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Bubble point of aqueous mixtures of sugar-based deep eutectic solvents and their individual components: Experimental study and modeling
Journal of Molecular Liquids 296 (2019) 111876
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Bubble point of aqueous mixtures of sugar-based deep eutectic solvents and their individual components: Experimental study and modeling Sahar Gholami, Aliakbar Roosta* Department of Chemical, Petroleum and Gas Engineering, Shiraz University of Technology, Shiraz, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 August 2019 Received in revised form 3 September 2019 Accepted 3 October 2019 Available online 9 October 2019
The bubble pressure of aqueous solutions of two natural deep eutectic solvents (DESs) was measured at temperatures 303.15 Ke343.15 K and different concentration of the DESs. DESs were formed with combining choline chloride and a sugar (glucose or fructose) in a choline chloride: sugar molar ratio of 2:1. In addition, the bubble pressure of aqueous solutions of fructose, glucose, and choline chloride, were measured to define the impact of the individual components of the DESs on the bubble pressure. According to the results, choline chloride had the greatest effect and sugars had the least effect on the bubble pressure. Experimental bubble point data was used to estimate the binary interaction parameters of the NRTL activity model. An overall AARD of 1.91% indicates the good accuracy of the model in estimating the bubble pressure of aqueous solutions studied in this work. To verify the calculated binary interaction parameters, the NRTL model was used to estimate the solubility of glucose and fructose in water and the results were compared with the experimental solubility available in the literature. © 2019 Elsevier B.V. All rights reserved.
Keywords: Bubble pressure Deep eutectic solvent Choline chloride Sugar Activity coefficient
1. Introduction Deep eutectic solvents (DESs) are a new type of green solvents which are usually liquid at room temperature [1,2]. These solvents were introduced by Abbott et al. for the first time in 2003 [3]. DESs are usually formed by combining an ammonium salt such as choline chloride as a hydrogen-bond acceptor with a hydrogenbond donor like amines, amides, alcohols and carboxylic acids in certain molar ratios [4,5]. The melting point of the DESs is significantly lower than the melting point of its pure constituents due to the hydrogen-bonding interactions between the constituents [6,7]. DESs are mostly non-toxic, non-flammable, low cost, accessible, biodegradable, environmentally friendly, and nonvolatile [8,9]. Recently, DESs have been in attention of many researchers in many applications such as gas absorption [8,10,11], liquid-liquid extraction [12,13], extraction of biological compounds [14e16], desulfurization [17e19], pharmaceuticals [20,21], distillation [22,23] and etc. A type of DESs that is formed by combining biological compounds is categorized in natural deep eutectic solvents (NADESs). The hydrogen-bond donor component of these solvents can be a
* Corresponding author. E-mail address: [email protected] (A. Roosta). https://doi.org/10.1016/j.molliq.2019.111876 0167-7322/© 2019 Elsevier B.V. All rights reserved.
sugar, an alcohol, an amine, or an amino acid [24,25]. Glucose and fructose are both natural, and edible hydrogen-bond donors which are found in plants [26]. In addition, choline chloride is a noteworthy hydrogen-bond acceptor in NADESs because of its nutritional value which is used as a complement for reducing the cholesterol level [27] and it is considered as a component of Bcomplex vitamins [28]. Therefore, the DESs of the choline chlorideglucose or choline chloride-fructose are of interest in many kinds of research. Despite the many advantages of the DESs, high viscosity values of DESs is an obstacle for industrial applications of these green solvents. To solve this problem, DESs are usually used as aqueous solutions [29]. Thus, the physical properties of the aqueous solution of DESs are important for a deeper comprehension of DES’s behaviors. One of the important thermodynamic properties of the aqueous solution of DESs is the activity coefficient of the individual components. The activity coefficient is a thermodynamic property that represents the deviation from the ideality in liquid solutions. It is widely used in phase equilibria calculations. The activity coefficients can be calculated with phase equilibrium data, such as the bubble point of the mixture. Therefore, in this study, the bubble pressure of aqueous solutions of two sugar-based NADESs was measured at different temperatures and different concentrations. Furthermore,
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the aqueous solutions of the individual components of the DESs (fructose, glucose and choline chloride) were studied to determine the contribution of each component to the non-ideality of the aqueous systems. Then, the experimental data were employed to estimate the activity coefficient of water in the binary and ternary systems. Finally, the NRTLactivity model was used to correlate the activity. 2. Materials and method
denote the mole fraction of water in the liquid phase and vapor phase, respectively. Due to the very low vapor pressure of glucose, fructose, choline chloride, and DESs, y was assumed to be the unity. Also, psat is the saturation vapor pressure of water which was gathered from the literature [31], and f is the fugacity coefficient of pure water in the vapor phase which was calculated by using the Peng-Robinson equation of state Eqs. (2)e(8) [32].
P¼
The chemicals used in this study are listed in Table 1. Choline chloride with the purity of more than 99% and D-(þ)-glucose with the purity of more than 99.5% were purchased from Sigma-Aldrich, and fructose with the purity of more than 99% was provided by Surechem Product. Furthermore, double deionized water was used in this experimental test.
RT
nb
aðTÞ
(2)
nðn þ bÞ þ bðn bÞ
b ¼ 0:07780
RTc Pc
(3)
aðTÞ ¼ aðTc Þ aðTÞ
(4)
2.1. Experiment In this study, the bubble pressure of aqueous solutions of two sugar-based DES was measured as well as the bubble pressure of aqueous solutions of their individual components, including choline chloride, fructose, and glucose. Choline chloride was placed in an oven at 100 C to remove its water content, while the other chemicals were used as received, without any purification. To prepare DESs, dried choline chloride and sugar (glucose or fructose) were mixed at a molar ratio of 2:1. Weighting the materials was conducted with an analytical balance (HR-200, A&D) with a precision of 104 g. To prevent moisture absorption of choline chloride, the mixture of choline chloride and sugar was placed on a hot plate at 100 C to form a clear liquid. Then, the formed liquid was combined with double deionized water at different ratios by using the analytical balance to prepare aqueous solutions of DESs with desired concentrations. Next, 3 mL of the aqueous solution was withdrawn by using a 10 mL glass syringe (Becton Dickinson). To remove the dissolved gas in the solution, the syringe was closed by a cap and a vacuum was created by pulling the piston of the syringe. Then, the released gases were removed from the syringe by pushing the piston. The procedure was repeated several times to ensure complete removal of the dissolved gas. The syringe was connected to a vacuum meter (VC-9200, Lutron) with a precision of 0.1 kPa. After that, the set of the syringe and vacuum meter was placed into a water bath (with a temperature precision of 0.1 K) in a given temperature. In addition, suction was applied to the syringe to create bubble conditions of the mixture. At least, 4 h were allowed to achieve temperature equilibrium, and consequently thermodynamic equilibrium. Finally, the bubble pressure was recorded by the vacuum meter.
aðTc Þ ¼ 0:45724
ðRTc Þ2 Pc
"
qffiffiffiffiffiffiffiffiffi aðTÞ ¼ 1 þ b 1 T=T c
First, the experimental bubble pressure data were used to estimate the water activity coefficient (gw) using the vapor-liquid equilibria (VLE) equation [30] as follows:
yw Pfw ¼ xw psat w gw
(1)
where P is the pressure, the subscript w stands for water and x, y
!#2 (6)
b ¼ 0:37464 þ 1:54226u 0:26992u2
(7)
0 pffiffiffi 1 v þ 1 þ 2 b Pv pðv bÞ a 1 ln pffiffiffi lnf ¼ ln@ pffiffiffi A RT RT 2 2bRT vþ 1 2 b
(8) where P, R, T, v, u are the pressure, gas constant, temperature, molar volume, and acentric factor respectively; and the subscript c denotes the critical point, a(T) and a(T) are parameters of the PengRobinson equation which depend on temperature, b and b are temperature independent parameters of the equation of state. Despite glucose and fructose, choline chloride is an ionic solute; however, it is assumed to be a nonionic solute in this study. Thus, the activity coefficient can be fitted to the NRTL activity model Eq. (9) [30] instead of e-NRTL activity model for all studied systems. m P
lngi ¼
tji Gji xj
j¼1 m P
Gji xj
j¼1
2.2. Model
(5)
13 m P tkj Gkj xk C7 B 6 xj Gij B C7 k¼1 þ 6m Btij m C7 P 4P @ A5 j¼1 Gkj xk Gkj xk 2
0
m 6 X
Gij ¼ exp atij ;
k¼1
tij ¼
k¼1
(9)
Dgij RT
where a is the non-randomness parameter (a) was considered to be constant of 0.3 in this study, tij and Gij are calculated by the binary parameters of the NRTL model (Dgij) which is an energy parameter and were correlated as a linear function of temperature in this study, as shown by Eq. (10).
Table 1 Source and purity of chemicals. Chemical Name
Source
Initial mass fraction purity
Purification method
Final mass fraction purity
Analysis method
D()Fructose D-(þ)-Glucose Choline chloride
Surechem Products Sigma-Aldrich Sigma-Aldrich
>0.99 >0.995 >0.99
None None Drying in oven
e e 0.99
e e Karl-Fischer
S. Gholami, A. Roosta / Journal of Molecular Liquids 296 (2019) 111876
Dgij ¼ Aij þ Bij T
(10)
where Aij and Bij are the constants of Eq. (10) which are estimated by the experimental data. The binary interaction parameters between water and the ingredients of DESs (fructose, glucose, and choline chloride) was calculated using the water activity coefficient in the binary mixtures (aqueous solutions of fructose, glucose, and choline chloride). Then, the binary parameters between the ingredients of DESs (between choline chloride and glucose, and between choline chloride and fructose) were correlated to the temperature using the experimental bubble pressure of the aqueous solutions of DESs and using the estimated parameters between the DES’s ingredients and water. To verify the accuracy of the binary interaction parameters, the bubble pressure of the studied systems was calculated and was compared to the experimental bubble pressure with calculating the average absolute relative deviation (AARD) as follows:
AARD% ¼
N Pcalc;i Pexp;i 100 X N i¼1 Pexp;i
(11)
where subscripts exp and calc denote experimental and calculated, and N is the number of data. The activity coefficient of the nonvolatile solutes (glucose, fructose, choline chloride, and DESs) is not applied to the bubble point calculation of current systems, and only the activity coefficient of water is used in these calculations. Therefore, a criterion is required to measure the accuracy of the NRTL model in calculating the activity coefficient of these solutes. The NRTL model with the obtained binary interaction parameters was used to predict the solubility of solutes (specifically glucose and fructose whose solubility data are available from the literature) in the water at different temperatures and the results were compared with the available data in the literature. Eq. (12) [30] was used to calculate the solubility of glucose and fructose in water.
Dhfus Tt 1 ¼ exp x 1 g1 RTt T 1
3
Table 3 Experimental bubble pressure of the glucose-water system at different temperatures and concentrations.a mole fraction of glucose (x)
T/K 303.15
313.15
323.15
333.15
343.15
0.02 0.04 0.06 0.075
4.1 3.9 3.7 3.5
7.0 6.8 6.6 6.3
12.0 11.2 11.0 10.4
19.4 18.7 17.8 17.2
30.0 28.8 27.2 26.3
a Standard uncertainty for temperature is u(T) ¼ 0.5 K. Relative standard uncertainties for mole fraction and pressures are ur(x) ¼ 0.04 and ur (P) ¼ 0.05, respectively.
pure water was gathered from the literature [34] and was added to this table to show the effect of choline chloride on reducing the water vapor pressure. For instance, the bubble pressure of a 0.3 (mol/mol) choline chloride solution is lower than 60% of the vapor pressure of pure water which represents a negative deviation from Raoult’s law. This is due to the hydrogen bond between choline chloride and water. The bubble pressure of aqueous solutions of glucose and fructose are collected in Tables 3 and 4, respectively. The maximum concentrations of glucose (0.075) and fructose (0.2) were selected based on the solubility of these sugars in the water at the lowest temperature (303.15 K). According to the results, the bubble pressure of a 0.075 (mol/mol) glucose solution is about 85% of the vapor pressure of pure water, while the bubble pressure of a 0.1 (mol/mol) fructose solution is about 88% of the vapor pressure of pure water. This indicates that adding glucose to water caused a more bubble pressure drop in comparison with fructose. Consequently, the deviation from Raoult’s law in glucose solution is more negative than the fructose solution under the same conditions. Tables 5 and 6 contain the bubble pressure of aqueous solutions of glucose-choline chloride and fructose-choline chloride DESs, respectively. As seen in these tables, increasing the DESs concentration cause a decrease in the bubble pressure. Furthermore, it can be concluded from the results of these tables that the effect of both
(12)
where x1 is the mole fraction of the solute (glucose or fructose) in water, Tt is the triple point temperature of the solute (419.65 K for glucose, and 385.85 K for fructose) and Dhfus is the molar enthalpy of the solute (33.40 kJ mol1 for glucose, 27.76 kJ mol1) [33].
3. Results and discussion
Table 4 Experimental bubble pressure of the fructose-water system at different temperatures and concentrations.a mole fraction of fructose (x)
T/K 303.15
313.15
323.15
333.15
343.15
0.05 0.10 0.15 0.20
4.0 3.7 3.5 3.3
6.9 6.5 6.0 5.7
11.4 10.7 9.9 9.5
18.4 17.4 16.6 15.2
28.9 27.4 25.7 24.0
The bubble pressure of aqueous solutions of choline chloride at 303.15 Ke323.15 K and mole fraction of choline chloride between 0.1 and 0.3 are listed in Table 2. In addition, the vapor pressure of
a Standard uncertainty for temperature is u(T) ¼ 0.5 K. Relative standard uncertainties for mole fraction and pressures are ur(x) ¼ 0.04 and ur (P) ¼ 0.05, respectively.
Table 2 Experimental bubble pressure of the choline chloride-water system at different temperatures and concentrations.a
Table 5 Experimental bubble pressure of DES (choline chloride þ glucose)-water system at different temperatures and concentrations.a
mole fraction of choline chloride (x)
T/K 303.15
313.15
323.15
333.15
343.15
mole fraction of DES (x)
0 0.10 0.15 0.20 0.30
4.2 3.5 3.2 2.8 2.0
7.4 5.9 5.3 4.7 3.1
12.3 9.8 8.7 7.2 4.7
19.9 16.1 13.9 11.9 7.9
31.2 24.8 21.9 18.5 12.8
a Standard uncertainty for temperature is u(T) ¼ 0.5 K. Relative standard uncertainties for mole fraction and pressures are ur(x) ¼ 0.04 and ur (P) ¼ 0.05, respectively.
0.05 0.10 0.15 0.20 0.25
T/K 303.15
313.15
323.15
333.15
343.15
4.0 3.7 3.3 3.0 2.7
6.9 6.3 5.8 5.4 4.7
11.3 10.5 9.2 8.3 7.5
18.3 16.9 15.0 13.6 11.9
28.4 26.1 23.8 21.4 17.7
a Standard uncertainty for temperature is u(T) ¼ 0.5 K. Relative standard uncertainties for mole fraction and pressures are ur(x) ¼ 0.04 and ur (P) ¼ 0.05, respectively.
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Table 6 Experimental bubble pressure of DES (choline chloride þ fructose)-water system at different temperatures and concentrations.a mole fraction of DES (x)
T/K 303.15
313.15
323.15
333.15
343.15
0.05 0.10 0.15 0.20 0.25
3.9 3.6 3.4 3.1 2.7
6.5 6.2 5.5 5.0 4.5
11.2 10.3 9.1 8.3 7.5
18.4 16.9 14.9 13.2 12.0
28.4 25.7 23.6 20.5 18.0
a Standard uncertainty for temperature is u(T) ¼ 0.5 K. Relative standard uncertainties for mole fraction and pressures are ur(x) ¼ 0.04 and ur (P) ¼ 0.05, respectively.
DESs on the bubble pressure is almost the same. A comparison between the results of Tables 2e6 indicates that adding sugars, choline chloride or DESs to the water decreased the bubble pressure. However, the effect of choline chloride on the bubble pressure reduction was more significant. Also, sugars had the least effect on the bubble pressure. To evaluate the present experimental bubble pressure, the experiments obtained in this study were compared to the bubble pressures available in the literature as illustrated in Fig. 1. Fig. 1(a)
shows the bubble pressure of the glucose-water system at different temperatures and different glucose concentrations. In this study, the bubble pressure of the glucose-water system was measured at temperatures of 303.15 K, 313.15 K, 323.15 K, 333.15 K, and 343.15 K; while the experiments of literature are available at 308.15 K and 318.15 K [35]. According to the results of Fig. 1(a), the trend of the bubble pressure with temperature in this study is in good agreement with the literature data [35]. Fig. 1(b) compares the bubble pressures of a 0.2 (mol/mol) choline chloride solution obtained in this study with the bubble pressures reported by Francisco et al. [36]. In addition, a comparison between the current bubble pressure of the aqueous choline chloride at a temperature of 323.15 K and the literature data [37] was made in Fig. 1(c). As discussed previously, the binary interaction parameters of the NRTL model were correlated to temperature (Eq. (10)) using the bubble pressure data. The estimated binary parameters are listed in Table 7. The obtained binary interaction parameters were used to calculate the bubble pressure of aqueous solutions studied in this work. The AARDs of the model in the estimation of bubble pressure are listed in Table 8. The maximum AARD was 3.22% for the glucosewater system, and the overall AARD was 1.91% for the five studied systems. The low deviation between the results of the model and the experiments is a reliable validation of the model capability in calculating the activity coefficient of water in the current systems.
Fig. 1. A comparison between current experimental data and literature data. (a): bubble pressure of glucose-water system [35], (b): bubble pressure of choline chloride (20% mol/ mol)þwater system [36], (c): bubble pressure of choline chloride þ water system at 323.15 K [37].
S. Gholami, A. Roosta / Journal of Molecular Liquids 296 (2019) 111876
5
Table 7 Estimated NRTL binary parameters (A and B of Eq. (10) where a ¼ 0.3). System
NRTL binary parameters
choline chloride (i)- water (j) glucose (i)- water (j) fructose (i)-water(j) choline chloride (i)- glucose (j) choline chloride (i)- fructose (j)
Aij (J.mol1)
Bij (J.mol1.K1)
Aji (J.mol1)
Bji (J.mol1.K1)
10076.1 1058.8 85.7 18.0 49254.9
9.1 21.8 0.93 11.5 127.2
75638.9 2853.7 14.5 511.9 1035.6
224.9 8.0 11.2 24.4 4.0
Table 8 AARD of the model in estimating bubble pressure of studied systems in the temperatures range of 303.15 Ke343.15 K. System
AARD%
choline chloride-water glucose-water fructose-water DES (choline chloride þ glucose)-water DES (choline chloride þ fructose)-water Overall
2.09 3.22 1.17 1.38 1.82 1.91
Furthermore, the results of the model are compared with the experimental bubble pressure in Figs. 2e6. As seen in these figures, the model has successfully estimated the effect of solute concentration and temperature on the bubble pressure for all studied systems. It should be mentioned that the binary parameters of the NRTL model were calculated with the experimental data in the temperature range of 303.15 Ke343.15 K. Thus, the parameters are at least valid for the mentioned conditions. Moreover, the NRTL model was used for predicting the solubility of glucose and fructose in the water at different temperatures. Fig. 7 compared the results of the model with the literature data [38,39]. The solubility of glucose in water was available in the temperature range of 303.15 Ke343.15 K [38], while we found only one data on the solubility of fructose in water (at 343.15 K) [39]. According to the results, the AARDs were obtained to be 3.01% and 4.29% for the solubility of glucose and fructose in water, respectively. Furthermore, the minimum and maximum deviation percentages for the
Fig. 2. Bubble pressure of choline chloride-water system as a function of temperature for different concentrations of choline chloride. : 0% (mol/mol), ,: 10% (mol/mol), △: 15% (mol/mol), :20% (mol/mol),▽:30% (mol/mol). Solid lines: model.
⋄
Fig. 3. Bubble pressure of glucose-water system as a function of water mole fraction at different temperatures. : 343.15 K, ,: 333.15 K, △: 323.15 K, :313.15 K,▽:303.15 K. Solid lines: model.
⋄
solubility of glucose in water were obtained to be 0.16% and 9.19%, respectively. Although no solubility data was used in the calculation of the binary interaction parameters, the solubility data were successfully predicted by the model. It indicates the reliable accuracy of the binary interaction parameters obtained in this study.
Fig. 4. Bubble pressure of fructose-water system as a function of water mole fraction at different temperatures. : 343.15 K, ,: 333.15 K, △: 323.15 K, :313.15 K,▽:303.15 K. Solid lines: model.
⋄
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S. Gholami, A. Roosta / Journal of Molecular Liquids 296 (2019) 111876
Fig. 5. Bubble pressure of DES (glucose þ choline chloride)-water system as a function of water mole fraction at different temperatures. : 343.15 K, ,: 333.15 K, △: 323.15 K, :313.15 K,▽:303.15 K. Solid lines: model.
⋄
Fig. 7. A comparison between experimental solubility of sugars in water and the solubility estimated by the NRTL model. : solubility of glucose [38], ,: solubility of fructose [39], line: model.
can successfully correlate the bubble point of the mixtures. Besides, the binary interaction coefficients of the NRTL model led to accurate prediction of sugars solubility in water. References
Fig. 6. Bubble pressure of DES (fructose þ choline chloride)-water system as a function of water mole fraction at different temperatures. : 343.15 K, ,: 333.15 K, △: 323.15 K, :313.15 K,▽:303.15 K. Solid lines: model.
⋄
4. Conclusions Recent increasing attention to the application of natural deep eutectic solvents in different research fields encouraged us to study important physical properties of these green solvents. One of the important thermodynamic properties of mixtures is the activity coefficient of the individual components that can be calculated by phase equilibrium data. In this study, the bubble pressure of aqueous solutions of two sugar-based DESs and aqueous solutions of their individual components was measured at different temperatures and different concentrations. According to the results, adding sugars, choline chloride or DESs to the water decreased the bubble pressure. The order of bubble pressure reduction was choline chloride > DESs > sugars. The experimental data were used to calculate the activity coefficient and also the binary interaction coefficients of the NRTL model. The results showed that the model
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Bubble point of aqueous mixtures of sugar-based deep eutectic solvents and their individual components: Experimental study and modeling Sahar Gholami, Aliakbar Roosta* Department of Chemical, Petroleum and Gas Engineering, Shiraz University of Technology, Shiraz, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 August 2019 Received in revised form 3 September 2019 Accepted 3 October 2019 Available online 9 October 2019
The bubble pressure of aqueous solutions of two natural deep eutectic solvents (DESs) was measured at temperatures 303.15 Ke343.15 K and different concentration of the DESs. DESs were formed with combining choline chloride and a sugar (glucose or fructose) in a choline chloride: sugar molar ratio of 2:1. In addition, the bubble pressure of aqueous solutions of fructose, glucose, and choline chloride, were measured to define the impact of the individual components of the DESs on the bubble pressure. According to the results, choline chloride had the greatest effect and sugars had the least effect on the bubble pressure. Experimental bubble point data was used to estimate the binary interaction parameters of the NRTL activity model. An overall AARD of 1.91% indicates the good accuracy of the model in estimating the bubble pressure of aqueous solutions studied in this work. To verify the calculated binary interaction parameters, the NRTL model was used to estimate the solubility of glucose and fructose in water and the results were compared with the experimental solubility available in the literature. © 2019 Elsevier B.V. All rights reserved.
Keywords: Bubble pressure Deep eutectic solvent Choline chloride Sugar Activity coefficient
1. Introduction Deep eutectic solvents (DESs) are a new type of green solvents which are usually liquid at room temperature [1,2]. These solvents were introduced by Abbott et al. for the first time in 2003 [3]. DESs are usually formed by combining an ammonium salt such as choline chloride as a hydrogen-bond acceptor with a hydrogenbond donor like amines, amides, alcohols and carboxylic acids in certain molar ratios [4,5]. The melting point of the DESs is significantly lower than the melting point of its pure constituents due to the hydrogen-bonding interactions between the constituents [6,7]. DESs are mostly non-toxic, non-flammable, low cost, accessible, biodegradable, environmentally friendly, and nonvolatile [8,9]. Recently, DESs have been in attention of many researchers in many applications such as gas absorption [8,10,11], liquid-liquid extraction [12,13], extraction of biological compounds [14e16], desulfurization [17e19], pharmaceuticals [20,21], distillation [22,23] and etc. A type of DESs that is formed by combining biological compounds is categorized in natural deep eutectic solvents (NADESs). The hydrogen-bond donor component of these solvents can be a
* Corresponding author. E-mail address: [email protected] (A. Roosta). https://doi.org/10.1016/j.molliq.2019.111876 0167-7322/© 2019 Elsevier B.V. All rights reserved.
sugar, an alcohol, an amine, or an amino acid [24,25]. Glucose and fructose are both natural, and edible hydrogen-bond donors which are found in plants [26]. In addition, choline chloride is a noteworthy hydrogen-bond acceptor in NADESs because of its nutritional value which is used as a complement for reducing the cholesterol level [27] and it is considered as a component of Bcomplex vitamins [28]. Therefore, the DESs of the choline chlorideglucose or choline chloride-fructose are of interest in many kinds of research. Despite the many advantages of the DESs, high viscosity values of DESs is an obstacle for industrial applications of these green solvents. To solve this problem, DESs are usually used as aqueous solutions [29]. Thus, the physical properties of the aqueous solution of DESs are important for a deeper comprehension of DES’s behaviors. One of the important thermodynamic properties of the aqueous solution of DESs is the activity coefficient of the individual components. The activity coefficient is a thermodynamic property that represents the deviation from the ideality in liquid solutions. It is widely used in phase equilibria calculations. The activity coefficients can be calculated with phase equilibrium data, such as the bubble point of the mixture. Therefore, in this study, the bubble pressure of aqueous solutions of two sugar-based NADESs was measured at different temperatures and different concentrations. Furthermore,
2
S. Gholami, A. Roosta / Journal of Molecular Liquids 296 (2019) 111876
the aqueous solutions of the individual components of the DESs (fructose, glucose and choline chloride) were studied to determine the contribution of each component to the non-ideality of the aqueous systems. Then, the experimental data were employed to estimate the activity coefficient of water in the binary and ternary systems. Finally, the NRTLactivity model was used to correlate the activity. 2. Materials and method
denote the mole fraction of water in the liquid phase and vapor phase, respectively. Due to the very low vapor pressure of glucose, fructose, choline chloride, and DESs, y was assumed to be the unity. Also, psat is the saturation vapor pressure of water which was gathered from the literature [31], and f is the fugacity coefficient of pure water in the vapor phase which was calculated by using the Peng-Robinson equation of state Eqs. (2)e(8) [32].
P¼
The chemicals used in this study are listed in Table 1. Choline chloride with the purity of more than 99% and D-(þ)-glucose with the purity of more than 99.5% were purchased from Sigma-Aldrich, and fructose with the purity of more than 99% was provided by Surechem Product. Furthermore, double deionized water was used in this experimental test.
RT
nb
aðTÞ
(2)
nðn þ bÞ þ bðn bÞ
b ¼ 0:07780
RTc Pc
(3)
aðTÞ ¼ aðTc Þ aðTÞ
(4)
2.1. Experiment In this study, the bubble pressure of aqueous solutions of two sugar-based DES was measured as well as the bubble pressure of aqueous solutions of their individual components, including choline chloride, fructose, and glucose. Choline chloride was placed in an oven at 100 C to remove its water content, while the other chemicals were used as received, without any purification. To prepare DESs, dried choline chloride and sugar (glucose or fructose) were mixed at a molar ratio of 2:1. Weighting the materials was conducted with an analytical balance (HR-200, A&D) with a precision of 104 g. To prevent moisture absorption of choline chloride, the mixture of choline chloride and sugar was placed on a hot plate at 100 C to form a clear liquid. Then, the formed liquid was combined with double deionized water at different ratios by using the analytical balance to prepare aqueous solutions of DESs with desired concentrations. Next, 3 mL of the aqueous solution was withdrawn by using a 10 mL glass syringe (Becton Dickinson). To remove the dissolved gas in the solution, the syringe was closed by a cap and a vacuum was created by pulling the piston of the syringe. Then, the released gases were removed from the syringe by pushing the piston. The procedure was repeated several times to ensure complete removal of the dissolved gas. The syringe was connected to a vacuum meter (VC-9200, Lutron) with a precision of 0.1 kPa. After that, the set of the syringe and vacuum meter was placed into a water bath (with a temperature precision of 0.1 K) in a given temperature. In addition, suction was applied to the syringe to create bubble conditions of the mixture. At least, 4 h were allowed to achieve temperature equilibrium, and consequently thermodynamic equilibrium. Finally, the bubble pressure was recorded by the vacuum meter.
aðTc Þ ¼ 0:45724
ðRTc Þ2 Pc
"
qffiffiffiffiffiffiffiffiffi aðTÞ ¼ 1 þ b 1 T=T c
First, the experimental bubble pressure data were used to estimate the water activity coefficient (gw) using the vapor-liquid equilibria (VLE) equation [30] as follows:
yw Pfw ¼ xw psat w gw
(1)
where P is the pressure, the subscript w stands for water and x, y
!#2 (6)
b ¼ 0:37464 þ 1:54226u 0:26992u2
(7)
0 pffiffiffi 1 v þ 1 þ 2 b Pv pðv bÞ a 1 ln pffiffiffi lnf ¼ ln@ pffiffiffi A RT RT 2 2bRT vþ 1 2 b
(8) where P, R, T, v, u are the pressure, gas constant, temperature, molar volume, and acentric factor respectively; and the subscript c denotes the critical point, a(T) and a(T) are parameters of the PengRobinson equation which depend on temperature, b and b are temperature independent parameters of the equation of state. Despite glucose and fructose, choline chloride is an ionic solute; however, it is assumed to be a nonionic solute in this study. Thus, the activity coefficient can be fitted to the NRTL activity model Eq. (9) [30] instead of e-NRTL activity model for all studied systems. m P
lngi ¼
tji Gji xj
j¼1 m P
Gji xj
j¼1
2.2. Model
(5)
13 m P tkj Gkj xk C7 B 6 xj Gij B C7 k¼1 þ 6m Btij m C7 P 4P @ A5 j¼1 Gkj xk Gkj xk 2
0
m 6 X
Gij ¼ exp atij ;
k¼1
tij ¼
k¼1
(9)
Dgij RT
where a is the non-randomness parameter (a) was considered to be constant of 0.3 in this study, tij and Gij are calculated by the binary parameters of the NRTL model (Dgij) which is an energy parameter and were correlated as a linear function of temperature in this study, as shown by Eq. (10).
Table 1 Source and purity of chemicals. Chemical Name
Source
Initial mass fraction purity
Purification method
Final mass fraction purity
Analysis method
D()Fructose D-(þ)-Glucose Choline chloride
Surechem Products Sigma-Aldrich Sigma-Aldrich
>0.99 >0.995 >0.99
None None Drying in oven
e e 0.99
e e Karl-Fischer
S. Gholami, A. Roosta / Journal of Molecular Liquids 296 (2019) 111876
Dgij ¼ Aij þ Bij T
(10)
where Aij and Bij are the constants of Eq. (10) which are estimated by the experimental data. The binary interaction parameters between water and the ingredients of DESs (fructose, glucose, and choline chloride) was calculated using the water activity coefficient in the binary mixtures (aqueous solutions of fructose, glucose, and choline chloride). Then, the binary parameters between the ingredients of DESs (between choline chloride and glucose, and between choline chloride and fructose) were correlated to the temperature using the experimental bubble pressure of the aqueous solutions of DESs and using the estimated parameters between the DES’s ingredients and water. To verify the accuracy of the binary interaction parameters, the bubble pressure of the studied systems was calculated and was compared to the experimental bubble pressure with calculating the average absolute relative deviation (AARD) as follows:
AARD% ¼
N Pcalc;i Pexp;i 100 X N i¼1 Pexp;i
(11)
where subscripts exp and calc denote experimental and calculated, and N is the number of data. The activity coefficient of the nonvolatile solutes (glucose, fructose, choline chloride, and DESs) is not applied to the bubble point calculation of current systems, and only the activity coefficient of water is used in these calculations. Therefore, a criterion is required to measure the accuracy of the NRTL model in calculating the activity coefficient of these solutes. The NRTL model with the obtained binary interaction parameters was used to predict the solubility of solutes (specifically glucose and fructose whose solubility data are available from the literature) in the water at different temperatures and the results were compared with the available data in the literature. Eq. (12) [30] was used to calculate the solubility of glucose and fructose in water.
Dhfus Tt 1 ¼ exp x 1 g1 RTt T 1
3
Table 3 Experimental bubble pressure of the glucose-water system at different temperatures and concentrations.a mole fraction of glucose (x)
T/K 303.15
313.15
323.15
333.15
343.15
0.02 0.04 0.06 0.075
4.1 3.9 3.7 3.5
7.0 6.8 6.6 6.3
12.0 11.2 11.0 10.4
19.4 18.7 17.8 17.2
30.0 28.8 27.2 26.3
a Standard uncertainty for temperature is u(T) ¼ 0.5 K. Relative standard uncertainties for mole fraction and pressures are ur(x) ¼ 0.04 and ur (P) ¼ 0.05, respectively.
pure water was gathered from the literature [34] and was added to this table to show the effect of choline chloride on reducing the water vapor pressure. For instance, the bubble pressure of a 0.3 (mol/mol) choline chloride solution is lower than 60% of the vapor pressure of pure water which represents a negative deviation from Raoult’s law. This is due to the hydrogen bond between choline chloride and water. The bubble pressure of aqueous solutions of glucose and fructose are collected in Tables 3 and 4, respectively. The maximum concentrations of glucose (0.075) and fructose (0.2) were selected based on the solubility of these sugars in the water at the lowest temperature (303.15 K). According to the results, the bubble pressure of a 0.075 (mol/mol) glucose solution is about 85% of the vapor pressure of pure water, while the bubble pressure of a 0.1 (mol/mol) fructose solution is about 88% of the vapor pressure of pure water. This indicates that adding glucose to water caused a more bubble pressure drop in comparison with fructose. Consequently, the deviation from Raoult’s law in glucose solution is more negative than the fructose solution under the same conditions. Tables 5 and 6 contain the bubble pressure of aqueous solutions of glucose-choline chloride and fructose-choline chloride DESs, respectively. As seen in these tables, increasing the DESs concentration cause a decrease in the bubble pressure. Furthermore, it can be concluded from the results of these tables that the effect of both
(12)
where x1 is the mole fraction of the solute (glucose or fructose) in water, Tt is the triple point temperature of the solute (419.65 K for glucose, and 385.85 K for fructose) and Dhfus is the molar enthalpy of the solute (33.40 kJ mol1 for glucose, 27.76 kJ mol1) [33].
3. Results and discussion
Table 4 Experimental bubble pressure of the fructose-water system at different temperatures and concentrations.a mole fraction of fructose (x)
T/K 303.15
313.15
323.15
333.15
343.15
0.05 0.10 0.15 0.20
4.0 3.7 3.5 3.3
6.9 6.5 6.0 5.7
11.4 10.7 9.9 9.5
18.4 17.4 16.6 15.2
28.9 27.4 25.7 24.0
The bubble pressure of aqueous solutions of choline chloride at 303.15 Ke323.15 K and mole fraction of choline chloride between 0.1 and 0.3 are listed in Table 2. In addition, the vapor pressure of
a Standard uncertainty for temperature is u(T) ¼ 0.5 K. Relative standard uncertainties for mole fraction and pressures are ur(x) ¼ 0.04 and ur (P) ¼ 0.05, respectively.
Table 2 Experimental bubble pressure of the choline chloride-water system at different temperatures and concentrations.a
Table 5 Experimental bubble pressure of DES (choline chloride þ glucose)-water system at different temperatures and concentrations.a
mole fraction of choline chloride (x)
T/K 303.15
313.15
323.15
333.15
343.15
mole fraction of DES (x)
0 0.10 0.15 0.20 0.30
4.2 3.5 3.2 2.8 2.0
7.4 5.9 5.3 4.7 3.1
12.3 9.8 8.7 7.2 4.7
19.9 16.1 13.9 11.9 7.9
31.2 24.8 21.9 18.5 12.8
a Standard uncertainty for temperature is u(T) ¼ 0.5 K. Relative standard uncertainties for mole fraction and pressures are ur(x) ¼ 0.04 and ur (P) ¼ 0.05, respectively.
0.05 0.10 0.15 0.20 0.25
T/K 303.15
313.15
323.15
333.15
343.15
4.0 3.7 3.3 3.0 2.7
6.9 6.3 5.8 5.4 4.7
11.3 10.5 9.2 8.3 7.5
18.3 16.9 15.0 13.6 11.9
28.4 26.1 23.8 21.4 17.7
a Standard uncertainty for temperature is u(T) ¼ 0.5 K. Relative standard uncertainties for mole fraction and pressures are ur(x) ¼ 0.04 and ur (P) ¼ 0.05, respectively.
4
S. Gholami, A. Roosta / Journal of Molecular Liquids 296 (2019) 111876
Table 6 Experimental bubble pressure of DES (choline chloride þ fructose)-water system at different temperatures and concentrations.a mole fraction of DES (x)
T/K 303.15
313.15
323.15
333.15
343.15
0.05 0.10 0.15 0.20 0.25
3.9 3.6 3.4 3.1 2.7
6.5 6.2 5.5 5.0 4.5
11.2 10.3 9.1 8.3 7.5
18.4 16.9 14.9 13.2 12.0
28.4 25.7 23.6 20.5 18.0
a Standard uncertainty for temperature is u(T) ¼ 0.5 K. Relative standard uncertainties for mole fraction and pressures are ur(x) ¼ 0.04 and ur (P) ¼ 0.05, respectively.
DESs on the bubble pressure is almost the same. A comparison between the results of Tables 2e6 indicates that adding sugars, choline chloride or DESs to the water decreased the bubble pressure. However, the effect of choline chloride on the bubble pressure reduction was more significant. Also, sugars had the least effect on the bubble pressure. To evaluate the present experimental bubble pressure, the experiments obtained in this study were compared to the bubble pressures available in the literature as illustrated in Fig. 1. Fig. 1(a)
shows the bubble pressure of the glucose-water system at different temperatures and different glucose concentrations. In this study, the bubble pressure of the glucose-water system was measured at temperatures of 303.15 K, 313.15 K, 323.15 K, 333.15 K, and 343.15 K; while the experiments of literature are available at 308.15 K and 318.15 K [35]. According to the results of Fig. 1(a), the trend of the bubble pressure with temperature in this study is in good agreement with the literature data [35]. Fig. 1(b) compares the bubble pressures of a 0.2 (mol/mol) choline chloride solution obtained in this study with the bubble pressures reported by Francisco et al. [36]. In addition, a comparison between the current bubble pressure of the aqueous choline chloride at a temperature of 323.15 K and the literature data [37] was made in Fig. 1(c). As discussed previously, the binary interaction parameters of the NRTL model were correlated to temperature (Eq. (10)) using the bubble pressure data. The estimated binary parameters are listed in Table 7. The obtained binary interaction parameters were used to calculate the bubble pressure of aqueous solutions studied in this work. The AARDs of the model in the estimation of bubble pressure are listed in Table 8. The maximum AARD was 3.22% for the glucosewater system, and the overall AARD was 1.91% for the five studied systems. The low deviation between the results of the model and the experiments is a reliable validation of the model capability in calculating the activity coefficient of water in the current systems.
Fig. 1. A comparison between current experimental data and literature data. (a): bubble pressure of glucose-water system [35], (b): bubble pressure of choline chloride (20% mol/ mol)þwater system [36], (c): bubble pressure of choline chloride þ water system at 323.15 K [37].
S. Gholami, A. Roosta / Journal of Molecular Liquids 296 (2019) 111876
5
Table 7 Estimated NRTL binary parameters (A and B of Eq. (10) where a ¼ 0.3). System
NRTL binary parameters
choline chloride (i)- water (j) glucose (i)- water (j) fructose (i)-water(j) choline chloride (i)- glucose (j) choline chloride (i)- fructose (j)
Aij (J.mol1)
Bij (J.mol1.K1)
Aji (J.mol1)
Bji (J.mol1.K1)
10076.1 1058.8 85.7 18.0 49254.9
9.1 21.8 0.93 11.5 127.2
75638.9 2853.7 14.5 511.9 1035.6
224.9 8.0 11.2 24.4 4.0
Table 8 AARD of the model in estimating bubble pressure of studied systems in the temperatures range of 303.15 Ke343.15 K. System
AARD%
choline chloride-water glucose-water fructose-water DES (choline chloride þ glucose)-water DES (choline chloride þ fructose)-water Overall
2.09 3.22 1.17 1.38 1.82 1.91
Furthermore, the results of the model are compared with the experimental bubble pressure in Figs. 2e6. As seen in these figures, the model has successfully estimated the effect of solute concentration and temperature on the bubble pressure for all studied systems. It should be mentioned that the binary parameters of the NRTL model were calculated with the experimental data in the temperature range of 303.15 Ke343.15 K. Thus, the parameters are at least valid for the mentioned conditions. Moreover, the NRTL model was used for predicting the solubility of glucose and fructose in the water at different temperatures. Fig. 7 compared the results of the model with the literature data [38,39]. The solubility of glucose in water was available in the temperature range of 303.15 Ke343.15 K [38], while we found only one data on the solubility of fructose in water (at 343.15 K) [39]. According to the results, the AARDs were obtained to be 3.01% and 4.29% for the solubility of glucose and fructose in water, respectively. Furthermore, the minimum and maximum deviation percentages for the
Fig. 2. Bubble pressure of choline chloride-water system as a function of temperature for different concentrations of choline chloride. : 0% (mol/mol), ,: 10% (mol/mol), △: 15% (mol/mol), :20% (mol/mol),▽:30% (mol/mol). Solid lines: model.
⋄
Fig. 3. Bubble pressure of glucose-water system as a function of water mole fraction at different temperatures. : 343.15 K, ,: 333.15 K, △: 323.15 K, :313.15 K,▽:303.15 K. Solid lines: model.
⋄
solubility of glucose in water were obtained to be 0.16% and 9.19%, respectively. Although no solubility data was used in the calculation of the binary interaction parameters, the solubility data were successfully predicted by the model. It indicates the reliable accuracy of the binary interaction parameters obtained in this study.
Fig. 4. Bubble pressure of fructose-water system as a function of water mole fraction at different temperatures. : 343.15 K, ,: 333.15 K, △: 323.15 K, :313.15 K,▽:303.15 K. Solid lines: model.
⋄
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S. Gholami, A. Roosta / Journal of Molecular Liquids 296 (2019) 111876
Fig. 5. Bubble pressure of DES (glucose þ choline chloride)-water system as a function of water mole fraction at different temperatures. : 343.15 K, ,: 333.15 K, △: 323.15 K, :313.15 K,▽:303.15 K. Solid lines: model.
⋄
Fig. 7. A comparison between experimental solubility of sugars in water and the solubility estimated by the NRTL model. : solubility of glucose [38], ,: solubility of fructose [39], line: model.
can successfully correlate the bubble point of the mixtures. Besides, the binary interaction coefficients of the NRTL model led to accurate prediction of sugars solubility in water. References
Fig. 6. Bubble pressure of DES (fructose þ choline chloride)-water system as a function of water mole fraction at different temperatures. : 343.15 K, ,: 333.15 K, △: 323.15 K, :313.15 K,▽:303.15 K. Solid lines: model.
⋄
4. Conclusions Recent increasing attention to the application of natural deep eutectic solvents in different research fields encouraged us to study important physical properties of these green solvents. One of the important thermodynamic properties of mixtures is the activity coefficient of the individual components that can be calculated by phase equilibrium data. In this study, the bubble pressure of aqueous solutions of two sugar-based DESs and aqueous solutions of their individual components was measured at different temperatures and different concentrations. According to the results, adding sugars, choline chloride or DESs to the water decreased the bubble pressure. The order of bubble pressure reduction was choline chloride > DESs > sugars. The experimental data were used to calculate the activity coefficient and also the binary interaction coefficients of the NRTL model. The results showed that the model
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