Formation of Deep Eutectic Solvents by Phenols and Choline Chloride

Article pubs.acs.org/jced

Formation of Deep Eutectic Solvents by Phenols and Choline Chloride and Their Physical Properties Wujie Guo,† Yucui Hou,‡ Shuhang Ren,† Shidong Tian,† and Weize Wu*,† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Department of Chemistry, Taiyuan Normal University, Taiyuan 030031, China

‡

S Supporting Information *

ABSTRACT: Novel deep eutectic solvents (DES) based on three different hydrogen-bond donors (HBD), namely phenol, o-cresol, and 2,3-xylenol, and choline chloride (ChCl) were successfully synthesized with different mole ratios of HBD to ChCl. Melting temperature of these DES were measured. Compared with an ideal mixture of the two components, the freezing temperature of the DES depresses greatly from (120 to 127) K. The physical properties, such as density, viscosity, and conductivity of phenolbased and o-cresol-based DES were determined at atmospheric pressure and temperatures from (293.2 to 318.2) K at an interval of 5 K. The results show that the type of HBD, the mole ratio of HBD to ChCl, and temperature have great influences on the physical properties of DES. Densities and viscosities of DES formed by phenol and ChCl decrease with increases of temperature and phenol content. The conductivities of the DES are from (1.40 to 7.06) mS·cm−1, similar to that of room temperature ionic liquids. The conductivities of the DES increase with an increase of temperature, and reach the highest values at phenol to ChCl mole ratios of 4.00 to 5.00. The temperature dependence of densities and conductivities for these DES were correlated by an empirical second-order polynomial with relative deviations less than 0.91 %, and the viscosities were fitted to the VTF equation with relative deviations less than 0.52 %.

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INTRODUCTION Phenols, one kind of the major industrial organic chemicals, are used to produce phenolic resins, bisphenol A, adipic acid, and so on.1 It is well-known that phenols are produced from petroleum, coal tar, coal liquefaction oil, and biomass pyrolysis oil.2−4 Phenols are first separated from these phenol-containing oils, and then the obtained phenols mixtures are refined for further applications. The present industrial method to separate phenols from oils mixtures is chemical extraction using aqueous alkaline solutions (such as aqueous NaOH) and then acidification of the extract by mineral acids (such as aqueous H2SO4) to recover the phenols. The disadvantages of this process are the use of large amounts of both strong alkalis and acids and the production of excessive amounts of wastewater containing phenols. The separation approach based on forming reversible chemical complexations is of high efficiency and high selectivity for separating polar organic solutes in dilute solutions.5 However, researches on extraction of phenolics from coal liquefaction oil and purification of crude phenol have not been reported on reversible chemical complexation because the chemical complexations are miscible with oil. Therefore, alternative methods to separate phenols from oil mixtures using a nonaqueous and oil-immiscible solvent were necessary. A deep eutectic is a eutectic mixture formed by compounds (usually solids) mixed together in proper ratios where the eutectic temperature is considerably lower, often by several 100 © 2013 American Chemical Society

K, than would be predicted from the known enthalpies of fusion of the pure compounds using ideal solution theory. As early as 2003, Abbott et al. reported that the mixtures of amides with quaternary ammonium salts could form low melting point eutectics that had unusual solvent properties.6 Later, the research group7,8 indicated that deep eutectic solvents (DES) could be formed between hydrogen bond donors (HBD) such as carboxylic acids as well as alcohols besides amides and a variety of quaternary ammonium salts. Formation and characterization of DES based on choline chloride and resorcinol, levulinic acid, and sugar-based polyols were also described.9,10 DES show good mediums for deposition of metals, such as Ag, Cu, Cr, Sn, and Zn, which have potential applications for electro-plating of metals.11,12 In addition, DES have been used as a reaction media in the process of synthesis of lanthanide-organic frameworks as well as polymer and related materials.13,14 Recently, DES have been used in separation process, because they are nontoxic, nonreactive with water, biodegradable, and easy to prepare.7,15 DES based on methyltriphenyl phosphunium bromide were used for the separation of glycerol from palm oil-based biodiesel, where glycerol is a HBD.16 The uses of DES as extraction agents for Received: September 1, 2012 Accepted: March 5, 2013 Published: March 15, 2013 866

dx.doi.org/10.1021/je300997v | J. Chem. Eng. Data 2013, 58, 866−872

Journal of Chemical & Engineering Data

Article

glycerol from biodiesel based on rapeseeds and soybeans was disclosed for the first time in 2007 by Abbott et al.8 In 2010, Hayyan et al.17 reported that DES based on a quaternary ammonium salt and glycerol could be also applied to extract the total glycerol from palm oil-based biodiesel. The physical properties such as conductivity and viscosity of these DES were similar to room temperature ionic liquids (ILs). At present, many researchers have focused on the physicochemical characterization of DES because of their promising applications.18 Lately, our research group found that choline chloride (ChCl) could be used to efficiently separate phenolic compounds from model oils by forming DES.19,20 When ChCl with an equimolar amount to phenol were added to phenol containing model toluene oils at 293 K, phenolextraction efficiency at an initial phenol concentration of 178 g·L−1 were found to be 93.4 % and the cresol-extraction efficiencies at an initial phenol concentration of about 200 g·L−1 were found to be 92.5 % (o-cresol), 94.8 % (m-cresol), and 94.7 % (p-cresol), respectively. When the DES method is employed for separations of phenols from oils, it is very important to understand the properties of DES phases for their significant influence on mass transfer, diffusion, phase separation efficiency, and so forth. However, data on the DES phase properties during separations of phenols from oils have not been reported in the literature. Therefore, this work focused on the formation of DES between phenolic compounds and ChCl, and measurements of melting temperature, density, viscosity, and conductivity of ChCl-based DES as a function of temperature and composition. Phenol, ocresol and 2,3-xylenol are dominant phenolic compounds in coal tar or coal liquefaction oil, and our previous work19 demonstrated that they could be extracted by ChCl by forming DES. Hence they were selected as HBD in this work.

Table 1. Compositions and Abbreviations for DES Used in This Worka salt

HBD

mole ratio of HBD to salt

abbreviation

water content/%

ChCl ChCl ChCl ChCl ChCl ChCl ChCl ChCl ChCl ChCl

phenol phenol phenol phenol phenol o-cresol o-cresol o-cresol o-cresol o-cresol

2.00 3.00 4.00 5.00 6.00 2.00 3.00 3.91 5.00 6.00

DES1 DES2 DES3 DES4 DES5 DES6 DES7 DES8 DES9 DES10

0.256 0.358 0.130 0.186 0.094 0.107 0.098 0.090 0.097 0.249

a

The standard uncertainty of mole ratio of HBD to Salt is 0.001, and the standard uncertainty of water content is 0.010 %.

of the pycnometer was calibrated by distilled water at different temperatures before experiment. Densities of the samples were measured from (293.2 to 318.2) K at an interval of 5 K. All of the measurements were carried out in a constant-temperature water bath, in which the temperature was maintained within ± 0.1 K by a temperature controller (model A2, Beijing Changliu Scientific Instrument Co., Ltd.). The reproducibility of the measurements was better than ± 0.05 %, and it was estimated that the uncertainty of the density was not more than ± 0.1 %. Viscosity Determination. The viscosities of the DES were determined using a traditional technique, Ubbelohde viscometer that was based on Poiseuiele Law. The standard oils of different viscosities provided by National Standard Bureau of China were used to calibrate the viscometer. 22 The reproducibility of the measurements was better than ± 1 %, and the uncertainty of the viscosity was less than ± 3 %. Conductivity Determination. Conductivities of the DES were determined via a conductivity meter (DDSJ-308A, Shanghai Leici Co., Ltd., Shanghai, China). The meter was calibrated with aqueous solutions of KCl of known conductivities at different concentrations. The reproducibility of the measurements was no more than ± 0.5 %, and the uncertainties of data were ± 1 %.

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EXPERIMENTAL SECTION Materials. ChCl, o-cresol, and 2,3-xylenol were obtained from Aladdin Chemical Co., Ltd. (Shanghai, China) with mass fraction purities more than 0.99. Phenol with a mass fraction purity of 0.98 was purchased from Beijing Chemical Plant (Beijing, China). All reagents were of analytical reagent grade and used for the synthesis of DES with no further purification. Synthesis of DES. In this work, novel DES based on ChCl and three HBD, namely phenol, o-cresol, and 2,3-xylenol, were synthesized with several mole ratios of ChCl to HBD. A 250 cm3 jacketed glass vessel with a mechanical stirrer was used to mix ChCl with HBD in different mole ratios at 40 rpm and 353.2 K until a homogeneous liquid appeared. The water contents of DES, detected by a Karl Fischer titration (ZDJ400S, Beijing Xianqu Weifeng Technology Development Co., Ltd.), were less than 0.4 % mass fraction. The compositions of the DES synthesized in this work along with their abbreviations and water contents are shown in Table 1. Freezing Point Determination. The freezing point of each DES was determined by a differential scanning calorimeter (Netzsch DSC 204 F1) in a temperature range of (150.2 to 353.2) K. A sample was sealed in an aluminum pan, and then it was heated and cooled at a scan rate of 10 K min−1 under a flow of nitrogen. The thermal data were collected during the second heating−cooling scan.21 It was estimated that the uncertainty of the freezing point was 8.1 K. Density Determination. The densities of the DES were determined via a pycnometer of 5 cm3 in volume. The volume

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RESULTS AND DISCUSSION Deep Eutectic Solvents. The freezing temperatures for DES of ChCl with phenolic compounds as a function of composition are shown in Table 2 and graphically in Figure 1. As can be seen from Figure 1, a deep eutectic is formed at about 75 % of phenolic compounds. Compared with an ideal mixture of the two components, the freezing temperature of the DES depresses largely, for example, 127 K for the phenol− ChCl system, 123 K for the phenol−o-cresol system and 120 K for the 2,3-xylenol−ChCl system. The depression of freezing point is not as large as the ChCl−oxalic acids (212 K)7 and the ChCl-ZnCl2 system (272 K),23 possibly because of the covalent bonds existing in the ChCl−oxalic acids and ChCl−ZnCl2. The crystal structure of ChCl, oxalic acids and ZnCl2 collapsed to a great extent when the covalent bonds were formed, resulting in the decrease of lattice energies for the DES with ChCl and oxalic acid as well as ZnCl2. Like conventional ILs, the formation of liquid phase for DES is governed by the freezing point of ionic compounds. The lattice energies of the quaternary ammonium salt and HBD as well as the entropy changes forming a liquid have a significant 867



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transition temperature indicated that the crystal structure of ChCl and phenol collapsed resulting from the strong hydrogen bonding between phenol and ChCl. Interestingly, for some phenol-based DES, it is difficult to decide freezing temperatures from DSC traces, as shown in Figure S1 in the ESI; however, the freezing temperatures of DES with a variety of salts and HBD have been widely reported.6,18,26 We also observed that some solid materials appeared and were suspended in the phenol-based DES liquid (such as CP1:1.5) with the decrease of temperature in the tested range. Therefore, the freezing temperatures of some phenol-based DES were measured by gradual cooling until the first solid appeared. Both o-cresolbased DES and 2,3-xylenol-based DES produced the characteristic broad single peaks at the deep eutectic composition and the characteristic double peaks at the other composition, which is similar to published eutectic systems.27,28 According to the literature,27 the broad single peaks could be illustrated by the characteristic peaks of deep eutectic composition. Systems with compositions beyond the eutectic point can be viewed as a mixture of a eutectic composition and HBD or ChCl. The left peaks for eutectic systems (such as CD1:2) at either side of the deep eutectic composition resulted from the eutectic composition. The right peaks derived from HBD dissolved in the eutectic composition. Density. The densities of DES formed between ChCl and phenol are listed in Table 3 and graphically shown in Figure 2a.

Table 2. Freezing Points of DES Based on ChCl and Three Different HBD at Pressure p = 0.101 MPa for Different HBD to ChCl Mole Fractionsa DES (ChCl + phenol)

DES (ChCl + o-cresol)

DES (ChCl + 2,3xylenol)

xHBD/%

Tf/K

xHBD/%

Tf/K

xHBD/%

Tf/K

0.0b 54.6 60.0 75.0 80.0 83.3 88.1 90.0 100.0c

577.2 335.2 294.2 253.1 260.2 255.6 279.3 297.5 313.8

0.0 54.6 60.1 64.3 75.0 79.7 83.4 85.7 90.9 100.0c

577.2 348.3 321.8 295.3 249.4 250.3 251.4 279.5 283.0 304.1

0.0 60.0 66.8 75.0 80.0 83.3 90.0 100.0c

577.2 330.5 323.6 290.8 316.3 325.0 339.8 348.2

a Standard uncertainties u are u(x) = 0.001, u(p) = 1 kPa, and the combined expanded uncertainty Uc is Uc(Tf) = 8.1 K. bThe Tf of ChCl was obtained from ref 18. cThe Tf of phenols was obtained from the NIST Chemistry WebBook.

Table 3. Densities of the DES at Different Phenol to ChCl Mole Ratios and at Pressure p = 0.101 MPa and Temperature T = (293.2 to 318.2) Ka ρ/g·cm−3

Figure 1. Freezing points Tf of ChCl with phenol (■), o-cresol (●), and 2,3-xylenol (▲) as a function of composition xHBD. The melting point of pure ChCl (at xHBD = 0) is 577.2 K.

effect on the freezing temperature of DES.4 The larger the ion size and the smaller the ion charge, the less energy is needed to break the bond, resulting in a depression of the freezing point.24 Due to the low lattice energy of such DES, they may exist as liquids at room temperature. It can be inferred that to form the eutectic, phenol molecules are required to complex each chloride ion. Hence, we conclude that the chloride ion is complexed by the associated phenol in a manner analogous to the urea/ChCl system reported previously.6 Figure 1 also indicates that the freezing points of phenolbased DES and o-cresol-based DES are lower than those of 2,3xylenol-based DES. Phenol and o-cresol may form more stable complex anions with chloride anions of ChCl than 2,3-xylenol, due to steric hindrance of 2,3-xylenol. Phenolic compounds can serve as HBD to interact with chloride anions, but produce different lattice energy of DES, resulting in different freezing points. Glass transition temperatures (Tgs) of the selected DES with phenols and ChCl could be also found in the representative DSC curves, showed in Figure S1 in the ESI. Tgs are significantly lower compared with the freezing temperatures of the starting materials, so Tgs are called ultralow glass transition temperatures. The Tgs of phenol-based DES were obvious and similar to low transition temperature mixtures described in the literature.25 The ultralow glass

T/K

DES1

DES2

DES3

DES4

DES5

293.2 298.2 303.2 308.2 313.2 318.2

1.0995 1.0967 1.0930 1.0901 1.0873 1.0843

1.0948 1.0921 1.0890 1.0858 1.0829 1.0795

1.0918 1.0893 1.0860 1.0819 1.0795 1.0763

1.0898 1.0870 1.0838 1.0803 1.0761 1.0736

1.0885 1.0852 1.0818 1.0782 1.0745 1.0717

a Standard uncertainties u are u(T) = 0.1 K, u(p) = 1 kPa, u(x) = 0.001, and the combined expanded uncertainty Uc is Uc(ρ) = 0.0010 g·cm−3.

Table 4 shows the density of the DES with different ChCl to ocresol mole ratios and at pressure p = 0.101 MPa and temperature T = 298.2 K. The density of DES is expected to decrease with an increase of temperatures at a fixed mole ratio of phenol to ChCl. At a given temperature, the density of DES decreases with an increase of phenol composition in the DES mixture. Phenol interacts with the anion of ChCl to form DES, which has the potential to decrease the DES density. Figure 2b shows the density of DES of phenol and ChCl as a function of HBD to ChCl mole ratio compared with that of ocresol and ChCl. The density of 3-xylenol-based DES was not measured due to their high freezing temperature. Obviously the density of DES of ChCl and phenol is greater than that of ChCl and o-cresol. The reason is possibly that the molecule of phenol is smaller than that of o-cresol, and phenol has the smaller stereohindrance effect than o-cresol. The densities as a function of temperature were fitted using the following polynomial eq 1 according to the literature.29 ρ = k 0 + k1T + k 2T 2 868

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Figure 2. (a) Densities (ρ) of ChCl-based DES as a function of temperature (T) in the range of (293.2 to 318.2) K with the polynomial fit curves. The mole ratio of phenol to ChCl: ■, 2.00; □, 3.00; ▲, 4.00; ○, 5.00; ●, 6.00. (b) Densities (ρ) of ChCl-based DES as a function of HBD to ChCl molar ratio at 298.2 K. HBD: ■, phenol; □, o-cresol.

Table 4. Densities of the DES at Different o-Cresol to ChCl Mole Ratios and at Pressure p = 0.101 MPa and Temperature T = 298.2 Ka

Table 6. Viscosities of the DES at Different Phenol to ChCl Mole Ratios at Pressure p = 0.101 MPa and Temperature T = (293.2 to 318.2) Ka

ρ/g·cm−3

η/mPa·s

T/K

DES6

DES7

DES8

DES9

DES10

T/K

DES1

DES2

DES3

DES4

DES5

298.2

1.0776

1.0707

1.0664

1.0631

1.0606

293.2 298.2 303.2 308.2 313.2 318.2

120.77 90.33 68.41 53.43 42.42 34.34

57.84 44.64 35.17 28.22 23.08 19.14

40.23 31.55 25.20 20.25 16.71 14.00

31.96 25.25 19.75 16.16 13.44 11.26

27.03 21.43 16.82 13.76 11.45 9.46

a

Standard uncertainties u are u(T) = 0.1 K, u(p) = 10 kPa, u(x) = 0.001, and the combined expanded uncertainty Uc is Uc(ρ) = 0.001 g·cm−3.

where ρ is the density in g·cm−3, k0, k1, and k2 are the fitting parameters that depend on the type of DES, and T is the temperature in K. The values of k0, k1, and k2 of eq 1 are

a

Standard uncertainties u are u(T) = 0.1 K, u(p) = 1 kPa, u(x) = 0.001, and the combined expanded uncertainty Uc is Uc(η) = 0.03η.

instance, at 298.2 K, the viscosity of DES with 2.00 mol ratio of phenol to ChCl is 90.33 mPa·S; and it is as low as 21.43 mPa·S for the DES with 6.00 mol ratio of phenol to ChCl. Phenol added to ChCl not only acts as a HBD to form a DES, but also as an organic solvent to decrease DES viscosity, which is similar to adding solvents (such as water) to ILs. Viscosity reflects the molecular interaction in the solution. Phenol added to DES can decrease the molecular interaction of DES, such as Coulomb attraction of cations and anions. In addition, the minimum of the viscosity for DES does not necessarily appear at the deep eutectic temperature (DES of ChCl and phenol, 253.1 K; DES of ChCl and o-cresol, 249.4 K) as can be inferred from Figure 3. These results are similar to the one reported previously for the DES of ChCl with ethylene glycol.31 Figure 3b shows viscosities of DES of phenol and ChCl at different ChCl to HBD mole ratios compared with that of DES of o-cresol and ChCl. Although o-cresol and phenol both can form DES with ChCl sharing almost the same freezing points, the viscosity of DES of phenol and ChCl is much lower than that of o-cresol and ChCl. The viscosity of 2,3-xylenol-based DES could not be measured because of their high freezing temperature. The Vogel−Tamman−Fulcher (VTF) equation, which is shown in eq 3,32 is used to fit the viscosities of DES as a function of temperature.

Table 5. Fit Parameters k0, k1, k2 and Relative Deviation σ for the Empirical Correlation of Density DES

k0

103k1

106k2

σ/%

DES1 DES2 DES3 DES4 DES5

1.45 1.10 1.26 1.10 1.38

−1.74 0.54 −0.55 0.57 −1.26

1.84 −1.88 −0.15 −2.02 0.94

0.12 0.01 0.03 0.07 0.07

obtained by correlation and shown in Table 5. The table also contains the relative deviation σ defined as30 σ (%) =

1 n

n

∑ i

|ρcalc, i − ρexp , i | ρexp , i

× 100 (2)

where ρcalc,i is the value of the density calculated by eq 1, ρexp,i is the value of the density measurement, and n is the number of experimental points. Table 5 shows that the relative deviations are very small, not more than 0.12 %, which means that eq 1 can be well correlated to the experimental data. Viscosity. The viscosity of phenol-based DES is presented in Table 6 and illustrated in Figure 3a. Table 7 shows the viscosity of the DES with different ChCl to o-cresol mole ratios at pressure p = 0.101 MPa and temperature T = 298.2 K. The viscosity values for these DES are found to cover a range from (9 to 121) mPa·s. As expected, the viscosity of phenol-based DES decreases with an increase of temperature. The viscosity of the DES with a low content of phenol is more sensitive to temperature. At a fixed temperature, the viscosity of the DES decreases with an increase of phenol content in DES. For

⎛ −k ⎞ η = AT 0.5 exp⎜ ⎟ ⎝ T − T0 ⎠

(3)

where k is the so-called pseudoactivation energy, T is the temperature and T0 the ideal glass transition temperature in K, 869



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Figure 3. (a) Viscosity (η) of ChCl-based DES as a function of temperature (T) at the range of (293.2 to 318.2) K with the VTF fit curves. Mole ratio of phenol to ChCl: ■, 2.00; □, 3.00; ▲, 4.00; ○, 5.00; ●, 6.00. (b) Viscosities (η) of ChCl-based DES as a function of HBD to ChCl molar ratio at 298.2 K. HBD: ■, phenol; □, o-cresol.

Table 7. Viscosities of the DES at Different o-Cresol to ChCl Mole Ratios and at Pressure p = 0.101 MPa and Temperature T = 298.2 Ka

Table 10. Electrical Conductivity of the DES at Different oCresol to ChCl Mole Ratios at Pressure p = 0.101 MPa and Temperature T = 298.2 Ka κ/mS·cm−1

η/mPa·s T/K

DES6

DES7

DES8

DES9

DES10

T/K

DES6

DES7

DES8

DES9

DES10

298.2

207.41

77.65

46.95

34.90

27.82

298.2

0.83

1.21

1.41

1.45

1.38

a

a

Standard uncertainties u are u(T) = 0.1 K, u(p) = 1 kPa, u(x) = 0.001, and the combined expanded uncertainty Uc is Uc(η) = 0.03η.

Standard uncertainties u are u(T) = 0.1 K, u(p) = 1 kPa, u(x) = 0.001, and the combined expanded uncertainty Uc is Uc(κ) = 0.01κ.

and A the high-temperature viscosity limiting value. All coefficients are listed in Table 8.

the DES with different ChCl to o-cresol mole ratios and at pressure p = 0.101 MPa and temperature T = 298.2 K.These DES have conductivities in the range from (1.40 to 7.06) mS·cm−1 changing with composition and temperature. The electrical conductivities of these DES are graphically shown in Figure 4a, which as expected increases with the increase of temperature. At fixed temperatures, the effect of phenol to ChCl mole ratio on the electrical conductivity is not monotone like on density and viscosity of the DES. The electrical conductivity of DES reaches a maximum at phenol to ChCl mole ratio of 4.00 or 5.00. The electrical conductivity of DES is influenced by its viscosity and ion concentration. As shown above, the viscosity of the DES is decreased with the increase of phenol to ChCl mole ratio, but the increase of phenol to ChCl mole ratio decreases the ion concentration supplied by ChCl. Therefore, at high phenol to ChCl mole ratios, such as phenol to ChCl mole ratio = 6.00, the decreases of the ion concentration become the dominant effect, resulting in the lower electrical conductivity of DES. Figure 4b shows that electrical conductivities of phenol-based DES are larger than those of o-cresol-based DES, which results from the effect of viscosity of DES as shown in Figure 3b. The electrical conductivities of 2,3-xylenol-based DES were not measured due to their high freezing temperature. The majority of 2,3-xylenol-based DES was solid at low freezing temperature (< 303 K), therefore, the conductivity of DES based on 2,3xylenol was not measured in the investigated temperatures. Electrical conductivities of both phenol-based DES and ocresol-based DES have maximum values at a HBD to ChCl mole ratio of about 5. The ChCl-based DES are potential solvents in the electrochemical field due to their relatively high conductivities from (1.40 to 7.06) mS·cm−1. The experimental conductivity (κ) was correlated using33

Table 8. Regression Parameters for Viscosity VTF Equation and Relative Deviation σ DES

103A

k

T0/K

σ/%


4.27 5.71 2.59 5.27 1.86

−869.03 −727.96 −882.76 −644.69 −874.04

175.93 179.14 163.59 183.42 163.60

0.20 0.09 0.26 0.52 0.49

The correlated results are shown in Figure 3a as curves, and the relative deviations are shown in Table 8, which indicates that the relative deviations are less than 0.523 %. Electrical Conductivity. Table 9 shows the electrical conductivity of the DES with different ChCl to phenol mole ratios and at pressure p = 0.101 MPa and different temperatures. Table 10 shows the electrical conductivity of Table 9. Electrical Conductivity of the DES at Different Phenol to ChCl Mole Ratios at Pressure p = 0.101 MPa and Temperature T = (293.2 to 318.2) Ka κ/mS·cm−1 T/K

DES1

DES2

DES3

DES4

DES5

293.2 298.2 303.2 308.2 313.2 318.2

1.40 2.05 2.65 3.37 4.16 5.09

2.38 3.14 3.88 4.77 5.76 6.77

2.75 3.45 4.21 5.07 6.02 7.06

2.81 3.47 4.25 5.08 5.99 6.96

2.73 3.38 4.09 4.87 5.74 6.66

a

Standard uncertainties u are u(T) = 0.1 K, u(p) = 1 kPa, u(x) = 0.001, and the combined expanded uncertainty Uc is Uc(κ) = 0.01κ.

κ = k 0 + k1(T − 273.2) + k 2(T − 273.2)2 870

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Figure 4. (a) Conductivity (κ) of ChCl-based DES as a function of temperature (T) at the range of (293.2 to 318.2) K with the polynomial fit curves. Mole ratio of phenol to ChCl: ■, 2.00; □, 3.00; ▲, 4.00; ○, 5.00; ●, 6.00. (b) Conductivities (κ) of ChCl-based DES as a function of HBD to ChCl molar ratio at 298.2 K. HBD: ■, phenol; □, o-cresol.

■

where κ is specific conductivity in mS·cm−1, T is the temperature in K, k0, k1, and k2 are fitting parameters. All coefficients are listed in Table 11. The relative deviations of the

Corresponding Author

*Tel./Fax: +86 10 64427603. E-mail: [email protected]. Funding

Table 11. Fit Parameters k0, k1, k2 and Relative Deviation σ for the Empirical Correlation of Conductivity DES

k0

k1

103k2

σ/%


−0.12 0.32 0.89 0.81 0.86

0.047 0.072 0.058 0.070 0.065

1.52 1.60 1.76 1.49 1.42

0.91 0.50 0.15 0.17 0.10

This work is financially supported by the National Basic Research Program of China (2011CB201303) and Shanxi Scholarship Council of China (2011−086). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Professors Zhenyu Liu and Qingya Liu at Beijing University of Chemical Technology for discussion and suggestions.

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correlated results are not more than 0.91 %. The correlated results are shown in Figure 4a as curves, which indicate that eq 4 can correlate well the conductivity of DES.

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REFERENCES

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CONCLUSIONS A variety of DES were synthesized by mixing ChCl with different hydrogen bond donors, phenol, o-cresol and 2,3xylenol. Freezing temperatures of the DES were measured, and the physical properties, including density, viscosity, and conductivity, of phenol-based and o-cresol-based DES were studied at ambient pressure and different temperatures from (293.2 to 318.2) K. The results show that the mole ratio and hydrogen bond donor have strong effects on the physical properties of DES. Densities and viscosities of DES of phenol and ChCl decrease with an increase of temperature and an increase of phenol content. Conductivities of DES of phenol and ChCl increase with an increase of temperature, and reach maximum values at phenol to ChCl mole ratios of 4.00 to 5.00. The phenol-based DES have higher densities and electrical conductivities and lower viscosities than those of o-cresol-based DES. An empirical second-order polynomial could be used to correlate density and conductivity as a function of the temperature. Reasonable correlations of the temperature dependence were achieved by fitting the viscosity to the VTF equation.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

Glass transition temperatures of selected DES; DSC curves for some representative mixtures. This material is available free of charge via the Internet at http://pubs.acs.org. 871



Article

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Formation of Deep Eutectic Solvents by Phenols and Choline Chloride

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