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Design, synthesis and properties of acidic deep eutectic solvents based on choline chloride
Accepted Manuscript Design, synthesis and properties of acidic deep eutectic solvents based on choline chloride
Yingna Cui, Changping Li, Jingmei Yin, Shenmin Li, Yingping Jia, Ming Bao PII: DOI: Reference:
S0167-7322(17)30318-5 doi: 10.1016/j.molliq.2017.04.052 MOLLIQ 7208
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
23 January 2017 12 April 2017 13 April 2017
Please cite this article as: Yingna Cui, Changping Li, Jingmei Yin, Shenmin Li, Yingping Jia, Ming Bao , Design, synthesis and properties of acidic deep eutectic solvents based on choline chloride. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Molliq(2017), doi: 10.1016/j.molliq.2017.04.052
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ACCEPTED MANUSCRIPT
Design, Synthesis and Properties of Acidic Deep Eutectic Solvents Based on Choline Chloride
a
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Yingna Cui,a, b Changping Li,*c Jingmei Yin,b Shenmin Li,b Yingping Jia,b Ming Bao*a
State Key Laboratory of Fine Chemicals, Dalian University of Technology, 116023,
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Dalian, China
Department of Chemical Engineering, Dalian University, 116622, Dalian, China
c
School of Environment and Civil Engineering, Dongguan University of Technology,
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b
MA
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523808, Dongguan, China,
Abstract
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Four deep eutectic solvents (DESs) based on different acids as hydrogen bond donors
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(HBDs), namely p-toluenesulfonic acid (PTSA), trichloroacetic acid (TCA), monochloroacetic acid (MCA), and propionic acid (PA) with hydrogen bond acceptor (HBA) choline chloride (ChCl) are synthesized with a mole ratios of HBD to HBA is
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2:1. The density, electrical conductivity, dynamic viscosity and refractive index of the
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four DESs were determined at atmospheric pressure and temperatures from (288.15 to 338.15) K at an interval of 5 K. The results show that the temperature has great influences on the physical properties of DESs. The thermal expansion coefficient, molecular volume, standard molar entropy, and lattice energy were calculated by empirical equation. The molar conductivity was determined from the data of density .
*Corresponding authors. Changping Li, Tel: +86-411-87402436; Fax: +86-411-87402449; Email address: [email protected]; Ming Bao, Tel: +86-411-84986180; Fax: +86-411-84986180; Email address: [email protected];
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and conductivity. The temperature dependence of electrical conductivities and dynamic viscosities for the DESs were fitted by Vogel–Fulcher–Tamman (VFT) and Arrhenius equation. The relationship of the molar conductivity and viscosity was determined using the Walden rule. The acidities of the above DESs were measured by Hammett acidic
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functions. The present study will help to understand the properties of DESs and provide
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a guide for further applications of DESs.
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Keywords: Ionic liquids analogues; Deep eutectic solvents; Density; Electrical
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conductivity; Dynamic viscosity; Refractive index; Acidity
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1. Introduction
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Nowadays, deep eutectic solvents (DESs), as a new generation of green solvents,
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have been attracting scientific and technological attention due to their unique properties such as the simple synthesis and purification process, low cost of raw materials and renewable, good biocompatibility, etc.1,
2
DESs have been applied in many fields,
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including solvents or catalysts for chemical reactions,3-6 electrochemistry,7,
8
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pharmaceuticals,9 separation process10-12 etc. The physicochemical properties of DESs are indispensable for its industrial and engineering applications. However, the lack of physicochemical properties data has hindered its applications to a great extent. Therefore, a comprehensive evaluation of the physicochemical properties of DESs is necessary and valuable for future applications. Generally, DESs can be prepared by two or more cheap and green components through establishment of hydrogen bond between the hydrogen bond donor (HBD) and the
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hydrogen bond acceptor (HBA).13, 14 The mostly DESs synthesized so far usually used choline chloride (ChCl), quaternary ammonium salt and phosphate salt as HBAs. However, the choice of HBD is relatively abundant, for example, alcohols, polyols, carboxylic acids, amides, amines etc. can be used as HBDs.15 DESs with different
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characters can be obtained through the choice of different HBDs. Series of DESs were designed, synthesized and applied for the removal of organic 17
Novel deep desulfurization
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sulfide in fuels by our group for the first time.16,
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technology was set up. Acidic DESs was proved an efficient catalyst, solvents and extractants. It was found that HBDs had greater influence on the desulfurization process
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than HBAs, and the acidity of DESs is the main factor that determines its catalytic
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capability.
A series of DESs was designed and synthesized according to the acidity of HBD,
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with which DESs’ acidity can be controlled concisely. In this way, some typical acidic
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DESs with different acidity gradient were got. Choline chloride (ChCl) was chosen as a typical HBA, which was a very cheap, non-toxic and biodegradable material.
18
And p-
toluenesulfonic acid (PTSA), trichloroacetic acid (TCA), monochloroacetic acid
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(MCA), propionic acid (PA) were chosen as HBDs. The main physicochemical,
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including the density, dynamic viscosity, electrical conductivity, and refractive index were characterized in temperature range of (288.15 to 338.15) K. The relationship of the density, viscosity and conductivity was built in terms of the Walden rule. The relative acidity was investigated by Hammett acidic functions.
2. Experimentals
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2.1 Materials The chemicals used in the work were supplied by Aladdin Chemicals Co., Ltd, Shanghai, China. Purities of the chemicals are shown as follows: Choline chloride , AR, 98%; p-toluenesulfonic acid monohydrate, AR≥98.5%; trichloroacetic acid , AR, 99%;
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monochloroacetic acid, AR, 99%; propionic acid, AR≥99.5%. All reagents were used with further purification or drying.
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2.2 Synthesis of DESs
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The purified compounds of ChCl and PTSA (or TCA, MCA, PA) were mixed with
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the molar ratio 1: 2. Then the deep eutectic solvents were prepared by different methods: The ChCl and PTSA were stirred at 80 °C for 4 hours; The ChCl and TCA (or
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MCA) were grinded in a mortar with a pestle at room temperature until a homogeneous liquid was formed.19 The ChCl and PA were stirred at room temperature until a
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homogeneous liquid was formed. The forming of ester impurity between ChCl and
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carboxylic acid can be avoided through grinding method and stirring method at room temperature. The purities of DESs were confirmed by 1H NMR as shown in S.Fig.1 of
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the Supplementary materials.
2.3 Density, Dynamic Viscosity, Conductivity and Acidity Measurements
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The density and dynamic viscosity were determined using an Anton paar Viscometer SVM 3000 in the temperature range of (288.15 to 338.15) K ± 0.02 K. The experimental error of density was within ± 0.0005 g·cm-3 and the uncertainties of dynamic viscosity were estimated to be ± 1%. Conductivities of the DESs were determined via a MP522 conductivity instrument with the cell constants of 1 cm-1 (the cell was calibrated with the aqueous KCl solution) in the temperature range of (288.15 to 338.15) ± 0.05 K and the uncertainties were estimated to be ± 1%. The measurement
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frequency and voltage of the electrical conductivity are 50 Hz and 9 V, respectively. 20, 21
The refractive index data were carried on a WAY-2S Abbe refractometer (Shanghai
Shen Guang Instrument Co., P. R. China). Hammett acidic functions were measured for different DESs on an Hitachi U3900-H UV–Vis spectroscopy using 4-nitroaniline as the
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basic indicator and water as solvent according to literatures.22 The melting points were determined by DSC 204HP (Netzsc, Germany).
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All the experiments in this study were performed in triplicate to determine its
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reproducibility, and the experimental errors were within 3%. The above experimental values were listed in S.Tables 1 to 4 of the supplementary materials. There is no melting
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point in our limited experimental conditions above -80 ℃, while the glass transition
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temperature of the DESs are -55 ℃ for [ChCl:2PTSA], -53 ℃for [ChCl:2TCA], -40 ℃ for [ChCl:2MCA], -64 ℃[ChCl:2PA], respectively. The results were shown in S.Fig. 2
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3. Results and discussions
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to 5 of the supplementary materials.
3.1 Density
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The temperature dependence on density are plotted in Fig. 1. It can be seen that the density decrease with the temperature increasing, and the line of density is linear with
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the increase of the temperature. For density, sequence of the four DESs is [ChCl:2PTSA]>[TBAC:2TCA]>[TBAC:2MCA]>[TBAC:2PA].
The
tendency
is
consistent with the molecular mass of DESs. The DESs exhibit the high density which has the relative large molecule mass.
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1.5
1.4
ChCl:2PTSA ChCl:2TCA ChCl:2MCA ChCl:2PA
1.2
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ρ/g•cm-3
1.3
1.0 280
290
300
310
320
330
340
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T/K
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1.1
Fig.1 Density versus temperature plots for DESs
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The thermal expansion coefficient, α, can be obtained by the lnρ versus T fitted according to a following straight line.
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ln ρ = b – α·T
(1)
where b is an empirical constant, α is the thermal expansion coefficient (Table 1). The
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values are in the range of 5×10-4 and 7×10-4 K-1 which obtained by Jacquemin.23
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The molecular volume, Vm, standard molar entropy, S, and lattice energy, UPOT, also
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can be calculated from density values by the following empirical equations.24 Vm =M/(N·ρ)
(2)
S0 = 1246.5·(Vm) + 29.5
(3)
UPOT = 1981.2·(ρ/M)1/3 + 103.8
(4)
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Table 1 Estimated physicochemical property values of DESs at 298.15 K at atmospheric pressure [ChCl:2PTSA]
[ChCl:2TCA]
[ChCl:2MCA]
[ChCl:2PA]
MW/g·mol–1
520.07
466.40
328.62
287.78
Vm/nm3
0.6469
0.5302
0.4269
0.4445
104α/K–1
5.17
6.48
V/cm–3·mol–1
389.4
319.2
S0/J·K–1·mol–1
835.9
690.4
Upot/kJ·mol–1
375
394
PT
Property
6.19
257.0
267.6
561.6
583.6
415
411
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5.98
3.2 Molar Electrical Conductivity
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The molar electrical conductivities can be determined according to the following equation from density and electrical conductivity: Λ = σMρ–1
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(5)
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where Λ is the molar electrical conductivity, σ is the electrical conductivity, M is the molar mass, and ρ is the density. The molar electrical conductivity values are listed in 2.
The
order
of
the
molar
electrical
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Table
conductivity
for
DESs
is
[ChCl:2PTSA]>[TBAC:2PA]>[TBAC:2MCA]>[TBAC:2TCA]. The p-toluenesulfonic
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acid based DESs exhibit the higher electrical conductivity than other acid based EDSs.
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Table 2 Calculated molar electrical conductivity values for DESs in the temperature range of (288.15 to 338.15) K [ChCl:2PTSA]
[ChCl:2TCA]
[ChCl:2MCA]
[ChCl:2PA]
288.15
0.123
0.014
0.057
0.082
293.15
0.163
0.018
0.069
0.094
298.15
0.212
0.023
0.081
0.105
303.15
0.272
0.028
0.095
0.118
308.15
0.344
0.034
0.109
0.130
313.15
0.429
0.041
318.15
0.528
0.049
323.15
0.640
328.15
0.770
333.15
0.908
338.15
1.068
0.145
0.141
0.158
0.057
0.159
0.173
0.065
0.176
0.189
0.075
0.194
0.204
0.085
0.214
0.221
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0.125
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PT
T/K
3.3 Electrical Conductivity
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Electrical conductivity versus temperature plots for DESs are listed in Fig.2. From Fig.2, it can be seen that for the electrical conductivity, the line is non-linear with
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temperature increasing, and the electrical conductivity increase with the temperature increasing. Generally, the Vogel−Fulcher−Tamman (VFT) and Arrhenius equations
19
can be used for the fitting of the electrical conductivity values, σ, versus temperature, T: σ= σ0·exp(–B/(T–T0))
(6)
σ = σ∞·exp(–Eσ/(kBT))
(7)
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here σ is the electrical conductivity; σ0, B are fitting parameters; Eσ is the activation energy, which indicates the energy needed for an ion to hop to a free hole; σ∞ is the maximum electrical conductivity, and kB is the Boltzmann constant.
3.5 3.0
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2.0 1.5 1.0
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σ/mS·cm-1
2.5
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ChCl:2PTSA ChCl:2TCA ChCl:2MCA ChCl:2PA
0.0 280
290
300
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0.5 310
320
330
340
T/K
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Fig.2 Electrical conductivity versus temperature plots for DESs
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The fitted values of electrical conductivity versus temperature are listed in Table 3. It can be seen that the correlation coefficient, R, is more than 0.9999, which indicates that
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the VFT equation can be well used for the electrical conductivity fitting. By the Arrhenius equation, the Lnσ vs 1000/T was plotted for the DESs in Fig. 3. According to
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the equation, the experimental points should follow a straight line, however, they are not on the straight line in the Fig. 3. (The red lines are straight lines) That is, the electrical conductivity does not meet the Arrhenius behavior well. For electrical conductivity, the VFT equation and Arrhenius equation can be combined according to the Vila et al.
25
researchand the final version of VFT equation
is: σ = σ∞·exp(–Eσ/(kB(T–T0))
9
(8)
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here, σ0 = σ∞ and B = Eσ/kB. Then the values of electrical conductivity activation energies can be calculated by the equation (Table 3). Table 3 The fitted values of electrical conductivity versus temperature for DESs by VFT and final version VFT equations [ChCl:2PTSA]
[ChCl:2TCA]
[ChCl:2MCA]
PT
Property
[ChCl:2PA]
0.697
0.012
0.013
0.015
B/K
957.7
600.06
435.25
592.62
103Eσ/eV
82.6
51.8
37.6
51.1
T0/K
161.8
180.6
R
0.99997
0.99995
1.0 0.5
-2.0
180.7
134.9
0.99995
0.99995
D
ChCl:2PTSA ChCl:2TCA ChCl:2MCA ChCl:2PA
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-0.5
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Lnσ/ mS cm-1
0.0
-1.5
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MA
1.5
-1.0
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σ0/S·cm−1
-2.5 -3.0
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2.9
3.0
3.1
3.2
3.3
3.4
3.5
1000/T(K)
Fig.3 Plot of Lnσ versus 1000/T for DESs
3.4 Dynamic Viscosity The temperature dependence on dynamic viscosity is listed in Fig. 4. The dynamic viscosity decrease with the temperature increasing, and the tendency of the four DESs is [ChCl:2PTSA] > [ChCl:2TCA] >[ChCl:2MCA] > [ChCl:2PA]. The small molecule exhibits the better liquidity than the relative big molecule. [ChCl:2PA] has the better
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liquidity than other DESs. The reason maybe that the force between ChCl and PA is relatively small, while the flow resistance become weaker, which led to the lower viscosity of [ChCl:2PA].
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2000 ChCl:2PTSA ChCl:2TCA ChCl:2MCA ChCl:2PA
1800 1600
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1400
1000
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η/mPa·s
1200
800 600
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400 200
280
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0 290
300
310
320
330
340
T /K
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Fig.4 Dynamic viscosity versus temperature plots for DESs
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The VFT and Arrhenius equations are also used for the fitting of the dynamic
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viscosity, η, versus temperature, T.
η = η0·exp(B/(T–T0))
(9)
η = η∞·exp(Eη/(kBT))
(10)
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where η is the dynamic viscosity; η0, B are fitting parameters; Eη is the activation energy for dynamic viscosity; η∞ is the maximum dynamic viscosity, and kB is the Boltzmann constant. The fitted values of dynamic viscosity versus temperature are shown in Table 4. From the correlation coefficient, R, are more than 0.999, it can be concluded that the VFT equation can be used for the dynamic viscosity fitting. According to equation (10), the Lnη vs 1000/T is described in Fig.5. The results indicate that the experimental dynamic viscosity does not follow the Arrhenius behavior
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well, which is similar to the electrical conductivity. The solid lines are smooth curves for Lnη vs 1000/T, the red lines are straight lines for fitting of Lnη vs 1000/T. The same for dynamic viscosity, the final version of VFT equation is: η = η∞·exp(Ea/(kB (T–T0))
(11)
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here, η0 = σ∞ and B = Eη/kB. The dynamic viscosity activation energies of DESs are shown in Table 4.
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Table 4 The fitted values of dynamic viscosity versus temperature of DESs by VFT and
[ChCl:2PTSA]
[ChCl:2TCA]
[ChCl:2MCA]
[ChCl:2PA]
η0/mPa·s
0.1298
0.0629
0.1248
0.1061
B/K
847.7
1033.0
787.7
855.4
103Eη/eV
73.1
89.1
68.0
73.82
T0/K
199.5
181.3
183.2
161.7
R
0.99985
0.99999
0.99999
0.99999
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D
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Property
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final version VFT equations
8.0
CE
7.5 7.0
Lnη/ mPa s
AC
6.5
ChCl:2PTSA ChCl:2TCA ChCl:2MCA ChCl:2PA
6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5
2.9
3.0
3.1
3.2
3.3
3.4
3.5
1000/T(K)
Fig.5 Plot of Lnη versus 1000/T for four DESs
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3.5 Walden Product According to the Walden rule, the relationship of molar conductivity and dynamic viscosity can be described by the the following equation: Ληα = k
(12)
logΛ = logk + αlogη-1
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(13)
where Λ is the molar conductivity, η is the dynamic viscosity, k is a temperature
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dependent constant. The variation of LogΛ versus logη-1 are showed in Fig.6. The
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slopes, α, are obtained and the values are 0.628 for [ChCl:2PTSA], 0.586 for [ChCl:2TCA], 0.544 for [ChCl:2MCA], and 0.514 for [ChCl:2PA], respectively. It can
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be seen from Fig.6 that all of the Walden plots are under the ideal line except
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[ChCl:2PTSA], the ideal line is obtained using aqueous KCl solutions at high dilution.26 The lines of the DESs lie below the ideal KCl line, which mean that the DESs are “subionic”.27, 28 However, the line of [ChCl:2PTSA] lie near the ideal line, it can be
2.0
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1.5
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classified as “ionic”.
0.5 0.0
AC
2
-1
logΛ(S•cm •mol )
1.0
ChCl:2PTSA ChCl:2TCA ChCl:2MCA ChCl:2PA
-0.5 -1.0 -1.5 -2.0 -2.0
-1.5
-1.0
-0.5
0.0
0.5
logη /(10 Pa•s) -1
-1
1.0
1.5
2.0
-1
Fig.6 Plot of LogΛ versus logη-1 for DESs from 288.15 to 338.15 K. The solid straight line is the ideal line for aqueous KCl solutions.
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3.6 Refractive Index The plot of nD versus T for DESs is listed in Fig. 7. It can be seen that the refractive index decrease with the temperature increasing, and lines of refractive index are linear. The
sequence
of
the
DESs
is
[ChCl:2PTSA]>
PT
[ChCl:2TCA]>[ChCl:2MCA]>[ChCl:2PA].
following linear equation:
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nD= A + CT
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The relationship of refractive index and temperature can be described by the
(14)
where nD is the refractive index. The fitted values of refractive index versus temperature
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are listed in Table 5. The results of table 5 show that the correlation coefficients of the
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DESs are higher than 0.999.
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1.52
ChCl:2PTSA ChCl:2TCA ChCl:2MCA ChCl:2PA
D
1.54
nD
1.50 1.48
CE
1.46
AC
1.44
280
290
300
310
320
330
T/K
Fig.7 Plot of nD versus T for DESs
14
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Table 5 The fitted values of refractive index versus temperature of DESs A
−C
R
[ChCl:2PTSA]
1.5873
2.37×10–4
0.9994
[ChCl:2TCA]
1.5703
2.67×10–4
0.9999
[ChCl:2MCA]
1.5414
2.33×10–4
0.9998
[ChCl:2AP]
1.5042
1.88×10–4
0.9997
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PT
DESs
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3.7 Acidities of the Different DESs
The maximal absorbance of the un-protonated form of 4-nitroaniline is at 380 nm in
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water. The maximal absorbance of the un-protonated form of the indicator molecules
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was decreased with increasing the concentration of the acidic DESs. Therefore, the absorbance strength of the indicator in pure water was taken as the initial reference to
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determine the protonated and unprotonated forms of the indicator.
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Then the Hammett acidity function (H0) can be calculated by the equation: H0 = pK(I)aq + log ([I]s/[IH+]s)
(15)
where I stands for indicator pK(I)aq=pKa=0.99, [I]s is the molarity of the unprotonated
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form of the indicator, [IH+]s is the molarity of the protonated form of the indicator. Fig.8
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shows the absorbance spectra of 4-nitroaniline in the presence of different DESs in water at the concentration of 40.0 mmol/l and 293.15 K. The indicator were dissolved in water with the concentrations of 7.24×10-2 mmol/L. According to equation (15), the H0 values of different acidic DESs are showed in Table 6. The sequence of acidities of the DESs is [ChCl:2PTSA]> [ChCl:2TCA]>[ChCl:2MCA]>[ChCl:2PA]. It is well known that there exists hydrogen bond between HBD and HBA, and the accepted hydrogen
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will enhance the acidity of the DES, so the [ChCl:2PTSA] has the highest acidity owning to the higher acidity of PTSA.
1.2 blank ChCl:2PTSA ChCl:2TCA ChCl:2MCA ChCl:2PA
PT
1.0
0.6
RI
Abs.
0.8
0.2
350
400 nm
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0.0 300
SC
0.4
450
500
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Fig.8 UV-vis absorbance spectra of 4-nitroaniline in the presence of different DESs in water at the concentration of 40.0 mmol/l and 293.15K
Amax
[I](%)
[IH] (%)
H0
1.055
100
0
-
0.456
43.2
56.8
0.87
[ChCl:2TCA]
0.508
48.2
51.8
0.96
[ChCl:2MCA]
0.790
74.9
25.1
1.46
[ChCl:2AP]
0.998
94.6
5.4
2.23
-
AC
CE
[ChCl:2PTSA]
PT E
DESs
D
Table 6 Hammett acidity functions of different DESs (40.0 mmol/l) in water at 293.15K
4. Conclusions In this work, deep eutectic solvents were synthesized by mixing choline chloride with different organic acids used as hydrogen bond donors. The physical properties (density,
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electrical conductivity, dynamic viscosity, and refractive index) were studied in the range of temperatures from 288.15 to 338.15 K at atmospheric pressure. The thermal expansion coefficient, molecular volume, standard molar entropy, lattice energy of the DESs were estimated by density values. The VFT equation can be used for fitting of the
PT
electrical conductivity and dynamic viscosity to temperature very well. The relationship of the density, electrical conductivity, and dynamic viscosity was built by the Walden
sequence
of
the
acidity
of
the
DESs
is
[ChCl:2PTSA]>
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The
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rule. The temperature dependence on refractive index was fitted by the linear equation.
[ChCl:2TCA]>[ChCl:2MCA]>[ChCl:2PA]. With the same HBA, the changing of HBD
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influences the physical properties of the DESs greatly.
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Acknowledgments
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Sincere thanks should be given to National Natural Science Foundation of China (NSFC Grant No. 21546007 and 21133055), Dalian Outstanding Scholar Project (Grant No. 2016RJ11), Dalian Technology Star Project (Grant No. 2016RQ079) and Natural
CE
Science Foundation of Liaoning Provice of China (Grant No. 201602019) for financial
AC
support of this project.
Supplementary materials
The experimental values of density, electrical conductivity, dynamic viscosity, and refractive index are listed in Tables S1 to S4 of the the Supplementary materials. This
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information
is
available
free
of
charge
via
the
internet
at
https://www.journals.elsevier.com/journal-of-molecular-liquids.
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(6) P. Liu, J.W. Hao, L.P. Mo, Z.H. Zhang, RSC Adv. 5 (2015) 48675-48704. (7) A.P. Abbott, K.E. Ttaib, G. Frisch, K.S. Ryder, D. Weston, Phys. Chem. Chem. Phys. 14 (2012) 2443-2449.
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(8) H.R. Jhong, D.S.H. Wong, C.C.Wan, Y.Y. Wang, T.C. Wei, Electrochem. Commun.
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11 (2009) 209-211.
(9) G. Morrison, C.C. Sun, S. Neervannan, Int. J. Pharm. 378 (2009) 136-139. (10) A. García, E. Rodríguez-Juan, G. Rodríguez-Gutiérrez, J.J. Rios, J. FernándezBolaños, Food Chem. 197 (2016) 554-561. (11) M.A. Kareem, F.S. Mjalli, M.A. Hashim, I.M. AlNashef, Fluid Phase Equilib. 314 (2012) 52-59.
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(12) Y. Wang, Y.C. Hou, W.Z. Wu, D.D. Liu, Y.A. Ji, S.H. Ren, Green Chem. 18 (2016) 3089-3097. (13) Q.H. Zhang, K.O. Vigier, S. Royer, F. Jérôme, Chem. Soc. Rev. 41 (2012) 71087146.
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(14) M. Francisco, A.V.D. Bruinhorst, M.C. Kroon, Angew. Chem. Int. Ed. 52 (2013) 3074-3085.
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(15) B. Tang, K.H. Row, Monatsh Chem.144 (2013) 1427-1454.
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(16) J.M. Yin, J.P. Wang, Z. Li, D. Li, G. Yang, Y.N. Cui, A.L.Wang, C.P. Li, Green Chem.17 (2015) 4552-4559.
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Green Chem. 15 (2013) 2793-2799.
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(17) C.P. Li, D. Li, S.S. Zou, Z. Li, J.M.Yin, A.L.Wang, Y.N. Cui, Z.L.Yao, Q. Zhao,
(18) G.F. Li, C.Y. Yan, B.B. Cao, J.Y. Jiang, W.C. Zhao, J.F. Wang, T. C. Mu, Green
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Chem. 18 (2016) 2522-2527.
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(19) C. Florindo, F.S. Oliveira, L.P.N. Rebelo, Ana M. Fernandes, I.M. Marrucho, ACS Sustain. Chem. & Eng. 2 (2014) 2416. (20) Q.S. Liu, P.P. Li, U. Welz-Biermann, J. Chen, X.X. Liu, J. Chem. Thermodyn. 66
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(2013) 88-94.
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(21) H.Z. Su, Q.S. Liu, J.M. Yin, C.P. Li, Acta Phys. -Chim. Sin. 31 (2015) 1468-1473. (22) A.L. Zhu, Q.Q. Li, L.J. Li, J.J. Wang, Catal. Lett.143 (2013) 463-468. (23) J. Jacquemin, P. Husson, A.A.H. Padua, V. Majer, Green Chem. 8 (2006) 172180. (24) L. Glasser, Thermochim. Acta. 421 (2004) 87-93. (25) J. Vila, P. Ginés, J.M. Pico, C. Franjo, E. Jiménez, L.M. Varela, O. Cabeza, Fluid Phase Equilibr. 242 (2006) 141-146.
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(26) C. Schreiner, S. Zugmann, R. Hartl, H.J. Gores, J. Chem. Eng. Data. 55 (2010) 1784-1788. (27) J.P. Belieres, C.A. Angell, J. Phys. Chem. B 111 (2007) 4926-4937.
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(28) C.A. Angell, N. Byrne, J.P. Belieres , Acc. Chem. Res. 40 (2007) 1228-1236.
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Supplementary materials
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S. Fig.1 1H NMR spectra of four DESs
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S. Fig.2 The DSC spectrum of [ChCl:2PTSA]
S. Fig.3 The DSC spectrum of [ChCl:2TCA]
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S. Fig.4 The DSC spectrum of [ChCl:2MCA]
S. Fig.5 The DSC spectrum of [ChCl:2PA]
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S.Table 1. The values of density, ρ/ g·cm-3, for DESs under atmospheric pressure [ChCl:2PTSA]
[ChCl:2TCA]
[ChCl:2MCA]
[ChCl:2PA]
288.15
1.24910
1.4712
1.2863
1.0821
293.15
1.24608
1.4662
1.2824
1.0788
298.15
1.24281
1.4612
1.2786
1.0755
303.15
1.23957
1.4563
1.2748
1.0721
308.15
1.23641
1.4516
1.2710
1.0688
313.15
1.23325
1.4470
1.2673
1.0655
318.15
1.23002
1.4424
1.2635
1.0622
323.15
1.22678
1.4378
1.2597
1.0589
328.15
1.22357
1.4332
333.15
1.22057
1.4287
338.15
1.21737
1.4240
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T/K
1.2559
1.0557
1.2521
1.0524
1.2483
1.0491
The uncertainty is ± 0.0005 g·cm-3 for density. The uncertainty is ± 0.02 K for
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temperature.
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[ChCl:2TCA]
[ChCl:2MCA]
[ChCl:2PA]
288.15
0.361
0.0449
0.223
0.310
293.15
0.477
0.0568
0.269
0.352
298.15
0.619
0.0712
0.317
0.393
303.15
0.791
0.0873
0.368
0.438
308.15
0.998
0.1061
0.422
0.484
313.15
1.243
0.1273
0.481
0.535
318.15
1.523
0.1503
0.542
0.585
323.15
1.844
0.1755
328.15
2.21
0.201
333.15
2.60
0.230
338.15
3.05
0.261
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T/K
0.608
0.638
0.671
0.692
0.738
0.745
0.812
0.805
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The uncertainty is ± 1% for electrical conductivity. The uncertainty is ± 0.05 K for
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temperature.
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S.Table 3. The values of dynamic viscosity, η/ mPa·s, for DESs under atmospheric pressure [ChCl:2PTSA]
[ChCl:2TCA]
[ChCl:2MCA]
[ChCl:2PA]
288.15
1838*
994.3
226.39
91.74
293.15
1102*
645.94
161.04
70.981
298.15
697.9
434.99
117.93
55.909
303.15
456.4
302.46
88.587
44.789
308.15
317.4
216.09
68.109
36.448
313.15
227.4
158.65
53.442
30.057
318.15
161.3
119.25
42.653
25.092
323.15
124.0
91.406
328.15
94.1
71.542
333.15
72.3
57.011
338.15
57.0
46.372
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T/K
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* the calculated value
34.648
21.186
28.565
18.071
23.859
15.557
20.220
13.536
The uncertainty is ± 1% for dynamic viscosity. The uncertainty is ± 0.02 K for
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temperature.
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[ChCl:2PTSA]
[ChCl:2TCA]
[ChCl:2MCA]
[ChCl:2PA]
288.15
1.5189
1.4934
1.4744
1.4499
293.15
1.5178
1.4920
1.4732
1.4491
298.15
1.5167
1.4907
1.4720
1.4482
303.15
1.5155
1.4894
1.4709
1.4472
308.15
1.5144
1.4881
1.4697
1.4463
313.15
1.5132
1.4868
1.4687
1.4453
318.15
1.5120
1.4854
1.4674
1.4444
323.15
1.5108
1.4841
1.4663
1.4434
328.15
1.5096
1.4827
1.4651
1.4425
333.15
1.5084
1.4814
1.4639
1.4415
338.15
1.5070
1.4800
1.4627
1.4406
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T/K
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S.Table 4. The values ofrefractive index, nD, for DESs under atmospheric pressure
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The uncertainty is ± 0.0002for refractive index. The uncertainty is ± 0.05 K for
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temperature.
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Graphical Abstrate
2.0 1.5
-1
logΛ(S•cm •mol )
1.0
ChCl:2PTSA ChCl:2TCA ChCl:2MCA ChCl:2PA
2
0.5 0.0
-1.0 -1.5 -1.5
-1.0
-0.5
0.0
0.5
logη /(10 Pa•s) -1
-1
1.0
-1
1.5
2.0
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-2.0 -2.0
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-0.5
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The density, dynamic viscosity, electrical conductivity, refractive index and acidity of
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DESs were characterized.
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Highlights 1) Acidic deep eutectic solvents were synthesized by mixing choline chloride with different organic acids. 2) The concise control of DESs’ acidity was realized successfully.
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3) The density, electrical conductivity, dynamic viscosity and refractive index of the four DESs were determined.
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4) The acidity of the four DESs was detected by Hammett acidic functions.
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Yingna Cui, Changping Li, Jingmei Yin, Shenmin Li, Yingping Jia, Ming Bao PII: DOI: Reference:
S0167-7322(17)30318-5 doi: 10.1016/j.molliq.2017.04.052 MOLLIQ 7208
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
23 January 2017 12 April 2017 13 April 2017
Please cite this article as: Yingna Cui, Changping Li, Jingmei Yin, Shenmin Li, Yingping Jia, Ming Bao , Design, synthesis and properties of acidic deep eutectic solvents based on choline chloride. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Molliq(2017), doi: 10.1016/j.molliq.2017.04.052
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Design, Synthesis and Properties of Acidic Deep Eutectic Solvents Based on Choline Chloride
a
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Yingna Cui,a, b Changping Li,*c Jingmei Yin,b Shenmin Li,b Yingping Jia,b Ming Bao*a
State Key Laboratory of Fine Chemicals, Dalian University of Technology, 116023,
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Dalian, China
Department of Chemical Engineering, Dalian University, 116622, Dalian, China
c
School of Environment and Civil Engineering, Dongguan University of Technology,
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b
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523808, Dongguan, China,
Abstract
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Four deep eutectic solvents (DESs) based on different acids as hydrogen bond donors
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(HBDs), namely p-toluenesulfonic acid (PTSA), trichloroacetic acid (TCA), monochloroacetic acid (MCA), and propionic acid (PA) with hydrogen bond acceptor (HBA) choline chloride (ChCl) are synthesized with a mole ratios of HBD to HBA is
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2:1. The density, electrical conductivity, dynamic viscosity and refractive index of the
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four DESs were determined at atmospheric pressure and temperatures from (288.15 to 338.15) K at an interval of 5 K. The results show that the temperature has great influences on the physical properties of DESs. The thermal expansion coefficient, molecular volume, standard molar entropy, and lattice energy were calculated by empirical equation. The molar conductivity was determined from the data of density .
*Corresponding authors. Changping Li, Tel: +86-411-87402436; Fax: +86-411-87402449; Email address: [email protected]; Ming Bao, Tel: +86-411-84986180; Fax: +86-411-84986180; Email address: [email protected];
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and conductivity. The temperature dependence of electrical conductivities and dynamic viscosities for the DESs were fitted by Vogel–Fulcher–Tamman (VFT) and Arrhenius equation. The relationship of the molar conductivity and viscosity was determined using the Walden rule. The acidities of the above DESs were measured by Hammett acidic
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functions. The present study will help to understand the properties of DESs and provide
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a guide for further applications of DESs.
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Keywords: Ionic liquids analogues; Deep eutectic solvents; Density; Electrical
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conductivity; Dynamic viscosity; Refractive index; Acidity
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1. Introduction
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Nowadays, deep eutectic solvents (DESs), as a new generation of green solvents,
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have been attracting scientific and technological attention due to their unique properties such as the simple synthesis and purification process, low cost of raw materials and renewable, good biocompatibility, etc.1,
2
DESs have been applied in many fields,
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including solvents or catalysts for chemical reactions,3-6 electrochemistry,7,
8
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pharmaceuticals,9 separation process10-12 etc. The physicochemical properties of DESs are indispensable for its industrial and engineering applications. However, the lack of physicochemical properties data has hindered its applications to a great extent. Therefore, a comprehensive evaluation of the physicochemical properties of DESs is necessary and valuable for future applications. Generally, DESs can be prepared by two or more cheap and green components through establishment of hydrogen bond between the hydrogen bond donor (HBD) and the
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hydrogen bond acceptor (HBA).13, 14 The mostly DESs synthesized so far usually used choline chloride (ChCl), quaternary ammonium salt and phosphate salt as HBAs. However, the choice of HBD is relatively abundant, for example, alcohols, polyols, carboxylic acids, amides, amines etc. can be used as HBDs.15 DESs with different
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characters can be obtained through the choice of different HBDs. Series of DESs were designed, synthesized and applied for the removal of organic 17
Novel deep desulfurization
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sulfide in fuels by our group for the first time.16,
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technology was set up. Acidic DESs was proved an efficient catalyst, solvents and extractants. It was found that HBDs had greater influence on the desulfurization process
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than HBAs, and the acidity of DESs is the main factor that determines its catalytic
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capability.
A series of DESs was designed and synthesized according to the acidity of HBD,
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with which DESs’ acidity can be controlled concisely. In this way, some typical acidic
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DESs with different acidity gradient were got. Choline chloride (ChCl) was chosen as a typical HBA, which was a very cheap, non-toxic and biodegradable material.
18
And p-
toluenesulfonic acid (PTSA), trichloroacetic acid (TCA), monochloroacetic acid
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(MCA), propionic acid (PA) were chosen as HBDs. The main physicochemical,
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including the density, dynamic viscosity, electrical conductivity, and refractive index were characterized in temperature range of (288.15 to 338.15) K. The relationship of the density, viscosity and conductivity was built in terms of the Walden rule. The relative acidity was investigated by Hammett acidic functions.
2. Experimentals
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2.1 Materials The chemicals used in the work were supplied by Aladdin Chemicals Co., Ltd, Shanghai, China. Purities of the chemicals are shown as follows: Choline chloride , AR, 98%; p-toluenesulfonic acid monohydrate, AR≥98.5%; trichloroacetic acid , AR, 99%;
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monochloroacetic acid, AR, 99%; propionic acid, AR≥99.5%. All reagents were used with further purification or drying.
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2.2 Synthesis of DESs
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The purified compounds of ChCl and PTSA (or TCA, MCA, PA) were mixed with
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the molar ratio 1: 2. Then the deep eutectic solvents were prepared by different methods: The ChCl and PTSA were stirred at 80 °C for 4 hours; The ChCl and TCA (or
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MCA) were grinded in a mortar with a pestle at room temperature until a homogeneous liquid was formed.19 The ChCl and PA were stirred at room temperature until a
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homogeneous liquid was formed. The forming of ester impurity between ChCl and
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carboxylic acid can be avoided through grinding method and stirring method at room temperature. The purities of DESs were confirmed by 1H NMR as shown in S.Fig.1 of
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the Supplementary materials.
2.3 Density, Dynamic Viscosity, Conductivity and Acidity Measurements
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The density and dynamic viscosity were determined using an Anton paar Viscometer SVM 3000 in the temperature range of (288.15 to 338.15) K ± 0.02 K. The experimental error of density was within ± 0.0005 g·cm-3 and the uncertainties of dynamic viscosity were estimated to be ± 1%. Conductivities of the DESs were determined via a MP522 conductivity instrument with the cell constants of 1 cm-1 (the cell was calibrated with the aqueous KCl solution) in the temperature range of (288.15 to 338.15) ± 0.05 K and the uncertainties were estimated to be ± 1%. The measurement
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frequency and voltage of the electrical conductivity are 50 Hz and 9 V, respectively. 20, 21
The refractive index data were carried on a WAY-2S Abbe refractometer (Shanghai
Shen Guang Instrument Co., P. R. China). Hammett acidic functions were measured for different DESs on an Hitachi U3900-H UV–Vis spectroscopy using 4-nitroaniline as the
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basic indicator and water as solvent according to literatures.22 The melting points were determined by DSC 204HP (Netzsc, Germany).
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All the experiments in this study were performed in triplicate to determine its
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reproducibility, and the experimental errors were within 3%. The above experimental values were listed in S.Tables 1 to 4 of the supplementary materials. There is no melting
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point in our limited experimental conditions above -80 ℃, while the glass transition
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temperature of the DESs are -55 ℃ for [ChCl:2PTSA], -53 ℃for [ChCl:2TCA], -40 ℃ for [ChCl:2MCA], -64 ℃[ChCl:2PA], respectively. The results were shown in S.Fig. 2
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3. Results and discussions
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to 5 of the supplementary materials.
3.1 Density
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The temperature dependence on density are plotted in Fig. 1. It can be seen that the density decrease with the temperature increasing, and the line of density is linear with
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the increase of the temperature. For density, sequence of the four DESs is [ChCl:2PTSA]>[TBAC:2TCA]>[TBAC:2MCA]>[TBAC:2PA].
The
tendency
is
consistent with the molecular mass of DESs. The DESs exhibit the high density which has the relative large molecule mass.
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1.5
1.4
ChCl:2PTSA ChCl:2TCA ChCl:2MCA ChCl:2PA
1.2
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ρ/g•cm-3
1.3
1.0 280
290
300
310
320
330
340
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T/K
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1.1
Fig.1 Density versus temperature plots for DESs
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The thermal expansion coefficient, α, can be obtained by the lnρ versus T fitted according to a following straight line.
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ln ρ = b – α·T
(1)
where b is an empirical constant, α is the thermal expansion coefficient (Table 1). The
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values are in the range of 5×10-4 and 7×10-4 K-1 which obtained by Jacquemin.23
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The molecular volume, Vm, standard molar entropy, S, and lattice energy, UPOT, also
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can be calculated from density values by the following empirical equations.24 Vm =M/(N·ρ)
(2)
S0 = 1246.5·(Vm) + 29.5
(3)
UPOT = 1981.2·(ρ/M)1/3 + 103.8
(4)
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Table 1 Estimated physicochemical property values of DESs at 298.15 K at atmospheric pressure [ChCl:2PTSA]
[ChCl:2TCA]
[ChCl:2MCA]
[ChCl:2PA]
MW/g·mol–1
520.07
466.40
328.62
287.78
Vm/nm3
0.6469
0.5302
0.4269
0.4445
104α/K–1
5.17
6.48
V/cm–3·mol–1
389.4
319.2
S0/J·K–1·mol–1
835.9
690.4
Upot/kJ·mol–1
375
394
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Property
6.19
257.0
267.6
561.6
583.6
415
411
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5.98
3.2 Molar Electrical Conductivity
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The molar electrical conductivities can be determined according to the following equation from density and electrical conductivity: Λ = σMρ–1
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(5)
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where Λ is the molar electrical conductivity, σ is the electrical conductivity, M is the molar mass, and ρ is the density. The molar electrical conductivity values are listed in 2.
The
order
of
the
molar
electrical
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Table
conductivity
for
DESs
is
[ChCl:2PTSA]>[TBAC:2PA]>[TBAC:2MCA]>[TBAC:2TCA]. The p-toluenesulfonic
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acid based DESs exhibit the higher electrical conductivity than other acid based EDSs.
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Table 2 Calculated molar electrical conductivity values for DESs in the temperature range of (288.15 to 338.15) K [ChCl:2PTSA]
[ChCl:2TCA]
[ChCl:2MCA]
[ChCl:2PA]
288.15
0.123
0.014
0.057
0.082
293.15
0.163
0.018
0.069
0.094
298.15
0.212
0.023
0.081
0.105
303.15
0.272
0.028
0.095
0.118
308.15
0.344
0.034
0.109
0.130
313.15
0.429
0.041
318.15
0.528
0.049
323.15
0.640
328.15
0.770
333.15
0.908
338.15
1.068
0.145
0.141
0.158
0.057
0.159
0.173
0.065
0.176
0.189
0.075
0.194
0.204
0.085
0.214
0.221
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0.125
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T/K
3.3 Electrical Conductivity
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Electrical conductivity versus temperature plots for DESs are listed in Fig.2. From Fig.2, it can be seen that for the electrical conductivity, the line is non-linear with
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temperature increasing, and the electrical conductivity increase with the temperature increasing. Generally, the Vogel−Fulcher−Tamman (VFT) and Arrhenius equations
19
can be used for the fitting of the electrical conductivity values, σ, versus temperature, T: σ= σ0·exp(–B/(T–T0))
(6)
σ = σ∞·exp(–Eσ/(kBT))
(7)
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here σ is the electrical conductivity; σ0, B are fitting parameters; Eσ is the activation energy, which indicates the energy needed for an ion to hop to a free hole; σ∞ is the maximum electrical conductivity, and kB is the Boltzmann constant.
3.5 3.0
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2.0 1.5 1.0
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σ/mS·cm-1
2.5
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ChCl:2PTSA ChCl:2TCA ChCl:2MCA ChCl:2PA
0.0 280
290
300
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0.5 310
320
330
340
T/K
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Fig.2 Electrical conductivity versus temperature plots for DESs
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The fitted values of electrical conductivity versus temperature are listed in Table 3. It can be seen that the correlation coefficient, R, is more than 0.9999, which indicates that
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the VFT equation can be well used for the electrical conductivity fitting. By the Arrhenius equation, the Lnσ vs 1000/T was plotted for the DESs in Fig. 3. According to
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the equation, the experimental points should follow a straight line, however, they are not on the straight line in the Fig. 3. (The red lines are straight lines) That is, the electrical conductivity does not meet the Arrhenius behavior well. For electrical conductivity, the VFT equation and Arrhenius equation can be combined according to the Vila et al.
25
researchand the final version of VFT equation
is: σ = σ∞·exp(–Eσ/(kB(T–T0))
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(8)
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here, σ0 = σ∞ and B = Eσ/kB. Then the values of electrical conductivity activation energies can be calculated by the equation (Table 3). Table 3 The fitted values of electrical conductivity versus temperature for DESs by VFT and final version VFT equations [ChCl:2PTSA]
[ChCl:2TCA]
[ChCl:2MCA]
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Property
[ChCl:2PA]
0.697
0.012
0.013
0.015
B/K
957.7
600.06
435.25
592.62
103Eσ/eV
82.6
51.8
37.6
51.1
T0/K
161.8
180.6
R
0.99997
0.99995
1.0 0.5
-2.0
180.7
134.9
0.99995
0.99995
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ChCl:2PTSA ChCl:2TCA ChCl:2MCA ChCl:2PA
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-0.5
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Lnσ/ mS cm-1
0.0
-1.5
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1.5
-1.0
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σ0/S·cm−1
-2.5 -3.0
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2.9
3.0
3.1
3.2
3.3
3.4
3.5
1000/T(K)
Fig.3 Plot of Lnσ versus 1000/T for DESs
3.4 Dynamic Viscosity The temperature dependence on dynamic viscosity is listed in Fig. 4. The dynamic viscosity decrease with the temperature increasing, and the tendency of the four DESs is [ChCl:2PTSA] > [ChCl:2TCA] >[ChCl:2MCA] > [ChCl:2PA]. The small molecule exhibits the better liquidity than the relative big molecule. [ChCl:2PA] has the better
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liquidity than other DESs. The reason maybe that the force between ChCl and PA is relatively small, while the flow resistance become weaker, which led to the lower viscosity of [ChCl:2PA].
PT
2000 ChCl:2PTSA ChCl:2TCA ChCl:2MCA ChCl:2PA
1800 1600
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1400
1000
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η/mPa·s
1200
800 600
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400 200
280
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0 290
300
310
320
330
340
T /K
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Fig.4 Dynamic viscosity versus temperature plots for DESs
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The VFT and Arrhenius equations are also used for the fitting of the dynamic
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viscosity, η, versus temperature, T.
η = η0·exp(B/(T–T0))
(9)
η = η∞·exp(Eη/(kBT))
(10)
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where η is the dynamic viscosity; η0, B are fitting parameters; Eη is the activation energy for dynamic viscosity; η∞ is the maximum dynamic viscosity, and kB is the Boltzmann constant. The fitted values of dynamic viscosity versus temperature are shown in Table 4. From the correlation coefficient, R, are more than 0.999, it can be concluded that the VFT equation can be used for the dynamic viscosity fitting. According to equation (10), the Lnη vs 1000/T is described in Fig.5. The results indicate that the experimental dynamic viscosity does not follow the Arrhenius behavior
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well, which is similar to the electrical conductivity. The solid lines are smooth curves for Lnη vs 1000/T, the red lines are straight lines for fitting of Lnη vs 1000/T. The same for dynamic viscosity, the final version of VFT equation is: η = η∞·exp(Ea/(kB (T–T0))
(11)
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here, η0 = σ∞ and B = Eη/kB. The dynamic viscosity activation energies of DESs are shown in Table 4.
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Table 4 The fitted values of dynamic viscosity versus temperature of DESs by VFT and
[ChCl:2PTSA]
[ChCl:2TCA]
[ChCl:2MCA]
[ChCl:2PA]
η0/mPa·s
0.1298
0.0629
0.1248
0.1061
B/K
847.7
1033.0
787.7
855.4
103Eη/eV
73.1
89.1
68.0
73.82
T0/K
199.5
181.3
183.2
161.7
R
0.99985
0.99999
0.99999
0.99999
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D
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Property
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final version VFT equations
8.0
CE
7.5 7.0
Lnη/ mPa s
AC
6.5
ChCl:2PTSA ChCl:2TCA ChCl:2MCA ChCl:2PA
6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5
2.9
3.0
3.1
3.2
3.3
3.4
3.5
1000/T(K)
Fig.5 Plot of Lnη versus 1000/T for four DESs
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3.5 Walden Product According to the Walden rule, the relationship of molar conductivity and dynamic viscosity can be described by the the following equation: Ληα = k
(12)
logΛ = logk + αlogη-1
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(13)
where Λ is the molar conductivity, η is the dynamic viscosity, k is a temperature
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dependent constant. The variation of LogΛ versus logη-1 are showed in Fig.6. The
SC
slopes, α, are obtained and the values are 0.628 for [ChCl:2PTSA], 0.586 for [ChCl:2TCA], 0.544 for [ChCl:2MCA], and 0.514 for [ChCl:2PA], respectively. It can
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be seen from Fig.6 that all of the Walden plots are under the ideal line except
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[ChCl:2PTSA], the ideal line is obtained using aqueous KCl solutions at high dilution.26 The lines of the DESs lie below the ideal KCl line, which mean that the DESs are “subionic”.27, 28 However, the line of [ChCl:2PTSA] lie near the ideal line, it can be
2.0
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1.5
PT E
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classified as “ionic”.
0.5 0.0
AC
2
-1
logΛ(S•cm •mol )
1.0
ChCl:2PTSA ChCl:2TCA ChCl:2MCA ChCl:2PA
-0.5 -1.0 -1.5 -2.0 -2.0
-1.5
-1.0
-0.5
0.0
0.5
logη /(10 Pa•s) -1
-1
1.0
1.5
2.0
-1
Fig.6 Plot of LogΛ versus logη-1 for DESs from 288.15 to 338.15 K. The solid straight line is the ideal line for aqueous KCl solutions.
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3.6 Refractive Index The plot of nD versus T for DESs is listed in Fig. 7. It can be seen that the refractive index decrease with the temperature increasing, and lines of refractive index are linear. The
sequence
of
the
DESs
is
[ChCl:2PTSA]>
PT
[ChCl:2TCA]>[ChCl:2MCA]>[ChCl:2PA].
following linear equation:
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nD= A + CT
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The relationship of refractive index and temperature can be described by the
(14)
where nD is the refractive index. The fitted values of refractive index versus temperature
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are listed in Table 5. The results of table 5 show that the correlation coefficients of the
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DESs are higher than 0.999.
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1.52
ChCl:2PTSA ChCl:2TCA ChCl:2MCA ChCl:2PA
D
1.54
nD
1.50 1.48
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1.46
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1.44
280
290
300
310
320
330
T/K
Fig.7 Plot of nD versus T for DESs
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Table 5 The fitted values of refractive index versus temperature of DESs A
−C
R
[ChCl:2PTSA]
1.5873
2.37×10–4
0.9994
[ChCl:2TCA]
1.5703
2.67×10–4
0.9999
[ChCl:2MCA]
1.5414
2.33×10–4
0.9998
[ChCl:2AP]
1.5042
1.88×10–4
0.9997
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PT
DESs
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3.7 Acidities of the Different DESs
The maximal absorbance of the un-protonated form of 4-nitroaniline is at 380 nm in
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water. The maximal absorbance of the un-protonated form of the indicator molecules
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was decreased with increasing the concentration of the acidic DESs. Therefore, the absorbance strength of the indicator in pure water was taken as the initial reference to
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determine the protonated and unprotonated forms of the indicator.
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Then the Hammett acidity function (H0) can be calculated by the equation: H0 = pK(I)aq + log ([I]s/[IH+]s)
(15)
where I stands for indicator pK(I)aq=pKa=0.99, [I]s is the molarity of the unprotonated
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form of the indicator, [IH+]s is the molarity of the protonated form of the indicator. Fig.8
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shows the absorbance spectra of 4-nitroaniline in the presence of different DESs in water at the concentration of 40.0 mmol/l and 293.15 K. The indicator were dissolved in water with the concentrations of 7.24×10-2 mmol/L. According to equation (15), the H0 values of different acidic DESs are showed in Table 6. The sequence of acidities of the DESs is [ChCl:2PTSA]> [ChCl:2TCA]>[ChCl:2MCA]>[ChCl:2PA]. It is well known that there exists hydrogen bond between HBD and HBA, and the accepted hydrogen
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will enhance the acidity of the DES, so the [ChCl:2PTSA] has the highest acidity owning to the higher acidity of PTSA.
1.2 blank ChCl:2PTSA ChCl:2TCA ChCl:2MCA ChCl:2PA
PT
1.0
0.6
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Abs.
0.8
0.2
350
400 nm
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0.0 300
SC
0.4
450
500
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Fig.8 UV-vis absorbance spectra of 4-nitroaniline in the presence of different DESs in water at the concentration of 40.0 mmol/l and 293.15K
Amax
[I](%)
[IH] (%)
H0
1.055
100
0
-
0.456
43.2
56.8
0.87
[ChCl:2TCA]
0.508
48.2
51.8
0.96
[ChCl:2MCA]
0.790
74.9
25.1
1.46
[ChCl:2AP]
0.998
94.6
5.4
2.23
-
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[ChCl:2PTSA]
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DESs
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Table 6 Hammett acidity functions of different DESs (40.0 mmol/l) in water at 293.15K
4. Conclusions In this work, deep eutectic solvents were synthesized by mixing choline chloride with different organic acids used as hydrogen bond donors. The physical properties (density,
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electrical conductivity, dynamic viscosity, and refractive index) were studied in the range of temperatures from 288.15 to 338.15 K at atmospheric pressure. The thermal expansion coefficient, molecular volume, standard molar entropy, lattice energy of the DESs were estimated by density values. The VFT equation can be used for fitting of the
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electrical conductivity and dynamic viscosity to temperature very well. The relationship of the density, electrical conductivity, and dynamic viscosity was built by the Walden
sequence
of
the
acidity
of
the
DESs
is
[ChCl:2PTSA]>
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The
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rule. The temperature dependence on refractive index was fitted by the linear equation.
[ChCl:2TCA]>[ChCl:2MCA]>[ChCl:2PA]. With the same HBA, the changing of HBD
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influences the physical properties of the DESs greatly.
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Acknowledgments
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Sincere thanks should be given to National Natural Science Foundation of China (NSFC Grant No. 21546007 and 21133055), Dalian Outstanding Scholar Project (Grant No. 2016RJ11), Dalian Technology Star Project (Grant No. 2016RQ079) and Natural
CE
Science Foundation of Liaoning Provice of China (Grant No. 201602019) for financial
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support of this project.
Supplementary materials
The experimental values of density, electrical conductivity, dynamic viscosity, and refractive index are listed in Tables S1 to S4 of the the Supplementary materials. This
17
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information
is
available
free
of
charge
via
the
internet
at
https://www.journals.elsevier.com/journal-of-molecular-liquids.
References
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(1) M. Francisco, A.V.D. Bruinhorst, M. C. Kroon, Green Chem., 14 (2012) 2153-2157. (2) A.P. Abbott, J.C. Barron, G. Frisch, S. Gurman, K.S. Ryder, Phys. Chem. Chem.
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Phys. 13 (2011) 10224-10231.
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(3) M.C. Serrano, M.C. Gutierrez, R. Jiménez, M.L. Ferrer, M.F. Del, Chem. Commun., 48 (2012) 579-581.
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(4) Z.H. Zhang, X.N. Zhang, L.P. Mo, Y.X. Li, F.P. Ma, Green Chem.43 (2012) 1502-
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(5) E. Massolo, S. Palmieri, M. Benaglia, V. Capriati, F.M. Perna, Green Chem. 18
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(6) P. Liu, J.W. Hao, L.P. Mo, Z.H. Zhang, RSC Adv. 5 (2015) 48675-48704. (7) A.P. Abbott, K.E. Ttaib, G. Frisch, K.S. Ryder, D. Weston, Phys. Chem. Chem. Phys. 14 (2012) 2443-2449.
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(8) H.R. Jhong, D.S.H. Wong, C.C.Wan, Y.Y. Wang, T.C. Wei, Electrochem. Commun.
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(9) G. Morrison, C.C. Sun, S. Neervannan, Int. J. Pharm. 378 (2009) 136-139. (10) A. García, E. Rodríguez-Juan, G. Rodríguez-Gutiérrez, J.J. Rios, J. FernándezBolaños, Food Chem. 197 (2016) 554-561. (11) M.A. Kareem, F.S. Mjalli, M.A. Hashim, I.M. AlNashef, Fluid Phase Equilib. 314 (2012) 52-59.
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(12) Y. Wang, Y.C. Hou, W.Z. Wu, D.D. Liu, Y.A. Ji, S.H. Ren, Green Chem. 18 (2016) 3089-3097. (13) Q.H. Zhang, K.O. Vigier, S. Royer, F. Jérôme, Chem. Soc. Rev. 41 (2012) 71087146.
PT
(14) M. Francisco, A.V.D. Bruinhorst, M.C. Kroon, Angew. Chem. Int. Ed. 52 (2013) 3074-3085.
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(15) B. Tang, K.H. Row, Monatsh Chem.144 (2013) 1427-1454.
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(16) J.M. Yin, J.P. Wang, Z. Li, D. Li, G. Yang, Y.N. Cui, A.L.Wang, C.P. Li, Green Chem.17 (2015) 4552-4559.
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Green Chem. 15 (2013) 2793-2799.
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(17) C.P. Li, D. Li, S.S. Zou, Z. Li, J.M.Yin, A.L.Wang, Y.N. Cui, Z.L.Yao, Q. Zhao,
(18) G.F. Li, C.Y. Yan, B.B. Cao, J.Y. Jiang, W.C. Zhao, J.F. Wang, T. C. Mu, Green
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Chem. 18 (2016) 2522-2527.
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(19) C. Florindo, F.S. Oliveira, L.P.N. Rebelo, Ana M. Fernandes, I.M. Marrucho, ACS Sustain. Chem. & Eng. 2 (2014) 2416. (20) Q.S. Liu, P.P. Li, U. Welz-Biermann, J. Chen, X.X. Liu, J. Chem. Thermodyn. 66
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(2013) 88-94.
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(21) H.Z. Su, Q.S. Liu, J.M. Yin, C.P. Li, Acta Phys. -Chim. Sin. 31 (2015) 1468-1473. (22) A.L. Zhu, Q.Q. Li, L.J. Li, J.J. Wang, Catal. Lett.143 (2013) 463-468. (23) J. Jacquemin, P. Husson, A.A.H. Padua, V. Majer, Green Chem. 8 (2006) 172180. (24) L. Glasser, Thermochim. Acta. 421 (2004) 87-93. (25) J. Vila, P. Ginés, J.M. Pico, C. Franjo, E. Jiménez, L.M. Varela, O. Cabeza, Fluid Phase Equilibr. 242 (2006) 141-146.
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(26) C. Schreiner, S. Zugmann, R. Hartl, H.J. Gores, J. Chem. Eng. Data. 55 (2010) 1784-1788. (27) J.P. Belieres, C.A. Angell, J. Phys. Chem. B 111 (2007) 4926-4937.
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(28) C.A. Angell, N. Byrne, J.P. Belieres , Acc. Chem. Res. 40 (2007) 1228-1236.
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Supplementary materials
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S. Fig.1 1H NMR spectra of four DESs
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S. Fig.2 The DSC spectrum of [ChCl:2PTSA]
S. Fig.3 The DSC spectrum of [ChCl:2TCA]
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S. Fig.4 The DSC spectrum of [ChCl:2MCA]
S. Fig.5 The DSC spectrum of [ChCl:2PA]
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S.Table 1. The values of density, ρ/ g·cm-3, for DESs under atmospheric pressure [ChCl:2PTSA]
[ChCl:2TCA]
[ChCl:2MCA]
[ChCl:2PA]
288.15
1.24910
1.4712
1.2863
1.0821
293.15
1.24608
1.4662
1.2824
1.0788
298.15
1.24281
1.4612
1.2786
1.0755
303.15
1.23957
1.4563
1.2748
1.0721
308.15
1.23641
1.4516
1.2710
1.0688
313.15
1.23325
1.4470
1.2673
1.0655
318.15
1.23002
1.4424
1.2635
1.0622
323.15
1.22678
1.4378
1.2597
1.0589
328.15
1.22357
1.4332
333.15
1.22057
1.4287
338.15
1.21737
1.4240
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SC
RI
PT
T/K
1.2559
1.0557
1.2521
1.0524
1.2483
1.0491
The uncertainty is ± 0.0005 g·cm-3 for density. The uncertainty is ± 0.02 K for
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CE
PT E
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temperature.
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ACCEPTED MANUSCRIPT S.Table 2. The electrical conductivity values, σ/ mS·cm–1, for DESs under atmospheric pressure [ChCl:2PTSA]
[ChCl:2TCA]
[ChCl:2MCA]
[ChCl:2PA]
288.15
0.361
0.0449
0.223
0.310
293.15
0.477
0.0568
0.269
0.352
298.15
0.619
0.0712
0.317
0.393
303.15
0.791
0.0873
0.368
0.438
308.15
0.998
0.1061
0.422
0.484
313.15
1.243
0.1273
0.481
0.535
318.15
1.523
0.1503
0.542
0.585
323.15
1.844
0.1755
328.15
2.21
0.201
333.15
2.60
0.230
338.15
3.05
0.261
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SC
RI
PT
T/K
0.608
0.638
0.671
0.692
0.738
0.745
0.812
0.805
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The uncertainty is ± 1% for electrical conductivity. The uncertainty is ± 0.05 K for
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CE
PT E
D
temperature.
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S.Table 3. The values of dynamic viscosity, η/ mPa·s, for DESs under atmospheric pressure [ChCl:2PTSA]
[ChCl:2TCA]
[ChCl:2MCA]
[ChCl:2PA]
288.15
1838*
994.3
226.39
91.74
293.15
1102*
645.94
161.04
70.981
298.15
697.9
434.99
117.93
55.909
303.15
456.4
302.46
88.587
44.789
308.15
317.4
216.09
68.109
36.448
313.15
227.4
158.65
53.442
30.057
318.15
161.3
119.25
42.653
25.092
323.15
124.0
91.406
328.15
94.1
71.542
333.15
72.3
57.011
338.15
57.0
46.372
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SC
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PT
T/K
MA
* the calculated value
34.648
21.186
28.565
18.071
23.859
15.557
20.220
13.536
The uncertainty is ± 1% for dynamic viscosity. The uncertainty is ± 0.02 K for
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CE
PT E
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temperature.
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[ChCl:2PTSA]
[ChCl:2TCA]
[ChCl:2MCA]
[ChCl:2PA]
288.15
1.5189
1.4934
1.4744
1.4499
293.15
1.5178
1.4920
1.4732
1.4491
298.15
1.5167
1.4907
1.4720
1.4482
303.15
1.5155
1.4894
1.4709
1.4472
308.15
1.5144
1.4881
1.4697
1.4463
313.15
1.5132
1.4868
1.4687
1.4453
318.15
1.5120
1.4854
1.4674
1.4444
323.15
1.5108
1.4841
1.4663
1.4434
328.15
1.5096
1.4827
1.4651
1.4425
333.15
1.5084
1.4814
1.4639
1.4415
338.15
1.5070
1.4800
1.4627
1.4406
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SC
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T/K
RI
S.Table 4. The values ofrefractive index, nD, for DESs under atmospheric pressure
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The uncertainty is ± 0.0002for refractive index. The uncertainty is ± 0.05 K for
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CE
PT E
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temperature.
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Graphical Abstrate
2.0 1.5
-1
logΛ(S•cm •mol )
1.0
ChCl:2PTSA ChCl:2TCA ChCl:2MCA ChCl:2PA
2
0.5 0.0
-1.0 -1.5 -1.5
-1.0
-0.5
0.0
0.5
logη /(10 Pa•s) -1
-1
1.0
-1
1.5
2.0
RI
-2.0 -2.0
PT
-0.5
SC
The density, dynamic viscosity, electrical conductivity, refractive index and acidity of
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CE
PT E
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DESs were characterized.
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Highlights 1) Acidic deep eutectic solvents were synthesized by mixing choline chloride with different organic acids. 2) The concise control of DESs’ acidity was realized successfully.
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3) The density, electrical conductivity, dynamic viscosity and refractive index of the four DESs were determined.
AC
CE
PT E
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MA
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SC
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4) The acidity of the four DESs was detected by Hammett acidic functions.
30