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Synthesis of citric acid monohydrate-choline chloride based deep eutectic solvents (DES) and characterization of their physicochemical properties
Journal of Molecular Liquids 288 (2019) 111081
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Synthesis of citric acid monohydrate-choline chloride based deep eutectic solvents (DES) and characterization of their physicochemical properties Muhammad Hakimin Shafie a, Rizana Yusof b, Chee-Yuen Gan a,⁎ a b
Analytical Biochemistry Research Centre (ABrC), Universiti Sains Malaysia, 11800 USM, Penang, Malaysia Department of Chemistry, Faculty of Applied Sciences, Universiti Teknologi MARA, Perlis Branch, 02600 Arau, Perlis, Malaysia
a r t i c l e
i n f o
Article history: Received 15 March 2019 Received in revised form 7 May 2019 Accepted 27 May 2019 Available online 29 May 2019 Keywords: Deep eutectic solvent Choline chloride Citric acid monohydrate Synthesis Characterization
a b s t r a c t The objective of the study was to synthesize deep eutectic solvents (DES) using choline chloride and citric acid monohydrate at different molar ratios (i.e. DES 3:1, 2:1, 1:1, 1:2 and 1:3) followed by physicochemical characterization. The DES produced were clear and homogenized liquids under a microscopic observation, and DES 1:1 was found to meet the eutectic point with the lowest melting temperature. Fourier-transform infrared spectroscopy (FTIR) analysis showed the hydrogen bond interaction between the components in DES where the wavenumbers of functional groups were shifted. Results also showed that when the molar ratio of citric acid monohydrate increased in DES, higher values of viscosity, surface tension and density were observed. It was also found that the DES produced were classified as hydrophilic DES because of their solubility in water, highly polar and most semi polar solvents. In summary, the DES synthesized was suggested to be used as a medium for the extraction of hydrophilic components from plant or animal materials. © 2019 Elsevier B.V. All rights reserved.
1. Introduction The deep eutectic solvents (DES) are another class of ionic liquids. The term “deep eutectic solvent” refers to liquids near to the eutectic composition of two or more compound mixtures. The formation of deep eutectic solvents (DES) must be at least two compounds that consist of quaternary ammonium salts and hydrogen bond donor (i.e. amides, amines, alcohols, and carboxylic acids) in certain ratios. The DES are produced when the mixtures arise at the eutectic point which has a low melting point. The DES was introduced in 2003 by Abbott et al. [1] on their successful to synthesize the eutectic mixture of choline chloride with urea at molar ratio 1:2. The DES can be classified into four types and the Type III of DES has the most applications due to their low cost and low eco-toxicity. This system consists of a cation and an anion from quaternary ammonium salt (i.e. choline chloride) and hydrogen bond donating species from a hydrogen bond donor (i.e. citric acid monohydrate). DES are structurally different from ionic liquids because of DES are not only have cations and anions but also non-ionic species. The quaternary ammonium salt (e.g. choline chloride) consists of choline cation and chloride anion. Meanwhile, the hydrogen bond donor, such as citric acid monohydrate, is the source of non-ionic ⁎ Corresponding author. E-mail address: [email protected] (C.-Y. Gan).
https://doi.org/10.1016/j.molliq.2019.111081 0167-7322/© 2019 Elsevier B.V. All rights reserved.
species. According to Ghaedi et al. [2], DES are easy to prepare, nontoxic, and biodegradability. In addition, the authors also reported that DES are now a class of solvents that possess desirable characteristics such as a wide liquid range, lower melting point than the constituents of the mixture, nonreactive with water, non-volatile, thermally stable, highly conductive, and cheaper price compare to ionic liquid. In view of these unique features, DES are used for a wide range of application such as metal oxide processing [3], biodiesel purification [4], extractions [5], electrochemistry [6], nanomaterials [7], carbon dioxide absorption [8], organic synthesis [9], biochemistry [10], biocatalysis [11], polymer synthesis [12], sulphur dioxide absorption [13], nitric oxide absorption [14], and ammonia absorption [15]. In this work, we have used choline chloride as the quaternary ammonium salt and citric acid monohydrate as hydrogen bond donors. In addition, their molar ratios play a key role in controlling the melting point of the DES. It is similar to viscosity, density, and surface tension of DES, is strongly influenced by the atomic structure of its components, the molar ratio by which it is prepared, the operating temperature, and the water content. The ionic size of components, void volume and interaction forces between components, such as electrostatic and Van der Waals, all significantly affected the properties of DES [16]. In this study, DES were prepared at five different molar ratios of 3:1, 2:1, 1:1, 1:2 and 1:3 by mixing of choline chloride and citric acid monohydrate. The aim of this work has been to investigate the physicochemical
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2. Materials and methods
pure DES could not be detected. The force tensiometer was equipped with the Attension sigma software for data collection and automatic control. The Wilhelmy plate was cleaned by flaming before beginning each measurement. The surface tensions of the DES were measured at 25 °C and for each measurement three replicates were conducted.
2.1. Chemicals
2.8. Density measurement
Choline chloride (purity N99%: C5H14ClNO) was obtained from Acros Organics and citric acid monohydrate (purity N99.5%: C6H8O7.H2O) was purchased from Fluka. All chemicals used in this study were of analytical grade and used as received. Choline chloride was kept in a desiccator to prevent moisture absorption.
The analysis was carried out according to a simple calculation of density by pouring the DES into 10 mL of volumetric flask until calibrated mark. The measurement was performed by measure the weight of the DES in a volumetric flask at 25 °C. The density was calculated as follows:
properties (i.e. functional group, melting point, viscosity, surface tension, density and solubility) of DES affected by using different molar ratio of choline chloride and citric acid monohydrate.
2.2. Synthesis of deep eutectic solvents (DES) The preparation of deep eutectic solvents (DES) was adopted from Abbott et al. [1] by mixing of the quaternary ammonium salt (choline chloride) and hydrogen bond donor (citric acid monohydrate) at different molar ratios (i.e. 3:1, 2:1, 1:1, 1:2, and 1:3). The mixture of compounds was stirred at 80 °C until a colorless liquid was formed, and maintained as a liquid after leaving about 24 h at room temperature. The DES were then stored in a desiccator before various characterizations were carried out to prevent moisture absorption. 2.3. Polarized light microscopy (POM) analysis A droplet of the DES was dropped in a microscope slide for observation at a magnification of 10×. Visual characterization of the DES was carried out at 25 °C by POM using the Olympus transmission microscope (SZX7, Olympus, Japan) coupled with a Lumenera Infinity camera and Infinity Analyzer software.
Density ðρÞ ¼
m V
ð1Þ
where m was the weight (g) of the DES in volumetric flask and V was the volume of the DES (mL). 2.9. Solubility measurements DES were initially added to the test tubes and subsequently marked at the level. Different solvents (i.e. hexane, diethyl ether, dichloromethane, butanol, chloroform, toluene, methanol, ethanol, acetic acid, dimethyl sulfoxide and water) were then added into the test tubes. The mixtures were vortexed at 25 °C and the solubility was determined by observing the separation of the DES and organic solvent. The presence of separate layer at the marked sign indicated that the DES was non-miscible with the added solvent. If the layer formed above the marked sign indicated that they were partially miscible whereas no layer formed indicated that the DES and organic solvent were completely miscible. 2.10. Statistical analysis
2.4. Fourier transform infrared (FTIR) spectroscopy analysis The attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy (Fortier, PerkinElmer, USA) was used in FTIR analysis to determine the functional groups that presence in the DES and to disclose the interaction between choline chloride with citric acid monohydrate. Each sample was scanned for 16 times in the range of 600 to 4000 cm−1.
An average of three replicates of the analyses was taken for each DES and analyzed in a one-way analysis of variance (ANOVA). The statistical analysis of the data was performed using SPSS statistical 24.0 software, and significant difference among the treatment at a 95% confidence interval means was analyzed using Duncan's multiple range test. 3. Results and discussion
2.5. Differential scanning calorimetry (DSC) analysis
3.1. POM analysis
The melting point of the DES were analyzed using a DSC (DSC 6, Perkin Elmer, USA) under a constant stream of nitrogen at a flow rate 40 mL/min. All the DES were tightly sealed in aluminum pans. These analyses were carried out in the temperature ranging from −50 to 300 °C at a heating rate of 5 °C/min to obtain the melting point.
Table 1 are summarized the appearance of the synthesized DES at various ratios. The formation of clear liquid mixtures was observed using POM imaging (Fig. 1). The POM imaging shows no residues or crystals were formed. In addition, the DES were found to be stable as a clear and viscous liquid over the course of time. A research conducted by Aroso et al. [17] reported that the successful synthesized DES for choline chloride-tartaric acid at molar ratios of 2:1, 1:1 and 1:2 were confirmed using the same approach and the mixtures did not contain any crystals.
2.6. Viscosity measurements The viscosity of the DES at three different temperatures (i.e. 25, 35, and 45 °C) were measured using rheometer (AR 1000-N, TA instruments, USA) fitted with parallel plate geometry with 20 mm of diameter and gap 1 mm. The measurements were performed by equilibrating the sample temperature for 2 min and applying shear rate 1 s−1 for 30 s. The thixotropic properties of DES were measured by imposing a continuous shear rate ramp cycle from 0.1 to 450 s−1 and decreasing back to 0.1 s−1. All the samples were analyzed in three replicates. 2.7. Surface tension analysis The surface tension measurements of all DES were carried out using a force tensiometer (Sigma 700, Attension, Sweden) and the DES were diluted to 90% (w/v) prior analysis because the surface tensiom for the
3.2. FTIR analysis FTIR analysis is widely used to study the interaction of molecules and is an excellent method to characterize a sample by confirming the presence of functional groups of atoms or bonds within molecules. Hence, the formation of a new eutectic DES through hydrogen bonding between the chloride ion and citric acid monohydrate can be determined as well as formation of all the functional groups in a DES. The details on the FTIR wavenumbers for all the synthesized DES are listed in Table 2. According to a study by Alomar et al. [18] on glycerol-based deep eutectic solvents, the bands between 3200 and 3600 cm−1 was indicating the presence of O\\H stretching and for choline chloride based DES, whereas N\\H stretching was detected to be overlapped with O\\H between 3000
M.H. Shafie et al. / Journal of Molecular Liquids 288 (2019) 111081 Table 1 The appearance of DES with different molar ratio at 25 °C. Molar ratio of DES
Abbreviation
Appearance
Observation After heating
After 24 h
3:1
DES 3:1
Liquid
Liquid
2:1
DES 2:1
Liquid
Liquid
1:1
DES 1:1
Liquid
Liquid
3
DES 3:1 (3298.76 cm−1) and DES 2:1 (3295.23 cm−1) which caused by more chlorine ion (Cl−) in choline chloride that induced stronger hydrogen bond interactions with hydroxyl group in citric acid monohydrate. However, the wavenumber shifted to lower values for DES 1:1, DES 1:2 and DES 1:3 (2935.33 cm−1, 2941.74 cm−1, and 2945.95 cm−1, respectively). This can be concluded that the formation of hydrogen bonds interactions in the DES occurred. During the formation of the DES, the hydroxyl groups (O\\H) on citric acid monohydrate was attracted to chlorine anion (Cl−) in choline chloride hence producing an O-H—Cl bond. The interactions were expected to be dissimilar with the variations of molar ratios of both choline chloride and citric acid monohydrate. This could be due to the fact that the presence of quaternary ammonium salt and hydrogen bond donor forms the interaction of hydrogen bonds (in particular with the hydroxyl functional groups), and thus changed the strength of the O\\H bond. It was also observed that the C_O bond and C\\O stretching shifted to the lower wavenumber with increasing the molar ratio of citric acid monohydrate. However, the CH3 bending is shifted to higher wavenumber with the addition of citric acid monohydrate molar ratio. This might be also affected by the hydrogen bond interaction formation within the DES. A similar result was reported by Gajardo-Parra et al. [20] that the hydrogen bond interactions are noticeable in the FTIR spectrum of choline chloride-ethylene glycol DES at molar ratio 1:2, where the wavenumber are shifted to be higher when ethylene glycol are mixed with choline chloride. Hence, it can be seen that the hydrogen bond between eutectic mixtures were affected by the changing or increasing the molar ratio of citric acid monohydrate as a hydrogen bond donor. 3.3. DSC analysis
1:2
DES 1:2
Liquid
Liquid
1:3
DES 1:3
Liquid
Liquid
and 3400 cm−1. The peaks around 1400–1485 cm−1 were indicating the presence of CH2 bending whereas the presence of CH3 bending at peaks around 1355–1395 cm−1 [19]. The authors also stated that the saturated ester stretching bonds was exhibited at 1100–1300 cm−1. The analysis of both choline chloride and citric acid monohydrate were required to investigate the structural changes and interaction that occurred upon the successful formation of DES. Table 2 shows the shifts in the representative bands of the involved bonds in the FTIR spectra. In choline chloride spectrum, a broad band at 3224.58 cm−1 was indicating the presence of O\\H stretching. The CH2 bending, CH3 bending and CN stretching were indicated by the bands at 1480.45 cm−1, 1349.40 cm−1 and 1083.62 cm−1, respectively. In the spectrum of citric acid monohydrate, a broad band at 3004.42 cm−1 was representing the O\\H in carboxylic group. The C_O ester, CH2 bending and C-OH stretching was indicated at 1714.99 cm−1, 1400.19 cm−1 and 1204.30 cm−1, respectively. Refer to Table 2, the shift of O\\H stretching was observed after the formation of the DES to higher wavenumber for
The melting temperature of DES was investigated and the results were shown in Table 3. The melting point of DES is referred as the temperature that disrupts the intermolecular interaction between the cation and anion in choline chloride as well as the hydrogen bonds with citric acid monohydrate. The melting temperatures of choline chloride and citric acid monohydrate were dominated by the presence of endothermic peaks at 216.70 °C and 193.45 °C, respectively. As compared to Schilling et al. [21] Wyrzykowski et al. [22], who reported that the melting temperature of citric acid monohydrate were 158 °C and 160 °C, respectively, the melting temperature of citric acid monohydrate was found to be higher due to the formation thermodynamically stable complexes of citric acid monohydrate with aluminum in the DSC pan, in which tridentate ligand formation occurred [23,24]. Apart from that, it could also be seen that the melting temperatures of DES were significantly (p b 0.05) lower than individual component (i.e. citric acid monohydrate or chlorine chloride). It was suggested that the DES obtained by the complexation of a quaternary ammonium salt and a hydrogen bond donor, therefore, altering the properties of the choline chloride and citric acid monohydrate. This finding was in agreement with Zdanowicz et al. [25] and Chemat et al. [26] who reported that DES exhibited a lower melting temperature than their individual components. The increment of choline chloride in the molar ratio of DES gave a higher melting temperature. This can be explained by the more chlorine ion (Cl−) in choline chloride induced stronger hydrogen bond interactions within DES as aforementioned in Section 3.2, thus resulting more energy were required to break the interactions formed. The results showed significantly effect by decreasing molar ratio of choline chloride and increasing the molar ratio of citric acid monohydrate. The melting temperatures of DES were continuously increased with increasing the composition of citric acid monohydrate except for DES 1:1, which exhibit the lowest melting temperature [19]. Thus, this indicated that the eutectic point was 159.55 °C at molar ratio of 1:1. Commonly, the eutectic point was found at molar ratio of 1:2 or 2:1. For examples, the research conducted by Abbott et al. [1] and Hayyan et al. [27] reported that the eutectic point of choline chloride-urea and choline chloride-glucose exhibits the eutectic point at a molar ratio of 1:2 and
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Fig. 1. POM imaging of (a) DES 3:1, (b) DES 2:1, (c) DES 1:1, (d) DES 1:2, and (e) DES 1:3 at a magnification 10×.
2:1, respectively. In our study, eutectic point was however at molar ratio of 1:1. This might be due to the interaction between chlorine anion (Cl−) and the hydroxyl group (OH) was more favorable to allow one choline chloride molecule as well as one citric acid monohydrate molecule to form hydrogen bonds. 3.4. Viscosity The viscosity of DES was useful to understanding the internal resistance of a fluid to a shear stress as well as the nature of intermolecular
Table 3 The melting temperature of DES.
Table 2 Band assignments and wavenumber exhibited by DES. Molar ratio of DES
ChCl CA DES 3:1 DES 2:1 DES 1:1 DES 1:2 DES 1:3
Wavenumber (cm−1) O-H stretching 3224.58 3004.42 3298.76 3295.23 2935.33 2941.74 2945.95
interactions within choline chloride and citric acid monohydrate. The measurement at different molar ratio and temperature are significant to relate the fluidity of DES with composition and heat, respectively. Based on Table 4, DES 1:3 was recorded as the most viscous DES (1742.67 Pa.s) at 25 °C among the synthesized DES due to the existence of more hydroxyl groups in citric acid monohydrate. The result shows the viscosity of DES increased with an increasing the molar ratio of citric acid monohydrate. The presence of more citric acid monohydrate causes more hydrogen bond interactions arise within DES, which increasing the attractive force and decreasing the free volume of DES. Therefore, the
C=O bond
CH2 bending
CH3 bending
C-O stretching
C-N stretching
ND 1714.99 1720.07 1719.85 1718.97 1709.88 1708.78
1480.45 1400.19 1477.04 1477.11 1477.04 1476.68 1476.67
1349.40 ND 1381.26 1383.62 1384.19 1388.23 1389.05
ND 1204.30 1176.45 1175.15 1174.35 1174.67 1175.67
1083.62 ND 1081.15 1080.45 1079.64 1078.44 1078.73
ND = Not detected. Note: ChCl was choline chloride, CA was citric acid monohydrate, DES 3:1, DES 2:1, DES 1:1, DES 1:2, and DES 1:3 were DES at molar ratio 3:1, 2:1, 1:1, 1:2, and 1:3 respectively.
Sample
Melting temperature (°C)
ChCl CA DES 3:1 DES 2:1 DES 1:1 DES 1:2 DES 1:3
216.70 ± 7.39f 193.45 ± 8.27e 189.97 ± 5.27de 182.31 ± 1.39d 159.55 ± 7.41a 165.04 ± 1.12b 173.80 ± 2.23c
Note: ChCl was choline chloride, CA was citric acid monohydrate, DES 3:1, DES 2:1, DES 1:1, DES 1:2, and DES 1:3 were DES at molar ratio 3:1, 2:1, 1:1, 1:2, and 1:3 respectively. Data reported are average values with error bars indicating the standard deviation of 3 replicates (n = 3). Values with different letters are significant different (p b 0.05) according Duncan's multiple test.
M.H. Shafie et al. / Journal of Molecular Liquids 288 (2019) 111081 Table 4 The viscosity of DES at different temperature. Sample
DES 3:1 DES 2:1 DES 1:1 DES 1:2 DES 1:3
Temperature (°C) 25
35
45
35.79 ± 0.98a 53.39 ± 2.08a 131.00 ± 16.65b 1083.00 ± 34.83c 1742.67 ± 52.08d
14.20 ± 1.96a 22.31 ± 2.67a 58.80 ± 0.20b 208.37 ± 21.51c 273.23 ± 14.68d
7.27 ± 0.23a 10.95 ± 0.07a 19.58 ± 0.89b 49.38 ± 2.31c 54.16 ± 6.60c
Note: DES 3:1, DES 2:1, DES 1:1, DES 1:2, and DES 1:3 were DES at molar ratio 3:1, 2:1, 1:1, 1:2, and 1:3 respectively. Data reported are average values with error bars indicating the standard deviation of 3 replicates (n = 3). Values with different letters are significant different (p b 0.05) according Duncan's multiple test.
higher viscosity was obtained from DES 1:3. A similar result was reported by Chemat et al. [26], which the density of synthesized DES increased (748.09 to 5532.80 mPa.s) with the increasing the molar ratio of Larginine (0 to 0.20) at 25 °C. In addition, the effect of the temperature was recorded and presented in Table 4. As the viscosity of DES significantly (p b 0.05) decreased from 25 °C to 45 °C for each molar ratio of DES from 35.79 to 7.27 Pa.s, 53.39 to 7.95 Pa.s, 131.00 to 19.58 Pa.s, 1083.00 to 49.38 Pa.s, and 1742.67 to 54.16 Pa.s for DES 3:1, DES 2:1, DES 1:1, DES 1:2 and DES 1:3, respectively. As mentioned by Ghaedi et al. [2], the increasing temperature were inducing a lower viscosity of DES because of the internal resistance within DES decreases at a higher temperature, resulted the DES were less viscous and flow easily. Zhang et al. [8] reported that the viscosity of the DES from the mixture of choline chloride and urea at molar ratio 1:2 has decreased from 0.75 to 0.17 Pa.s with the increasing of temperature from 25 °C to 40 °C, respectively. 3.5. Surface tension The surface tension was defined as the required energy to increase the surface area of a liquid by a specific amount. The measurement of surface tension was important for interpretation of the energy required to increase the surface area of liquid as well as the intermolecular force intensity between the choline chloride and citric acid monohydrate in DES. According to Ghaedi et al. [2], the discontinuity of the electrostatic forces on the surface caused by the intensity of interaction molecules in DES. Fig. 2 shows the surface tension values for DES 3:1, DES 2:1, DES 1:1, DES 1:2 and DES 1:3 were 37.72, 40.87, 41.04, 41.36 and 41.58 mN/m, respectively. The highest surface tension was exhibited by DES 1:2 and DES 1:3. The surface tension increased continuously with increasing the molar ratio of citric acid monohydrate. The temperature, interaction between quaternary ammonium salt and hydrogen bond donor, alkyl chain, viscosity and molecular weight are the main factors affecting the surface tension of DES [26]. The more hydrogen bonding interaction within DES associates the formation of cohesive forces. Hence, the cohesive forces occurred, causing the surface to produce stronger mutual
Fig. 2. The surface tension of (a) DES 3:1, (b) DES 2:1, (c) DES 1:1, (d) DES 1:2, and (e) DES 1:3.
5
attractive forces to resist external forces and arises the surface molecules to resist surface breakage. This can be explained by the increasing the molar ratio of citric acid monohydrate as a hydrogen bond donor in DES induced more hydrogen bonding interaction formed in DES as aforementioned. A similar result was reported by Mjalli et al. [28], which the surface tension of choline chloride-monoethanolamine DES increased (from 67.04 to 72.07 mN/m) with increasing the molar ratio (from 5 to 8) of monoethanolamine as a hydrogen bond donor. 3.6. Density The type of the quaternary ammonium salt and a hydrogen bond donor as well as the molar ratio was affecting the density of DES. The measurements of the DES densities are presented in Fig. 3. It was noted that DES 3:1 (2.64 g/mL) has the lowest density followed by DES 2:1, DES 1:1, DES 1:2 and finally DES 1:3 with the highest density. The density of DES increased dramatically from 2.64 to 3.11 g/mL when the molar ratio of citric acid monohydrate increased. The increasing the molar ratio of citric acid monohydrate contributed to a reduction in the free space between the DES, thus increasing the density. This result was similar to the study by Singh et al. [29] which reported that the amount of hydrogen bond donor increased in DES and resulted in a higher density. Furthermore, the densities of DES were significantly decreased with the increment of the molar ratio of choline chloride. This could be explained by the interaction of hydroxyl groups in citric acid monohydrate complexed with chlorine anion (Cl−) was induced with the addition of citric acid monohydrate molar ratio. It was also responsible for the size of the formed DES. The packing structure and density of DES were also affected. The significantly (p b 0.05) increase in the density of DES, with increasing ratios of citric acid monohydrate, indicating the complexation that occurred in DES. It was due to more hydroxyl groups of citric acid monohydrate to make the interaction with the chlorine ion (Cl−) in choline chloride, which caused more hydrogen bond interaction between the anion of choline chloride and the hydroxyl groups in citric acid monohydrate. Different sizes of complexes were formed and changed the density of DES. According to Abbott et al. [30], the “hole” theory can be applied to explain the density of DES, which depends on the packing or the molecular structure of the DES. When the choline chloride and citric acid monohydrate were mixed resulted the average hole radius decreases, thus increasing the density of DES. The results obtained were proved that the complexation of DES was strongly affected by the molar ratio of citric acid monohydrate. 3.7. Solubility The solubility of DES in various organic solvents with different polarity was investigated to evaluate the ability of DES to solubilize a solute. The unique feature of DES with the existence of not only cation and anion but also non-ionic species has improved their ability to dissolve in a wide range of solvents. Table 5 demonstrates each molar ratio of
Fig. 3. The density of (a) DES 3:1, (b) DES 2:1, (c) DES 1:1, (d) DES 1:2, and (e) DES 1:3.
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Table 5 The solubility of DES. Solvent Hexane Toluene Diethyl ether Dichloromethane Butanol Chloroform Methanol Ethanol Acetic acid Dimethyl sulfoxide Water
DES 3:1
DES 2:1
DES 1:1
DES 1:2
DES 1:3
nm nm nm nm nm nm m m m m m
nm nm nm nm nm nm m m m m m
nm nm nm nm nm nm m m m m m
nm nm nm nm nm nm m m m m m
nm nm nm nm nm nm m m m m m
m = miscible; nm = nonmiscible. Note: DES 3:1, DES 2:1, DES 1:1, DES 1:2, and DES 1:3 were DES at molar ratio 3:1, 2:1, 1:1, 1:2, and 1:3 respectively.
DES were soluble (miscible) in water, highly polar and most semi polar solvents. Therefore, the results proved that the DES produced could be classified as hydrophilic DES due to the hygroscopic behavior of choline chloride and high polarity of citric acid monohydrate. Dai et al. [31] stated that the polarity of DES was strongly affecting its capability and ability to solubilize solute or any constituents. As reported by Malaeke et al. [28], the lignin exhibited higher solubility about 48.15% (w/w) in DES of choline chloride-resorcinol at the molar ratio of 1:1. This result suggested that the ionic fragments of DES will not only be able to diffuse into a compacted structure of a matrix, but also can interact favorably, change the intermolecular interaction and finally disrupt highly complex compound interactions [32]. Therefore, the synthesized DES could be used as a medium for extraction of hydrophilic biopolymer and bioactive compounds.
4. Conclusions The DES were successfully produced using choline chloride and citric acid monohydrate. All the synthesized DES were in liquid form at room temperature. The DES with molar ratio of 1:1 was found to meet the eutectic point, which had the lowest melting temperature. The different molar ratios of choline chloride and citric acid monohydrate had significantly affected the physicochemical properties of the DES. Results showed that the addition of citric acid monohydrate molar ratio gave a higher value of viscosity, surface tension and density. However, the DES with higher choline chloride molar ratio showed a high melting temperature. Meanwhile, all the DES were soluble in water, highly polar and semi polar organic solvents. Therefore, it was suggested that this physicochemical study will be useful for the development of acidbased DES application in a broad range of industrial applications, such as the extraction medium for hydrophilic components from plant or animal materials.
Acknowledgement The authors would like to acknowledge the financial support from Universiti Sains Malaysia RUI Grant (Grant number: 1001/CABR/ 8011045). The authors would also like to acknowledge the financial support from USM Fellowship. References [1] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, V. Tambyrajah, Novel solvent properties of choline chloride/urea mixtures, Chem. Commun. (2003) 70–71. [2] H. Ghaedi, M. Ayoub, S. Su, A.M. Shariff, B. Lal, The study on temperature dependence of viscosity and surface tension of several phosphonium-based deep eutectic solvents, J. Mol. Liq. 241 (2017) 500–510.
[3] A.P. Abbott, G. Capper, D.L. Davies, K.J. Mckenzie, S.U. Obi, Solubility of metal oxides in deep eutectic solvents based on choline chloride, J. Chem. Eng. 51 (2006) 1280–1282. [4] K. Shahbaz, F.S. Mjalli, M.A. Hashim, I.M. AlNashef, Using deep eutectic solvents for the removal of glycerol from palm-oil based biodiesel, J. Od Appl. Sci. 10 (2010) 3354. [5] S.P. Saravana, Y. Cho, H.-C. Woo, B. Chun, Green and efficient extraction of polysaccharides from brown seaweed by adding deep eutectic solvent in subcritical water hydrolysis, J. Clean. Prod. 198 (2018) 1474–1484. [6] Q. Xu, Y.N. Ji, L.Y. Qin, P.K. Leung, A.A. Shah, Y.S. Li, H.N. Su, H.M. Li, Effect of carbon dioxide additive on the characteristics of a deep eutectic solvent (DES) electrolyte for non-aqueous redox flow batteries, Chem. Phys. Lett. 708 (2018) 48–53. [7] K. Xu, Y. Wang, X. Ding, Y. Huang, N. Li, Q. Wen, Magnetic solid-phase extraction of protein with deep eutectic solvent immobilized magnetic graphene oxide nanoparticles, Talanta 148 (2016) 153–162. [8] Q. Zhang, K.D.O. Vigier, R. Sebastien, F. Jerome, Deep eutectic solvents: syntheses, properties and application, Chem. Soc. Rev. 41 (2012) 7108–7146. [9] L. Hu, L. Chen, Y. Fang, A. Wang, C. Chen, Z. Yan, Facile synthesis of zeolitic imidazolate framework-8 (ZIF-8) by forming imidazole-based deep eutectic solvent, Microporous Mesoporous Mater. 268 (2018) 207–215. [10] N. Li, Y. Wang, K. Xu, Y. Huang, Q. Wen, X. Ding, Development of green betainebased deep eutectic solvent aqueous two-phase system for the extraction of protein, Talanta 152 (2016) 23–32. [11] V. Gotor-fernández, C.E. Paul, Deep eutectic solvents for redox biocatalysis, J. Biotechnol. 293 (2019) 24–35. [12] P.F. Pereira, C.T. Andrade, Optimized pH-responsive film based on a eutectic mixture-plasticized chitosan, Carbohydr. Polym. 165 (2017) 238–246. [13] Y. Chen, B. Jiang, H. Dou, L. Zhang, X. Tantai, Y. Sun, H. Zhang, Highly efficient and reversible capture of low partial pressure SO2 by functional deep eutectic solvents, Energy Fuel 32 (2018) 10737–10744. [14] J. Dou, Y. Zhao, F. Yin, H. Li, J. Yu, Mechanistic study of selective absorption of NO in flue gas using EG-TBAB deep eutectic solvents, Environ. Sci. Technol. 53 (2019) 1031–1038. [15] D. Deng, B. Gao, C. Zhang, X. Duan, Y. Cui, J. Ning, Investigation of protic NH4SCNbased deep eutectic solvents as highly efficient and reversible NH3 absorbents, Chem. Eng. J. 358 (2019) 936–943. [16] A.P. Abbott, C.A. Eardley, Solvent properties of liquid and supercritical hydrofluorocarbons, J. Phys. Chem. 103 (1999) 2504–2509. [17] I.M. Aroso, A. Paiva, R.L. Reis, A. Rita, C. Duarte, Natural deep eutectic solvents from choline chloride and betaine – physicochemical properties, J. Mol. Liq. 241 (2017) 654–661. [18] M.K. Alomar, M. Hayyan, M.A. Alsaadi, S. Akib, A. Hayyan, M.A. Hashim, Glycerolbased deep eutectic solvents: physical properties, J. Mol. Liq. 215 (2016) 98–103. [19] R.K. Ibrahim, M. Hayyan, M. Abdulhakim, S. Ibrahim, A. Hayyan, M.A. Hashim, Physical properties of ethylene glycol-based deep eutectic solvents, J. Mol. Liq. 276 (2018) 794–800. [20] N.F. Gajardo-parra, M.J. Lubben, J.M. Winnert, Á. Leiva, J.F. Brennecke, R.I. Canales, Physicochemical properties of choline chloride- based deep eutectic solvents and excess properties of their pseudo-binary mixtures with 1-butanol, J. Chem. Thermodyn. 133 (2019) 272–284. [21] S.U. Schilling, C.D. Bruce, N.H. Shah, A.W. Malick, J.W. McGinity, Citric acid monohydrate as a release-modifying agent in melt extruded matrix tablets, Int. J. Pharm. 361 (2008) 158–168. [22] D. Wyrzykowski, E. Hebanowska, G. Nowak-Wick, M. Makowski, L. Chmurzynski, Thermal behavior of citric acid and isomeric aconitic acids, J. Therm. Anal. Calorim. 104 (2011) 731—735. [23] M. Matzapetakis, C.P. Raptopoulou, A. Terziz, A. Lakatos, T. Kiss, A. Salifoglou, Synthesis, structural characterization, and solution behavior of the first mononuclear, aqueous aluminum citrate complex, Inorg. Chem. 38 (1999) 618–619. [24] D. Wyrzykowski, L. Chmurzynski, Thermodynamics of citrate complexation with Mn2+, Co2+, Ni2+ and Zn2+ ions, J. Therm. Anal. Calorim. 102 (2010) 61–64. [25] M. Zdanowicz, K. Wilpiszewska, T. Spychaj, Deep eutectic solvents for polysaccharides processing. A review, Carbohydr. Polym. 200 (2018) 361–380. [26] F. Chemat, H. Anjum, A. Md Shariff, P. Kumar, T. Murugesan, Thermal and physical properties of (choline chloride + urea + L -arginine) deep eutectic solvents, J. Mol. Liq. 218 (2016) 301–308. [27] A. Hayyan, F.S. Mjalli, I.M. Alnashef, Y.M. Al-wahaibi, T. Al-wahaibi, M.H. Ali, Glucose-based deep eutectic solvents : physical properties, J. Mol. Liq. 178 (2013) 137–141. [28] F.S. Mjalli, G. Murshid, S. Al-zakwani, A. Hayyan, Fluid phase equilibria monoethanolamine-based deep eutectic solvents, their synthesis and characterization, Fluid Phase Equilib. 448 (2017) 30–40. [29] A. Singh, R. Walvekar, M. Khalid, W. Yin, T.C.S.M. Gupta, Thermophysical properties of glycerol and polyethylene glycol (PEG 600) based DES, J. Mol. Liq. 252 (2018) 439–444. [30] A.P. Abbott, R.C. Harris, K.S. Ryder, Application of hole theory to define ionic liquids by their transport properties, J. Phys. Chem. B 111 (2007) 4910–4913. [31] Y. Dai, G. Witkamp, R. Verpoorte, Y. Hae, Tailoring properties of natural deep eutectic solvents with water to facilitate their applications, Food Chem. 187 (2015) 14–19. [32] H. Malaeke, M.R. Housaindokht, H. Monhemi, M. Izadyar, Deep eutectic solvent as an efficient molecular liquid for lignin solubilization and wood delignification, J. Mol. Liq. 263 (2018) 193–199.
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Synthesis of citric acid monohydrate-choline chloride based deep eutectic solvents (DES) and characterization of their physicochemical properties Muhammad Hakimin Shafie a, Rizana Yusof b, Chee-Yuen Gan a,⁎ a b
Analytical Biochemistry Research Centre (ABrC), Universiti Sains Malaysia, 11800 USM, Penang, Malaysia Department of Chemistry, Faculty of Applied Sciences, Universiti Teknologi MARA, Perlis Branch, 02600 Arau, Perlis, Malaysia
a r t i c l e
i n f o
Article history: Received 15 March 2019 Received in revised form 7 May 2019 Accepted 27 May 2019 Available online 29 May 2019 Keywords: Deep eutectic solvent Choline chloride Citric acid monohydrate Synthesis Characterization
a b s t r a c t The objective of the study was to synthesize deep eutectic solvents (DES) using choline chloride and citric acid monohydrate at different molar ratios (i.e. DES 3:1, 2:1, 1:1, 1:2 and 1:3) followed by physicochemical characterization. The DES produced were clear and homogenized liquids under a microscopic observation, and DES 1:1 was found to meet the eutectic point with the lowest melting temperature. Fourier-transform infrared spectroscopy (FTIR) analysis showed the hydrogen bond interaction between the components in DES where the wavenumbers of functional groups were shifted. Results also showed that when the molar ratio of citric acid monohydrate increased in DES, higher values of viscosity, surface tension and density were observed. It was also found that the DES produced were classified as hydrophilic DES because of their solubility in water, highly polar and most semi polar solvents. In summary, the DES synthesized was suggested to be used as a medium for the extraction of hydrophilic components from plant or animal materials. © 2019 Elsevier B.V. All rights reserved.
1. Introduction The deep eutectic solvents (DES) are another class of ionic liquids. The term “deep eutectic solvent” refers to liquids near to the eutectic composition of two or more compound mixtures. The formation of deep eutectic solvents (DES) must be at least two compounds that consist of quaternary ammonium salts and hydrogen bond donor (i.e. amides, amines, alcohols, and carboxylic acids) in certain ratios. The DES are produced when the mixtures arise at the eutectic point which has a low melting point. The DES was introduced in 2003 by Abbott et al. [1] on their successful to synthesize the eutectic mixture of choline chloride with urea at molar ratio 1:2. The DES can be classified into four types and the Type III of DES has the most applications due to their low cost and low eco-toxicity. This system consists of a cation and an anion from quaternary ammonium salt (i.e. choline chloride) and hydrogen bond donating species from a hydrogen bond donor (i.e. citric acid monohydrate). DES are structurally different from ionic liquids because of DES are not only have cations and anions but also non-ionic species. The quaternary ammonium salt (e.g. choline chloride) consists of choline cation and chloride anion. Meanwhile, the hydrogen bond donor, such as citric acid monohydrate, is the source of non-ionic ⁎ Corresponding author. E-mail address: [email protected] (C.-Y. Gan).
https://doi.org/10.1016/j.molliq.2019.111081 0167-7322/© 2019 Elsevier B.V. All rights reserved.
species. According to Ghaedi et al. [2], DES are easy to prepare, nontoxic, and biodegradability. In addition, the authors also reported that DES are now a class of solvents that possess desirable characteristics such as a wide liquid range, lower melting point than the constituents of the mixture, nonreactive with water, non-volatile, thermally stable, highly conductive, and cheaper price compare to ionic liquid. In view of these unique features, DES are used for a wide range of application such as metal oxide processing [3], biodiesel purification [4], extractions [5], electrochemistry [6], nanomaterials [7], carbon dioxide absorption [8], organic synthesis [9], biochemistry [10], biocatalysis [11], polymer synthesis [12], sulphur dioxide absorption [13], nitric oxide absorption [14], and ammonia absorption [15]. In this work, we have used choline chloride as the quaternary ammonium salt and citric acid monohydrate as hydrogen bond donors. In addition, their molar ratios play a key role in controlling the melting point of the DES. It is similar to viscosity, density, and surface tension of DES, is strongly influenced by the atomic structure of its components, the molar ratio by which it is prepared, the operating temperature, and the water content. The ionic size of components, void volume and interaction forces between components, such as electrostatic and Van der Waals, all significantly affected the properties of DES [16]. In this study, DES were prepared at five different molar ratios of 3:1, 2:1, 1:1, 1:2 and 1:3 by mixing of choline chloride and citric acid monohydrate. The aim of this work has been to investigate the physicochemical
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2. Materials and methods
pure DES could not be detected. The force tensiometer was equipped with the Attension sigma software for data collection and automatic control. The Wilhelmy plate was cleaned by flaming before beginning each measurement. The surface tensions of the DES were measured at 25 °C and for each measurement three replicates were conducted.
2.1. Chemicals
2.8. Density measurement
Choline chloride (purity N99%: C5H14ClNO) was obtained from Acros Organics and citric acid monohydrate (purity N99.5%: C6H8O7.H2O) was purchased from Fluka. All chemicals used in this study were of analytical grade and used as received. Choline chloride was kept in a desiccator to prevent moisture absorption.
The analysis was carried out according to a simple calculation of density by pouring the DES into 10 mL of volumetric flask until calibrated mark. The measurement was performed by measure the weight of the DES in a volumetric flask at 25 °C. The density was calculated as follows:
properties (i.e. functional group, melting point, viscosity, surface tension, density and solubility) of DES affected by using different molar ratio of choline chloride and citric acid monohydrate.
2.2. Synthesis of deep eutectic solvents (DES) The preparation of deep eutectic solvents (DES) was adopted from Abbott et al. [1] by mixing of the quaternary ammonium salt (choline chloride) and hydrogen bond donor (citric acid monohydrate) at different molar ratios (i.e. 3:1, 2:1, 1:1, 1:2, and 1:3). The mixture of compounds was stirred at 80 °C until a colorless liquid was formed, and maintained as a liquid after leaving about 24 h at room temperature. The DES were then stored in a desiccator before various characterizations were carried out to prevent moisture absorption. 2.3. Polarized light microscopy (POM) analysis A droplet of the DES was dropped in a microscope slide for observation at a magnification of 10×. Visual characterization of the DES was carried out at 25 °C by POM using the Olympus transmission microscope (SZX7, Olympus, Japan) coupled with a Lumenera Infinity camera and Infinity Analyzer software.
Density ðρÞ ¼
m V
ð1Þ
where m was the weight (g) of the DES in volumetric flask and V was the volume of the DES (mL). 2.9. Solubility measurements DES were initially added to the test tubes and subsequently marked at the level. Different solvents (i.e. hexane, diethyl ether, dichloromethane, butanol, chloroform, toluene, methanol, ethanol, acetic acid, dimethyl sulfoxide and water) were then added into the test tubes. The mixtures were vortexed at 25 °C and the solubility was determined by observing the separation of the DES and organic solvent. The presence of separate layer at the marked sign indicated that the DES was non-miscible with the added solvent. If the layer formed above the marked sign indicated that they were partially miscible whereas no layer formed indicated that the DES and organic solvent were completely miscible. 2.10. Statistical analysis
2.4. Fourier transform infrared (FTIR) spectroscopy analysis The attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy (Fortier, PerkinElmer, USA) was used in FTIR analysis to determine the functional groups that presence in the DES and to disclose the interaction between choline chloride with citric acid monohydrate. Each sample was scanned for 16 times in the range of 600 to 4000 cm−1.
An average of three replicates of the analyses was taken for each DES and analyzed in a one-way analysis of variance (ANOVA). The statistical analysis of the data was performed using SPSS statistical 24.0 software, and significant difference among the treatment at a 95% confidence interval means was analyzed using Duncan's multiple range test. 3. Results and discussion
2.5. Differential scanning calorimetry (DSC) analysis
3.1. POM analysis
The melting point of the DES were analyzed using a DSC (DSC 6, Perkin Elmer, USA) under a constant stream of nitrogen at a flow rate 40 mL/min. All the DES were tightly sealed in aluminum pans. These analyses were carried out in the temperature ranging from −50 to 300 °C at a heating rate of 5 °C/min to obtain the melting point.
Table 1 are summarized the appearance of the synthesized DES at various ratios. The formation of clear liquid mixtures was observed using POM imaging (Fig. 1). The POM imaging shows no residues or crystals were formed. In addition, the DES were found to be stable as a clear and viscous liquid over the course of time. A research conducted by Aroso et al. [17] reported that the successful synthesized DES for choline chloride-tartaric acid at molar ratios of 2:1, 1:1 and 1:2 were confirmed using the same approach and the mixtures did not contain any crystals.
2.6. Viscosity measurements The viscosity of the DES at three different temperatures (i.e. 25, 35, and 45 °C) were measured using rheometer (AR 1000-N, TA instruments, USA) fitted with parallel plate geometry with 20 mm of diameter and gap 1 mm. The measurements were performed by equilibrating the sample temperature for 2 min and applying shear rate 1 s−1 for 30 s. The thixotropic properties of DES were measured by imposing a continuous shear rate ramp cycle from 0.1 to 450 s−1 and decreasing back to 0.1 s−1. All the samples were analyzed in three replicates. 2.7. Surface tension analysis The surface tension measurements of all DES were carried out using a force tensiometer (Sigma 700, Attension, Sweden) and the DES were diluted to 90% (w/v) prior analysis because the surface tensiom for the
3.2. FTIR analysis FTIR analysis is widely used to study the interaction of molecules and is an excellent method to characterize a sample by confirming the presence of functional groups of atoms or bonds within molecules. Hence, the formation of a new eutectic DES through hydrogen bonding between the chloride ion and citric acid monohydrate can be determined as well as formation of all the functional groups in a DES. The details on the FTIR wavenumbers for all the synthesized DES are listed in Table 2. According to a study by Alomar et al. [18] on glycerol-based deep eutectic solvents, the bands between 3200 and 3600 cm−1 was indicating the presence of O\\H stretching and for choline chloride based DES, whereas N\\H stretching was detected to be overlapped with O\\H between 3000
M.H. Shafie et al. / Journal of Molecular Liquids 288 (2019) 111081 Table 1 The appearance of DES with different molar ratio at 25 °C. Molar ratio of DES
Abbreviation
Appearance
Observation After heating
After 24 h
3:1
DES 3:1
Liquid
Liquid
2:1
DES 2:1
Liquid
Liquid
1:1
DES 1:1
Liquid
Liquid
3
DES 3:1 (3298.76 cm−1) and DES 2:1 (3295.23 cm−1) which caused by more chlorine ion (Cl−) in choline chloride that induced stronger hydrogen bond interactions with hydroxyl group in citric acid monohydrate. However, the wavenumber shifted to lower values for DES 1:1, DES 1:2 and DES 1:3 (2935.33 cm−1, 2941.74 cm−1, and 2945.95 cm−1, respectively). This can be concluded that the formation of hydrogen bonds interactions in the DES occurred. During the formation of the DES, the hydroxyl groups (O\\H) on citric acid monohydrate was attracted to chlorine anion (Cl−) in choline chloride hence producing an O-H—Cl bond. The interactions were expected to be dissimilar with the variations of molar ratios of both choline chloride and citric acid monohydrate. This could be due to the fact that the presence of quaternary ammonium salt and hydrogen bond donor forms the interaction of hydrogen bonds (in particular with the hydroxyl functional groups), and thus changed the strength of the O\\H bond. It was also observed that the C_O bond and C\\O stretching shifted to the lower wavenumber with increasing the molar ratio of citric acid monohydrate. However, the CH3 bending is shifted to higher wavenumber with the addition of citric acid monohydrate molar ratio. This might be also affected by the hydrogen bond interaction formation within the DES. A similar result was reported by Gajardo-Parra et al. [20] that the hydrogen bond interactions are noticeable in the FTIR spectrum of choline chloride-ethylene glycol DES at molar ratio 1:2, where the wavenumber are shifted to be higher when ethylene glycol are mixed with choline chloride. Hence, it can be seen that the hydrogen bond between eutectic mixtures were affected by the changing or increasing the molar ratio of citric acid monohydrate as a hydrogen bond donor. 3.3. DSC analysis
1:2
DES 1:2
Liquid
Liquid
1:3
DES 1:3
Liquid
Liquid
and 3400 cm−1. The peaks around 1400–1485 cm−1 were indicating the presence of CH2 bending whereas the presence of CH3 bending at peaks around 1355–1395 cm−1 [19]. The authors also stated that the saturated ester stretching bonds was exhibited at 1100–1300 cm−1. The analysis of both choline chloride and citric acid monohydrate were required to investigate the structural changes and interaction that occurred upon the successful formation of DES. Table 2 shows the shifts in the representative bands of the involved bonds in the FTIR spectra. In choline chloride spectrum, a broad band at 3224.58 cm−1 was indicating the presence of O\\H stretching. The CH2 bending, CH3 bending and CN stretching were indicated by the bands at 1480.45 cm−1, 1349.40 cm−1 and 1083.62 cm−1, respectively. In the spectrum of citric acid monohydrate, a broad band at 3004.42 cm−1 was representing the O\\H in carboxylic group. The C_O ester, CH2 bending and C-OH stretching was indicated at 1714.99 cm−1, 1400.19 cm−1 and 1204.30 cm−1, respectively. Refer to Table 2, the shift of O\\H stretching was observed after the formation of the DES to higher wavenumber for
The melting temperature of DES was investigated and the results were shown in Table 3. The melting point of DES is referred as the temperature that disrupts the intermolecular interaction between the cation and anion in choline chloride as well as the hydrogen bonds with citric acid monohydrate. The melting temperatures of choline chloride and citric acid monohydrate were dominated by the presence of endothermic peaks at 216.70 °C and 193.45 °C, respectively. As compared to Schilling et al. [21] Wyrzykowski et al. [22], who reported that the melting temperature of citric acid monohydrate were 158 °C and 160 °C, respectively, the melting temperature of citric acid monohydrate was found to be higher due to the formation thermodynamically stable complexes of citric acid monohydrate with aluminum in the DSC pan, in which tridentate ligand formation occurred [23,24]. Apart from that, it could also be seen that the melting temperatures of DES were significantly (p b 0.05) lower than individual component (i.e. citric acid monohydrate or chlorine chloride). It was suggested that the DES obtained by the complexation of a quaternary ammonium salt and a hydrogen bond donor, therefore, altering the properties of the choline chloride and citric acid monohydrate. This finding was in agreement with Zdanowicz et al. [25] and Chemat et al. [26] who reported that DES exhibited a lower melting temperature than their individual components. The increment of choline chloride in the molar ratio of DES gave a higher melting temperature. This can be explained by the more chlorine ion (Cl−) in choline chloride induced stronger hydrogen bond interactions within DES as aforementioned in Section 3.2, thus resulting more energy were required to break the interactions formed. The results showed significantly effect by decreasing molar ratio of choline chloride and increasing the molar ratio of citric acid monohydrate. The melting temperatures of DES were continuously increased with increasing the composition of citric acid monohydrate except for DES 1:1, which exhibit the lowest melting temperature [19]. Thus, this indicated that the eutectic point was 159.55 °C at molar ratio of 1:1. Commonly, the eutectic point was found at molar ratio of 1:2 or 2:1. For examples, the research conducted by Abbott et al. [1] and Hayyan et al. [27] reported that the eutectic point of choline chloride-urea and choline chloride-glucose exhibits the eutectic point at a molar ratio of 1:2 and
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M.H. Shafie et al. / Journal of Molecular Liquids 288 (2019) 111081
Fig. 1. POM imaging of (a) DES 3:1, (b) DES 2:1, (c) DES 1:1, (d) DES 1:2, and (e) DES 1:3 at a magnification 10×.
2:1, respectively. In our study, eutectic point was however at molar ratio of 1:1. This might be due to the interaction between chlorine anion (Cl−) and the hydroxyl group (OH) was more favorable to allow one choline chloride molecule as well as one citric acid monohydrate molecule to form hydrogen bonds. 3.4. Viscosity The viscosity of DES was useful to understanding the internal resistance of a fluid to a shear stress as well as the nature of intermolecular
Table 3 The melting temperature of DES.
Table 2 Band assignments and wavenumber exhibited by DES. Molar ratio of DES
ChCl CA DES 3:1 DES 2:1 DES 1:1 DES 1:2 DES 1:3
Wavenumber (cm−1) O-H stretching 3224.58 3004.42 3298.76 3295.23 2935.33 2941.74 2945.95
interactions within choline chloride and citric acid monohydrate. The measurement at different molar ratio and temperature are significant to relate the fluidity of DES with composition and heat, respectively. Based on Table 4, DES 1:3 was recorded as the most viscous DES (1742.67 Pa.s) at 25 °C among the synthesized DES due to the existence of more hydroxyl groups in citric acid monohydrate. The result shows the viscosity of DES increased with an increasing the molar ratio of citric acid monohydrate. The presence of more citric acid monohydrate causes more hydrogen bond interactions arise within DES, which increasing the attractive force and decreasing the free volume of DES. Therefore, the
C=O bond
CH2 bending
CH3 bending
C-O stretching
C-N stretching
ND 1714.99 1720.07 1719.85 1718.97 1709.88 1708.78
1480.45 1400.19 1477.04 1477.11 1477.04 1476.68 1476.67
1349.40 ND 1381.26 1383.62 1384.19 1388.23 1389.05
ND 1204.30 1176.45 1175.15 1174.35 1174.67 1175.67
1083.62 ND 1081.15 1080.45 1079.64 1078.44 1078.73
ND = Not detected. Note: ChCl was choline chloride, CA was citric acid monohydrate, DES 3:1, DES 2:1, DES 1:1, DES 1:2, and DES 1:3 were DES at molar ratio 3:1, 2:1, 1:1, 1:2, and 1:3 respectively.
Sample
Melting temperature (°C)
ChCl CA DES 3:1 DES 2:1 DES 1:1 DES 1:2 DES 1:3
216.70 ± 7.39f 193.45 ± 8.27e 189.97 ± 5.27de 182.31 ± 1.39d 159.55 ± 7.41a 165.04 ± 1.12b 173.80 ± 2.23c
Note: ChCl was choline chloride, CA was citric acid monohydrate, DES 3:1, DES 2:1, DES 1:1, DES 1:2, and DES 1:3 were DES at molar ratio 3:1, 2:1, 1:1, 1:2, and 1:3 respectively. Data reported are average values with error bars indicating the standard deviation of 3 replicates (n = 3). Values with different letters are significant different (p b 0.05) according Duncan's multiple test.
M.H. Shafie et al. / Journal of Molecular Liquids 288 (2019) 111081 Table 4 The viscosity of DES at different temperature. Sample
DES 3:1 DES 2:1 DES 1:1 DES 1:2 DES 1:3
Temperature (°C) 25
35
45
35.79 ± 0.98a 53.39 ± 2.08a 131.00 ± 16.65b 1083.00 ± 34.83c 1742.67 ± 52.08d
14.20 ± 1.96a 22.31 ± 2.67a 58.80 ± 0.20b 208.37 ± 21.51c 273.23 ± 14.68d
7.27 ± 0.23a 10.95 ± 0.07a 19.58 ± 0.89b 49.38 ± 2.31c 54.16 ± 6.60c
Note: DES 3:1, DES 2:1, DES 1:1, DES 1:2, and DES 1:3 were DES at molar ratio 3:1, 2:1, 1:1, 1:2, and 1:3 respectively. Data reported are average values with error bars indicating the standard deviation of 3 replicates (n = 3). Values with different letters are significant different (p b 0.05) according Duncan's multiple test.
higher viscosity was obtained from DES 1:3. A similar result was reported by Chemat et al. [26], which the density of synthesized DES increased (748.09 to 5532.80 mPa.s) with the increasing the molar ratio of Larginine (0 to 0.20) at 25 °C. In addition, the effect of the temperature was recorded and presented in Table 4. As the viscosity of DES significantly (p b 0.05) decreased from 25 °C to 45 °C for each molar ratio of DES from 35.79 to 7.27 Pa.s, 53.39 to 7.95 Pa.s, 131.00 to 19.58 Pa.s, 1083.00 to 49.38 Pa.s, and 1742.67 to 54.16 Pa.s for DES 3:1, DES 2:1, DES 1:1, DES 1:2 and DES 1:3, respectively. As mentioned by Ghaedi et al. [2], the increasing temperature were inducing a lower viscosity of DES because of the internal resistance within DES decreases at a higher temperature, resulted the DES were less viscous and flow easily. Zhang et al. [8] reported that the viscosity of the DES from the mixture of choline chloride and urea at molar ratio 1:2 has decreased from 0.75 to 0.17 Pa.s with the increasing of temperature from 25 °C to 40 °C, respectively. 3.5. Surface tension The surface tension was defined as the required energy to increase the surface area of a liquid by a specific amount. The measurement of surface tension was important for interpretation of the energy required to increase the surface area of liquid as well as the intermolecular force intensity between the choline chloride and citric acid monohydrate in DES. According to Ghaedi et al. [2], the discontinuity of the electrostatic forces on the surface caused by the intensity of interaction molecules in DES. Fig. 2 shows the surface tension values for DES 3:1, DES 2:1, DES 1:1, DES 1:2 and DES 1:3 were 37.72, 40.87, 41.04, 41.36 and 41.58 mN/m, respectively. The highest surface tension was exhibited by DES 1:2 and DES 1:3. The surface tension increased continuously with increasing the molar ratio of citric acid monohydrate. The temperature, interaction between quaternary ammonium salt and hydrogen bond donor, alkyl chain, viscosity and molecular weight are the main factors affecting the surface tension of DES [26]. The more hydrogen bonding interaction within DES associates the formation of cohesive forces. Hence, the cohesive forces occurred, causing the surface to produce stronger mutual
Fig. 2. The surface tension of (a) DES 3:1, (b) DES 2:1, (c) DES 1:1, (d) DES 1:2, and (e) DES 1:3.
5
attractive forces to resist external forces and arises the surface molecules to resist surface breakage. This can be explained by the increasing the molar ratio of citric acid monohydrate as a hydrogen bond donor in DES induced more hydrogen bonding interaction formed in DES as aforementioned. A similar result was reported by Mjalli et al. [28], which the surface tension of choline chloride-monoethanolamine DES increased (from 67.04 to 72.07 mN/m) with increasing the molar ratio (from 5 to 8) of monoethanolamine as a hydrogen bond donor. 3.6. Density The type of the quaternary ammonium salt and a hydrogen bond donor as well as the molar ratio was affecting the density of DES. The measurements of the DES densities are presented in Fig. 3. It was noted that DES 3:1 (2.64 g/mL) has the lowest density followed by DES 2:1, DES 1:1, DES 1:2 and finally DES 1:3 with the highest density. The density of DES increased dramatically from 2.64 to 3.11 g/mL when the molar ratio of citric acid monohydrate increased. The increasing the molar ratio of citric acid monohydrate contributed to a reduction in the free space between the DES, thus increasing the density. This result was similar to the study by Singh et al. [29] which reported that the amount of hydrogen bond donor increased in DES and resulted in a higher density. Furthermore, the densities of DES were significantly decreased with the increment of the molar ratio of choline chloride. This could be explained by the interaction of hydroxyl groups in citric acid monohydrate complexed with chlorine anion (Cl−) was induced with the addition of citric acid monohydrate molar ratio. It was also responsible for the size of the formed DES. The packing structure and density of DES were also affected. The significantly (p b 0.05) increase in the density of DES, with increasing ratios of citric acid monohydrate, indicating the complexation that occurred in DES. It was due to more hydroxyl groups of citric acid monohydrate to make the interaction with the chlorine ion (Cl−) in choline chloride, which caused more hydrogen bond interaction between the anion of choline chloride and the hydroxyl groups in citric acid monohydrate. Different sizes of complexes were formed and changed the density of DES. According to Abbott et al. [30], the “hole” theory can be applied to explain the density of DES, which depends on the packing or the molecular structure of the DES. When the choline chloride and citric acid monohydrate were mixed resulted the average hole radius decreases, thus increasing the density of DES. The results obtained were proved that the complexation of DES was strongly affected by the molar ratio of citric acid monohydrate. 3.7. Solubility The solubility of DES in various organic solvents with different polarity was investigated to evaluate the ability of DES to solubilize a solute. The unique feature of DES with the existence of not only cation and anion but also non-ionic species has improved their ability to dissolve in a wide range of solvents. Table 5 demonstrates each molar ratio of
Fig. 3. The density of (a) DES 3:1, (b) DES 2:1, (c) DES 1:1, (d) DES 1:2, and (e) DES 1:3.
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M.H. Shafie et al. / Journal of Molecular Liquids 288 (2019) 111081
Table 5 The solubility of DES. Solvent Hexane Toluene Diethyl ether Dichloromethane Butanol Chloroform Methanol Ethanol Acetic acid Dimethyl sulfoxide Water
DES 3:1
DES 2:1
DES 1:1
DES 1:2
DES 1:3
nm nm nm nm nm nm m m m m m
nm nm nm nm nm nm m m m m m
nm nm nm nm nm nm m m m m m
nm nm nm nm nm nm m m m m m
nm nm nm nm nm nm m m m m m
m = miscible; nm = nonmiscible. Note: DES 3:1, DES 2:1, DES 1:1, DES 1:2, and DES 1:3 were DES at molar ratio 3:1, 2:1, 1:1, 1:2, and 1:3 respectively.
DES were soluble (miscible) in water, highly polar and most semi polar solvents. Therefore, the results proved that the DES produced could be classified as hydrophilic DES due to the hygroscopic behavior of choline chloride and high polarity of citric acid monohydrate. Dai et al. [31] stated that the polarity of DES was strongly affecting its capability and ability to solubilize solute or any constituents. As reported by Malaeke et al. [28], the lignin exhibited higher solubility about 48.15% (w/w) in DES of choline chloride-resorcinol at the molar ratio of 1:1. This result suggested that the ionic fragments of DES will not only be able to diffuse into a compacted structure of a matrix, but also can interact favorably, change the intermolecular interaction and finally disrupt highly complex compound interactions [32]. Therefore, the synthesized DES could be used as a medium for extraction of hydrophilic biopolymer and bioactive compounds.
4. Conclusions The DES were successfully produced using choline chloride and citric acid monohydrate. All the synthesized DES were in liquid form at room temperature. The DES with molar ratio of 1:1 was found to meet the eutectic point, which had the lowest melting temperature. The different molar ratios of choline chloride and citric acid monohydrate had significantly affected the physicochemical properties of the DES. Results showed that the addition of citric acid monohydrate molar ratio gave a higher value of viscosity, surface tension and density. However, the DES with higher choline chloride molar ratio showed a high melting temperature. Meanwhile, all the DES were soluble in water, highly polar and semi polar organic solvents. Therefore, it was suggested that this physicochemical study will be useful for the development of acidbased DES application in a broad range of industrial applications, such as the extraction medium for hydrophilic components from plant or animal materials.
Acknowledgement The authors would like to acknowledge the financial support from Universiti Sains Malaysia RUI Grant (Grant number: 1001/CABR/ 8011045). The authors would also like to acknowledge the financial support from USM Fellowship. References [1] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, V. Tambyrajah, Novel solvent properties of choline chloride/urea mixtures, Chem. Commun. (2003) 70–71. [2] H. Ghaedi, M. Ayoub, S. Su, A.M. Shariff, B. Lal, The study on temperature dependence of viscosity and surface tension of several phosphonium-based deep eutectic solvents, J. Mol. Liq. 241 (2017) 500–510.
[3] A.P. Abbott, G. Capper, D.L. Davies, K.J. Mckenzie, S.U. Obi, Solubility of metal oxides in deep eutectic solvents based on choline chloride, J. Chem. Eng. 51 (2006) 1280–1282. [4] K. Shahbaz, F.S. Mjalli, M.A. Hashim, I.M. AlNashef, Using deep eutectic solvents for the removal of glycerol from palm-oil based biodiesel, J. Od Appl. Sci. 10 (2010) 3354. [5] S.P. Saravana, Y. Cho, H.-C. Woo, B. Chun, Green and efficient extraction of polysaccharides from brown seaweed by adding deep eutectic solvent in subcritical water hydrolysis, J. Clean. Prod. 198 (2018) 1474–1484. [6] Q. Xu, Y.N. Ji, L.Y. Qin, P.K. Leung, A.A. Shah, Y.S. Li, H.N. Su, H.M. Li, Effect of carbon dioxide additive on the characteristics of a deep eutectic solvent (DES) electrolyte for non-aqueous redox flow batteries, Chem. Phys. Lett. 708 (2018) 48–53. [7] K. Xu, Y. Wang, X. Ding, Y. Huang, N. Li, Q. Wen, Magnetic solid-phase extraction of protein with deep eutectic solvent immobilized magnetic graphene oxide nanoparticles, Talanta 148 (2016) 153–162. [8] Q. Zhang, K.D.O. Vigier, R. Sebastien, F. Jerome, Deep eutectic solvents: syntheses, properties and application, Chem. Soc. Rev. 41 (2012) 7108–7146. [9] L. Hu, L. Chen, Y. Fang, A. Wang, C. Chen, Z. Yan, Facile synthesis of zeolitic imidazolate framework-8 (ZIF-8) by forming imidazole-based deep eutectic solvent, Microporous Mesoporous Mater. 268 (2018) 207–215. [10] N. Li, Y. Wang, K. Xu, Y. Huang, Q. Wen, X. Ding, Development of green betainebased deep eutectic solvent aqueous two-phase system for the extraction of protein, Talanta 152 (2016) 23–32. [11] V. Gotor-fernández, C.E. Paul, Deep eutectic solvents for redox biocatalysis, J. Biotechnol. 293 (2019) 24–35. [12] P.F. Pereira, C.T. Andrade, Optimized pH-responsive film based on a eutectic mixture-plasticized chitosan, Carbohydr. Polym. 165 (2017) 238–246. [13] Y. Chen, B. Jiang, H. Dou, L. Zhang, X. Tantai, Y. Sun, H. Zhang, Highly efficient and reversible capture of low partial pressure SO2 by functional deep eutectic solvents, Energy Fuel 32 (2018) 10737–10744. [14] J. Dou, Y. Zhao, F. Yin, H. Li, J. Yu, Mechanistic study of selective absorption of NO in flue gas using EG-TBAB deep eutectic solvents, Environ. Sci. Technol. 53 (2019) 1031–1038. [15] D. Deng, B. Gao, C. Zhang, X. Duan, Y. Cui, J. Ning, Investigation of protic NH4SCNbased deep eutectic solvents as highly efficient and reversible NH3 absorbents, Chem. Eng. J. 358 (2019) 936–943. [16] A.P. Abbott, C.A. Eardley, Solvent properties of liquid and supercritical hydrofluorocarbons, J. Phys. Chem. 103 (1999) 2504–2509. [17] I.M. Aroso, A. Paiva, R.L. Reis, A. Rita, C. Duarte, Natural deep eutectic solvents from choline chloride and betaine – physicochemical properties, J. Mol. Liq. 241 (2017) 654–661. [18] M.K. Alomar, M. Hayyan, M.A. Alsaadi, S. Akib, A. Hayyan, M.A. Hashim, Glycerolbased deep eutectic solvents: physical properties, J. Mol. Liq. 215 (2016) 98–103. [19] R.K. Ibrahim, M. Hayyan, M. Abdulhakim, S. Ibrahim, A. Hayyan, M.A. Hashim, Physical properties of ethylene glycol-based deep eutectic solvents, J. Mol. Liq. 276 (2018) 794–800. [20] N.F. Gajardo-parra, M.J. Lubben, J.M. Winnert, Á. Leiva, J.F. Brennecke, R.I. Canales, Physicochemical properties of choline chloride- based deep eutectic solvents and excess properties of their pseudo-binary mixtures with 1-butanol, J. Chem. Thermodyn. 133 (2019) 272–284. [21] S.U. Schilling, C.D. Bruce, N.H. Shah, A.W. Malick, J.W. McGinity, Citric acid monohydrate as a release-modifying agent in melt extruded matrix tablets, Int. J. Pharm. 361 (2008) 158–168. [22] D. Wyrzykowski, E. Hebanowska, G. Nowak-Wick, M. Makowski, L. Chmurzynski, Thermal behavior of citric acid and isomeric aconitic acids, J. Therm. Anal. Calorim. 104 (2011) 731—735. [23] M. Matzapetakis, C.P. Raptopoulou, A. Terziz, A. Lakatos, T. Kiss, A. Salifoglou, Synthesis, structural characterization, and solution behavior of the first mononuclear, aqueous aluminum citrate complex, Inorg. Chem. 38 (1999) 618–619. [24] D. Wyrzykowski, L. Chmurzynski, Thermodynamics of citrate complexation with Mn2+, Co2+, Ni2+ and Zn2+ ions, J. Therm. Anal. Calorim. 102 (2010) 61–64. [25] M. Zdanowicz, K. Wilpiszewska, T. Spychaj, Deep eutectic solvents for polysaccharides processing. A review, Carbohydr. Polym. 200 (2018) 361–380. [26] F. Chemat, H. Anjum, A. Md Shariff, P. Kumar, T. Murugesan, Thermal and physical properties of (choline chloride + urea + L -arginine) deep eutectic solvents, J. Mol. Liq. 218 (2016) 301–308. [27] A. Hayyan, F.S. Mjalli, I.M. Alnashef, Y.M. Al-wahaibi, T. Al-wahaibi, M.H. Ali, Glucose-based deep eutectic solvents : physical properties, J. Mol. Liq. 178 (2013) 137–141. [28] F.S. Mjalli, G. Murshid, S. Al-zakwani, A. Hayyan, Fluid phase equilibria monoethanolamine-based deep eutectic solvents, their synthesis and characterization, Fluid Phase Equilib. 448 (2017) 30–40. [29] A. Singh, R. Walvekar, M. Khalid, W. Yin, T.C.S.M. Gupta, Thermophysical properties of glycerol and polyethylene glycol (PEG 600) based DES, J. Mol. Liq. 252 (2018) 439–444. [30] A.P. Abbott, R.C. Harris, K.S. Ryder, Application of hole theory to define ionic liquids by their transport properties, J. Phys. Chem. B 111 (2007) 4910–4913. [31] Y. Dai, G. Witkamp, R. Verpoorte, Y. Hae, Tailoring properties of natural deep eutectic solvents with water to facilitate their applications, Food Chem. 187 (2015) 14–19. [32] H. Malaeke, M.R. Housaindokht, H. Monhemi, M. Izadyar, Deep eutectic solvent as an efficient molecular liquid for lignin solubilization and wood delignification, J. Mol. Liq. 263 (2018) 193–199.