Excess Molar Enthalpies of Deep Eutectic Solvents (DESs) Composed

Article pubs.acs.org/jced

Excess Molar Enthalpies of Deep Eutectic Solvents (DESs) Composed of Quaternary Ammonium Salts and Glycerol or Ethylene Glycol Pablo López-Porfiri,† Joan F. Brennecke,‡ and Maria Gonzalez-Miquel*,§ †

Laboratorio de Termodinámica de Procesos, Departamento de Ingeniería Química y Ambiental, Universidad Técnica Federico Santa María, Avda. España 1680, Valparaíso, Chile ‡ Department of Chemical and Biomolecular Engineering. University of Notre Dame, Notre Dame, Indiana 46556, United States § School of Chemical Engineering and Analytical Science, The University of Manchester, Manchester M13 9PL, United Kingdom ABSTRACT: Molar enthalpies of mixing (HE) were measured for the following deep eutectic solvents (DESs): {choline chloride + glycerol}, {choline chloride + ethylene glycol}, {tetrabutylammonium chloride + glycerol}, and {tetrabutylammonium chloride + ethylene glycol} at 323.15 K and molar ratios of 1:4, 1:3, 1:2 and 1:1. Results show that all systems are endothermic, with HE values ranging from 1.90 to 5.35 kJ·mol−1. Results indicate that the intermolecular interactions between the molecules of the pure components are stronger than those of the DESs complexes. To shed some light on the mutual interactions between the molecules within the mixtures, effects of the hydrogen bond acceptor structure (HBA), hydrogen bond donor structure (HBD), and concentration (HBA:HBD molar ratio) were analyzed. The nature of the HBA salt is the most important: choline chloride-based systems required almost twice as much energy as tetrabutylammonium chloride-based systems in order to form the DES mixture, most likely because of a higher enthalpy of fusion of the choline-based HBA salt. Choline chloride is more stable than tetrabutylammonium chloride because of its hydroxyl group; consequently, more energy is needed to break the choline chloride interactions in order to form DES mixtures with glycerol or ethylene glycol. Other effects suggest a competition in the formation of hydrogen bonds among the pure species (like molecular interactions) and the DES complexes (unlike molecular interactions). Overall, this work reports a systematic evaluation of HE for a series of representative DESs that elucidates the roles of HBD and HBA in the energy penalty required for DES formation, which is critical for assessing their potential in practical applications on an industrial scale.

1. INTRODUCTION Deep eutectic solvents (DESs) are fluids composed of different Lewis or Brønsted acids and bases, which form liquids at low temperature (lower than 100 °C). DESs share many features with ionic liquids (ILs), as they have low vapor pressures, wide liquid ranges, and low flammability and are frequently considered to be alternative solvents in a wide variety of separation processes and synthesis applications.1 However, the main characteristic of the DESs is the decrease in the melting point of the mixture relative to the melting points of each component in the mixture. Figure 1 shows a schematic representation of the phase diagram of a eutectic mixture of two components, which illustrates the temperature difference (ΔTf) between the freezing points of the theoretically ideal and the real mixtures as well as the eutectic point (i.e., the lowest freezing point of the real mixture). A common class of DESs is composed of a quaternary ammonium salt acting as the hydrogen bond acceptor (HBA) and a metal salt or other hydrogen bond donor (HBD). The interactions between the molecules of the mixture promote a charge delocalization through hydrogen bonding, causing a decrease in the melting point.2 Figure 2 illustrates the charge © XXXX American Chemical Society

Figure 1. Schematic phase diagram of a eutectic mixture of two components.

Special Issue: Proceedings of PPEPPD 2016 Received: July 8, 2016 Accepted: September 30, 2016

A

DOI: 10.1021/acs.jced.6b00608 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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have reported the simulation of the molecular interactions of several DES systems,18−20 focusing on the charge-transfer interaction promoting SO2 capture in a DES composed of choline chloride and glycerol;18 studying the hydrogen bonding interactions occurring between the ions of choline chloride and urea to explain the structural characteristics and the low melting point of the eutectic mixture;19 and analyzing the different intermolecular and intramolecular hydrogen bonding interactions occurring among the components of DESs involving choline chloride and three different HBDs.20 However, there is a lack of relevant experimental data to provide a comprehensive thermodynamic understanding of the system behavior. Excess enthalpy is a particularly valuable thermodynamic property for understanding the origin of the nonideality of mixtures while providing a sensitive measure of the intermolecular interactions within the system. During mixing, energy is required to break the like−like interactions; then new interactions are formed between unlike species releasing energy. The differences in the absorbed and released energies can be quantified through the enthalpies of mixing in order to determine the endothermic or exothermic nature of the solvent formation process. Several publications have reported experimental excess enthalpies for ionic systems involving ILs and various solvents, including water,21,22 alcohols,23 ketones,24 amines,25 and alkanediols.26 However, there is a significant research gap relative to the excess enthalpies of DESs, which is essential for gaining insight into the behavior of the compounds within the systems and analyzing the nature of the molecular interactions within the eutectic mixtures. The main objective of this work is to perform a systematic experimental characterization of the intermolecular interactions of a series of DESs through measurements of the excess enthalpies for representative HBA and HBD combinations. In particular, the enthalpies of mixing of various DES systems composed of quaternary ammonium salts (choline chloride or tetrabutylammonium chloride) and polyols (glycerol or ethylene glycol) in different HBA:HBD ratios (from 1:4 to 1:1) are evaluated to provide a comprehensive understanding of the effect of the chemical nature and composition of the eutectic mixtures. Furthermore, the fact that choline chloride has a hydroxyl group in its structure, as opposed to tetrabutylamonium chloride, and that the polyols have different numbers of hydroxyl groups will provide further insight into the role of the hydrogen bonding interactions on the DES behavior. The selected DES systems and their abbreviations are collected in Table 1, and the available experimental freezing point data is

Figure 2. Charge delocalization through hydrogen bonding of the DES system.

delocalization through hydrogen bonding for a mixture composed of an HBA salt (choline chloride) and an HBD, where the acidic hydrogen of the hydroxyl group pertaining to the HBD molecule is able to interact through hydrogen bonding with the lone electron pairs of the chlorine atom of the HBA salt. Therefore, it is expected that the DES behavior relies heavily on the nature of the HBA:HBD pair comprising the eutectic mixture as well as the specific nature of the molecular interactions between those compounds. As with ILs, the use of DESs as alternative solvents for industrial applications becomes an interesting opportunity to develop new technologies. Moreover, the fact that DESs can be made from readily available and biodegradable materials, which have lower toxicity and cost than many ILs, makes them promising candidates to develop efficient and sustainable processes in a variety of fields. Among these applications is their use as a solvent for the deposition or dissolution of metals.1,3,4 Furthermore, because of their environmentally friendly properties,5 DESs have been proposed for synthesis processes,1,6−8 carbon dioxide absorption,1,9,10 and biotransformations.1,11−13 An example of the latter is related to biodiesel manufacturing and purification, where choline chloride (a quaternary ammonium salt) is used to extract the glycerol (an HBD) formed as a byproduct of the transesterification process, hence improving the final biofuel quality.14 In addition, DESs formed from choline chloride and an HBD such as glycerol or ethylene glycol have been studied for the removal of residual alkali metals in biodiesel to avoid engine damage.15 DES systems based on quaternary ammonium halide salts are the most common in the literature. Among them, choline chloride is often the salt of choice because of its high biodegradability and biocompatibility, low toxicity, and wide availability because it is produced in large quantities for use as an animal feed supplement.16 Recently, the physical characterization of systems composed of tetrabutylammonium chloride as an alternative quaternary ammonium salt (HBA) and glycerol or ethylene glycol (HBD) has been reported for DES formation.17 Usually, the quaternary ammonium salts are combined with amides, alcohols, or carboxylic acids, which act as the HBD compounds. However, among these systems, the DESs based on polyols such as glycerol or ethylene glycol generally present lower freezing temperatures and are even liquid at room temperature. To date, most of the research related to DESs has focused on the characterization of their physical properties (mainly freezing points) or their application in particular processes. The measurement of the thermodynamic phenomena involved in the formation of these solvents, which is critical for understanding the system behavior in order to enhance their performance in potential applications as well as promote the design of task-specific DES for particular processes, has not been adequately investigated. Recently, a few theoretical studies

Table 1. Deep Eutectic Solvent Systems Studied in this Work system N

HBD salt

HBD

acronym

1 2 3 4

choline chloride choline chloride tetrabutylammonium chloride tetrabutylammonium chloride

glycerol ethylene glycol glycerol ethylene glycol

ChCl:G ChCl:EG TBAC:G TBAC:EG

illustrated schematically in Figure 3. (Note that the experimental decomposition temperature of choline chloride has been represented in Figure 3 for illustrative purposes; further explanations of the theoretical melting point of choline chloride are provided in detail in Section 3.1.) B



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⎡ ΔHE = ⎢H − ⎢⎣

N

⎤

⎡

N

⎤

i=1

⎥⎦

⎢⎣

i=1

⎥⎦

∑ xiHi⎥ − ⎢H id − ∑ xiHi⎥ = HE (2)

In this work, excess enthalpy is calculated on the basis of the total enthalpy of mixing recorded by the calorimeter, which is obtained by the numerical integration of the heat flow peaks as shown in eq 3 Q total =

t

dQ dt dt

(3)

where dQ/dt is the specific heat flow and Qtotal is the total heat released. In particular, the excess enthalpies of DES have been determined by calorimetry, using the aforementioned Setaram C80 calorimeter, following the methodology described in previous studies.21,22,25,27 As reported, the method was verified using organic systems under a wide temperature range to obtain results in agreement with the literature.27 Moreover, the excess enthalpy measurements for different systems containing (ILs + water) as well as (ILs + monoethanolamine) have been supported by further quantum chemical calculations.21,25 Therefore, the suitability of the method to obtain sensible measurements of excess enthalpies for ionic systems, some of them showing relevant hydrogen bonding molecular interactions, has been appropriately verified in previous studies. All measurements were made at least in triplicate. The combined expanded uncertainties (Ucomb) with a 95.45% confidence level were estimated according to Chirico et al.28 The experimental errors reported consider the inherited error of each variable used with its propagation in subsequent calculations.

Figure 3. Phase diagram of DES systems. Experimental data points: (□) choline chloride + glycerol,15 (○) choline chloride + ethylene glycol,15 (△) tetrabutylammonium chloride + glycerol,17 and (◊) tetrabutylammonium chloride + ethylene glycol.17 (---) Lines are drawn to guide the eye.

2. EXPERIMENTAL SECTION 2.1. Materials. The compounds used for DES synthesis were as follows: choline chloride (CAS no. 67-48-1), tetrabutylammonium chloride (CAS no. 1112-67-0), glycerol (CAS no. 56-81-5), and ethylene glycol (CAS no. 107-21-1). The supplier and purity of the compounds are shown in Table 2. No further purification steps were performed. To remove water, all compounds were dried under vacuum (0.980 >0.99 ≥0.99

none none none none

0.99 >0.980 >0.99 ≥0.99

Ta Ta GCb GCb

Titration. bGas chromatography. C



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Table 3. Compound Properties

a

Decomposition temperature. bEstimated on the basis of the results of this work. cAccording to the suppliers shown in Table 2. dReference 30.

an average of Ucomb(HE) = 19 J·mol−1 with a maximum of Ucomb(HE) = 35 J·mol−1. Because the mixing of the two components to form the DESs is spontaneous but requires the addition of energy, the entropy change on mixing is an important factor. Although the polyols are the HBD of the DES, alcohols can function as both hydrogen bond donors and hydrogen bond acceptors. Likewise, the −OH group on choline chloride can serve as both a hydrogen bond donor and a hydrogen bond acceptor. Therefore, during the mixing process, a variety of like-molecule hydrogen bonds will be broken and many new hydrogen bonds will be formed between the HBD and HBA. This rearrangement results in a significant entropy increase, even though, as the results show, the overall attractive forces among the pure compounds are stronger than those within the DES complexes. It is possible to identify three main effects on the behavior of the series of deep eutectic solvents studied: (1) effect of hydrogen bonding acceptor salt; (2) effect of hydrogen bond donor; and (3) effect of concentration (i.e., HBA:HBD molar ratio). To preliminarily assess the behavior of the DES systems studied, the significance of each of these three factors on the response obtained (i.e., excess enthalpy) was analyzed in terms of the variance: ANOVA29 with 5% significance. The results are summarized in Table 5. On the basis of the experimental data

Figure 4. Experimental excess molar enthalpies of DESs at 323.15 K and atmospheric pressure.

Table 4. Experimental Excess Enthalpies of Deep Eutectic Solvent Mixtures at T = 323.15 K and p = 0.1 MPa and Combined Expanded Uncertainties (Ucomb) with a 95.45% Confidence Levela xsalt salt:HBD molar ratio 1:4 1:3 1:2 1:1 1:4 1:3 1:2 1:1 1:4 1:3 1:4 1:3 1:2 a

no. of replicates

Ucomb(xsalt)

mol· mol−1

HE

Ucomb(HE) J· mol−1

Choline Chloride:Glycerol 3 0.2003 0.0007 3689 4 0.2492 0.0010 4919 3 0.3321 0.0017 4005 3 0.5017 0.0034 5107 Choline Chloride:Ethylene Glycol 3 0.2002 0.0008 3954 4 0.2522 0.0012 4590 5 0.3369 0.0022 5353 3 0.4991 0.0032 4309 Tetrabutylammonium Chloride:Glycerol 3 0.1999 0.0009 2106 3 0.2508 0.0010 2142 Tetrabutylammonium Chloride:Ethylene Glycol 3 0.2005 0.0007 1900 3 0.2505 0.0011 2267 3 0.3334 0.0016 2599

Table 5. Effects and Levels in ANOVA effect

13 20 21 35 16 21 35 28

low level

high level

(1) effect of HBA salt (2) effect of HBD

ChCl as an HBA salt glycerol as an HBD

(3) effect of concentration

1:4

TBAC as an HBA salt ethylene glycol as an HBD 1:3

and the results of the ANOVA analysis of the primary effects shown in Figure 5, it is clear that the structure of the HBA salt has the most significant influence on the energy required for DES formation, followed by the molar concentration of the mixture (i.e., HBA:HBD molar ratio). The HBD does not appear to have a significant influence on the energy required for the formation of the DES. Excess enthalpies for the DES formation process may be decomposed into two independent phenomena: salt melting and species mixing, according to eq 4:

10 10 7 11 13

u(T) = 0.01 K; u(p) = 0.001 MPa.

HE = x HBA ΔH SL + ΔH mix D

(4) DOI: 10.1021/acs.jced.6b00608 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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Figure 5. Response of main effects in ANOVA analysis for excess enthalpy.

Figure 6. Estimated molar enthalpies of mixing of DESs at 323.15 K and atmospheric pressure.

significantly more endothermic than TBAC-based systems, with excess enthalpies of approximately 4500 and 2200 J·mol−1, respectively. As discussed above, it is likely that choline chloride has a significantly higher enthalpy of fusion, reflected in its high decomposition temperature (576 K), which occurs prior to melting, versus tetrabutylammonium chloride’s much lower melting point (343 K). This is consistent with ChCl having a lower-energy, more stable crystalline structure that is difficult to break. Although TBAC is a much larger molecule, ChCl has a hydroxyl group, which establishes a hydrogen bond with Cl− in its most stable configuration18,19 (Figure 7). No studies on

From this perspective, it seems reasonable to infer that part of the total energy absorbed by the system is used to melt the salt in an endothermic process, whereas the remaining energy balance is the result of the rearrangement of the molecules to form the DES complexes during mixing; consequently, the latter can be an endothermic or an exothermic process, which can be estimated as a function of the measured excess enthalpies and the enthalpy of fusion of the HBA salts. In the case of tetrabutylammonium chloride, the enthalpy of fusion is 73.8 J·g−1;30 however, for choline chloride, no values of the enthalpy of fusion were found in the literature because it decomposes at 576 K after suffering a crystallographic transition at about 351 K,31 although it can be assumed that the theoretical melting point would be higher than the decomposition temperature. Therefore, in order to estimate the enthalpy of fusion for choline chloride, the enthalpies of mixing were assumed to be approximately equal for both HBA salt-based systems. This yields an estimated enthalpy of fusion for choline chloride of ΔHSL(ChCl) ≈ 213.1 J·g−1 (estimate). Note that this is not a measured value. It is based on a crude assumption of the enthalpy of mixing. The estimated enthalpy of fusion of choline chloride is significantly higher than the experimentally determined enthalpy of fusion for tetrabutylammonium chloride, ΔHSL(TBAC) = 73.8 J·g−1, which is expected on the basis of the theoretically much higher melting point of choline chloride, as shown in Table 3. Under this premise, it is now possible to analyze the mixing process phenomena relative to DES formation: negative values of ΔHmix were obtained as depicted in Figure 6, revealing that mixing is an exothermic process driven by the formation of new interactions between the HBA and HBD components. These results suggest a much less pronounced effect for the type of HBA salt but still no significant trend for the type of HBD; however, it is made clear that the HBA:HBD concentration has the main impact on the mixing enthalpy of DES (i.e., increasing exothermic mixing enthalpy with increasing HBA salt concentration). A more detailed discussion is given below to further explain the different impacts of the three main factors (structure of HBA, structure of HBD, and HBA:HBD molar ratio) on the overall behavior of the formation of the DES complexes studied here. 3.2. Effect of HBA Salt. Regardless of the HBD molecule forming the DES, the results show that ChCl-based systems are

Figure 7. Most stable schematic structure of interactions of choline chloride obtained by the M06-2X density functional18 and molecular dynamic simulation.19

tetrabutylammonium chloride’s configuration were found. According to the structure of the HBA salts used in the present study as DESs complexes, the main interactions are C− H···Cl− and O−H···Cl− hydrogen bonds, where the interaction between the Cl− anion with a hydroxyl group is stronger than the interaction between the Cl− anion with C−H groups.19 Consequently, choline chloride constitutes stronger intramolecular hydrogen bonds than TBAC, making it more difficult to promote the charge delocalization for the eutectic solvent formation. This process requires more energy. 3.3. Effect of HBD. Two hydrogen bond donors with different numbers of hydroxyl groups were studied, namely, ethylene glycol (two hydroxyl groups) and glycerol (three hydroxyl groups). The difference between the excess enthalpies when mixing a given salt with the two different HBDs ranges from 125 to 1348 J·mol−1. The insertion of the HBD species within the mixture disrupts the structure of the HBA salts by promoting HB interactions between the anion of the salt and the HBD molecule, as a detriment to the HB interactions between the cation and the anion of the salt. Makowska et al.32 reported the miscibility of ionic liquids with polyhydric alcohols, observing a trend toward an improvement in E



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system. However, if the energy input required to melt the salt is removed and the focus is placed on the mixing process, then the attractive molecular interactions between unlike species are predominant and increase with increasing HBA concentration, driving the formation of the resulting DES systems. Therefore, above a critical concentration there will not be enough hydrogen bond donors to break the crystalline structure of the HBA salt. Nevertheless, the energy requirements to break the salt network under specific operating conditions and a concentration range should also be considered to acquire a comprehensive understanding of DES behavior.

miscibility and a greater interaction of unlike species with an increasing number of hydroxyl groups. Also, they noticed enhanced miscibility with the vicinal positioning of OH groups.32 However, this trend is not observed for the DES systems studied in this work. In fact, it appears not to be a general rule driving the behavior of the DES systems composed of glycerol or ethylene glycol in terms of the number of hydroxyl groups within the HBD molecule. Among the possible interactions, two phenomena may be competing: HB interactions between the HBD and the HBA salts and the HB interactions among the molecules of the HBD neat solvents. Increasing the number of OH− groups in the HBD molecule leads to an increase in the HB interactions between the HBD and the HBA salts and also enhances the HB interactions among the molecules of the pure HBD solvent. Glycerol is more able than ethylene glycol to establish HB with the salt as a result of the larger number of OH groups; however, the larger number of OH groups, particularly in vicinal positions, also promotes stronger interactions among the molecules of the neat solvent so that the affinity among like molecules within glycerol is greater than in ethylene glycol. Therefore, although glycerol should present a higher solvation capacity than ethylene glycol to attract and associate with the HBA salt molecules, it also has stronger like-molecule interactions that are more difficult to break, which is necessary to form the DES. The effect of the HBD on the formation of new interactions between like species was explored in recent papers reporting densities of aqueous mixtures of ChCl:EG33 as well as densities and dynamic viscosities of aqueous mixtures of ChCl:G.34 Results suggested the presence of stronger, preferably hydrogen bonding, interspecies interaction (between water and DES) rather than intraspecies interaction (among water molecules or among DES molecules) due to the capability of both the OH group and the chloride ion of ChCl and OH groups of glycerol and ethylene glycol to form hydrogen bonds with water; this supports the critical role of the HBD in promoting charge delocalization within the salt network and the formation of new interactions between unlike molecules. 3.4. Effect of Concentration. Mixtures of ChCl:G and ChCl:EG form DESs over the entire range of concentration studied, (0.2−0.5) mol·mol−1, under the reported operating conditions (323.15 K and atmospheric pressure). However, although TBAC:G and TBAC:EG systems showed eutectic behavior, liquid solutions were not obtained with mixtures of TBAC:G and TBAC:EG at 1:1 or with TBAC:G at 1:2 under the specified operating conditions. These results are in agreement with the literature.17 Those cases in which a liquid solution is not obtained may be related to the necessity to provide a higher concentration of HBD to break the salt network but also to the fact that it may not be possible to provide sufficient energy to break the salt network at temperatures at which the vapor pressure of the HBD is not too high (e.g., near its boiling point). Within each system, a relation between the trend in excess enthalpies and freezing points is observed, although a general behavior cannot be established: around the eutectic point, the excess enthalpies decrease in glycerol-based systems while the excess enthalpies increase for ethylene glycol-based systems. As the HBA salt concentration is increased, two types of interactions compete: on the one hand, more energy is required to melt the salt; and on the other hand, more hydrogen bonds between the HBA and the HBD are formed, releasing more energy into the

4. CONCLUSIONS Molar enthalpies of mixing (or, equivalently, HE) were measured for a series of four DES systems composed of quaternary ammonium salts and polyols, considering different representative hydrogen bond acceptor salts (choline chloride and tetrabutylammonium chloride) and hydrogen bond donor molecules (ethylene glycol and glycerol) in molar ratios of 1:4, 1:3, 1:2, and 1:1 at 323.15 K. All systems were found to be endothermic, meaning that attractive forces of the pure compounds are stronger than those of DES complexes; therefore, it is necessary to provide energy to break the interactions within like molecules and allow new interactions between the HBA salts and the HBD compounds to form DES solvents. The stability of the pure HBA salt has proven to be a dominant factor, with choline chloride-based systems requiring almost twice as much energy as tetrabutylammonium chloridebased systems for DES formation, mainly as a result of the higher energy input required to melt the HBA salt. Furthermore, the effect of the HBD structure suggests a competition in the formation of hydrogen bonds for HBA− HBD (to allow DES complex formation) and HBD−HBD (intermolecular interactions within the neat solvent). Both effects were further supported by the analysis of the HBA:HBD molar ratio with respect to the excess enthalpy required for DES solvent formation. Consequently, within mixtures composed of HBA salts and HBD molecules for deep eutectic solvent formation, there is a constant competition between like and unlike species to form the more stable complexes. However, it was demonstrated that, focusing the energy analysis purely on the mixing process, the attractive molecular interactions between the HBA salt and HBD compounds are dominant, which drives the formation of DES systems. Overall, this work provides novel insights into the nature of deep eutectic solvents in terms of their enthalpy of mixing and the associated energy required for solvent formation, which is critical for assessing potential practical applications. Important differences in behavior were observed at the level of intermolecular interactions of the systems studied, illustrating the complexity of the phenomena involved with DES formation.

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AUTHOR INFORMATION

Corresponding Author

*Tel. +44 (0) 161 306 4396. E-mail: maria.gonzalezmiquel@ manchester.ac.uk. Funding

This work was financed by the Chilean agency FONDECYT (Regular project 115-0822) and supported by the University of Notre Dame Incropera-Remick Endowment for Excellence. F



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Notes

(15) Shahbaz, K.; Mjalli, F. S.; Hashim, M. A.; AlNashef, I. M. Eutectic solvents for the removal of residual palm oil-based biodiesel catalyst. Sep. Purif. Technol. 2011, 81, 216−222. (16) European Food Safety Authority.. Scientific Opinion on safety and efficacy of choline chloride as a feed additive for all animal species. EFSA J. 2011, 9, 2353−2368. (17) Mjalli, F. S.; Naser, J.; Jibril, B.; Alizadeh, V.; Gano, Z. Tetrabutylammonium Chloride Based Ionic Liquid Analogues and Their Physical Properties. J. Chem. Eng. Data 2014, 59, 2242−2251. (18) Li, H.; et al. Theoretical evidence of charge transfer interaction between SO2 and deep eutectic solvents formed by choline chloride and glycerol. Phys. Chem. Chem. Phys. 2015, 17, 28729−28742. (19) 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. (20) Perkins, S. L.; Painter, P.; Colina, C. M. Experimental and Computational Studies of Choline Chloride-Based Deep Eutectic Solvents. J. Chem. Eng. Data 2014, 59, 3652−3662. (21) Ficke, L. E.; Brennecke, J. F. Interactions of ionic liquids and water. J. Phys. Chem. B 2010, 114, 10496−10501. (22) Ficke, L. E.; Novak, R. R.; Brennecke, J. F. Thermodynamic and thermophysical properties of ionic liquid + water systems. J. Chem. Eng. Data 2010, 55, 4946−4950. (23) Paduszyński, K.; Królikowski, M.; Domańska, U. Excess enthalpies of mixing of piperidinium ionic liquids with short-chain alcohols: Measurements and PC-SAFT modeling. J. Phys. Chem. B 2013, 117, 3884−3891. (24) Bhagour, S.; Solanki, S.; Hooda, N.; Sharma, D.; Sharma, V. K. Thermodynamic properties of binary mixtures of the ionic liquid [emim][BF4] with acetone and dimethylsulfoxide. J. Chem. Thermodyn. 2013, 60, 76−86. (25) Gonzalez-Miquel, M.; Massel, M.; DeSilva, M.; Palomar, J.; Rodriguez, F.; Brennecke, J. F. Excess Enthalpy of Monoethanolamine + Ionic Liquid Mixtures: How Good are COSMO-RS Predictions? J. Phys. Chem. B 2014, 118, 11512−11522. (26) Domanska, U.; Papis, P.; Szydłowski, J.; Krolikowska, M.; Krolikowski, M. Excess Enthalpies of Mixing, Effect of Temperature and Composition on the Density, and Viscosity and Thermodynamic Properties of Binary Systems of {Ammonium-Based Ionic Liquid + Alkanediol}. J. Phys. Chem. B 2014, 118, 12692−12705. (27) Ficke, L. E.; Rodríguez, H.; Brennecke, J. F. Heat Capacities and Excess Enthalpies of 1-Ethyl-3-methylimidazolium-Based Ionic Liquids and Water. J. Chem. Eng. Data 2008, 53, 2112−2119. (28) Chirico, R. D.; Frenkel, M.; Diky, V. V. ThermoML−An XMLBased approach for storage and exchange of experimental and critically evaluated thermophysical and thermochemical property data. 2. Uncertainties. J. Chem. Eng. Data 2003, 48, 1344−1359. (29) Box, G. E. P.; Hunter, J. S.; Hunter, W. G. Statistics for Experimenters, 2nd ed.; Wiley-Interscience: Hoboken, NJ, 2005. (30) Coker, T. G.; Ambrose, J.; Janz, G. J. Fusion Properties of Some Ionic Quaternary Ammonium Compounds. J. Am. Chem. Soc. 1970, 92, 5293−5297. (31) Petrouleas, V.; Lemmon, R. M. Calorimetric studies of choline chloride, bromide, and iodide. J. Chem. Phys. 1978, 69, 1315−1316. (32) Makowska, A.; Dyoniziak, E.; Siporska, A.; Szydłowski, J. Miscibility of Ionic Liquids with Polyhydric Alcohols. J. Phys. Chem. B 2010, 114, 2504−2508. (33) Yadav, A.; Kar, J. R.; Verma, M.; Naqvi, S.; Pandey, S. Densities of aqueous mixtures of (choline chloride + ethylene glycol) and (choline chloride + malonic acid) deep eutectic solvents in temperature range 283.15 to 363.15 K. Thermochim. Acta 2015, 600, 95−101. (34) Yadav, A.; Trivedi, S.; Rai, R.; Pandey, S. Densities and dynamic viscosities of (choline chloride + glycerol) deep eutectic solvent and its aqueous mixtures in the temperature range (283.15−363.15) K. Fluid Phase Equilib. 2014, 367, 135−142.

The authors declare no competing financial interest.

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T Tf H x Q δQ̇ n u U

LIST OF SYMBOLS absolute temperature (K) freezing temperature (K) molar enthalpy (J·mol−1) molar fraction (mol·mol−1) total heat of mixing (J) heat flow (mW) amount of substance (mol) uncertainty expanded uncertainty

Superscripts

E id mix SL

excess property ideal property mixing property solid−liquid phase change

Subscripts

i comb HBA HBD

■

molecular species combined standard hydrogen bond acceptor hydrogen bond donor

REFERENCES

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Excess Molar Enthalpies of Deep Eutectic Solvents (DESs) Composed

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