Urea Deep Eutectic

Article pubs.acs.org/jced

Molar Enthalpy of Mixing for Choline Chloride/Urea Deep Eutectic Solvent + Water System Chunyan Ma,†,‡ Yanhua Guo,† Dongxue Li,† Jianpeng Zong,† Xiaoyan Ji,‡ Chang Liu,*,† and Xiaohua Lu† †

College of Chemical Engineering, Nanjing Tech University, 210009 Nanjing, China Energy Engineering, Division of Energy Science, Luleå University of Technology, 97187 Luleå, Sweden

‡

ABSTRACT: The molar enthalpies of mixing for binary systems of choline chloride (chcl)/urea deep eutectic solvents (mole ratios of 1:1.5, 1:2, and 1:2.5) with water were measured at 308.15 and 318.15 K under atmospheric pressure with an isothermal calorimeter. The binary mixture of (chcl/urea (1:2.5) + water) showed endothermic behavior over the entire range of compositions, while the binary mixtures of (chcl/urea (1:1.5) + water) and (chcl/urea (1:2) + water) showed endothermic behavior first and then was changed to be exothermic with increasing content of deep eutectic solvents. The Redlich−Kister (RK) equation and the nonrandom twoliquid (NRTL) model were used to fit experimental molar enthalpies of mixing. The NRTL model with the fitted parameters was further used to predict the vapor pressure for the three systems and was compared with the experimental data from literature. For the binary mixtures of (chcl/urea (1:2) + water), the predicted vapor pressure agreed well with the experimental data only when the temperature was lower than 333.15 K and the mole fraction of chcl/urea (1:2) was lower than 0.1. Otherwise, the deviation increased greatly with an increase of the amount of chcl/urea (1:2).

1. INTRODUCTION Deep eutectic solvents (DESs) have received much attention because of their favorable properties of nonvolatility, thermal stability, water compatibility, biocompability, biodegradability, and diversity of ionic structures.1−5 However, their viscosity is generally higher than other organic solvents,4 which leads to a great challenge in practical processing. It has been reported that the addition of water can significantly decrease the viscosity of DESs and improve the transport properties with little effect on other properties.6,7 To develop and design a novel process based on aqueous DESs, the fundamental understandings of the physicochemical properties of pure DESs and their aqueous mixtures are necessary. Therefore, a few research groups have performed systematic measurements of their thermodynamic properties.6−17 Li et al. systematically studied the physical and chemical properties of pure DESs and their aqueous solutions including densities, viscosities, refractive indices, vapor pressure, molar heat capacities, electrical conductivities, and CO2 solubility.8,10,12−17 The molar enthalpy of mixing (Hm) is a fundamental thermodynamic property of solutions and it is critically important in designing chemical processes and developing and validating thermodynamic models. It characterizes the nonideal behavior of real mixtures and can be used to investigate the molecular interactions and macroscopic behavior of fluid mixtures.18,19 As an environmentally favorable solvent, the choline chloride/urea deep eutectic solvents2 have been widely used © XXXX American Chemical Society

in different areas such as synthesis of microporous crystalline zeolites,20 catalytic chemical reactions,21 CO2 capture process,22 and other potential for other processes.4,5 To the best of our knowledge, Hm values have not been reported so far in these DESs/H2O systems. In this work, the Hm for the binary system of choline chloride/urea (mole ratios of 1:1.5, 1:2, and 1:2.5) with water at 308.15 and 318.15 K under atmospheric pressure was measured with an isothermal calorimeter. The specific mole ratios of choline chloride in DESs were chosen according to the literature.22 The Redlich−Kister equation and the nonrandom two-liquid (NRTL) model were used to fit the experimental data measured in this work. The NRTL model with the fitted parameters were used to predict the vapor pressure of aqueous DESs solutions and validated by experimental data.

2. EXPERIMENTAL SECTION 2.1. Materials. The molecular formula, CAS number, purity, and source of the chemicals used in this work are listed in Table 1. The chemicals were analytical reagent (A.R.) grade and used as received. Choline chloride (ChCl) and urea were used for the synthesis of the DESs. Potassium chloride and tris(hydroxymethyl) aminomethane (THAM) were used to verify the accuracy of the calorimeter. High-quality deionized water was used to prepare the DESs/H2O mixture in this work. Special Issue: Proceedings of PPEPPD 2016 Received: July 1, 2016 Accepted: October 14, 2016

A

DOI: 10.1021/acs.jced.6b00569 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Chemicals Used in This Work

a

chemical name

molecular formula

cas number

mass fractiona

source

purification method

ChCl urea potassium chloride hydrochloric acid THAM

C5H14ClNO CH4N2O KCl HCl C4H11NO3

67-48-1 57-13-6 7447-40-7 7647-01-0 77-86-1

≥0.98 ≥0.99 0.998 0.36−0.38 ≥0.999

Sinopharm Chemical Reagent Co.,Ltd. Xilong Chemical Co., Ltd. Aladdin Shanghai Lingfeng Chemical Reagent Co., Ltd. Aladdin

none none none none none

As stated by suppliers.

The DESs were prepared by stirring ChCl and urea at 353.15 K until a homogeneous colorless liquid was formed.2 Each DES was dried under vacuum at 333 K for 72 h to remove volatile impurities and stored in a drybox for further use. The DESs were prepared by weighing with an analytical balance (Sartorius BSA124S) with a precision of ±10−4 g. The corresponding uncertainty for DES composition, that is, mole ratio of the components of the DESs, is within ±4 × 10−3. The purity of the made DESs is higher than 0.984 (mass fraction). The impurity of the sample may introduce some errors in the measurement, but it is within the tolerance of the error of the measurement. The water contents of the three DESs were determined using Karl Fischer titrator, and the results of water content of all the DESs were less than 0.00265 (mass fraction). 2.2. Molar Enthalpy of Mixing Measurement. The molar enthalpy of mixing of the binary systems (i.e., ChCl-urea based DESs with water) was measured by the TAM Air isothermal microcalorimeter (TA Instruments, USA). The operating temperature was controlled by circulating air to keep the temperature within ±0.02 K. The parallel twin-chamber measuring channels were used for the measurements: one chamber was used for measurement and the other was for reference. In the measurement chamber, the DESs were placed in a 20 mL ampule and water was in the Hamilton syringe on the top of ampule. While in the reference chamber, the DES and water were premixed to the desired proportion and placed in a 20 mL ampule. During the measurement, the corresponding thermal power curve was recorded automatically by a computer. Water was injected into the ampule by a Hamilton syringe after the ampule temperature was stable; that is, the corresponding thermal power curve was stable. The reference chamber was used to eliminate the heat disturbance from the environment, and the heat of stirring was negligible. The accuracy of the calorimeter was verified by measuring the dissolution enthalpy of KCl in water and THAM in 0.1 M HCl at 298.15 K. The measured molar solution enthalpies of these two systems were 17.5314 ± 0.046 kJ·mol−1 and 29.7766 ± 0.02 kJ·mol−1, respectively. The results from literatures are 17.536 ± 0.009 kJ·mol−1 for KCl23,24 and (29.739 ± 0.01) kJ· mol−1 for THAM.24 The average deviations were 0.03% for KCl and 0.13% for THAM.

ARD% =

RMSD =

Yexp, i − Ycal, i Yexp,i

i=1

100 (2)

k

1

Hm(J·mol‐1) = x1x 2 ∑ a j(2x1 − 1)j ‐ 1 ,

aj =

j=1

∑ aji × T i i=0

(3)

where xi is the mole fraction of component i with x1 + x2 = 1 (note that further in the text we will denote the DES and water as 1 and 2, respectively). The coefficients aji are the lineally/ polynomial coefficients to represent aj, and T is the absolute temperature. The coefficients aji are determined by leastsquares analysis with all data points weighted equally, and k is the number of fitted parameters that was set to be 5 in this work. 3.2. Hm Correlation with the NRTL Model. According to Gibbs−Helmholtz equation (eq 4), the molar enthalpy of mixing is directly related to the temperature dependence of excess Gibbs energy. ⎡ ∂(GE /RT ) ⎤ ⎢ ⎥ ∂T ⎣ ⎦

=− x ,p

Hm RT 2

(4) −1

−1

where R is the molar gas constant, 8.314 J·mol ·K . The excess Gibbs energy is related to the activity coefficient (γi), as expressed in eq 5. GE = RT

2

∑ xi ln γi = x1 ln γ1 + x2 ln γ2

(5)

i=1 30

The nonrandom two-liquid model (NRTL) was used to calculate the activity coefficient. Applying NRTL, the derived expression of Hm was given in eq 6 representing the relationship of molar enthalpy of mixing with the parameters of the model. ⎡ x + x exp(ατ )(1 − ατ ) 1 21 21 Hm = RTx1x 2⎢ 2 τ21 (x 2 + x1 exp(ατ21))2 ⎣ +

x1 + x 2 exp(ατ12)(1 − ατ12) 2

(x1 + x 2 exp(ατ12))

⎤ τ12 ⎥ ⎦

(6)

with τ12 = m12 + n12 /T ,

N 2

∑ (Yexp,i − Ycal,i) i=1

∑

where N stands for the number of data points. 3.1. Hm Correlation with the Redlich−Kister Equation. The Redlich−Kister (RK) equation (eq 3)25−29 was used to fit the molar enthalpies of mixing.

3. THEORETICAL SECTION A summary of equations used to regress and/or predict the obtained experimental data is presented in this section. The root-mean-square deviation (RMSD) and average absolute relative deviation (ARD) were used to measure the correlation/ prediction performance of a given property Y (where Y = Hm in this work): 1 N

N

1 N

τ21 = m21 + n21/T

where m12, n12, m21, n21 and α are adjustable parameters and α is also the nonrandomness factor.

(1) B



Article

Table 2. Molar Enthalpy of Mixing (Hm) of (x1 DES + x2 H2O) at 308.15 and 318.15 K and at 101 kPa chcl/urea (1:1.5)+H2O

chcl/urea (1:2) + H2O

x1a

Hmb/(J·mol−1)

x1

0.0483 0.0910 0.1680 0.2542 0.3685 0.4300 0.6488 0.7600

110.29 121.97 52.02 −88.87 −182.43 −194.05 −153.95 −122.78

0.0265 0.0576 0.1190 0.2566 0.4240 0.6332 0.7700

0.0279 0.0411 0.0685 0.2948 0.3831 0.5471 0.7640

113.42 138.22 175.03 −195.09 −259.67 −279.05 −173.60

0.0280 0.0420 0.0684 0.2351 0.3670 0.5345 0.7689

chcl/urea (1:2.5) + H2O

Hm/(J·mol−1)

x1

Hm/(J·mol−1)

112.09 175.23 222.68 154.02 −54.61 −97.34 −66.99

0.0532 0.0999 0.1829 0.2699 0.4106 0.5753 0.7834

185.13 257.82 294.00 236.17 172.88 128.58 41.64

114.87 144.89 185.81 54.02 −42.66 −94.03 −87.08

0.0304 0.0442 0.0732 0.1641 0.2749 0.4755 0.6741 0.7771

138.14 178.74 235.24 229.32 168.40 67.89 24.70 10.07

308.15 K

318.15 K

x1 is the mole fraction of DESs in the mixture of (DES + water). x2 is the mole fraction of H2O in the mixture of (DES + water). x2 = 1 − x1. The abbreviation u(x) is used for the standard uncertainty of quantity x: u(T) = 0.02 K, u(p) = 3 kPa, u(x1) = 0.0002, u(x2) = 0.0002 and u(Hm) = 25 J· mol−1. The abbreviation ur(x) is used for the relative standard uncertainty of quantity x: ur(x1) = 0.0015, ur(x2) = 0.0003. bHm is the molar enthalpy of mixing in Joule per mol mixture. a

3.3. Vapor Pressure Prediction. The vapor pressure for the binary system (DESs (1) + H2O (2)) can be calculated using eq 7 considering the nonvolatility of DESs, that is, ps1 = 0. The vapor pressure of pure water was taken from the literature.31 According to the NRTL model, the activity coefficient of H2O (γ2) can be calculated by eq 8. 2

p=

∑ xiγipis

= x 2γ2p2s

(7)

i=1

2 ⎡ ⎤ τ12G12 τ21G21 ⎥ ln γ2 = x12⎢ + 2 2 (x1 + x 2G21) ⎦ ⎣ (x 2 + x1G12)

(8)

with G12 = exp( −ατ12),

G21 = exp( −ατ21)

where p and psi are the vapor pressures of liquid mixture and pure component i, respectively.

Figure 1. Molar enthalpy of mixing Hm for (x1 DES + x2 H2O), where x2 = 1 − x1. x1 is the mole fraction of DES and x2 is the mole fraction of H2O. Blue points, experimental data at 308.15 K: (◆) chcl/urea (1:2.5), (▲) chcl/urea (1:2), (●) chcl/urea (1:1.5). Black points, experimental data at 318.15 K: (◊) chcl/urea (1:2.5), (△) chcl/urea (1:2), (○) chcl/urea (1:1.5). Curves: calculated results from NRTL model.

4. RESULTS AND DISCUSSION Molar enthalpy of mixing (Hm) was defined as the isothermal enthalpy-change per mol solution when mixing DES and water. In this work, the Hm was measured by calorimetry at 308.15 and 318.15 K. The measured results are listed in Table 2. To the best of our knowledge, no data has been reported for the Hm of the investigated DESs. Thus, no comparison was made. According to the experimental Hm data in Table 2, the values of Hm are positive for the binary systems of (chcl/urea (1:2.5) + water) over the whole composition range at 308.15 and 318.15 K. This implies that the mixing processes are endothermic. As shown in Figure 1, Hm reaches the maximum for the binary systems of (chcl/urea (1:2.5) + water) when x1 is around 0.2. Considering that the positive Hm implicates weaker interaction upon mixing, it can be stated that the interaction forces between the same kind of ions or molecules (DES-DES and water−water interactions) are stronger than those between

dissimilar ones. While for the binary systems of (chcl/urea (1:2) + water) and (chcl/urea (1:1.5) + water), Hm exhibits an “S-shaped” behavior, that is, the value is positive at first and then gradually changes to negative with increasing DES concentration in the aqueous mixture. The “S-curve” implies that the mixing process for the (chcl/urea (1:2) + water) and (chcl/urea (1:1.5) + water) binary systems is endothermic at first, and then it changes to exothermic with increasing DES ratio. These results suggest that the molecular interactions of the DES−DES and water−water pairs are stronger than DES− water pair when water dominates in the mixture. With increasing DES amount in the mixture, DES−water interaction C



Article

Table 3. Parameters aji of the Redlich−Kister equation with ARD and RMSD for x1 DESs + x2 H2O at 308.15 and 318.15 K and at 101 kPa chcl/urea (1:1.5) + H2O



parameters

aj0

aj1

aj0

aj1

aj0

aj1

j=1 j=2 j=3 j=4 j=5 ARD/% RMSD/J·mol−1

10135.29 11977.07 24015.35 −41960.61 −114035.13 3.00 4.11

−35.38 −37.63 −76.58 127.75 376.47

−1619.98 −5609.00 95801.89 −37970.44 −166569.84 3.50 6.92

4.04 15.84 −302.76 117.26 538.82

10143.36 1635.88 27129.29 −29237.96 −86727.57 4.70 5.94

−31.03 −6.69 −84.84 87.02 282.40

Table 4. Parameters of NRTL Equation with ARD and RMSD for x1 DESs + x2 H2O DESs (1) + H2O (2)

m12

m21

n12

n21

α

ARD/%

RMSD/ J·mol−1

chcl/urea (1:1.5) + H2O chcl/urea (1:2) + H2O chcl/urea (1:2.5) + H2O

−4.08 4.51 0.98

2.60 13.48 10.27

1302.30 −1392.48 −223.29

24.99 −3533.82 −2596.85

0.69 0.68 0.69

11.03 15.85 9.84

17.60 18.35 8.59

Table 5. Vapor Pressure of (x1 DES + x2 H2O) from 303.15 to 343.15 K and at Different Concentrations

takes the dominant role and the mixture system shows exothermic behavior. When the temperature increases from 308.15 to 318.15 K, the turning point (Hm = 0) for both of (chcl/urea (1:2) + water) and (chcl/urea (1:1.5) + water) is left-shifted. The Hm values are higher for the mixture of (chcl/ urea (1:2.5) + water) than those of the mixtures of (chcl/urea (1:2) + water) and (chcl/urea (1:1.5) + water). In general, the Hm values decreases with increasing system temperature. The measured Hm values were correlated with the RK equation and the NRTL model. The fitted parameters are summarized in Tables 3 and 4 together with the ARD and RMSD. Figure 1 shows the Hm results (Hm − x1 diagrams) of the three binary systems together with the correlation results by using the NRTL model at 308.15 and 318.15 K. From ARD and RMSD results, it is apparent that the RK equation provides better regression results of Hm data than the NRTL model. However, the RK equation is merely a fitting equation, which is not capable of predicting other properties such as vapor pressure of solutions. While the NRTL model together with Gibbs−Helmholtz equation has this capability, and this is the reason we adopted this activity coefficient model in this paper. In the next part, the NRTL model with the fitted parameters was used to predict the vapor pressure and the results were compared with the available experimental data from literature. The vapor pressures of binary mixtures chcl/urea (1:2) + water, have been measured by Wu et al.17 The reported experimental data (pexp), predicted vapor pressures (ppred) in this work and the relative deviations (RD) between the predicted and experimental vapor pressures are shown in Table 5. To further illustrate the fitting accuracy, the predicted results were compared with the experimental data as shown in Figure 2. For the binary mixtures of (chcl/urea (1:2) + water), the predicted vapor pressure agreed with the experimental data in general with an ARD of 29% and the RD increased with the increase of chcl/urea (1:2) content. Specifically, when the content of chcl/urea (1:2) was lower than 0.1 and temperature was lower than 333.15 K, the experimental data agreed well with the predicted value. Otherwise, the deviation increased greatly with increasing amount of chcl/urea (1:2). It is worth mentioning that the temperature also affected the deviation greatly. When the temperature was 343.15 K, the deviation was largest at all levels of chcl/urea (1:2) concentration. The predicted vapor pressures showed deviation at the high




x1a

T/(K)

pexp,17/ (kPa)

ppred/ (kPa)

RD/%

ppred/ (kPa)

ppred/ (kPa)

0.0495

303.15 313.15 323.15 333.15 343.15 303.15 313.15 323.15 333.15 343.15 303.15 313.15 323.15 333.15 343.15 303.15 313.15 323.15 333.15 343.15

4.080 7.066 11.732 18.892 29.398 3.693 6.346 10.586 17.039 26.518 3.000 5.240 8.813 14.132 21.918 1.893 3.333 5.546 8.853 13.679

4.084 7.141 12.014 19.521 31.300 3.935 6.971 11.853 19.401 31.235 3.710 6.669 11.406 18.661 29.893 3.042 5.538 9.484 15.436 24.533

0.1 1.1 2.4 3.3 6.5 6.6 9.9 12.0 13.9 17.8 23.7 27.3 29.4 32.0 36.4 60.7 66.2 71.0 74.4 79.3

4.144 7.203 12.042 19.435 30.952 4.101 7.120 11.884 19.142 30.413 3.938 6.808 11.305 18.097 28.548 3.243 5.515 8.987 14.095 21.751

4.082 7.122 11.956 19.384 31.015 3.933 6.921 11.708 19.105 30.725 3.736 6.630 11.268 18.412 29.572 3.196 5.664 9.563 15.481 24.604

0.1218

0.2379

0.4543

a

x1 is the mole fraction of DESs. x2 is the mole fraction of H2O. x2 = 1 − x1 .

concentration of chcl/urea (1:2), and this was probably because of the complex molecular interaction between chcl/ urea (1:2) and water. For the binary mixtures of (chcl/urea (1:1.5) + water) and (chcl/urea (1:2.5) + water), the predicted vapor pressures are also illustrated in Table 5 and Figure 2. No vapor pressure data has been reported for these two systems, and therefore no comparison was made.

5. CONCLUSION The molar enthalpies of mixing for binary systems of choline chloride/urea deep eutectic solvents (mole ratios of 1:1.5, 1:2, and 1:2.5) with water were measured at 308.15 and 318.15 K under atmospheric pressure. The experimental results showed D



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Carboxylic Acids: Versatile Alternatives to Ionic Liquids. J. Am. Chem. Soc. 2004, 126, 9142−9147. (2) Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K.; Tambyrajah, V. Novel solvent properties of choline chloride/urea mixtures. Chem. Commun. (Cambridge, U. K.) 2003, 70−71. (3) García, G.; Aparicio, S.; Ullah, R.; Atilhan, M. Deep Eutectic Solvents: Physicochemical Properties and Gas Separation Applications. Energy Fuels 2015, 29, 2616−2644. (4) Smith, E. L.; Abbott, A. P.; Ryder, K. S. Deep eutectic solvents (DESs) and their applications. Chem. Rev. (Washington, DC, U. S.) 2014, 114, 11060−11082. (5) Zhang, Q.; De Oliveira Vigier, K.; Royer, S.; Jerome, F. Deep eutectic solvents: syntheses, properties and applications. Chem. Soc. Rev. 2012, 41, 7108−7146. (6) Shah, D.; Mjalli, F. S. Effect of water on the thermo-physical properties of Reline: An experimental and molecular simulation based approach. Phys. Chem. Chem. Phys. 2014, 16, 23900−23907. (7) Xie, Y.; Dong, H.; Zhang, S.; Lu, X.; Ji, X. Effect of Water on the Density, Viscosity, and CO2 Solubility in Choline Chloride/Urea. J. Chem. Eng. Data 2014, 59, 3344−3352. (8) Su, W. C.; Wong, D. S. H.; Li, M. H. Effect of Water on Solubility of Carbon Dioxide in (Aminomethanamide + 2-Hydroxy-N,N,Ntrimethylethanaminium Chloride). J. Chem. Eng. Data 2009, 54, 1951−1955. (9) Sun, H.; Li, Y.; Wu, X.; Li, G. Theoretical study on the structures and properties of mixtures of urea and choline chloride. J. Mol. Model. 2013, 19, 2433−2441. (10) Leron, R. B.; Li, M.-H. High-pressure density measurements for choline chloride: Urea deep eutectic solvent and its aqueous mixtures at T = (298.15 to 323.15)K and up to 50 MPa. J. Chem. Thermodyn. 2012, 54, 293−301. (11) Yadav, A.; Pandey, S. Densities and Viscosities of (Choline Chloride + Urea) Deep Eutectic Solvent and Its Aqueous Mixtures in the Temperature Range 293.15 to 363.15 K. J. Chem. Eng. Data 2014, 59, 2221−2229. (12) Leron, R. B.; Wong, D. S. H.; Li, M.-H. Densities of a deep eutectic solvent based on choline chloride and glycerol and its aqueous mixtures at elevated pressures. Fluid Phase Equilib. 2012, 335, 32−38. (13) Leron, R. B.; Li, M.-H. High-pressure volumetric properties of choline chloride−ethylene glycol based deep eutectic solvent and its mixtures with water. Thermochim. Acta 2012, 546, 54−60. (14) Leron, R. B.; Soriano, A. N.; Li, M.-H. Densities and refractive indices of the deep eutectic solvents (choline chloride + ethylene glycol or glycerol) and their aqueous mixtures at the temperature ranging from 298.15 to 333.15 K. J. Taiwan Inst. Chem. Eng. 2012, 43, 551−557. (15) Siongco, K. R.; Leron, R. B.; Caparanga, A. R.; Li, M.-H. Molar heat capacities and electrical conductivities of two ammonium-based deep eutectic solvents and their aqueous solutions. Thermochim. Acta 2013, 566, 50−56. (16) Siongco, K. R.; Leron, R. B.; Li, M.-H. Densities, refractive indices, and viscosities of N,N-diethylethanol ammonium chloride− glycerol or − ethylene glycol deep eutectic solvents and their aqueous solutions. J. Chem. Thermodyn. 2013, 65, 65−72. (17) Wu, S.-H.; Caparanga, A. R.; Leron, R. B.; Li, M.-H. Vapor pressure of aqueous choline chloride-based deep eutectic solvents (ethaline, glyceline, maline and reline) at 30−70°C. Thermochim. Acta 2012, 544, 1−5. (18) Gonzalez-Miquel, M.; Massel, M.; DeSilva, A.; Palomar, J.; Rodriguez, F.; Brennecke, J. F. Excess Enthalpy of Monoethanolamine plus Ionic Liquid Mixtures: How Good are COSMO-RS Predictions? J. Phys. Chem. B 2014, 118, 11512−11522. (19) Porcedda, S.; Marongiu, B.; Schirru, M.; Falconieri, D.; Piras, A. Excess enthalpy and excess volume for binary systems of two ionic liquids + water. J. Therm. Anal. Calorim. 2011, 103, 29−33. (20) Cooper, E. R.; Andrews, C. D.; Wheatley, P. S.; Webb, P. B.; Wormald, P.; Morris, R. E. Ionic liquids and eutectic mixtures as solvent and template in synthesis of zeolite analogues. Nature (London, U. K.) 2004, 430, 1012−1016.

Figure 2. Vapor pressure p for (x1 DES + x2 H2O), where x2 = 1 − x1. x1 is the mole fraction of DESs and x2 is the mole fraction of H2O. Symbols: experimental results for chcl/urea (1:2) by Wu et al.17 at temperatures (◆) 303.15 K, (○) 313.15 K, (●) 323.15 K, (△) 333.15 K, (▲) 343.15 K. Curves: Predicted for chcl/urea (1:2), chcl/urea (1:1.5), and chcl/urea (1:2.5) according the NRTL model and eq 7.

that the mixing process of chcl/urea (1:2.5) with water was endothermic and the Hm values reached the maximum when x1 is around 0.2. The mixing processes of chcl/urea (1:1.5) or chcl/urea (1:2) with water were endothermic at first and then changed to exothermic with increasing DESs content. When the temperature changes from 308.15 to 318.15 K, the turning point (Hm = 0) for (chcl/urea (1:2) + water) and (chcl/urea (1:1.5) + water) is left-shifted. And the Hm values decreased with the increase of temperature for the three systems studied in this work. The Hm values for the mixture of (chcl/urea (1:2.5) + water) were higher than those for the mixtures of (chcl/urea (1:2) + water) and (chcl/urea (1:1.5) + water). The Redlich−Kister (RK) equation and the nonrandom two-liquid (NRTL) model were used to fit experimental results. Using the RK equation provides better regression results than using the NRTL model in correlating Hm data in these binary mixtures. The NRTL model with fitted parameters was used to predict the vapor pressure as validated by comparing with the experimental data from literature. In general, the predicted vapor pressures for the binary mixtures of (chcl/urea (1:2) + water) agreed well with the experimental data only when the temperature was lower than 333.15 K and the mole fraction of chcl/urea (1:2) was lower than 0.1. Otherwise, the deviation increased greatly with increasing of the amount of chcl/urea (1:2)

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AUTHOR INFORMATION

Corresponding Author

*Tel.:+86-2583587205. E-mail: [email protected]. Funding

This work was supported by Chinese MOST 973 project (2013CB733501), National Natural Science Foundation of China (21136004, 21476106), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Abbott, A. P.; Boothby, D.; Capper, G.; Davies, D. L.; Rasheed, R. K. Deep Eutectic Solvents Formed between Choline Chloride and E



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Urea Deep Eutectic

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