,
ACCEPTED MANUSCRIPT
This is an early electronic version of an as-received manuscript that has been accepted for publication in the Journal of the Serbian Chemical Society but has not yet been subjected to the editing process and publishing procedure applied by the JSCS Editorial Office.
Please cite this article as B. S. Đorđević, D. Z. Troter, V. B. Veljković, M. Lj. Kijevčanin, I. R. Radović, Z. B. Todorović, J. Serb. Chem. Soc. (2020)
https://doi.org/10.2298/JSC200425050D
1
The physicochemical properties of the deep eutectic solvents
with triethanolamine as a major component
BILJANA S. ĐORĐEVIĆ1, DRAGAN Z. TROTER1, VLADA B. VELJKOVIĆ1,3, MIRJANA LJ. KIJEVČANIN2, IVONA R. RADOVIĆ2 and ZORAN B. TODOROVIĆ1*
1University of Niš, Faculty of Technology, Bulevar Oslobođenja 124, 16000 Leskovac, Serbia; 2Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade,
Serbia; 3Serbian Academy of Sciences and Arts, Knez Mihailova 35, 11000 Belgrade, Serbia
(Received 25 April, revised 16 July; accepted 27 August 2020) Abstract: Different deep eutectic solvents (DESs) of triethanolamine (TEOA) and oxalic acid (OA), glacial acetic acid (AA), L-(+)-lactic acid (LA), oleic acid (OLA), glycerol (G), ethylene glycol (EG), propylene glycol (PEG), choline chloride (ChCl) or 1,3-dimethylurea (DMU) were prepared and characterized re-garding their physicochemical (density, viscosity, electrical conductivity, refract-tive index, coefficient of volume expansion, molecular volume, lattice energy, and heat capacity) properties over the temperature range of 293.15-363.15 K at 101.325 kPa. For all tested DESs, the density, viscosity and refractive index decreased with increasing the temperature while the electrical conductivity increased. Those temperature dependence for viscosity and electrical conducti-vity are described by the Vogel-Tamman-Fulcher equations. The viscosity and molar conductivity, which exhibited a linear behavior, were correlated by the fractional Walden rule. Besides, the Fourier transform infrared spectroscopy (FTIR) was used to study the functional groups of these DESs while thermogra-vimetric analysis (TGA) and differential scanning calorimetry (DSC) provided the information about their stability. The tested DESs of TEOA possess desirable properties for use in various industrial processes, such as extractions, separations, chemical technology and biotechnology.
Keywords: DESs; density; viscosity; conductivity; refractive index. INTRODUCTION
Deep eutectic solvents (DESs) are a class of two-component or three-component liquid solutions, able to dissolve many other organic and inorganic compounds. Their advantages are a low cost, easy preparation, biodegradability, nontoxicity, wide liquid range, non-volatility, thermal stability and non-reactivity with water. Many different chemicals are used for their preparation, mostly substituted quaternary ammonium salts (usually choline chloride, ChCl) and hydrogen bond donors (HBDs) like alcohols, amides, carboxylic acids, esters,
*Corresponding author E-mail: [email protected] https://doi.org/10.2298/JSC200425050D
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ethers and hydrated metal salts of halides, nitrates and acetates, which provide mixtures with a lower melting point than the constituents.1
Despite the recommendation for various chemical reactions2,3 and the uses as a „green” solvent and in cosmetic formulations, polishes, paints, inks, etc., triethanolamine (TEOA) has been rarely used in the preparation of DESs. For instance, Shekaari et al.4 used ChCl:TEOA, as well as ChCl:monoethanolamine, ChCl:diethanolamine, for benzene and thiophene extraction from n -hexane/aro-matic mixtures. The amine-based DESs, such as ChCl:monoethanolamine, ChCl:diethanolamine and ChCl:methyldiethanolamine, have already been used in CO2 capture due to their high CO2 dissolution capability5–8 and in the pretre-atment of wheat straw for improving enzymatic hydrolysis.9 Liquid solutions based on organic solvents, particularly ionically conducting ones, have been applied for the electrolytic capacitors and batteries and in electrolytic processes of cathodic deposition of very electronegative metals.10,11,13 However, to the best of the authors’ knowledge, the physicochemical and thermodynamic properties of TEOA-based DESs have not been studied yet, except the ChCl-TEOA DES.12,13
In order to involve the TEOA-based DESs in the future laboratory and industrial processes, it is crucial for chemists and chemical engineers to know or be able to predict their physicochemical properties. The physical properties, such as density, viscosity, electrical conductivity and refractive index, could provide essential information on the purity of samples and the molecular interactions in the liquid, which will be useful for the design of the contacting equipment and modeling of the process.14
In this study, the physicochemical (density, viscosity, electrical conductivity, refractive index, thermal expansion coefficient, molecular volume, lattice energy, and heat capacity) properties of different DESs of TEOA as hydrogen bond acceptor (HBA) and oxalic acid (OA), glacial acetic acid (AA), L-(+)-lactic acid (LA), oleic acid (OLA), glycerol (G), ethylene glycol (EG), propylene glycol (PEG), ChCl or 1,3-dimethylurea (DMU) as HBDs were measured in the tempe-rature range of 293.15-363.15 K at 101.325 kPa for the first time. The study was performed in the region of liquid state of binary systems (melting points of tested DESs are in the range of 283.15-313.15 K, Table S-I, Supplementary material). The fractional Walden rule was employed for understanding the relationship between molar conductivity and viscosity of the tested DESs. Additionally, the functional groups of these DESs were studied by the FTIR while the TGA and DSC analyses were performed to get an insight into their stability in the tempe-rature range interesting for their industrial point of view.
EXPERIMENTAL Materials
TEOA (99.0 %), ChCl, EG and DMU (all ≥ 98.0 %) were obtained from Sigma Aldrich (St. Louis, USA). PEG and G (both Ph. Eur. grade) were provided from MeiLab (Belgrade,
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Serbia). AA was from Zorka (Šabac, Serbia). OLA (99.0 %) was obtained from Sigma Aldrich (St. Louis, USA). OA and LA (both 99.0 %) were purchased from MosLab (Belgrade, Serbia). All chemicals were used as purchased. Information on the used chemicals is presented in Table S-I of the Supplementary material.
Preparation of DESs
DESs were prepared by combining TEOA with the selected compound at the desired molar ratio (Table S-II of the Supplementary material) on a rotary evaporator at 70 °C, as described elsewhere.15 All weight measurements were performed by a digital balance (Adam Equipment, model NBL 214e) having an accuracy of ±1·10-4 g. The prepared DESs were kept in the well-closed glass bottles and stored in a desiccator containing CaCl2 before characterization to prevent moisture absorption. The water content of the dried DESs was determined by the Karl-Fischer method (Metrohm 73KF coulometer). A summary of the compositions, molar mass and water content (mass fraction) for the investigated DESs have been presented in Table S-II of the Supplementary material while their FTIR spectra are shown in Fig. S-1.
Physicochemical properties of DESs
All measurements were conducted at 101.325 kPa in the temperature range between 293.15 and 363.15 K after 15 min of thermostatting. Density was measured by using a DMA 4500 Anton Paar densitometer with the temperature measurement standard uncertainty of ±0.005 K. Viscosity was measured by a rotational viscometer (Visco Basic Plus, ver. 0.8, Fungilab S.A., Barcelona, Spain) while a B250 ProLine conductivity meter was used for the electrical conduc-tivity measurements. An automatic Atago refractometer A100 was applied to measure the re-fractive index. The densitometer was calibrated by measuring the density of doubly-distilled water and the results were compared to the reference values indicated in the calibration certificate of the standards. The rotational viscometer was calibrated by measuring the viscosity of standard silicon oils (Fungilab) certified for each used spindle. The refractometer was calibrated by measuring the certified refractive index liquids (Cargille Laboratories) and the refractive index of distilled water. The conductivity meter was calibrated by using the 0.001 M standard solution of KCl (Hach). Three replicates were carried out for each measurement of density, viscosity, conductivity and refractive index with the estimated standard uncertainty (u) of ±0.5 kg·m-3, 5 % of the measured value, ± 0.0001 S·m-1 and ± 0.00005, respectively.
TGA and DSC analyses
The TGA and DSC analyses were done in the temperature range of 298.15-273.15 K only for the liquid DESs, namely TEOA:G, TEOA:EG, TEOA:PEG, ChCl:TEOA, as well as the ChCl-based DESs with the same hydrogen bond donors for comparison. These analyses were performed using a STA449F5 Jupiter®, NETZSCH TGA-DSC instrument in an argon atmosphere with a total gas flow of 140 ml/min and a heating rate of 3 K/min. The calibration of the temperature and the enthalpy of the TGA-DSC instrument were carried out using the indium standard (purity > 99.999 %, Sigma-Aldrich) following the instrument supplier instructions.
RESULTS AND DISCUSSIONS Effect of temperature on density of DESs
The dependence of the density of the studied DESs on temperature is shown in Fig. 1. The experimental density values are given in Tables S-III–S-XI of the Supplementary material.
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Fig. 1. Temperature dependence of the density (ρ) of the tested DESs: TEOA:OA – , TEOA:AA – , TEOA:LA – , TEOA:OLA – , TEOA:G – , TEOA:EG – , TEOA:PEG – , ChCl:TEOA – and TEOA:DMU –
For the temperature range of 293.15-363.15 K, the density of the tested DESs were within the range of 1362.6-1042.1 kg∙m-3. At 313.15 K, the densities of the tested DESs follow the order: TEOA:PEG < TEOA:OLA < TEOA:EG < TEOA:DMU < ChCl:TEOA < TEOA:AA < TEOA:G < TEOA:LA < TEOA:OA.
Due to the lack of data for most of the tested TEOA-based DESs, their density was compared with the density of the DESs based on the same or similar donors. The density of DESs depends directly on the binding ability between their constitutional components. TEOA:G has a higher density than N,N -diethyl-ethanol ammonium chloride:G (1:2),16 tetrapropylammonium bromide:glycerol (1:3),17 choline chloride:G (1:2)18 because TEOA has the highest number of -OH groups and makes the stronger intermolecular H-bonds network than the other DESs with glycerol.19 For the same reason the density of TEOA:EG is higher than the density of the N,N-diethylethanol ammonium chloride:EG DES16 and ChCl:EG in the molar ratios of 1:1 and 1:213,18,20 but lower than the density of the ChCl:triethylene glycol DES in the 1:3 molar ratio21 and the density of TEOA:OA is higher than that of ChCl:OA (1:1)12,13 and N,N-diethylethanol ammonium chloride:OA (3:2).22
Although TEOA has a higher number of -OH groups than ChCl, the density of TEOA:PEG is lower than the density for ChCl:PEG (1:2)18. This phenomenon may be explained by the different molecular organization or packing of the DES accord-ing to the „hole theory”, which assumes that, after meltaccord-ing, the ionic material con-tains empty spaces arising from thermally generated local density fluctuations.19
The density of ChCl:TEOA is almost the same as those of some ChCl-based DESs.12,18,21,23,24 The present density values for ChCl:TEOA are lower than the values reported for ChCl:TEOA (1:2)12 but higher than that reported for ChCl:TEOA (1:1),13 ChCl:monoethanolamine (1:5–8)6 and ChCl:diethanolamine (1:4–6).25 For ChCl:TEOA, with the same molar ratio, the difference in the density values could be due to the experimental error. A decrease of the ChCl:TEOA molar
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ratio reduces the DES density because TEOA has a strong cohesive energy due to the presence of an important intermolecular H-bonds network and addition of ChCl was attributed to the partial rupture of this H-bonds network.19
Density was used for calculating the molecular volume (Vm), the lattice energy (Upot) and the heat capacity (Cp) using the already-published equations (see Supplementary material)13,26; their values at 313.15 K are listed in Table S-XIII. The lattice energy of the DESs was like those of the molten salts27 which explained their liquid state at this temperature. The molar volume of the TEOA:OLA (1:1) DES was the highest due to the highest molecular size of OLA. The greatest influence of the molecular size of the HBD/HBA on the molar volume was previously reported for different DESs.28 Hence, the TEOA:OLA (1:1) and ChCl:TEOA (1:2) DESs had the highest heat capacity values among all tested TEOA-based DESs.
Effect of temperature on viscosity of DESs
The viscosity of DESs depends strictly on their molecular size and the strength of the hydrogen bonding in their structures. The increased hydrocarbon chain size of a DES causes an increase in the viscosity.30 Also, the temperature, nature of HBD (alkyl chain length) and salt/HBD molar ratio greatly influence the viscosity of the tested DESs. The experimental viscosity values are presented in Tables S-III–S-XI of the Supplementary material.
The temperature dependence of the viscosity for the tested DESs, described by the Vogel-Tamman-Fulcher (VTF) equation, is shown in Fig. 2.
Fig. 2. VTF plot of the viscosity (η) for the following DESs:
TEOA:OA – , TEOA:AA – , TEOA:LA – , TEOA:OLA – , TEOA:G – , TEOA:EG – , TEOA:PEG – , ChCl:TEOA – and TEOA:DMU –
The VTF equations describing the viscosity-temperature relationship for the analyzed DESs are shown in Supplementary material. As can be seen, the viscosity decreased with increasing the temperature, probably due to the
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diminution of the intermolecular forces between the molecules as a result of diminishing of the hydrogen bonds in DESs.
For the temperature range of 293.15-363.15 K, the viscosity of the tested DESs lies in the range of 39.76-0.001 Pa∙s. At 313.15 K, the viscosity of the tested DESs follow the order: TEOA:EG < TEOA:PEG < TEOA:G < TEOA:OA < TEOA:AA < ChCl:TEOA < TEOA:LA < TEOA:DMU < TEOA:OLA.
As for the density, due to the lack of the published data, the viscosity of the tested TEOA-based DESs was compared with the viscosity of the DESs based on the same or similar donors. The viscosity of TEOA:G is lower than those reported for N,N-diethylethanol ammonium chloride:G,16 tetrapropyl ammonium bromide:G (1:2)17 and ChCl:G (1:2).18 The viscosity of TEOA:EG is almost the same to those of N,N-diethylethanol ammonium chloride:EG16 and ChCl:EG in molar ratios of 1:113 and 1:218 while lower than that for ChCl:triethylene glycol (1:3).21 The change transfer via H-bonds between PEG and chloride anion from ChCl is the main reason for the larger viscosity of ChCl:PEG (1:2) relative to TEOA:PEG.18 Although the TEOA:G has a higher density than N,N -diethylethanol ammonium chloride:G (1:2),16 tetrapropylammonium bromi-de:glycerol (1:3),17 choline chloride:G (1:2)18 it has lower value for viscosity than adove cited DESs. This might be due to a different molecular organization of the DESs. The viscosity of the DES depends on the chemical structure of the HBD. TEOA has a higher number of -OH groups than the other HBDs, but the factors like the alkyl chain length, the ion size and the electrostatic interactions may have a greater impact on the viscosity. At TEOA:EG the influence of these factors is smaller so this DES has almost the same viscosity as the DESs compared above.19
The viscosity of TEOA:OA is higher than those of ChCl:OA (1:1)12,13 and almost the same to those for N,N-diethylthanol ammonium chloride:OA (3:2).22 The viscosity of ChCl:TEOA is almost the same to that of ChCl:TEOA (1:2)12 and ChCl:diethanolamine (1:4–6)25 but higher than those of ChCl:TEOA (1:1)13 and ChCl:monoethanolamine (1:5-8).6 The ChCl:TEOA viscosity is almost the same to those reported for some ChCl-based DESs21,23 while the viscosities of TEOA:DMU and ChCl:DMU (1:2) are similar.18 The larger viscosities of the DESs with DMU or ChCl, compared to the DESs with polyols, are the result of the differences in the strengths of hydrogen bonds. Two methyl groups in DMU also show a major effect on the viscosity of its DESs, as seen with ChCl:DMU (1:2).18 Similarly, TEOA:OLA is the most viscous DES among all the DESs with carboxylic acids due to the largest number of methyl groups. DESs with longer alkyl chain and with carboxylic groups (TEOA:OLA; TEOA:DMU; TEOA:LA; TEOA:AA; TEOA:OA) have larger viscosities than DESs with polyols (TEOA:G; TEOA:PEG; TEOA:EG) with -OH groups. Obviously the presence of one extra carboxylic or longer alkyl chain leads to an increase in the viscosity.19
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Among the polyol-based DESs, TEOA:G has the highest viscosity since G forms stronger H-bonds than the other polyol-based DESs because it has one more -OH group in the molecule than EG and PEG. 19
In order to get a more detailed insight into the viscous flow and the thermodynamic functions of activation, the following expression for the viscosity of liquid mixtures is applied:13
* *
A
Δ Δ
ln ηV H S
hN = RT − R (1)
where η is the viscosity, V is the molar volume of the eutectic mixture (calculated as the ratio of average MDES and density of the mixture at desired temperature), h
is the Planck’s constant, NA is the Avogadro’s number, R is the universal gas constant, T is the absolute temperature, ΔH* is the activation enthalpy change of the viscous flow and ΔS* is the entropy change of the viscous flow. In Fig. 3, ln (ηV/hNA) is shown as a function of T-1, Eq. (1), for each of the tested DESs. The ΔH* and ΔS* values were calculated from the slope and the intercept of the straight lines, respectively. Table S-XV of the Supplementary material contains the values of the thermodynamic functions of activation at 313.15 K (all tested DESs are liquid at this temperature).
Because the DESs with TEOA have the properties like the DESs with ChCl,12,13,18,21,23 they can serve as their suitable replacement in the possible industrial applications and various chemical reactions. However, due to the high density and viscosity of several DESs with TEOA at room temperature, it is advisable to apply these DESs in the technological processes at temperatures higher than 313.15 K.
Fig. 3. Plots of ln (ηV/hNA) versus 1/T for the tested DESs: TEOA:OA – , TEOA:AA – , TEOA:LA – , TEOA:OLA – , TEOA:G – , TEOA:EG – , TEOA:PEG – , ChCl:TEOA – and TEOA:DMU –
Effect of temperature on the electrical conductivity of DESs
The experimental electrical conductivity values, as well as additional considerations, are presented in Tables S-III–S-XI of the Supplementary
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material. The electrical conductivity values for the tested DESs in the temperature range of 293.15-363.15 K lie in the range of 0.00015-9.003 S·m–1. At 313.15 K, the electrical conductivities of the tested DESs are in the following order: TEOA:PEG < TEOA:G < TEOA:DMU < TEOA:EG < TEOA:OLA < TEOA:LA < TEOA:AA < TEOA:OA < ChCl:TEOA.
TEOA:G has less electrical conductivity compared to the literature values for the ChCl:G (1:2).18,24 The same case is for the TEOA:EG, which electrical conductivity is lower than those reported for ChCl:EG in the molar ratios of 1:113 and 1:220 but higher than that of the ChCl:triethylene glycol (1:3) DES.21 The electrical conductivity of TEOA:PEG is lower than that of ChCl:PEG (1:2).18 The TEOA:OA electrical conductivity is lower than that of ChCl:OA (1:1)12 and almost the same to that of N,N-diethylethanol ammonium chloride:OA (3:2).22 The electrical conductivity of ChCl:TEOA is higher than the values reported for ChCl:TEOA (1:2)12 and ChCl:TEOA (1:1).13 The electrical conductivity of TEOA:DMU is lower than that of ChCl:DMU (1:2).18
The conductivity of DESs is affected by both the mobility and number of charged species. DESs with relatively high viscosities exhibit poor ionic conductivities. In relation to all tested DESs TEOA:G and TEOA:PEG had the lowest viscosity, so it is logical that they have the highest conductivity.19 Generally, all TEOA-based DESs have lower conductivity compared to the ChCl-based DESs because high weight ratio of the salt in ChCl contributes to the higher number of charge carriers of the ChCl relative to the TEOA. Also, the larger size of the TEOA compared to the ChCl results in both reduced ion mobility and lower conductivity.19
The temperature dependence of the electrical conductivity of the tested DESs described by the VTF equation is shown in Fig. 4.
Fig. 4. VTF plot of the electrical conductivity (κ) for the DESs: TEOA:OA – , TEOA:AA – , TEOA:LA – , TEOA:OLA – , TEOA:G – , TEOA:EG – , TEOA:PEG – , ChCl:TEOA – and TEOA:DMU –
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Molar conductivity and viscosity relationship
The temperature dependence of the molar conductivity of the tested DESs described by the VTF equation is shown in Fig. 5 (additional considerations are given in Supplementary material).
Fig. 5. VTF plot for the molar conductivity (κ) of the tested DESs: TEOA:OA – , TEOA:AA – , TEOA:LA – , TEOA:OLA – , TEOA:G – , TEOA:EG – , TEOA:PEG – , ChCl:TEOA – and TEOA:DMU –
The Walden rule is very useful for the DES categorization. Data for the dilute aqueous 0.01 M KCl solution is used for drawing the „ideal” reference line,31 which is representative for the independent ions with no interionic interactions.29 Solvents with the ability to form ions are located very close to the reference line while the solvents with this lower ability are further away.21,30 As can be seen in Fig. 6, the tested DESs belong to the category of the „subionic liquids”, indicating the non-ionic structure of the DESs.
Fig. 6. Walden plots for the tested DESs: TEOA:OA – , TEOA:AA – , TEOA:LA – , TEOA:OLA – , TEOA:G – , TEOA:EG – , TEOA:PEG – , ChCl:TEOA – and TEOA:DMU – .
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Effect of temperature on the refractive index of DESs
The linear dependence of the refractive index of the studied DESs on temperature is shown in Fig. 7 (additional considerations are given in Supple-mentary material).
Fig. 7. Temperature dependence of the refractive index (nD) of the tested DESs: TEOA:OA – , TEOA:AA – , TEOA:LA – , TEOA:OLA – , TEOA:G – , TEOA:EG – , TEOA:PEG – , ChCl:TEOA – and TEOA:DMU –
As expected, the refractive index of the pure DESs decreases linearly with increasing the temperature depending on the structure and nature of their components that are combined with TEOA. By heating, molecules are moving faster, resulting in the density and refractive index reduction. The refractive index of the tested DESs in the temperature range of 293.15-363.15 K lies within the range of 1.4991-1.4137. At 313.15 K, the refractive index of the tested DESs follows the order: TEOA:OLA < TEOA:PEG < TEOA:EG < TEOA:G < TEOA:AA < TEOA:OA < TEOA:LA < ChCl:TEOA < TEOA:DMU. The refractive index of these DESs agrees to those of the other DESs with ChCl.14 The experimental refractive index values are represented in Tables S-III–S-XI of the Supplementary material as well as additional considerations.
The refractive index of TEOA:G is almost the same to those of N,N -diethyl-ethanol ammonium chloride:G,16 tetrapropylammoniumbromide:G (1:2)17 and ChCl:G (1:2).18 Almost the same values of the refractive index of TEOA:PEG and ChCl:PEG (1:2),18 and ChCl:TEOA, ChCl:monoethanolamine6 and ChCl:dietha-nolamine (1:4–6)25 are related to their similar molecular structures. The refracttive index of TEOA:DMU and ChCl:DMU (1:2) are almost the same.18
TGA and DSC analyses
The TGA and DSC curves of the selected TEOA- and ChCl-based DESs with the same donors are shown in Fig. S-2 of the Supplementary material. The TGA and DSC analysis was performed over the temperature range of 298.15-373.15 K. Since the investigated systems are liquids, it is reasonable that no melting temperature have been observed in the DSC traces. Furthermore, even though the first derivatives of DSC and TGA curves show some peaks at certain
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temperatures, they refer neither to some special changes in the molecular structure of the mixtures nor to the phase transition. They indicate the stability of the DESs within the tested temperature range, which is favourable for their potential application in the industry.
CONCLUSIONS
The physicochemical properties (density, viscosity, electrical conductivity and refractive index) of several TEOA-based DESs were measured in the temperature range of 293.15-363.15 K at 101.325 kPa. The thermal expansion coefficient, the molecular volume, the lattice energy and the heat capacity of these DESs were calculated using the density and viscosity data. The Vogel-Tamman-Fulcher equations were used to characterize the temperature dependence of the viscosity and electrical conductivity for the investigated DESs. While density, viscosity and refractive index of the tested DESs decrease with increasing the temperature, their electrical conductivity increases. According to the Walden rule, these DESs are classified as „subionic liquids”. Among these DESs, those with polyols and lower carboxylic acids exhibit low viscosities at lower temperatures, so they can serve as solvents in the possible industrial applications and various chemical reactions, while it is recommended to heat up DESs with oleic acid and DMU before usage and apply them at temperatures higher than 313.15 K. The FTIR analysis showed no chemical changes occurred during their preparation while TGA and DSC analyses confirmed the stability of the DESs within the tested temperature range, which is favorable for their potential application in the industrial processes, such as extractions of different compounds, separations, chemical and biochemical technology.
SUPPLEMENTARY MATERIAL
Additional data and considerations are available electronically from
http://www.shd.org.rs/JSCS/, or from corresponding author on request.
Acknowledgements: This work has been funded by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Project assigned to the Faculty of Technology, Leskovac, University of Niš, researchers’ group III 45001, No. 451-03-68/2020-14/200133) and the Serbian Academy of Sciences and Arts (the project F-78).
ИЗВОД
ФИЗИЧКО-ХЕМИЈСКЕ И ТЕРМОДИНАМИЧКЕ ОСОБИНЕ ЕУТЕКТИЧКИХ РАСТВАРАЧА СА ТРИЕТАНОЛАМИНОМ
БИЉАНА С. ЂОРЂЕВИЋ1, ДРАГАН З. ТРОТЕР1, ВЛАДА Б. ВЕЉКОВИЋ1,3, МИРЈАНА Љ. КИЈЕВЧАНИН2, ИВОНА Р. РАДОВИЋ2 и ЗОРАН Б. ТОДОРОВИЋ1
1Технолошки факултет, Универзитет у Нишу, Булевар ослобођења 124, 16000 Лесковац; 2Технолошко металуршки факултет, Универзитет у Београду, Карнегијева 4, 11000 Београд; 3Српска академија науке
и уметности, Кнез Михајлова 35, 11000 Београд
У овом раду направљени су еутектички растварачи триетаноламинa sa оксалном киселином, глацијалном сирћетном киселином, млечном киселином, олеинском
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лином, глицеролом, етилен гликолом, пропилен гликолом, холин-хлоридом и 1,3-диме-тилуреом. Свим еутектичким растварачима одређене су физичко-хемијске особине (густина, вискозитет, електрична проводљивост и индекс рефракције) и израчунате термо-динамичке особине (коефицијент запреминског ширења, моларна запремина, енергија решетке и топлотни капацитет) у функцији од температуре у опсегу 293,15-363,15 К на 101,325 кPa. Код свих припремљених еутектичких растварача густина, вискозитет и индекс рефракције опадају са порастом температуре док електрична проводљивост расте. Ове температурне зависности вискозности и електричне проводљивости су описане Vogel-Tamman-Fulcher једначином. Вискозитет и моларна проводљивост, који показују линеарно понашање, били су у корелацији са фракционим Wаlden правилом. Инфрацрвена спектроскопија са Фуријеовом (Fourier) трансформацијом коришћена је за проучавање функционалних група еутектичких растварача, док су термогравиметријска анализа и диференцијална скенирајућа калориметрија коришћене за одређивање њихових стабилности. Утврђено је да анализирани еутектички растварачи поседују својства која им омогућавају примену у различите индустријске процесе попут екстракције, сепарације, хемијске технологије и биотехнологије.
(Примљено 25. априла; ревидирано 16. јула; прихваћено 27. августа 2020)
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S1
SUPPLEMENTARY MATERIAL TO
The physicochemical properties of the deep eutectic solvents
with triethanolamine as a major component
BILJANA S. ĐORĐEVIĆ1, DRAGAN Z. TROTER1, VLADA B. VELJKOVIĆ1,3, MIRJANA LJ. KIJEVČANIN2, IVONA R. RADOVIĆ2 and ZORAN B. TODOROVIĆ1
1University of Niš, Faculty of Technology, Bulevar Oslobođenja 124, 16000 Leskovac, Serbia; 2Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade,
Serbia; 3Serbian Academy of Sciences and Arts, Knez Mihailova 35, 11000 Belgrade, Serbia
TABLE S-I. Information on the used chemicals Chemical
name Abbreviation Formula
Molar mass, g·mol-1
Melting point, K
Chemical structure
CAS number
Triethanol-amine TEOA C6H15NO3 149.19 294.75 102-71-6
Oxalic acid OA C2H2O4 90.03 462 144-62-7
Glacial
acetic acid AA C2H4O2 60.05 289 64-19-7
L-(+)-Lactic
acid LA C3H6O3 90.08 291 50-21-5
Oleic acid OLA C18H34O2 282.47 286 112-80-1
Glycerol G C3H8O3 92.09 290.9 56-81-5
Ethylene
glycol EG C2H6O2 62.07 261.6 107-21-1
Propylene
glycol PEG C3H8O2 76.09 214 57-55-6
Choline
chloride ChCl C5H14ClNO 139.62 573.15 67-48-1
1,3-Dime-thylurea DMU C3H8N2O 88.11 377.5 96-31-1
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TABLE S-II.Prepared DESsa
DES Abbreviation Molar ratio Molar mass of DES*, g·mol-1
Water content (mass fraction) Triethanolamine :oxalic acid TEOA:OA 1:1 119.61 0.0007 Triethanolamine :acetic acid TEOA:AA 1:1 104.62 0.0006 Triethanolamine : L-(+)-lactic acid TEOA:LA 1:1 119.64 0.0004 Triethanolamine : oleic acid TEOA:OLA 1:1 215.83 0.0007 Triethanolamine : glycerol TEOA:G 1:2 110.93 0.0006 Triethanolamine : ethylene glycol TEOA:EG 1:2 90.82 0.0005 Triethanolamine :propylene glycol TEOA:PEG 1:2 100.21 0.0004 Choline chloride : triethanolamine ChCl:TEOA 1:2 146.03 0.0006 Triethanolamine :1,3-dimethylurea TEOA:DMU 1:2 108.27 0.0004 aHBA – as hydrogen bond acceptor, HBD – hydrogen bond donor. The molecular mass (M
DES) for
TEOA-based DESs is determined from Eq1:
HBA HBA HBD HBD DES
HBA HBD
x M x M
M
x x
+
= + ,
where MDES is the molecular mass of DES in g·mol−1, xHBA and MHBA are the mole ratio and molecular mass of the HBA in g·mol−1, respectively; x
HBD and MHBD are the mole ratio and molecular mass of the HBD in g·mol−1, in order.
TABLE S-III. The physicochemical properties of TEOA:OA DESa
T / K ρ / kg∙m-3 η / Pa∙s κ / S∙m-1 n
D
293.15 1362.2 0.7261 0.102 1.48016
303.15 1357.9 0.4869 0.120 1.47717
313.15 1350.8 0.2386 0.162 1.47391
323.15 1346.4 0.1088 0.251 1.47118
333.15 1342.4 0.0701 0.340 1.46816
343.15 1338.5 0.0441 0.467 1.46450
353.15 1334.7 0.0377 0.503 1.46219
363.15 1330.8 0.0363 0.564 1.45918
aUncertainties (u): (u)T = ±0.005 K; (u)ρ = ±0.5 kg∙m–3; (u) η = 5% of the measured value; (u)k = ± 0.0001 S∙m–1; (u)nD = ±0.00005.
TABLE S-IV. The physicochemical properties of TEOA:AA DESa
T / K ρ / kg∙m-3 η / Pa∙s κ / S∙m-1 n
D
293.15 1182.9 1.2054 0.0189 1.47778
303.15 1181.7 0.8143 0.0202 1.47585
313.15 1179.4 0.4114 0.029 1.47365
323.15 1176.6 0.1944 0.046 1.47171
333.15 1174.8 0.0991 0.086 1.46978
343.15 1173 0.0651 0.111 1.46769
353.15 1171.3 0.0534 0.159 1.46574
363.15 1168.9 0.0511 0.22 1.46385
aUncertainties (u): (u)T = ±0.005 K; (u)ρ = ±0.5 kg∙m–3; (u) η = 5% of the measured value; (u)k = ± 0.0001 S∙m–1; (u)nD = ±0.00005.
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TABLE S-V. The physicochemical properties of TEOA:LA DESa
T / K ρ / kg∙m-3 η / Pa∙s κ / S∙m-1 n
D
293.15 1265.4 1.7377 0.004 1.47999
303.15 1258.2 1.218 0.019 1.47874
313.15 1252.3 0.7572 0.026 1.47782
323.15 1244.6 0.2714 0.052 1.47647
333.15 1238.9 0.1922 0.095 1.47574
343.15 1230.2 0.1102 0.199 1.47482
353.15 1224.1 0.0406 0.356 1.47363
363.15 1216.9 0.0094 0.598 1.47291
aUncertainties (u): (u)T = ±0.005 K; (u)ρ = ±0.5 kg∙m–3; (u) η = 5 % of the measured value; (u)k = ± 0.0001 S∙m–1; (u)nD = ±0.00005.
TABLE S-VI. The physicochemical properties of TEOA:OLA DESa
T / K ρ / kg∙m-3 η / Pa∙s κ / S∙m-1 n
D
293.15 1114.9 39.76 0.00555 1.43984
303.15 1108.9 25.93 0.00682 1.43789
313.15 1102.9 12.64 0.0127 1.43372
323.15 1096.9 4.542 0.0327 1.42765
333.15 1090.9 0.1359 0.072 1.42499
343.15 1084.9 0.0949 0.121 1.41992
353.15 1080.1 0.0097 0.159 1.41785
363.15 1077.9 0.0046 0.1812 1.41374
aUncertainties (u): (u)T = ±0.005 K; (u)ρ = ±0.5 kg∙m–3; (u) η = 5 % of the measured value; (u)k = ± 0.0001 S∙m–1; (u)nD = ±0.00005.
TABLE S-VII. The physicochemical properties of TEOA:G DESa
T / K ρ / kg∙m-3 η / Pa∙s κ / S∙m-1 n
D
293.15 1249.2 0.3124 0.00056 1.46873
303.15 1242.7 0.1749 0.00089 1.46735
313.15 1234.4 0.0948 0.0013 1.46645
323.15 1230.9 0.0355 0.00251 1.46528
333.15 1224.2 0.0082 0.00446 1.46472
343.15 1218.7 0.0028 0.00667 1.46385
353.15 1212.5 0.0019 0.00824 1.46254
363.15 1206.7 0.0011 0.00945 1.46129
aUncertainties (u): (u)T = ±0.005 K; (u)ρ = ±0.5 kg∙m–3; (u) η = 5 % of the measured value; (u)k = ± 0.0001 S∙m
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TABLE S-VIII. The physicochemical properties of TEOA:EG DES.a
T / K ρ / kg∙m-3 η / Pa∙s κ / S∙m-1 n
D
293.15 1147.2 0.0795 0.00332 1.45585
303.15 1139.7 0.0542 0.00481 1.45536
313.15 1132.8 0.0369 0.00674 1.45472
323.15 1126.9 0.0238 0.00753 1.45421
333.15 1119.5 0.0112 0.00998 1.45387
343.15 1117.3 0.0056 0.01126 1.45325
353.15 1111.7 0.0025 0.01254 1.45282
363.15 1104.7 0.0008 0.01485 1.45234
aUncertainties (u): (u)T = ±0.005 K; (u)ρ = ±0.5 kg∙m–3; (u) η = 5 % of the measured value; (u)k = ± 0.0001 S∙m–1; (u)nD = ±0.00005.
TABLE S-IX. The physicochemical properties of TEOA:PG DESa
T / K ρ / kg∙m-3 η / Pa∙s κ / S∙m-1 n
D
293.15 1092.2 0.1027 0.00015 1.44928
303.15 1081.4 0.0657 0.0004 1.44869
313.15 1073.2 0.0386 0.00085 1.44788
323.15 1069.1 0.0138 0.00184 1.44599
333.15 1062.1 0.0086 0.00247 1.44501
343.15 1056.9 0.0048 0.00281 1.44422
353.15 1048.5 0.0023 0.0034 1.44356
363.15 1042.1 0.001 0.00699 1.44281
aUncertainties (u): (u)T = ±0.005 K; (u)ρ = ±0.5 kg∙m–3; (u) η = 5 % of the measured value; (u)k = ± 0.0001 S∙m–1; (u)nD = ±0.00005.
TABLE S-X. The physicochemical properties of TEOA:ChCl DESa
T / K ρ / kg∙m-3 η / Pa∙s κ / S∙m-1 n
D
293.15 1178.9 1.881 0.2196 1.4814
303.15 1173.8 0.704 0.301 1.4803
313.15 1168.6 0.501 0.5375 1.4789
323.15 1163.7 0.319 0.75 1.4781
333.15 1158.4 0.1742 1.56 1.4773
343.15 1153.3 0.0386 2.215 1.4764
353.15 1148.9 0.0272 4.773 1.4755
363.15 1145.2 0.0189 9.003 1.4743
aUncertainties (u): (u)T = ±0.005 K; (u)ρ = ±0.5 kg∙m–3; (u) η = 5 % of the measured value; (u)k = ± 0.0001 S∙m
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TABLE S-XI. The physicochemical properties of TEOA:DMU DESa
T / K ρ / kg∙m-3 η / Pa∙s κ / S∙m-1 n
D
293.15 1182.6 7.244 0.0018 1.4991
303.15 1173.4 2.623 0.0029 1.494
313.15 1163.7 1.1921 0.0041 1.4886
323.15 1153.2 0.3502 0.0093 1.4839
333.15 1143.3 0.0952 0.0197 1.478
343.15 1129.1 0.0232 0.0319 1.4745
353.15 1116.9 0.00677 0.0405 1.4687
363.15 1113.2 0.00289 0.0447 1.4642
aUncertainties (u): (u)T = ±0.005 K; (u)ρ = ±0.5 kg∙m–3; (u) η = 5 % of the measured value; (u)k = ± 0.0001 S∙m–1; (u)nD = ±0.00005.
Effect of temperature on density of DESs
The density of the tested DESs slightly decreases linearly which agrees with the previous reports.2,3-5 The density-temperature correlation can be outlined by a linear equation (1):
ρ = a + bT (1)
where ρ, T, a and b represent the density, the absolute temperature, the density at 0 K and the coefficient of volume expansion, respectively. The characteristic parameters a and b of Eq. (1), density ranges, mean relative percent deviation (MRPD) and the coefficient of determination (R2) are given in Table S-XII. A good linear dependence of density on temperature is confirmed by the low MRPD-values (<0.2 %) and the R2-values close to unity (>0.990).
TABLE S-XII.Parameters of Eq. (1) (293.15-363.15) K
DES Density range, kg·m-3 a / kg·m-3 b / kg·m-3·K-1 MRPD, % R2
TEOA:OA (1:1) 1362.2-1330.8 1492.6 -0.4485 0.07 0.990
TEOA:AA (1:1) 1182.9-1168.9 1242.9 -0.2036 0.08 0.995
TEOA:LA (1:1) 1265.4-1216.9 1468.7 -0.6929 0.04 0.999
TEOA:OLA (1:1) 1114.9-1077.9 1275.5 -0.5512 0.09 0.991
TEOA:G (1:2) 1249.2-1206.7 1423.6 -0.5980 0.05 0.997
TEOA:EG (1:2) 1147.2-1104.7 1316.9 -0.5850 0.10 0.990
TEOA:PEG (1:2) 1092.2-1042.1 1288.8 -0.6799 0.10 0.991 ChCl:TEOA (1:2) 1178.9-1145.2 1322.1 -0.4900 0.03 0.998
TEOA:DMU (1:2) 1182.6-1113.2 1491.5 -1.0500 0.15 0.993
Considering the values of the coefficient of volume expansion, the thermal sen-sitivity of these DESs is in the following order: TEOA:DMU ˃ TEOA:LA ˃ > TEOA:PEG ˃ TEOA:G ˃ TEOA:EG ˃ TEOA:OLA ˃ ChCl:TEOA ˃ > TEOA:OA ˃ TEOA:AA.
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The molecular volume (Vm), the lattice energy (Upot) and the heat capacity (Cp) were calculated by the means of Eq. (2), (3), (4), (5) and (6)2,6 and the values obtained at 313.15 K are listed in Table S-XIII.
DES m A M V N ρ
= (2)
where MDES is the molecular mass of DES in g·mol−1 and NA is Avogadro’s constant. 1/3
pot
DES
1981.2 ρ 103.8
U
M
= −
(3)
for TEOA:OA (1:1), TEOA:AA (1:1), TEOA:LA (1:1) and TEOA:OLA (1:1). 1/3
pot
DES
8375.6 ρ 178.8
U
M
= −
(4)
for TEOA:G (1:2), TEOA:EG (1:2), TEOA:PEG (1:2) and TEOA:DMU (1:2). 1/3
pot
DES
6763.3 ρ 365.4
U
M
= −
(5)
for ChCl:TEOA (1:2).
Cp = 1037Vm + 45 (6)
TABLE S-XIII. The values of molecular volume(Vm), lattice energy(Upot) and heat capacity (Cp)for the tested DESs at 313.15 K
DES Vm / nm3 Upot / kJ·mol-1 Cp / J·mol-1K-1
TEOA:OA (1:1) 0.147 548 197
TEOA:AA (1:1) 0.147 548 198
TEOA:LA (1:1) 0.159 537 209
TEOA:OLA (1:1) 0.325 445 382
TEOA:G (1:2) 0.149 1691 200
TEOA:EG (1:2) 0.133 1764 183
TEOA:PEG (1:2) 0.155 1667 206
ChCl:TEOA (1:2) 0.208 1718 260
TEOA:DMU (1:2) 0.154 1670 205
Effect of temperature on viscosity of DESs
The viscosity-temperature relationship of the analyzed DESs can be described by the VTF equation is more appropriate:7
η 0 0 exp B η η T T = − (7)
where T, η0, Bη, and T0 represent the absolute temperature, the adjustable parameter, the factor related to the activation energy and the so-called ideal glass-transition temperature, respectively. The values of these parameters, along with VTF equations, MRPD and R2 are shown in Table S-XIV.
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TABLE S-XIV. The parameters of the VTF equation, Eq. (7), for the tested DESs (293.15-363.15) K
DES η / Pa s range
Parameters of VTF eq.*
η0 / Pa s Bη/ K T0 / K MRPD, % R2
a B
TEOA:OA (1:1) 0.726-0.036 645.38 7.664 4.696·10-4 645.4 207 13.97 0.979
TEOA:AA (1:1) 1.205-0.051 995.25 8.773 1.548·10-4 995.3 184 29.61 0.977
TEOA:LA (1:1) 1.738-0.009 7161.3 23.983 3.840·10-11 7161.3 7 33.53 0.940
TEOA:OLA (1:1) 39.76-0.005 15048 46.791 4.774·10-21 15048 9 29.39 0.940
TEOA:G (1:2) 0.312-0.001 9343.1 32.746 6.006·10-15 9343.1 5 10.57 0.977
TEOA:EG (1:2) 0.080-0.001 6716.9 25.044 1.329·10-11 6716.9 8 7.80 0.941
TEOA:PEG (1:2) 0.103-0.001 7019.1 26.013 5.043·10-12 7019.1 11 4.29 0.985
ChCl:TEOA (1:2) 1.881-0.019 7134 23.681 5.194·10-11 7134 10 24.41 0.965
TEOA:DMU (1:2) 7.244-0.003 12191 39.425 7.550·10-18 12191 12 41.71 0.987 *ln η = a·(T-T0)-1- b
TABLE S-XV. The thermodynamic functions of activation of viscous flow, ΔH*, ΔS* and ΔG*, and R2 for the tested DESs at 313.15 K
DES Parameters of Eyring’s eq.* R2 ΔH* / kJ·mol-1 TΔS* / kJ·mol-1 ΔG* / kJ·mol-1
a B
TEOA:OA (1:1) 5003 5.143 0.960 41.59 13.34 28.20 TEOA:AA (1:1) 5355.9 5.774 0.962 44.52 15.03 29.49 TEOA:LA (1:1) 7427.1 11.921 0.930 61.75 31.04 30.71 TEOA:OLA (1:1) 15191.3 33.835 0.930 126.30 88.09 38.21 TEOA:G (1:2) 9354.9 20.364 0.972 77.78 53.02 24.76 TEOA:EG (1:2) 6707.3 12.734 0.930 55.76 33.15 22.61 TEOA:PEG (1:2) 6998.3 13.514 0.982 58.18 35.18 22.99 ChCl:TEOA (1:2) 7138.1 10.965 0.960 59.34 28.55 30.80 TEOA:DMU (1:2) 12252.8 27.011 0.985 101.87 70.32 31.54
*ln (ηV/hNA) = a·T-1 - b
The values of the molar enthalpy change of activation for the viscous flow are higher than the TΔS*values, implying that the energetic contribution corresponding to the molar enthalpy change of activation for the viscous flow is more important than the entropic contribution to the molar Gibbs energy change of activation. Effect of temperature on the electrical conductivity of DESs
Analogous to the viscosity, the electrical conductivity-temperature relationship is also described by the VTF equation.8
k 0
0
exp B k k
T T
=
− (8)
where κ0, Bk and T0 represent the preexponential factor, a factor correlated to the activation energy and the ideal glass-transition temperature, respectively. The fit-ting parameters of the VTF equation for the electrical conductivity of the tested DESs are summarized in Table S-XVI. The preexponential factor κ0 is correlated with the number of mobile charge carriers in the molecule. The highest κ0 value of ChCl:TEOA is due to its enhanced ion dissociation while the lowest κ0 of
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TEOA:PEG is explained by the formation of the polypropylene glycol that inhibits the ion mobility.
TABLE S-XVI. The parameters of the VTF equation, Eq. (7), for the electrical conductivity (κ) of the tested DESs (293.15-363.15) K
DES Parameters of VTF eq.*a b κ0 / S∙m-1 Bκ / K T0 / K MRPD, % R2
TEOA:OA (1:1) -1258.3 4.4946 89.5 1258.3 110 7.47 0.979 TEOA:AA (1:1) -4038.1 9.5925 14654.50 4038.1 11 3.72 0.975 TEOA:LA (1:1) -3282.3 12.1660 192143.90 3282.3 105 4.31 0.986 TEOA:OLA (1:1) -3621.5 11.0850 65186 3621.5 74 7.32 0.969 ChCl:TEOA (1:2) -5607.1 17.3960 35891103.20 5607.1 10 23.5 0.979 TEOA:DMU (1:2) -2390.9 6.5285 684.40 2390.9 109 3.89 0.979 TEOA:G (1:2) -1647.2 2.5920 13.40 1647.2 131 1.82 0.985 TEOA:EG (1:2) 219.03 -2.7029 0.08 219.03 220 0.58 0.995 TEOA:PEG (1:2) 310.9 -2.6100 0.07 310.9 243 2.11 0.983
*ln κ = a·(T-T0)-1 + b
Molar conductivity and viscosity relationship
The equation used for determining the molar conductivity (Λ) is: kM
Λ ρ
= (9)
where κ is the electrical conductivity, M is the molar mass and ρ is the density. The empirical VTF equation for the molar conductivity is as follows:
0
0
exp BΛ Λ Λ
T T
− =
− (10)
where Λ0, BΛ, T0 are the fitting parameters. Their values are given in Table S-XVII.
TABLE S-XVII. The parameters of VTF equation, Eq. (10), for the molar conductivity (Λ) of the tested DESs over the temperature range (293.15-363.15) K
DES Parameters of VTF eq.* Λ0 / S∙m2∙mol–1 BΛ/ K T0 / K MRPD,% R2
a b
TEOA:OA (1:1) -1285.8 -4.7336 0.009 1285.8 109 0.76 0.975 TEOA:AA (1:1) -4006.3 0.2439 1.276 4006.3 3 0.92 0.970 TEOA:LA (1:1) -3339.8 3.1104 22.430 3339.8 104 0.97 0.984 TEOA:OLA (1:1) -3566.3 2.5061 12.257 3566.3 77 1.78 0.964 TEOA:G (1:2) -1529.2 -6.9835 0.001 1529.2 139 0.68 0.984 TEOA:EG (1:2) -233.8 -12.0295 5.97·10–6 233.8 218 0.20 0.994 TEOA:PEG (1:2) -332.5 -11.7204 8.13·10–6 332.5 241 0.80 0.981 ChCl:TEOA (1:2) -5651.7 8.5519 5176.581 5651.7 1 1.93 0.971 TEOA:DMU (1:2) -2737.9 -1.8599 0.156 2737.9 96 1.19 0.972
*ln Λ = a·(T-T0)-1 + b
The Walden plot is useful for illustrating the conductivity-viscosity relationship for the pure ionic liquids.9 It describes the connection between the mobility of ions and the fluidity of their surrounding medium according to the following equation:10
Λη = k (11)
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where Λ is the molar conductivity, η is the viscosity and k are a temperature-dependent constant. Therefore, the Walden plot is used for classifying the ionic liquids as “good”, “poor”, “superionic”, etc.2 The logarithmic plot of Λ, representing the ion mobility, versus the fluidity φ (φ = η−1) is used for comparing the ions’ formation ability in non-aqueous electrolyte solutions, molten salts and ionic liquids.11 This so-called “fractional” Walden rule is written as follows:11
log Λ= log C+ α’log η−1 (12) where C is the Walden product, and α’ is the slope of the Walden plot line and reflects on the decoupling of the ions. The coefficients of the Walden equation for the DESs are given in Table S-XVIII.
TABLE S-XVIII.The coefficients of the Walden equation (12) along with MRPD and R2 for the tested DESs over the temperature range (293.15-363.15) K
DES Parameters of Walden equation α’ C MRPD, % R2
TEOA:OA (1:1) log Λ = 0.5722·log η-1 - 5.1623 0.5722 6.88·10–6 0.51 0.988
TEOA:AA (1:1) log Λ = 0.7397·log η-1 - 5.8138 0.7397 1.54·10–6 1.32 0.947
TEOA:LA (1:1) log Λ = 0.9027·log η-1- 5.8043 0.9027 1.57·10–6 3.21 0.897
TEOA:OLA (1:1) log Λ = 0.3836·log η-1 - 5.2010 0.3836 6.3·10–6 2.43 0.926
TEOA:G (1:2) log Λ = 0.4939·log η-1- 7.4646 0.4939 3.43·10–8 0.64 0.982
TEOA:EG (1:2) log Λ = 0.304·log η-1- 6.7715 0.304 1.69·10–7 1.06 0.878
TEOA:PEG (1:2) log Λ = 0.7439·log η-1- 8.3162 0.7439 4.83·10–9 1.80 0.900
ChCl:TEOA (1:2) log Λ = 0.7694·log η-1- 4.4159 0.7694 3.84·10–5 2.46 0.945
TEOA:DMU (1:2) log Λ = 0.443·log η-1 - 6.3386 0.4430 4.59·10–7 1.44 0.962 Effect of temperature on the refractive index of DESs
The parameters of the linear equations,12 refractive index ranges, MRPD and
R2 are listed in Table S-XIX.
TABLE S-XIX. The parameters of the refractive index (nD) equation for the tested DESs (293.15-363.15) K
DES nD range Intercept Slope MRPD, % R2
TEOA:OA (1:1) 1.4802-1.4592 1.5684 -0.0003 0.03 0.999 TEOA:AA (1:1) 1.4778-1.4639 1.5363 -0.0002 0.01 0.999 TEOA:LA (1:1) 1.4800-1.4729 1.5094 -0.0001 0.02 0.996 TEOA:OLA (1:1) 1.4398-1.4137 1.5547 -0.0004 0.25 0.991 TEOA:G (1:2) 1.4687-1.4613 1.4980 -0.0001 0.02 0.992 TEOA:EG (1:2) 1.4559-1.4523 1.4705 -0.0001 0.01 0.997 TEOA:PEG (1:2) 1.4493-1.4428 1.4783 -0.0001 0.03 0.979 ChCl:TEOA (1:2) 1.4814-1.4743 1.5098 -0.0001 0.05 0.994 TEOA:DMU (1:2) 1.4991-1.4642 1.6451 -0.0005 0.03 0.999
Table S-XX contains the ranges of the phase velocity (υ), the molar refractivity (A) and the free volume (fm) for the tested DESs, calculated as recommended elsewhere.1,13
The phase velocities are similar for all tested DESs, the highest being for the TEOA:OLA (1:1) DES due to its lowest refraction index in the applied temperature
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range. The molar refractivity, which is a measure of the polarizability of a substance, is mostly influenced by the molecular mass while the temperature, density and refraction index show a weak effect. A minor effect of the temperature on molar refractivity has already been reported.1 The free volume increases with heating, as expected.1 Among all tested DESs, TEOA:OLA (1:1) has the highest free volume due to the longest alkyl chain of oleic acid.1
At 313.15 K, the phase velocity of the tested DESs is in following order: TEOA:DMU < ChCl:TEOA < TEOA:LA < TEOA:OA < TEOA:AA < TEOA:G < < TEOA:EG < TEOA:PEG < TEOA:OLA.
TABLE S-XX. The phase velocity (υ), molar refractivity (A) and free volume (fm) ranges for the tested DESs (293.15-363.15) K.
DES υ / 107 m∙s-1 A / 10-6 m3 mol-1 f
m / 10-6 m3 mol-1
TEOA:OA (1:1) 20.27-20.56 24.57-24.95 62.29-6289 TEOA:AA (1:1) 24.69-25.03 49.38-50.05 64.81-63.42 TEOA:LA (1:1) 20.27-20.37 26-85-27-57 67.85-70.74 TEOA:OLA (1:1) 20.84-21.22 50.01-51.00 142.58-150.22
TEOA:G (1:2) 20.43-20.53 24.72-25.23 64.08-66.69 TEOA:EG (1:2) 20.61-20.66 21-51-22.19 57.65-60.02 TEOA:PEG (1:2) 20.70-20.79 35.27-35.85 88.59-91.66 ChCl:TEOA (1:2) 20.25-20.35 26.84-26.88 64.66-70.41 TEOA:DMU (1:2) 20.01-20.49 32.33-35-76 79.71-89.58
FTIR analysis
FTIR spectra of the DESs and their individual components are shown in Fig. S-1. The FTIR spectra of the tested DESs show a very strong and broad band at 3200-3500 cm-1 ascribed to ν(OH) stretching vibration, which confirms the existence of the hydrogen bonds in these mixtures. This band covers all bands belonging to the amine vibrations from TEOA and ChCl and has great intensity in every starting component of the DESs, except OLE, due to its long carbon chain that limits the formation of the intramolecular hydrogen bonds. The presence of ν(C-H) stretching bands at 2800-3000 cm-1 is obvious in the spectra of all DESs and their initial compounds,14 except OA. The band at 1550-1690 cm-1 in all spectra belongs to δ(OH) vibrations, overlapping the δ(NH3+) bands at 1660 and 1646 cm-1 in the spectra of TEOA and ChCl. The bands at 1403-1419 cm-1 and 1000-1117 cm-1 derived from δ(C-H) and ν(C-O) vibrations, respectively, are present in all spectra. The ν(C-N) bands at 1358,1350 and 1340 cm-1 present in the spectra of TEOA, ChCl and DMU are also present in all DESs that contain these components. In the spectra of AA, OA, LA and OLE, as well as of their DESs the ν(C=O) bands are at 1710-1730 cm-1.14 The FTIR analysis proved both the presence of the hydrogen bonds in these DESs and the characteristic functional groups of their constituents, showing no chemical changes (reactions) that occurred during their preparation.
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Fig. S-1. FTIR spectra of the DESs and their individual components at 25 °C in the region of 400-4000 cm-1.
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TGA and DSC analyses
The TGA and DSC curves of the selected TEOA- and ChCl-based DESs with the same donors are shown in Fig. S-2 abd S-3. As can be seen in Fig. S-2 and S-3, the TGA and DSC curves do not show any surprising behavior of the samples since no characteristic peaks are present, except the certain mass loss evident from the TGA curves, within which one part refers certainly to water evaporation, explained by the higher upper limit of the temperature range.
Fig. S-2. TGA and DSC curves recorded for: (a) TEOA:G, (b) TEOA:EG, (c) TEOA:PEG, (d) ChCl:TEOA 298.15
-
373.15 and 101.325 kPa. Note: – denotes TGA curve, –·– denotes thefirst derivative of the TGA function, – – denotes DSC curve and
–··– denotes the first derivative of DSC function.
Fig. S-3. GA and DSC curves recorded for: (e) ChCl:G, (f) ChCl:EG, (g) ChCl:PEG, at temperature range 298.15
-
373.15 K for (e) and 298.15-
343.15 K for (f) and (g) and 101.325 kPa. Note: – denotes TGA curve, –·– denotes the first derivative of the TGA function, – – denotes DSC curve and –··– denotes the first derivative of DSC function.On
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