Solubility of Sodium Salts in Ammonium-Based ... - ACS Publications

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Solubility of Sodium Salts in Ammonium-Based Deep Eutectic Solvents Fatemeh Saadat Ghareh Bagh,† Farouq S. Mjalli,‡,* Mohd Ali Hashim,† Mohamed Kamel Omar Hadj-Kali,§ and Inas M. AlNashef§ †

Chemical Engineering Department, University of Malaya, Kuala Lumpur, Malaysia Petroleum and Chemical Engineering Department, Sultan Qaboos University, Oman § Chemical Engineering Department, King Saud University, Saudi Arabia ‡

ABSTRACT: The solubility of sodium chloride (NaCl), sodium bromide (NaBr), and sodium carbonate (Na2CO3) was measured in nine ammoniumbased deep eutectic solvents (DESs). The aim of the study is to assess the potential use of these DESs as solvents and electrolytes for the separation of sodium metal from its salts. The studied DESs were prepared by combining ammonium salts with various hydrogen-bond donors (HBDs) or metal halides. It was found that the solubility of the sodium salts in DESs increases with temperature in many cases. The maximum solubility of NaCl achieved was 100 w = 80 in N,N-diethylethanolammonium chloride:zinc(II) chloride (molar ratio 1:3) at 90 °C. In addition, the solubility of the sodium salts was modeled successfully using the non-random two liquid (NRTL) model.

1. INTRODUCTION Sodium metal (alkali metal) is used in wide industrial applications including pharmaceutical industries, battery industries, and automobile manufacturing. It is produced by the electrolytic reduction of a mixture of sodium salts at a very high temperature. Alkali metals are known to be active reducing agents with high reactivity. This property renders traditional extractive methods inapplicable for alkali metals due to practical difficulties. For example, it is well-known that metals such as lithium and sodium cannot be separated from their oxides by normal reduction procedures. In addition, due to the reaction of metals with water forming the corresponding metal oxides, electrolysis cannot be used to extract the metals from their aqueous solutions.1 Moreover, sodium has a fast and exothermic reaction when it comes into contact with atmospheric oxygen or water. It reacts with water to form sodium hydroxide and releases flammable hydrogen (causing a fire in many times). Thus, the presence of any traces of water in the deep eutectic solvent (DES) is not favorable.2 The currently used process for the production of sodium and lithium from their salts on a worldwide basis is known as the “Downs” process. It is the major process for producing sodium metal as well as a minor source for producing industrial chlorine. CaCl2 is added to molten NaCl to decrease the operating temperature from 804 °C to 600 °C. The process utilizes an electrochemical cell in which sodium metal accumulates around an iron cathode and chlorine gas is released by a carbon anode.1,3,4 Operating an electrolytic process at such a temperature is difficult and presents a serious operational constraint. In addition, the process generates © 2013 American Chemical Society

pollutants and consumes large amounts of energy. Hence, it is of paramount importance to find green solvents that can replace molten salts as electrolytes in the Downs process which have all the capabilities of molten salts but can be used at moderate temperatures.1,3 The green solvents’ concept was adopted to represent the efforts spent to minimize the industrial impact on the environment by utilizing safer and friendlier solvents than the conventional ones. This adoption led to the development of four direct implementations for this green solvents’ concept. They are: (i) replacing the hazardous solvents by solvents of better environmental, health, and safety properties, (ii) to use biosolvents produced from renewable sources, (iii) to replace the volatile organic compounds used as solvents by benign solvents, and (iv) to use ionic liquids (ILs) which are nonvolatile by their nature (showed negligible vapor pressure).5 ILs are liquids consisting of ions only that exert no vapor pressure. They have a wide ranging liquid phase and are nonflammable and less toxic than conventional solvents.6 They are recyclable, which leads to the reduction in the cost of operation. They were found to have a high potential to dissolve metal salts, which augments their industrial importance. It was found that the solubility of various salts in the halometalate ILs is utterly high due to the high halide concentration in them.7−10 One drawback of utilizing ILs is that they are usually expensive and unavailable at the industrial scale. Received: January 6, 2013 Accepted: July 3, 2013 Published: July 18, 2013 2154

dx.doi.org/10.1021/je400045d | J. Chem. Eng. Data 2013, 58, 2154−2162

Journal of Chemical & Engineering Data

Article

Table 1. Summary of the DESs Studied in This Work and Their Abbreviations abbreviation

salt:HBD

DES1 DES2 DES3 DES4 DES5 DES6 DES7 DES8 DES9

choline chloride:ethylene glycol choline chloride:glycerol N,N-diethylethanolammonium chloride:ethylene glycol N,N-diethylethanolammonium chloride:glycerol choline chloride:zinc(II) chloride choline chloride:tin(II) chloride choline chloride:zinc(II) chloride:tin(II) chloride N,N-diethylethanolammonium chloride:zinc(II) chloride N,N-diethylethanolammonium chloride:ferric chloride

molar ratios

Abbott et al.11 developed a range of liquids formed from eutectic mixtures of salts and hydrogen bond donors. These liquids were termed as deep eutectic solvents (DESs) to differentiate them from ILs which contain only discrete anions. They were found to possess similar characteristics to ILs. Some research referred to DESs as analogues of ionic liquids due to the similarity in the properties.12 These IL analogues can compete strongly with ILs in terms of physical properties and more importantly in price.7,9,11,13 As a consequence, DESs were proposed as alternatives to common solvents and ILs.14 DESs are composed of a mixture of a salt and a hydrogen bond donor (HBD) which results in a liquid medium with a freezing point lower than the freezing points of the constituting compounds. DESs are advantageous in comparison to ILs because of their easy and low cost preparation. In addition, their components can be selected to be biodegradable and nontoxic. It was reported that most of these DESs are moisture-stable as well.13,15−17 Many research groups reported the synthesis and use of DESs in different applications. Professor Andrew Abbott and his group were the first to report the synthesis and use of ammonium-based DESs in different applications.7 It is reported that the industrial applications of DESs are very promising.18−23 The applications of DESs in various chemical processes are now more common in the literature than four years ago. Numerous uses and applications for DESs up to date are reported in a recent review paper by Zhang et al.24 The solubility of a salt in a solvent is explained by mutual interactions between the solvent and solute molecules. The dissolution of the salt is determined by two forces: (i) attraction forces between the complex of the solvent and the ions of the solute and (ii) attraction forces between oppositely charged ions of the salt. When the first forces are predominant, the salt is highly soluble in the solvent. Otherwise, the solubility of the salt in the solvent will be very low.25 In this paper we have studied the solubility of several commercially available sodium salts in ammonium based DESs at different temperatures. The main objective of this work is to explore the potential use of these solvents in the electrochemical production of sodium metal at moderate temperatures.

1:1.75 1:1.5 1:2.5 1:2.5 1:1 1:3 1:1:1 1:1 1:3

1:2 1:2 1:3 1:3 1:2

1:2.25 1:2.5 1:4 1:4 1:3

1:2.5 1:3 1:4.5

1:2

1:3

1:4

sodium bromide were supplied by Merck (Germany). All used chemicals are of high purity (> 99 %). No additional purification was done. This is a common procedure followed by many researches available in the literature.13,17,20,21 2.2. Synthesis of DESs. The eutectic mixtures were formed by following the method described previously in many publications.7,13 The salt and HBD were placed together in a jacketed vessel and vigorously mixed at a specific temperature and atmospheric pressure until a homogeneous and colorless liquid was formed. Synthesized DESs were placed in tight and humidity-safe screw-capped bottles and stored in a dehumidifier chamber to control their moisture content. The synthesized DESs in this work are ammonium based and are listed in Table 1. For each DES, different molar ratios of salt:HBD around eutectic point value were chosen. The selected ammonium salt was either choline chloride or N,Ndiethylethanolammonium chloride, while the used HBDs were ethylene glycol and glycerol. Zinc chloride, tin chloride, and ferric chloride were also used as Lewis acid HBDs. All of the DESs were synthesized at temperatures less than 100 °C. 2.3. Solubility. To measure the solubility of sodium salts in various DESs, three important factors were taken into consideration. The DES and sodium salt must be of high purity because any small amount of impurities will affect the measured solubility. In addition, the taken samples from the saturated solution must contain no precipitated salt, and the temperature must be controlled. Traditionally, the equilibrium solubility at a given temperature is determined by the shake flask method. According to this method, the solute is added to the solvent and shaken (stirred) for 24 h or longer. The saturation is reached and confirmed by observing the presence of undissolved solute in the solvent. Sometimes, solutions can hold a certain amount of excess solute in the solvent and are commonly called a “supersaturated solution”. This often occurs when the saturated solution is cooled slowly, while supersaturation for salts can be avoided by fast cooling and continuous shaking of the sample during cooling down. Both the mixing and settling of the solution must be performed at the same temperature. Dilution may be necessary in certain cases of sample preparation to prevent crystallization. After separation of the slurry, samples can be taken for chemical analysis. The shake-flask method which was used in this work is the most accurate method to determine solubility but it is timeconsuming. The solubility was measured by the addition of about 0.1 g of sodium salt (anhydrous) in 5 g of the DESs. The mixture is stirred for 24 h to 48 h at constant temperature. If the solution was able to dissolve all the 1 g of the sodium salt, more milligrams were added, and observation was focused to ensure that a saturated solution was achieved. Saturation means

2. EXPERIMENTAL METHODOLOGY The whole of the experimental work of this research work was carried out under a glovebox environment whereby the humidity was less than 0.4 ppm. 2.1. Chemicals. All chemicals used in this work, which are choline chloride, N,N-diethylethanolammonium chloride, ethylene glycol, glycerol, anhydrous zinc chloride, anhydrous tin chloride, anhydrous ferric chloride, sodium chloride, and 2155



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Figure 1. Solubility profiles of sodium chloride, sodium carbonate, and sodium bromide in DES1 as a function of temperature. The continuous line is drawn through the experimental data of the same system for visual clarity.

electrical conductivity of the resulting solution, and the stability of the produced sodium metal in the solution. In this work we investigated the solubility of commercially available sodium salts in selected ammonium-based DESs. Ionic interaction behavior of a dipolar sodium salt like NaCl in an unconventional solvent like DES can strongly affect the normal tendency of solubility with temperature. It is common knowledge that when a solvent undergoes additional heating, the particles of the solute in that solution tend to move more easily between the solid phase and the solution. This can be explained in the context of the second law of thermodynamics which predicts that the mixture will tend to be in a more disordered form causing the two components to be in a solution form. Several solubility measurements were done repeatedly to increase the reliability of the experimental results. The results are classified according to solvent structure, range of temperature and stability of the solution. Figures 1 through 8 depict the results of solubility in DESs at different temperatures. To make it easier for the reader the results were shown for each DES in an individual figure. The authors took ultimate care to make the data clear for the reader, and thus in some cases whereby the experimental results may overlap, different lines were drawn through the scattered points to distinguish different data series. The studied DESs according to their structure and salt:HBD molar ratios are shown in Table 1. The solubilities of NaCl, NaBr, and Na2CO3 in [choline chloride:ethylene glycol] DES, DES1, are tabulated in Table 2. Figure 1 shows plots for these solubilties as a function of temperature. This DES has a maximum solubility of 100 w = 0.25 of Na2CO3 while the solubility of NaCl and NaBr are 100 w = 0.06 and 100 w = 0.04, respectively. The solubility increases by increasing temperature from ambient to 35 °C and then increases slowly afterward up to a temperature of 60 °C. The slight decrease in solubility noticed at 50 °C for a few cases can be neglected and considered as experimental error. This is because it happened in certain cases and not in all. At temperatures higher than 60 °C the DES started to lose ethylene glycol and thus became unstable. There is no clear trend in the solubility of all three salts in DES1 with the change of salt:HBD molar ratio.

that the solvent is unable to dissolve more solute. The heating and stirring were achieved by Fischer brand hot plates and magnetic stirrers. The temperature accuracy was further assured by using an independent thermometer for each test in addition to the built-in thermometer for the hot plate stirrer. When saturation was reached, the solution was filtered out to remove the excess sodium salt. This filtration was done at the same temperature of stirring. A sample of the saturated solution was taken by heat-resistant syringes and placed into special falcons for analysis. The samples were diluted with deionized water and analyzed using Perkin-Elmer Optima 5300DV inductively coupled plasma-atomic emission spectrometer (ICP-AES). Each solubility measurement was repeated three times and the average was recorded. The uncertainty of the ICP is estimated to be (0.02 for solubility 100 w > 20 and 0.05 for solubility 100 w < 10). In this work, the solubility of three different low-cost sodium salts, namely NaCl, NaBr, and Na2CO3 was measured in various DESs at different temperatures (25 °C to 145 °C). The temperature variation was achieved by the hot plate stirrer. An oil bath was heated by the hot plate stirrer, and the vial of the solubility experiment was placed inside the oil bath. The temperature accuracy was ascertained by using external thermometer to compare the measurement of the stirrer. The uncertainty in the temperature of the experiments was ± 0.01 °C. 2.4. Stability. It is well-known that sodium metal is highly reactive when it is in its pure form. For this reason, sodium metal cannot be found pure in nature, but in the form of compounds after reacting with other elements or compounds to form sodium halides, nitrites, and oxides. The stability of sodium inside the proposed DESs was studied by placing approximately 0.5 g of pure sodium metal in each DES and monitoring the results of this addition over a time span of 24 h.

3. RESULTS AND DISCUSSION Three important factors must be taken into account when choosing the proper solvent for the production of sodium metal electrochemically. These factors are the high solubility of commercially available sodium salts in the solvent, reasonable 2156



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Table 2. Solubility (as 100 w) of Sodium Chloride, Sodium Carbonate, and Sodium Bromide in DES1 at Different Temperatures and a Pressure of 0.1 MPaa

a

temp

1:1.75

25 35 50 60

0.01 0.03 0.03 0.04

25 35 50 60

0.03 0.22 0.22 0.25

25 35 50 60

0.00 0.03 0.03 0.03

1:2 NaCl 0.01 0.05 0.04 0.05 Na2CO3 0.02 0.16 0.15 0.18 NaBr 0.00 0.04 0.04 0.05

1:2.25

1:2.5

0.00 0.05 0.05 0.06

0.01 0.07 0.06 0.07

0.01 0.15 0.14 0.16

0.01 0.18 0.15 0.18

0.00 0.04 0.04 0.04

0.01 0.15 0.12 0.14

Figure 2. Solubility profiles of NaCl and NaBr in DES2 as a function of temperature. NaCl series, for salt:HBD ratios 1:1.5 (●), 1:2 (○), 1:2.5 (◆) and 1:3 (◇). NaBr series, for salt:HBD ratios 1:1.5 (▼), 1:2 (▽), 1:2.5 (▲) and 1:3 (△).

Standard uncertainties u are u(t) = 0.01 °C and ur(S) = 0.05.

Table 4. Solubility (as 100 w) of Sodium Chloride in DES3 at Different Temperatures and a Pressure of 0.1 MPaa

For the [choline chloride:glycerol], DES2, the solubility of sodium salts decreases smoothly by increasing temperature, Table 3. DES2 dissolves a maximum of 100 w = 1.09 of NaCl Table 3. Solubility (as 100 w) of Sodium Chloride and Sodium Bromide in DES2 at Different Temperatures and a Pressure of 0.1 MPaa temp

1:1.5

25 50 75 100 120 140

0.37 0.36 0.33 0.32 0.31 0.31

25 50 75 100 120 140

0.33 0.32 0.30 0.29 0.28

1:2

1:2.5

1:3

0.87 0.86 0.84 0.80 0.80 0.78

0.37 0.36 0.33 0.32 0.31 0.31

0.78 0.76 0.72 0.72 0.66

0.98 0.97 0.94 0.88

NaCl

a

0.43 0.42 0.40 0.40 0.40 NaBr 0.38 0.38 0.36 0.36

a

temp

1:2.5

1:3

1:4

1:4.5

25 50 75 100 125 150

0.44 0.40 0.37 0.32 0.28 0.21

NaCl 0.53 0.52 0.51 0.49 0.44 0.41

1.04 0.98 0.95 0.86 0.80 0.70

1.15 1.07 1.06 0.99 0.89 0.80


increased the stability of the DES to much higher temperatures which enabled us to measure the solubility up to 150 °C. When the HBD in DES3 was replaced by glycerol, a new DES was formed and named as DES 4 (Table 1). The solubility of NaCl in DES4 was 100 w = 2.08 and 100 w = 2.53 at 25 °C and 150 °C, respectively (Table 5 and Figure 4). Moreover, the solubility of NaCl increased with the increase of temperature

Standard uncertainties u are u(t) = 0.01 °C, and ur(S) = 0.05.

and 100 w = 0.97 of NaBr as it can be seen in Figure 2. Thus, it is clear that the type of the HBD does affect the solubility of sodium salts in DES. To see the effect of the salt used in the synthesis the DES on the solubility of sodium chloride, choline chloride was replaced by N,N-diethylethanolammonium chloride with ethylene glycol as HBD, DES3. The results are listed in Table 4. The maximum solubility of NaCl at 55 °C was around 100 w = 0.55. This is higher than that for the [choline chloride:ethylene glycol], DES1, at the same conditions. The maximum solubility of NaCl in DES3 was 100 w = 1.15 at 25 °C for the salt:HBD molar ratio of 1:4. Surprisingly, the solubility of NaCl in DES3 decreased with the increase of temperature and with the increase of the HBD molar ratio in the DES from 2.5 to 4 (Figure 3). The use of N,N-diethylethanolammonium chloride

Figure 3. Solubility profiles of NaCl in DES3 as a function of temperature. NaCl series, for salt:HBD ratios 1:2.5 (●), 1:3 (○), 1:4 (◆) and 1:4.5 (◇). 2157



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Table 5. Solubility (as 100 w) of Sodium Chloride in DES4 at Different Temperatures and a Pressure of 0.1 MPaa temp

1:2.5

1:3

Table 6. Solubility (as 100 w) of Sodium Chloride and Sodium Bromide in DES5 at Different Temperatures and a Pressure of 0.1 MPaa

1:4

NaCl 25 50 75 100 125 150 a

1.05 1.10 1.18 1.20 1.27 1.31

1.14 1.20 1.26 1.33 1.37 1.43

2.05 2.23 2.36 2.40 2.45 2.53


temp

1:1

temp

1:2

1:3

50 60 75 110 125 135

42.56 43.68 44.85 46.00 45.24 44.95

NaCl 50 60 70 75 110 125 135

58.00 60.00 62.00 63.00 65.60 63.00 62.50

60.00 63.00 65.00 66.00 68.15 67.00 66.00 1:3

temp

1:1

50 55 60 70 90 120 130

41.50 42.30 43.00 44.00 44.70 44.20 44.10

1:2 NaBr

a

50.40 51.20 52.30 53.60 54.90 54.00 53.80

53.40 54.00 55.50 56.70 58.30 57.00 56.80


Figure 4. Solubility profiles of NaCl in DES4 as a function of temperature. NaCl series, for salt:HBD ratios 1:2.5 (●), 1:3 (○), 1:4 (◆).

and the decrease of HBD molar ratio in the DES. It is clear from Figures 1 to 4 that for the same salt, the solubility of NaCl in DESs using glycerol as HBD is higher than that when ethylene glycol is used as HBD. The relatively low NaCl solubility in the DESs studied so far motivated us to investigate the use of other types of DESs, namely the DESs resulting from mixing ammonium based salts with metal halides. This type of DESs was synthesized and used for different applications by Abbott et al.11 We started by measuring the solubility of NaCl and NaBr in [choline chloride:zinc chloride] DES, DES5, at different temperatures and different salt:metal halide ratio. The results are listed in Table 6. Interestingly, it was found that the solubility of NaCl and NaBr in DES5 is very high, reaching a maximum of 100 w = 67 and 100 w = 56 , respectively (Figure 5). Again the solubility of NaBr is less than that of NaCl at the same conditions for all ratios and temperatures. It can be also noted that the solubility of both NaCl and NaBr increased with the increase of metal halide molar ratio. On the other hand, the solubility increased with the increase of temperature until a certain temperature is reached then it started to decrease slowly. Special analytical techniques must be used in order to understand this behavior. This will be the subject of another specialized study. For the temperature effect on sodium salts’ solubility in DES5, it was observed that the solubility profiles have two distinct regions, increasing and decreasing regions. The region where by the solubility was increasing, is observed from 50 °C up to 90 °C. This is followed by a decreasing region which lasts

Figure 5. Solubility profiles of NaCl and NaBr in DES5 as a function of temperature. NaCl series, for salt:HBD ratios 1:1 (●), 1:2 (○), and 1:3 (◆). NaBr series, for salt:HBD ratios 1:1 (◇), 1:2 (▼), and 1:3 (▽).

until the maximum temperature used in this work which is 135 °C. This finding promises that a DES made from an ammonium salt and a metal halide instead of a typical HBD has two advantages: an increased solubility for sodium salts and a decreased temperature of application. These are very critical factors that can make the production of sodium in this DES medium a very profitable process than that followed by industry nowadays. If the variation of the metal halide’s molar ratio in the DES is to be discussed, it was found that the higher the metal halide’s ratio is, the highest is the solubility of sodium salt. This is to be further studied taking into consideration financial factors and other design restrictions. These interesting findings about incorporating metal halides in DESs instead of typical HBDs led us to further expand the research to the utilization of another metal halide, tin chloride, and study the solubility of different sodium salts in the resulting DESs. DES6 in Table 1 is the result of mixing choline chloride 2158



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with tin chloride, and as usual different molar ratios were investigated. In Table 7, the solubility of NaCl in DESs 5, 6, Table 7. Comparison of Solubility of Sodium Chloride in DESs 5, 6, and 7 at 60 °Ca DES and ratio DES5, DES5, DES5, DES6, DES7,

1:1 1:2 1:3 1:3 1:1:1

solubility (100 w) 43.68 60.00 63.00 02.08 04.27

Standard uncertainties u are u(t) = 0.01 °C, ur(S) = 0.02 for S 100 w > 20 and 0.05 for S 100 w < 10.

a

and 7 at 60 °C is given, and a plot for these data is given in Figure 6. From the figure, it can be inferred that the solubility of sodium chloride in tin chloride-based DES, 100 w = 2.07, is much lower than that in zinc chloride-based DES.

Figure 7. Solubility profiles of NaCl in DES8 as a function of temperature. NaCl series, for salt:HBD ratios 1:1 (●), 1:2 (○), 1:3 (▲) and 1:4 (△).

Table 8. Solubility (as 100 w) of Sodium Chloride in DES8 at Different Temperatures and a Pressure of 0.1 MPaa

a

temp

1:1

1:2

1:3

1:4

50 60 75 85 90 110 120

67.00 67.60 68.10 68.80 68.20 67.96 67.30

76.00 76.70 77.20 77.25 76.80 74.97 74.30

77.70 78.40 79.30 80.00 79.30 78.60 77.90

50.42 50.80 51.10 51.20 51.10 51.00 50.87


Table 9. Solubility (as 100 w) of Sodium Chloride in DES9 at Different Temperatures and a Pressure of 0.1 MPaa Figure 6. Comparison of solubility of NaCl in DES5, 6, and 7 at 60 °C.

Both ZnCl2 and SnCl2 were used with choline chloride in the synthesis of DES7. The solubility of NaCl in this DES at 60 °C is also shown in Figure 6. At approximately 100 w = 4.27, this is also not as good as DES5. Thus further investigation of NaCl’s solubility at different temperatures in this DES was not necessary. Because of the high sodium salt solubility in DES5, the other ammonium salt, N,N-diethylethanolammonium chloride, was chosen to synthesize a novel DES with zinc chloride, DES8. As in DES5, the solubility profiles of NaCl versus the temperature showed two regions of increase and decrease, from 50 °C to 85 °C and from 85 °C to 120 °C, respectively. Figure 7 shows these findings and the numerical data are tabulated in Table 8. The solubility of NaCl in DES8 has a maximum NaCl solubility of about 100 w = 80 . The solubility of NaCl in this DES increased when the metal halide’s molar ratio in this DES increased from 1 to 3; however, the solubility decreased when the metal halide’s molar ratio was further increased to 4. Following the discussion concerning the effect of metal halide on the solubility of NaCl, zinc chloride was replaced by iron(II) chloride to give a new DES, DES9. Table 9 lists the experimental solubilities of NaCl in this DES. Figure 8 shows a plot for the variation in the solubility of NaCl as the temperature increases. A maximum solubility of 100 w = 10.8 at 95 °C was observed, while in DES 8 with the same salt (N,N-

temp

1:3

44 55 77 95

3.57 4.72 10.21 10.40

Standard uncertainties u are u(t) = 0.01 °C, ur(S) = 0.02 for S 100 w > 20 and 0.05 for S 100 w < 10.

a

diethylethanolammonium chloride) but zinc chloride as metal halide, 100 w = 80 of solubility was observed. No further investigation was carried out on other salt:metal halide ratios of this DES. 3.1. Stability of Sodium Metal. As expected, sodium metal reacted with DESs containing ethylene glycol and glycerol as HBDs (DES1 to DES4). This is due to the presence of the hydroxide group in these HBDs and the fact that sodium metal in its pure state reacts instantly with oxygen, hydroxides, or halides. However, DESs synthesized by utilizing metal halides (DES5 to DES9) are stable and did not react with the sodium metal. Even though a halide group existed, it seems that the structure of the DES held it strongly and prevented it from reacting. These results can narrow down the search for a potential DES to conduct the task of sodium metal production. 3.2. Solubility Modeling. The basic equation for predicting the saturation mole fraction of a solid in a liquid is given by the general equation:25,26 2159



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The latent heat of fusion of pure sodium chloride and its melting temperature were taken from the DDB database (ΔHfus = 28.3 kJ/mol and Tm = 1073.95 K). The non-random two liquid (NRTL) model27 was used to calculate the activity coefficient at equilibrium. This model has three parameters, τij, τji, and αij, for each pair of components in the multicomponent mixture. The model development was achieved by utilizing the Simulis thermodynamics environment which is a thermophysical properties calculation server, provided by ProSim.28 The deep eutectic solvent was considered as a pseudocomponent. In this work, we have made the hypothesis that τij = τji and the nonrandomness parameter referred to as αij was taken equal to 0.20, a commonly used value. The binary interaction parameters τij were estimated from the N experimental data points at each temperature for NaCl with DES2, DES3, DES4, and DES5, respectively. An iterative procedure was used for each temperature by minimizing the squared relative error (criterion) between the calculated and the experimental solubilities:

Figure 8. Solubility profile of NaCl in DES9 (salt:HBD ratio 1:3) as a function of temperature.

ΔH̲ fus(Tm) ⎡ T ⎤ ⎥ ⎢1 − RT Tm ⎦ ⎣

ln(x1γ1) = − −

1 RT

∫T

T

ΔCP dT +

m

1 R

∫T

T

ΔCP dT T

m

1 criterion = N

τij = τij0 + τijT(T − 273.15)

z

⎪

1.5 2.0 2.5 3.0

⎪

(2)

2.5 3.0 4.0

This equation can be used based on the assumption of simple eutectic mixtures with complete miscibility in the liquid and immiscibility in the solid phases. However, due to the lack of suitable data representing the difference in heat capacity (ΔCp) between the solute in the two states, the simplified version of the solubility without the ΔCp term was applied: ⎪

⎪

⎛ T ⎞⎤⎫ Tr ΔCP ⎡ + ln⎜ r ⎟⎥⎬ ⎢1 − ⎝ T ⎠⎦⎭ RT ⎣ T ⎪

⎪

τij0

τijT

DES2 −623.9 −647.3 −764.9 −829.6 DES4 −785.3 −822.7 −1063.6

z

3.194 2.767 2.446 2.368

2.5 3.0 4.0 4.5

1.709 1.601 1.267

1.0 2.0 3.0

τij0 DES3 −668.5 −698.9 −816.2 −849.5 DES5 −3718.9 −4542.7 −4847.4

τijT 3.472 2.987 2.636 2.569 −4.531 −10.282 −12.939

of the calculated values by the model with experimental data is presented in Figure 9 for the solubility of NaCl in DES2, DES3, DES4, and DES5. The calculated solubilities by the model show a good agreement with experimental measured solubilities. It must be noticed that the modeling procedure has been tested and validated for NaCl with a few DESs, but it could be applied easily to fit NaBr and Na2CO3 solubilities with other DESs provided that the number of experimental data is representative.

⎧ ΔH̲ fus(T ) ⎡ T⎤ r ln x1 = −ln γ1 − ⎨ ⎢1 − ⎥ Tr ⎦ ⎩ RT ⎣ +

(5)

Table 10. NRTL Binary Interaction Parameters between NaCl and DES2 to DES5 for Different Molar Ratio (i ≡ NaCl and j ≡ DES)

⎪

⎪

T /K

The optimized parameters τij0 and τijT optimized for each DES and different molar ratios are listed in Table 10. A comparison

⎧ ΔH̲ fus(T ) ⎡ T ⎤ m ln(x1γ1) = −⎨ ⎥ ⎢1 − RT Tm ⎦ ⎣ ⎩ ⎛ T ⎞⎤⎫ Tm ΔCP ⎡ + ln⎜ m ⎟⎥⎬ ⎢1 − ⎝ T ⎠⎦⎭ RT ⎣ T

(4)

Linear temperature dependence is obtained for the binary interaction parameters expressed by the following correlation:

(1)

The subscript 1 denotes the solid solute, x1 and γ1 its molar composition (solubility at equilibrium) and activity coefficient in the mixture, respectively, Tm, the melting point temperature of the solid, T, the temperature of the system at equilibrium, ΔHfus and ΔCp the enthalpy and heat capacity changes from the solid to the liquid state of the solute. Without introducing appreciable error, we can assume that ΔCp is independent of temperature. So, the last equation becomes

+

⎛ x exp − x cal ⎞2 ∑ ⎜⎜ 1 exp 1 ⎟⎟ x1 ⎠ N ⎝

(3)

The expected error resulting from neglecting ΔCp usually depends on the compound under consideration. For conventional molecular compounds, the error does not exceed 2 %. If the liquid mixture is ideal, γ1 = 1 and the solubility can be computed from the thermodynamic data (ΔHfus and ΔCp) for the solid species near the melting point. For nonideal solutions, γ1 must be estimated from either experimental data or liquid solution models, like the NRTL or UNIFAC model.

4. CONCLUSIONS The solubility of selected sodium salts in different types of deep eutectic solvents were measured at different temperatures and for different salt:HBD mole ratios. In most cases, it was found that the solubility of the sodium salt increases with the increase of temperature and with the decrease of salt:HBD. The chemical structure of the DES has a big effect on the solubility 2160



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Figure 9. Calculated (NRTL) vs experimental solubilities of sodium chloride in choline chloride:glycerol (DES2), N,N-diethylethanolammonium chloride:ethylene glycol (DES3), N,N-diethylethanolammonium chloride:glycerol (DES4) and choline chloride:zinc(II) chloride (DES5) for different ratios and at different temperatures: z is the ratio of the HBD in the DES considering that the salt’s ratio is always 1.

and Chemical Engineering Department, Engineering Faculty in Sultan Qaboos University, Oman.

of sodium salts. The solubility of sodium salts in DESs that have ethylene glycol or glycerol as HBD, neutral molecules, is very small. On the other hand, the solubility of sodium salts in DESs containing ammonium salts and metal halides was very high especially for the metal halide zinc chloride (ZnCl2) where the solubility reached a maximum of 100 w = 67 and 100 w = 80 for choline chloride and N,N-diethylethanolammonium chloride, respectively. These values are much higher than the corresponding values reported in ionic liquids at the same conditions. The results showed clearly that certain types of DESs are potential candidates as solvents and electrolytes in the process of producing sodium metal at moderate temperatures. Finally, the NRTL activity coefficient model based on the local composition theory was successfully applied to model solubilities of NaCl in different DESs. The predicted solubility values showed good agreement with the experimental results.

■

Notes

The authors declare no competing financial interest.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +968-24142558. Fax: +968-24141354. Funding

This research was funded by University of Malaya Research Grant No. HIR-MOHE D000003-16001 and by the Deanship of Scientific Research at King Saud University through group Project No. RGP-VPP-108 in collaboration with the Petroleum 2161



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