.01 K. The viscometer was calibrated with deionized doubly distilled water. In order to minimize the kinetic energy corrections we used an Ubbelohde viscometer of relative long flow. Measurements of the liquid flow time of the liquids were performed, at least, twice at each temperature and the results were averaged. The uncertainty of the flow measurement was ±0.01 s and that of viscosity measurement was within ±0.003 Pa·s. Finally the kinematic
No.1 POPESCU Ana-Maria et al. 121
viscosities were converted into absolute viscosity. The electrical conductivity measurements were carried out
by means of a WTW-Germany multi-parameter instrument 350I provided with a conductivity cell TetraCon 325(k=0.475 cm–1). Before and after the analysis of the sample liquids, the meter was calibrated by water(double distilled) and yielded results were with errors less than ±0.2%. The ionic liquid was placed into a glass tube(introduced in a thermostated bath) with a ground joint and the measuring cell was well sealed to prevent moisture. The uncertainity of the measurements was within ±0.005 S/m. All the experiments were performed in air at ambient pressure.
The density, viscosity and conductivity values were given along with the best fitted equations in the investigated temperature range. For viscosity and conductivity the Arrhenius behavior was discussed and applicability of the Frenkel equa-tion was verified.
3 Results and Discussion
3.1 Density
The densities of all the three binary mixtures ChCl+A(A= urea, malonic acid and citric acid) were obtained as a function of temperature in a range from 329 K to 393 K. For the eutectic liquid ChCl+citric acid, the measurement was done in a tem-perature range from 354 K to 393 K due to the viscosity of this liquid within the range of the temperatures. The mixture densi-ties are related to those of co-solvent. The density values of the studied systems are summarized in Table 1. One can observe that densities decrease very slowly with increasing temperature in a range from 328 K to 373 K.
Table 1 Densities(ρ) for studied eutectic liquids with a molar ratio of 1:2 ChCl+A(A=urea, ma-lonic acid, citric acid)
In the case of mixtures involving ionic liquids, the correla-tion with temperature can be expressed by the following linear equation:
ρ=a+bT (1) where ρ(kg/m3) is the density and T(K) is the absolute temperature.
The data from Table 1 fit Eq.(1) well(σ=0.20 for ChCl+ urea, σ=0.32 for ChCl+malonic acid, and σ=0.70 for ChCl+ citric acid). And the characteristic parameters a and b from Eq.(1) were determined from the intercept and slope of the corresponding lines(see Table 2). Analyzing these par- ameters from Table 2 we found that parameter a increased linearly in the sequence of co-solvent urea<di-carboxylic acid<
tri-carboxylic acid. It is of interest to note that the density of the studied eu-
tectic liquids decreases linearly with increasing temperature but at a lower rate than that of molecular organic solvents[13]. The ionic liquids do not expand appreciably in the studied tempera-ture range
Table 2 Coefficients of Eq.(1) describing density and temperature ranges for eutectic liquids of ChCl+A(A=urea, malonic acid, citric acid) with a molar ratio of 1:2
Eutectic liquid Parameter a Parameter b R2 Temperaturerange/K
Unfortunately we could not find any data in the literature for the densities of those eutectic liquids in order to compare our results. The thermal expansion coefficients, Eq.(2), were easily obtained from linear fits of the density data(density of DES is a linear function of temperature, ρ=a+bT). The thermal expansion coefficients of studied DES are in a range of (4.0―5.5)×10–4 K–1.
1
pTρα
ρ∂⎛ ⎞= − ⎜ ⎟∂⎝ ⎠
(2)
where ρ is the density; T is the absolute temperature; and p is the pressure.
3.2 Viscosity
Viscosity is an important parameter for electrolysis processes. Literature[2,12,18,19] gave data on viscosity of ionic liquids, but very few for our studied systems. Viscosity mea-surements could only be made on ChCl+A(A=urea, malonic acid) as the mixture ChCl+citric acid was too viscous for our measurement device.
Table 3 presents the experimental values of viscosities for the two eutectic liquids studied. One can see that the ChCl+urea mixture has a lower viscosity. The viscosities of ILs are governed essentially by van der Waals interactions and H bonding that are also valuable for our studied eutectic liquids. Alkyl chain lengthening or fluorination makes the salt more viscous due to an increase in van der Waals interactions and hydrogen bonds[7].
Table 3 Viscosities(η) of ChCl+A(A=urea and malonic acid) eutectic liquids with a molar ratio of 1:2
The range of viscosity value is from 0.036 Pa s to 0.874 Pa s with a standard deviation of σ=0.287 for ChCl+urea and from 0.068 Pa s to 0.782 Pa s with σ=0.234 for ChCl+malonic acid. Those results indicate that the viscosities of the eutectic liquids listed in Table 3 are strongly dependent upon tempera-ture and change with the hydrogen bond donor type. These values are similar to those generally observed with ionic liquids[18—20].
We could not find any data in the literature to compare our results. Only a graph was presented by Abbott et al.[12] about the viscosity of ChCl+urea. And comparing the data with ours, we can say that our data have the same evolution with temperature as that of those of Abbott, but with little higher values, and we assumed that this was due to the fact that we used choline cholide and urea not purified and that maybe it was another device used for viscosity determination.
The change in viscosity(η) with temperature can be describe by the Arrhenius expression[21]:
lgη=lgη0+Eη/(2.303RT) (3) where η0 is a constant(the pre-exponential factor) and Eη is the activation energy for the viscous flow, T is absolute tempera-ture(K) and R is the universal gas constant(8.3145×10–3
kJ·mol–1·K–1). If a fluid obeys Eq.(3), then a plot of viscosity versus re-
ciprocal absolute temperature should be linear and Fig.2 shows that all the experimental data obey Eq.(3) well(σ≈0.031) and the slope can be used to determine the activation energy for the viscous flow(Eη). In this assignment, the viscosities of the two liquids are given over a certain temperature range. In Table 4 we present the calculated preexponential factors(Aη=lgη0) and activation energies(Eη) for the two studied eutectic liquids.
Fig.2 Plots of lgη vs. reciprocal of temperature for ChCl+A[A=urea(a) and malonic acid(b)] eu-tectic liquids with a molar ratio of 1:2
Table 4 Arrhenius equations of viscosity along with the correlation coefficient(R2) and corresponding calculated preexponetial factors(Aη), activation energies(Eη), and temperature ranges for ChCl+A(A=urea, malonic acid) eutectic liquids with a molar ratio of 1:2
Eutectic liquid Arrhenius equation of viscosity R2 Aη Eη/(kJ·mol–1) Temperature range/K ChCl+urea lg η= −9.3354 + 2780.82T–1 0.9939 4.62×10–10 53.24 299.15―353.15
ChCl+malonic acid lg η= −7.3113 + 2167.42 T–1 0.9949 4.8×108 41.52 301.15―353.15 It is already known that the viscosity of ionic liquids is
determined by van der Walls forces and hydrogen bonding. The observed difference between the two ionic liquids studied(with urea and malonic acid) must arise from the ability of urea to form hydrogen bonds via its two NH2 groups and from this the differences in density and viscosity.
3.3 Electrical Conductivity
Electrical conductivity of electrolytes is one of the most important properties for electrodeposition application. The electrolyte with a higher conductivity exhibits a lower ohmic drop during electrolysis and a lower cell voltage. And thus a higher energy efficiency is to be expected. That is why the conductivity data of ionic liquids and eutectic liquids are very important.
The eutectic liquids experimental values have conductivi-ties in a range of 0.001―0.613 S/m, which increase with in-creasing temperature as shown in Fig.3. The conductivity values were obtained with σ=0.016 for ChCl+urea, σ=0.005 for ChCl+malonic acid and σ=0.001 for ChCl+citric acid. These values are similar to those for other based ionic liquids[2,12,21―26], and those data support the hypothesis that those eutectic liquids are highly conducting, confirming that the ionic species are dissociated in the liquid and can move inde-pendently.
Analogous to the viscosity, the conductivity of eutectic liquids can be fitted to an Arrhenius relationship of the form:
lgκ=lgk0+ EΛ/(2.303RT) (4)
where lgk0=AΛ(preexponential factor) is a constant and EΛ is the activation energy for conduction. Fig.4 shows that the
Fig.3 Electrical conductivity vs. temperature for the 1:2 molar mixtures of ChCl+A[A=urea(a), malonic acid(b), citric acid(c)] eutectic liquids
Fig.4 Plots of lgκ vs. reciprocal of absolute temperature for the eutectic liquids a. ChCl+urea; b. ChCl+malonic acid; c. ChCl+citric acid.
No.1 POPESCU Ana-Maria et al. 123
conductivity data for all studied eutectic liquids fit Eq.(4) accurately.
The values for preexponential factor(AΛ) and conduction activation energy(EΛ) are summarized in Table 5. And like the corresponding activation energy for viscous flow, Eη, the acti-vation energies for conduction are larger than those for molten
salts. As for viscosity and also for conductivity data, we could not find literature data to be compared with our results. In this case we could only make a comparison of the data with some graphical representation of Abbott[12,14,19], and so we could see that our data show the same evolution as previously Abbott data.
Table 5 Arrhenius equations of conductivity along with the corresponding calculated coefficients and the temperature ranges for ChCl+A(A=urea, malonic acid and citric acid) eutectic liquids with a molar ratio of 1:2
A strong negative correlation clearly exists between EΛ and Eη(EΛ/Eη<1). And Fig.5 shows that experimental data for ChCl+urea and ChCl+malonic acid obey Frenkel equation [Eq.(5)] well(r<0.95), frequently used for molten salts and made after the Walden’s rule:
λn·η=Constant (5) The constant of proportionality is defined as n=EΛ/Eη and
is given as the slope of the straight line(Fig.5) and is found to be equal to –0.5899 for ChCl+urea and to –0.7161 for ChCl+malonic acid. Those results mean that in the two liquids there are not only charge carrying species simply formed by choline and chloride ions because in this case all slopes have to be similar.
Fig.5 Application of Frenkel equation for the studied eutectic liquids a. ChCl+urea; b. ChCl+malonic acid.
From all measurements we can conclude that only ChCl+urea and ChCl+malonic acid are suitable for electro-chemical studies.
3.4 Electrochemical Windows
The fundamental requirement for an ionic liquid to be useful in developing application in electrochemistry is that it should be able to offer a large electrochemical window. The electrochemical window for aqueous solutions is about 1.23 V at the standard state. Electrolytes for application in electro-chemical devices should be also resistant to electrochemical reduction and oxidation.
An example of a typical voltammogram recorded in ChCl+urea mixture(1:2, molar ratio) at 353 K is shown in Fig.6, indicating a potential window on Pt electrodes is relatively small, from +1.0 V to –1.2 V(electrode potentials vs. Ag qua-si-reference). On the cathodic branch of the volatmmogram, one wave appears with current amplitude that increases at faster scan rates. We assumed that the existence of such a wave with
limiting currents less than 1 mA/cm2 is due to the presence of small amount of H+ ions in binary liquid, resulted by dissocia-tion of water molecules present in our experiments.
Fig.6 Cyclic voltammogram for Pt electrode (0.08 cm2) immersed in a ChCl+urea mixture with a molar ratio of 1:2 T=353 K; v=200 mV/s.
On anodic branch, the current is almost zero in a potential range of –1.0 V to +1.0 V, with a continuous increasing at more positive values, corresponding to the possible oxidation of Cl– ions. Figs.7 and 8 present typical cyclic voltammograms of ChCl+malonic acid(1:2, molar ratio) and ChCl+citric acid(1:2, molar ratio) recorded under the same conditions for ChCl+urea. A potential window from +1.1 V to –0.7 V(electrode potential vs. Ag quasi-reference) was determined for the ChCl+malonic acid. In regard to the anodic process, it is clearly that over +1.1 V appears an anodic oxidation of Cl– ions. At this moment we cannot say anything about the chemical nature of the reduc-tion products of the large organic cations of acid complexes that exists in the ionic medium.
As for the ChCl+citric cyclic voltammogramm presented
Fig.7 Cyclic voltammogram for Pt electrode(0.08 cm2) immersed in a ChCl+malonic acid mixture with a molar ratio of 1:2 T=353 K; v=200 mV/s.
124 Chem. Res. Chin. Univ. Vol.30
Fig.8 Cyclic voltammogram for Pt electrode
(0.08 cm2) immersed in a ChCl+citric acid mixture with a molar ratio of 1:2 T=353 K; v=200 mV/s.
in Fig.8, one can say that because of the too many waves pre-sented on the cathodic region, this DES is not suitable for being an electrolyte. This result is in good correlation with a big vis-cosity and low conductivity found for this ionic liquid.
Knowing that the cathodic limit is important for electro-deposition of metals, we can conclude that only ChCl+A (A=urea, malonic acid) was found to have small electrochemi-cal windows on Pt electrode, which along with the high con-ductivities and low viscosities render them suitable for being good green electrolyte to some electrodeposition application of metals and alloys(e.g., Cu, Sn, Ni-Cu-Sn).
4 Conclusions New systematic study on density, viscosity and electrical
conductivity of the eutectic liquids mixtures of choline chlo-ride+A(A=urea, malonic acid, citric acid) in a molar ratio of 1:2 were reported as a function of temperature at atmospheric pressure. In the investigated temperature range, the density of the systems decreases linearly with temperature, while the vis-cosity and electrical conductivity change significantly with this parameter. The viscosity and electrical conductivity of the stu-died ionic liquids exhibit classical Arrhenius behaviour, being determined by the strength of the interactions in the mixture: van der Waals forces and hydrogen bonds. The easy synthesis, availability and biodegradability of the components of these eutectic liquids make them a versatile alternative to classical ionic liquids. Both eutectic liquids obtained from choline chlo-ride+urea and choline chloride+malonic acid are clearly color-less and have good transport properties(density and viscosity are decreasing, while the conductivity is rising with tempera-ture). These results along with the obtained electrochemical windows make these eutectic liquids promising candidates for electrodeposition media. Unfortunately the eutectic liquid choline chloride+citric acid mixture proved not to be a good electrolyte, in fact, it is more a gel than a liquid at room tem-perature. And its thermo-physical properties make it not suita-ble for electrochemical studies as it is too viscous and has a too low conductivity. However, the charge transport in these mix-tures is predominantly controlled by ionic mobility, which is in turn affected by the viscosity of the liquid. In a good agreement with literature data, choline chloride+urea system seems to exhibit the best thermo-physical properties from the three
eutectic liquids studied so as to be used for future electrochem-ical purposes.
References
[1] Abbott A. P., Boothby D., Capper G., Davies D. L., Rasheed R., J. Am. Chem. Soc., 2004, 126, 9142
[2] Wasserscheid P., Welton T., Ionic Liquids in Synthesis, Wiley-VCH Verlag, New York, 2003, 41
[3] Song C. E., Yoon M. Y., Choi D. S., Bull. Korean Chem. Soc., 2005, 26, 1321
[4] Jorapur Y. R., Chi D. Y., Bull. Korean Chem. Soc., 2006, 27, 345 [5] Wasserscheid P., Keim W., Angew. Chem. Int. Ed., 2000, 39, 3772 [6] Dyson P. J., Transition Met. Chem., 2002, 27, 353 [7] Bonhôte P., Dias A. P., Armand M., Papageorgiou N., Kalyana-
sundaram K., Grätzel M., Inorg. Chem., 1996, 35, 1168 [8] Singh G., Kumar A., Ind. J. Chem., 2008, 47, 495 [9] Buzzeo M. C., Evans R. G., Compton R. G., Chem. Phys. Chem.,
2004, 5, 1106 [10] Buzzeo M. C., Evans R. G., Compton R. G., Electrochemical Aspects
of Ionic Liquids, Wiley-Interscience, Hoboken, 2005, 173 [11] Webber A., Blomgren G. E.; Eds.: van Schalkwijk W. A., Scrosati B.,
Advances in Lithium-ion Batteries, Kluwer Academic/Plenum Pub-lishers, New York, 2002, 185
[12] Abbott A. P., Capper G., Davies D. L., Rasheed R. K., Tambyrajah V., Chem. Commun., 2003, (1), 70
[13] Zhang S. J., Sun N., He X. Z., Lu X. M., Zhang X. P., J. Phys. Chem. Ref. Data, 2006, 35, 1475
[14] Abbott A. P., Harris R. C., Ryder K. S., J. Phys. Chem., 2007, B111, 4910
[15] Zuca S., Borcan R., Rev. Roum. Chim., 1970, 15, 1681 [16] Janz G. J., J. Phys. & Chem. Ref. Data, 1980, 9, 794 [17] Popescu A. M., Constantin V., Olteanu M., Baran A., Iovescu A.,
Dutu M., Dumitrache R., Density, Viscosity and Conductivity of Some Ionic Liquids Based on Choline Chloride, Intern. Conf. Phys. Chem.-ROMPHYSCHEM 13, Bucharest, 2008, S5, 101
[18] Welton T., Chem. Rev., 1999, 99, 2071 [19] Abbott A. P., Barron J. C., Ryder K. S., Wilson D., Chem. Eur. J.,
2007, 13, 6495 [20] Popescu A. M., Constantin V., Olteanu M., Baran A., Iovescu A.,
Florea A., Duta M., 6th Preparation and Transport Properties of Three Choline-based Ionic Liquids, Int. Conf. of the Chem. Soc. of the South-Eastern Europ. Countr. (ICOSECS-6), Sofia, 2008, BA, 37
[21] Bockris J. O’M., Reddy A. R., Modern Electrochemistry, Plenum Press, New York, 1970, Vol1, Chapter 6, 547
[22] Geng Y. F., Chen S. L., Wang T. F., Yu D. H., Peng C. J., Liu H. L., Hu Y., J. Molec. Liq., 2008, 143, 100
[23] Johnson K. E., Driver G., Anodic Species in Liquid Onium Salts Not Necessarily Derived from the Cathion, Proc. Intern Symp. on Ionic Liquids in Honour of Marcelle Gaune-Escard, Carry Le Rouet, 2003, 1
[24] Popescu A. M., Cojocaru A., Donath C., Constantin V., Chem. Res. Chinese Universities, 2013, 29(5), 991
[25] Yang F. Z., Xu M. M., Yuan Y. X., Mei J. H., Yao J. L., Gu R. A., Chem. J. Chinese Universities, 2012, 33(9), 2047
[26] Gao G. H., Lu L., Zou T., Gao J. B., Liu Y., He M. Y., Chem. Res. Chinese Universities, 2007, 23(2), 169
