Molar Heat Capacities of Choline Chloride-based Deep Eutectic Solvents and Their Binary Mixtures...

Accepted Manuscript

Title: Molar heat capacities of choline chloride-based deepeutectic solvents and their binary mixtures with water

Authors: Rhoda B. Leron, Meng-Hui Li

PII: S0040-6031(11)00579-XDOI: doi:10.1016/j.tca.2011.11.036Reference: TCA 75896

To appear in: Thermochimica Acta

Received date: 5-10-2011Revised date: 26-11-2011Accepted date: 29-11-2011

Please cite this article as: R.B. Leron, M.-H. Li, Molar heat capacities ofcholine chloride-based deep eutectic solvents and their binary mixtures with water,Thermochimica Acta (2010), doi:10.1016/j.tca.2011.11.036

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dx.doi.org/doi:10.1016/j.tca.2011.11.036dx.doi.org/10.1016/j.tca.2011.11.036

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Highlights > The molar heat capacities for choline chloride-based deep eutectic solvents and their binary mixtures with water were measured. > The measured data were reported as functions of temperature and composition. > A RedlichKister type equation was used to correlate the data. > The applied correlation satisfactorily represented the data.

*Highlights

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Molar heat capacities of choline chloride-based deep

eutectic solvents and their binary mixtures with water

Rhoda B. Leron, Meng-Hui Li

*

R&D Center for Membrane Technology and Department of Chemical Engineering,

Chung Yuan Christian University, Chung Li, 32023, Taiwan, R.O.C.

*Corresponding author. Tel: + 886 3 265 4109; fax: + 886 3 265 4199; E-mail address:

[email protected] (M.-H. Li)

ABSTRACT

In this study, the molar heat capacities, CP, of three choline chloride-based deep eutectic solvents

(DESs); Reline, Ethaline and Glyceline, and their binary mixtures with water were determined. Using

a heat flow differential scanning calorimeter, the heat capacities were measured at standard pressure

from (303.2 to 353.2) K and over the complete range of composition. Results showed that the molar

heat capacity increased with increasing temperature and, for the binary systems, with increasing DES

concentration. The temperature-dependence of the pure DESs were successfully represented by a

second-order empirical correlation with an AAD% of 0.05. The excess molar heat capacities, CPE, of

the binary mixtures also determined and found generally negative over the whole composition range

while exhibiting negative temperature dependence. The CPE values were fitted to a Redlich-Kister type

equation to correlate them to the temperature and DES mole fraction and the molar heat capacities of

the binary mixtures were predicted. The applied models successfully correlated the experimental CP

data as functions of both temperature and composition.

*Manuscript

http://ees.elsevier.com/tca/viewRCResults.aspx?pdf=1&docID=5920&rev=1&fileID=162807&msid={254655AB-4D17-4490-816B-6A555428D9F7}

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KEYWORDS: Choline chloride, Deep eutectic solvents, Excess molar heat capacity, Molar heat

capacity, RedlichKister equation

1. Introduction

Room-temperature ionic liquids (RTILs) have attracted much attention over the past decades. Due

to their unique properties such as negligible vapor pressure, wide liquid range, high thermal stability,

high solvation capacity and non-flammability [1-7], their potential use in a variety of chemical and

industrial applications has been greatly explored. However, RTILs are too expensive to be used in bulk

applications. Some of them even have very low moisture tolerance and their toxicology remains

unclear [8-10].

Low-cost alternatives to RTILs are deep eutectic solvents (DESs). They belong to another class of

ionic liquids made by mixing a quaternary ammonium salt with a metal salt [11], a hydrated salt [12]

or a simple hydrogen bond donor (HDB) such as alcohol, amide and carboxylic acid [13, 14] as

complexing agent. This result in the formation of a eutectic mixture with a melting point that is

considerably lower than its original precursors; hence, it is called DES. DESs were found to have

similar characteristics with RTILs [15, 16]. They posses unusual solvent properties and have the

potential as tunable solvents due to the great number of possible complexing agents available [17, 18].

Furthermore, DESs overcome some disadvantages inherent to conventional RTILs. They are easy to

prepare in the pure state at relatively low cost, non reactive with water, with biodegradable

constituents and their toxicology is known [14, 19]. Because of these unusual characteristics, DESs

have become the interest of many recent undertakings. They were used as solvents in the

electrodeposition and electropolishing of metals, metal oxide processing and other synthetic processes

[20-23]. They are also considered as potential green solvents for biodiesel purification [24, 25],

polymer synthesis [26-28], drug solubilization [19] and gas (i.e. CO2) absorption [29, 30].

The evaluation of DESs as solvents for specific applications requires knowledge of their physical

and thermodynamic properties. The heat capacity, for instance, is one of the properties necessary to

assess the suitability of DESs as solvents for heat transfer applications. To date, however, there is

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practically no data available on the heat capacity of most deep eutectic solvents as well as for their

binary mixtures with water. Thus, in this work, the molar heat capacities, CP, of three DESs namely:

Reline, Ethaline and Glyceline and their aqueous mixtures were determined. All of which are based on

choline chloride as the quaternary ammonium salt being the most commonly used precursor for

eutectic based ionic liquids due to its biodegradability and low cost [31]. The measurements were done

over the temperature range (303.15 to 353.15) K and for the entire range of composition (DES mole

fraction, x1 = 0.1 0.9). For the pure DESs, the temperature-dependence of the CP was represented by

a second-order empirical equation. Also the excess molar heat capacity, CPE, for the binary systems

were determined. Using a RedlichKister type equation, the experimental CPE and the CP data were

correlated to the systems temperature and composition.

2. Experimental Section

2.1 Chemicals

The pure deep eutectic solvents Reline, Ethaline and Glyceline (purity > 98%) were obtained

from Scionix Ltd. Their specific compositions, molar masses and freezing points are given in Table 1.

All pure samples were used without further purification except for drying. Each pure sample was

vacuum dried at 343 K for at least 48h, in an evacuated oven, to remove any volatile impurities and

reduce the water content to minimum. The water contents of the samples after drying were measured

using a Mettler Toledo Karl-Fischer (model DL31) titrator and were 1.5 10-3

(mass fraction).

Distilled deionized water (Type I reagent-grade; resistivity = 183 Mcm; total organic carbon

content (TOC) < 15 ppb), processed in a Barnstead Thermolyne (model Easy Pure 1052) water system,

was used in all the experiments. All weight measurements were done using a Mettler Toledo (model

AL204) digital balance with an accuracy of 1 10-4

g.

2.2 Measurement of molar heat capacity

The heat capacity, CP, measurements were performed in a differential scanning calorimeter (DSC)

from TA Instruments (model DSC-2010) that was equipped with a thermal analysis controller. The

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repeatability of the temperature measurements is 0.1C. The DSC has a calorimetric sensitivity of 1

W (rms) and a calorimetric precision of 1% based on measurements of metal samples. The

apparatus and the experimental procedures were described in detail in the previous work of Chiu et al.

[32] and only pertinent details are discussed here.

For each measurement, a sample of mass (10 15) mg encapsulated in an aluminum hermetic

sample pan was used. The CP of the sample was measured by comparing the differential heat flow

curve of the sample with that of standard sapphire. Both curves were blank corrected. For each run, the

method used consisted a 5-min isothermal period at 20C, followed by a temperature ramp from (20C

to 90C) at a heating rate of 5 K/min, and then an isothermal stage at 90C for the final 5 min. The

purge gas used was nitrogen at a flow rate of 40 mL/min. For each sample, measurements were done in

three to five replicate runs. The method used was validated by measuring the CP of water of which the

details were discussed in our previous works [33, 34]. Based on tests performed with water and taking

into account the uncertainties in the weighing involved, the overall uncertainty in our CP measurements

was estimated to be 2%. In addition, periodic calibration (using indium as calibrant) of the DSC was

conducted to ensure the accuracy of the measurements.

3. Results and Discussion

The experimental molar heat capacities, CP, of the pure DESs Reline, Ethaline and Glyceline at

different temperatures are summarized in Table 2. Results show that the CP of the pure DESs increase

with temperature. It can also be noted that, at the same temperature, the CP values are in the order:

Reline < Ethaline < Glyceline, indicating that the CP increases with increasing DES molar mass. Such

results may be expected since CP is associated with the number of translational, vibrational, and

rotational energy storage modes in the molecule [35, 36]. It is also apparent that, at the same

temperature, the CP of each pure DES is greater than that of water, which can be attributed to the

higher molar mass of the DES compared to water. To our knowledge, there is no data on the heat

capacity of the pure DESs studied here except for a single data reported for the CP of Glyceline at

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298 K (cP = 1.71 0.03 Jg1

K1) [37] which appears to be lower than our reported measurements.

The difference in the purity and water content of the purchased DES may have contributed to such

deviation.

The experimental CP values for the pure DESs were correlated to the temperature using a

second-order empirical polynomial equation of the form

2-1 -1

P 0 1 2/ J mol K / K / KC a a T a T (1)

where T is the absolute temperature and a0, a1 and a2 are fitting parameters determined by the

least-squares method. The determined parameters and the AAD% of the fit are tabulated in Table 3.

The absolute average deviation (AAD) between the experimental and calculated data were determined

using the following equation

cal exp expi=1100

AAD / % /n

in (2)

where cal and exp are the experimental and calculated values, respectively. Based on the overall AAD

of 0.05 % for a total of 33 data points, it can be said that the proposed equation provides good

correlation of the experimental data. This is also shown by the excellent fit between the experimental

and calculated CP values given in Figure 1.

The molar heat capacities of the binary solutions of Reline, Ethaline and Glyceline with water at

different temperatures and DES mole fractions, x1 are presented in Tables 4, 5 and 6, respectively. To

clearly show the temperature- and composition-dependences of the obtained CP data, a representative

plot for the CP of the binary solutions of Reline (1) + H2O (2) as a function of temperature is presented

in Figure 2. Results show that for the aqueous binary systems, CP values slightly increase with

increasing temperature. This effect, however, decreases as the mole fraction of the DES decreases. It is

also clear that CP of the binary solutions increases as the concentration of the DES increases. Such

results are expected since the CP of the pure DESs are higher than that of water. Similar behaviors

were also observed for the binary solutions of the other DES systems (Ethaline and Glyceline)

investigated in this study. These trends have also been observed for many ionic liquids reported in

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literature [36, 38] and in our previous works [33, 34, 39-43]. Since DES is a class of ionic liquids, it is

not surprising that they exhibit behaviors similar to those of RTILs.

The excess molar heat capacity of a binary mixture, CPE, is generally defined as the difference

between the heat capacity of the real mixture and that of the ideal mixture. In this work, the CPE for the

aqueous DES solutions were calculated using the expression defined by Lide and Kehiaian [44] as

E

P P P, i i

i

C C x C (3)

where CP, i is the molar heat capacity of the pure component i. For liquid water, the CP data used in the

calculations were those reported by Osborne et al. [45].

The excess molar heat capacities, CPE, are shown as points on the curves in Figures 3 to 5. As

shown, for all three DES systems, the CPE for the binary solutions are generally negative over the

whole composition range suggesting the presence of stronger interactions between DES and water than

those in the pure liquids. Negative temperature dependence was also observed. For Reline + H2O

mixtures (Figure 3), a minimum occurred in the low DES concentration region (x1~ 0.3) while for

Ethaline + H2O (Figure 4) and Glyceline + H2O (Figure 5) systems the minima were virtually observed

at intermediate compositions. At high water concentrations and high temperature, a maximum was also

observed for Glyceline + H2O system which may be due to partial dissociation of the DES as a result

of dilution [46] and the weakening of the hydrogen bonds in the DES at elevated temperature [47, 48].

It is also noticeable that the trends in the CPE are similar for Ethaline + H2O and Glyceline + H2O

systems which may be attributed to the similarity in structure of the hydrogen bond donors comprising

the two DESs. The CPE values for these two systems were also relatively smaller than those of the

Reline + H2O solutions.

The excess molar heat capacity, CPE were correlated to the composition and temperature using a

RedlichKister type equation of the form

1E -1 -1

P 1 2 1 2

1

/ J mol Kn

i

i

i

C x x A x x

(4)

Ai was assumed to be temperature-dependent such that

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,0 ,1 / Ki i iA a a T (5)

where ai,0 and ai,1 are the fitting parameters whose values are summarized in Table 7, along with their

corresponding AAD. The predicted values are shown as smooth curves in Figures 3 to 5. With the

good agreement between the experimental and predicted data and the low overall AAD values of 0.08

and 4.3 % for Cp and CPE, respectively, it can be said that applied empirical equation successfully

correlated our experimental data to both temperature and composition.

4. Conclusions

The molar heat capacities of the deep eutectic solvents Reline, Ethaline and Glyceline and their

binary mixtures with water were measured in the temperature range (303.2 to 353.2) K and over a

complete range of composition. The CP of the pure DES varied slightly with temperature. This

temperature-dependence was suitably represented by a second-order empirical equation with three

fitting parameters.

For the DES + H2O systems, the CP increased with increasing temperature and DES mole fraction

in the solution. The excess molar heat capacities of the binary solutions were generally negative over

the whole composition range while exhibiting negative temperature dependence. The CPE was

successfully correlated to the temperature and composition using a RedlichKister type equation with

low overall AAD values of 0.08% for CP and 4.3% for CPE. It can therefore be concluded that the data

and correlations presented in this work can be used in engineering design calculations with sufficient

accuracy.

Acknowledgements

This research was supported by Grant, NSC 99-2221-E-033-044-MY3, of the National Science

Council of the Republic of China.

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Table 1

Choline chloride (ChCl)-based DESs studied, their compositions, molar masses and freezing points

DES trade namea

HBD Molar ratio

(ChCl:HBD)

Molar mass,

MDESb

(gmol-1

)

Freezing point

(oC)

Reline Urea 1:2 86.58 12 [13]

Ethaline Ethylene glycol 1:2 87.92 -66 [49]

Glyceline Glycerol 1:2 107.94 -35 [24]

a Trade/commercial name given to the DES by the manufacturer (Scionix Ltd).

b where x and M are mole fraction and molecular weight, respectively. ChCl ChCl HBD HBDM x M x M DES

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Table 2

Molar heat capacities of pure DESs

Molar heat capacity, CP (Jmol-1

K-1

)a

T (K) Reline Ethaline Glyceline

303.15 181.4 0.5 190.8 0.4 237.7 0.6

308.15 182.2 0.6 192.2 0.3 239.1 0.5

313.15 183.2 0.5 194.0 0.3 240.8 0.4

318.15 183.5 0.5 194.9 0.4 241.9 0.2

323.15 184.5 0.6 196.4 0.2 243.5 0.4

328.15 185.3 0.6 197.6 0.2 244.9 0.1

333.15 186.4 0.5 199.2 0.3 246.9 0.1

338.15 187.4 0.5 200.6 0.3 248.4 0.4

343.15 188.5 0.7 202.1 0.1 250.3 0.3

348.15 189.5 0.6 203.9 0.2 252.1 0.3

353.15 190.8 0.8 205.6 0.2 254.3 0.4

a J/molK refers to J/molDESK.

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Table 3

Fitting parameters for Eq. (1)a for the CP of the pure DES

parameter no. of data

points, n AAD

b (%)

DES a0 a1 a2

Reline 247.4 -0.5633 1.141 10-3

11 0.05

Ethaline 181.9 -0.1936 7.371 10-4

11 0.06

Glyceline 302.8 -0.6783 1.531 10-3

11 0.04

overall 33 0.05

a 2-1 -1

P 0 1 2/ J mol K / K / KC a a T a T .

b cald expt expti=1100

AAD / % /n

in .

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Table 4

Molar heat capacities of Reline (1) + H2O (2) solutions


K-1

)a

T (K) x1 = 0.0998 x1 = 0.2995 x1 = 0.4981 x1 = 0.6975 x1 = 0.8926

303.15 81.51 0.1b

100.5 0.3 122.1 0.2 145.0 0.8 168.3 0.2

308.15 81.93 0.1 101.1 0.3 122.9 0.2 146.1 0.9 169.2 0.1

313.15 82.32 0.1 101.7 0.3 123.8 0.3 147.1 0. 8 170.1 0.2

318.15 82.44 0.2 102.1 0.4 124.3 0.2 147.6 1.0 170. 6 0.3

323.15 82.79 0.2 102.8 0.4 125.2 0.2 148.5 0.8 171.4 0.3

328.15 83.09 0.2 103.4 0.4 125.9 0.3 149.2 1.0 172.3 0.4

333.15 83.51 0.2 104.1 0.4 126.8 0.3 150.3 1.0 173.4 0.6

338.15 83.82 0.2 104.8 0.5 127.7 0.3 151.1 1.0 174.2 0.4

343.15 84.17 0.2 105.6 0.5 128.5 0.5 152.3 1.1 175.3 0.5

348.15 84.62 0.1 106.3 0.6 129.6 0.4 153.3 1.2 176.5 0.4

353.15 85.15 0.1 107.4 0.6 130.7 0.4 154.6 0.9 177.8 0.5

a J/molK refers to J/molsolnK.

b Mean standard deviation.

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Table 5

Molar heat capacities of Ethaline (1) + H2O (2) solutions


K-1

)

T (K) x1 = 0.0997 x1 = 0.3005 x1 = 0.4965 x1 = 0.6937 x1 = 0.8893

303.15 84.33 0.6 104.7 0.4 126.8 0.2 151.0 0.3 175.9 0.3

308.15 84.93 0.5 105.8 0.4 128.1 0.4 152.5 0.3 177.3 0.2

313.15 85.45 0.5 106.8 0.4 129.4 0.4 153.7 0.2 178.9 0.3

318.15 85.68 0.5 107.5 0.4 130.2 0.4 154.6 0.3 179.8 0.2

323.15 86.10 0.5 108.5 0.4 131.3 0.5 155.9 0.3 181.2 0.4

328.15 86.37 0.4 109.3 0.3 132.3 0.6 157.1 0.3 182.4 0.3

333.15 86.89 0.4 110.2 0.3 133.6 0.6 158.4 0.3 183.9 0.3

338.15 87.21 0.3 111.1 0.3 134.7 0.7 159.6 0.3 185.2 0.3

343.15 87.56 0.3 112.0 0.3 135.9 0.7 161.0 0.3 186.6 0.3

348.15 87.95 0.1 112.9 0.4 137.1 0.7 162.3 0.1 188.2 0.1

353.15 88.34 0.1 114.0 0.2 138.4 0.7 163.9 0.3 189.8 0.5

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Table 6

Molar heat capacities of Glyceline (1) + H2O (2) solutions


K-1

)

T (K) x1 = 0.1000 x1 = 0.2995 x1 = 0.4947 x1 = 0.6993 x1 = 0.8980

303.15 88.93 0.1 119.2 0.1 150.4 0.4 184.4 0.4 218.8 0.6

308.15 89.42 0.1 120.2 0.2 151.6 0.5 185.7 0.4 220.4 0.7

313.15 89.92 0.1 121.2 0.1 153.0 0.7 187.1 0.6 221.9 0.5

318.15 90.19 0.1 122.0 0.1 153.8 0.6 188.1 0.6 223.0 0.6

323.15 90.69 0.1 123.0 0.1 155.2 0.6 189.4 0.5 224.5 0.7

328.15 91.02 0.1 123.9 0.1 156.2 0.7 190.5 0.4 225.8 0.9

333.15 91.66 0.1 125.0 0.1 157.8 0.7 192.3 0.5 227.6 0.9

338.15 92.08 0.1 126.0 0.1 159.0 0.7 193.8 0.5 229.1 0.9

343.15 92.56 0.1 127.0 0.1 160.3 0.7 195.3 0.6 230.9 1.1

348.15 93.10 0.1 128.1 0.1 161.7 0.8 196.8 0.8 232.6 0.9

353.15 93.51 0.1 129.3 0.1 163.3 0.9 198.7 0.6 234.7 1.2

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Table 7

Fitting parameters for Eq. (4) a

for the aqueous DES systems

parameters no. of data points

AAD (%)

System i ai,0 ai,1 CPE CP

Reline (1) + H2O (2) 1 -110.2 0.2855

55 3.5 0.09 2 63.55 -0.1542

3 -41.53 0.0919

Ethaline (1) + H2O (2) 1 -120.1 0.3219

55 4.7 0.08 2 74.78 -0.2344

3 8.112 -0.0269

Glyceline (1) + H2O (2) 1 -125.6 0.3502

55 4.8 0.06 2 83.75 -0.2695

3 -20.67 0.0471

overall 165 4.3 0.08

a 1E -1 -1

P 1 2 1 2

1

/ J mol Kn

i

i

i

C x x A x x

where ,0 ,1 / Ki i iA a a T .

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Figure Captions

Figure 1. Molar heat capacity of the pure DES as a function of temperature: , Reline; , Ethaline;

, Ethaline, data from this work; and lines, calculated using Eq. (1)

Figure 2. Molar heat capacity for Reline (1) + H2O (2) solutions as a function of temperature, at

different mole fractions: , x1 1.0; , x1 0.1; , x1 0.3; , x1 0.5; , x1 0.7; , x1 0.9,

data from this work; , liquid water, data from Osborne et al. [45]; and lines, calculated using Eqs. (3)

and (4)

Figure 3. Excess molar heat capacity, for Reline (1) + H2O (2) solutions as a function of DES mole

fraction, at different temperatures: , 303.2 K; , 313.2 K; , 323.2 K; , 333.2 K; , 343.2 K; ,

353.2 K; and lines, calculated using Eq. (4)

Figure 4. Excess molar heat capacity, for Ethaline (1) + H2O (2) solutions as a function of DES mole



Figure 5. Excess molar heat capacity, for Glyceline (1) + H2O (2) solutions as a function of DES mole



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Figure 1

Leron et al., 2011

300 310 320 330 340 350 360

120

160

200

240

280

CP/

Jm

ol-1K

-1

T/ K

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Figure 2

Leron et al., 2011

300 310 320 330 340 350 360

60

90

120

150

180

210

CP/

Jm

ole

-1K

-1

T/ K

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Figure 3

Leron et al., 2011

0.0 0.2 0.4 0.6 0.8 1.0

-8

-6

-4

-2

0

2

CP

E/

Jm

ol-1K

-1

x1

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Figure 4

Leron et al., 2011

0.0 0.2 0.4 0.6 0.8 1.0

-6

-4

-2

0

2

CP

E/

Jm

ol-1K

-1

x1

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Figure 5

Leron et al., 2011

0.0 0.2 0.4 0.6 0.8 1.0

-6

-4

-2

0

2

CP

E/

Jm

ol-1K

-1

x1

Molar Heat Capacities of Choline Chloride-based Deep Eutectic Solvents and Their Binary Mixtures...

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