Effect of Ethylene, Diethylene, and Triethylene Glycols and Glycerol on

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

pubs.acs.org/jced

Effect of Ethylene, Diethylene, and Triethylene Glycols and Glycerol on the Physicochemical Properties and Phase Behavior of Benzyltrimethyl and Benzyltributylammonium Chloride Based Deep Eutectic Solvents at 283.15−343.15 K Anusha Basaiahgari,† Somenath Panda,† and Ramesh L. Gardas* Department of Chemistry, Indian Institute of Technology Madras, Chennai - 600 036, India S Supporting Information *

ABSTRACT: Deep eutectic solvents (DESs) are regarded as a promising and emerging alternative for volatile organic compounds (VOCs) due to their cost-effectiveness and environmentally sustainable nature. Formulation of new DESs and thorough investigation of their physicochemical properties have been on the rise ever since potential applications of DES have been realized. The present study reports a new series of DES based on benzyltrimethylammonium chloride and benzyltributylammonium chloride as hydrogen bond acceptor (HBAs) with three ethylene glycols and glycerols as hydrogen bond donors (HBDs). The current work includes the synthesis, characterization, and investigation of the thermophysical properties of a series of DES systems. The studied properties include density, ρ, speed of sound, u, viscosity, η, refractive index, nD, and electrical conductivity, σ, for all DESs in the temperature range from 283.15 to 343.15 K. The observed trends in properties like density, speed of sound, viscosity, and refractive indices were correlated with appropriate equations, and some industrially useful parameters have also been derived. An insight about the internal association in these DESs has been analyzed through the rheological and Fourier-transform infrared (FT-IR) spectroscopy. The newly synthesized DESs have also been investigated for their efficiency in aqueous biphasic system (ABS) formation. This systematic study revealed the influence of both HBD/HBA and temperature on the physicochemical characteristics of the DESs. tunable nature6−8 as well as their resemblance to ILs with respect to their properties and applications. DESs are obtained by simple mixing of quaternary ammonium salts that act as hydrogen bond acceptors (HBAs) with suitable hydrogen bond donors (HBDs) in an appropriate mole ratio.6 The starting materials are usually of low cost and are capable of forming hydrogen bonding interactions. Hydrogen bonding interactions lead to charge delocalization, resulting in a product with a melting point lower than 373 K. This synthesis method is 100% atom economy reaction, and the resultant homogeneous liquid product does not require any further purification process. DESs exhibit properties similar to ILs like low vapor pressure, wide liquidus range, low toxicity, nonflammability, biodegradability, and compatibility with other solvents like water.7 The most studied DES combination is based on choline chloride as HBA and urea as HBD which was further extended

1. INTRODUCTION The scientific community is in constant search for solvents that can be alternatives for potentially harmful organic solvents. In order to avoid ill effects of pollution and global warming, environmentally sustainable solvent systems are essential. In due course of this search, ionic liquids (ILs) have emerged as solvents of unique and desirable properties like low melting point, negligible volatility in comparison to common organic solvents, wide liquidus and solubility range, etc. As a result, ILs are considered the most promising among other alternatives.1,2 However, ILs have been associated with some disadvantages like high cost of synthesis and purification, extensive usage of organic solvents in their synthesis, variable biodegradability and sustainability, etc., thus hindering their extensive applicability in chemical industries.1,3−5 In recent years, a novel class of solvents called deep eutectic solvents (DESs) has emerged as promising solvents by overcoming the limitations posed by traditional organic solvents as well as ILs. The research on DESs otherwise known as low melting mixtures has escalated significantly ever since their potential applications have been realized. DESs exhibited interesting properties and benefits such as relatively low cost of starting materials, easy synthesis methods, often biodegradable and natural constituents, and © XXXX American Chemical Society

Special Issue: Emerging Investigators Received: March 17, 2018 Accepted: April 17, 2018

A

DOI: 10.1021/acs.jced.8b00213 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Details of the Chemicals Used in the Present Study along with Their CAS Number, Source, and Purity

a

chemical name

CAS number

supplier

puritya (mass fraction)

ethylene glycol diethylene glycol triethylene glycol glycerol tripotassium phosphate benzyltrimethylammonium chloride benzyltributylammonium chloride

107-21-1 111-46-6 112-27-6 56-81-5 7778-53-2 56-93-9 23616-79-7

Merck Sigma-Aldrich Sigma-Aldrich Merck Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich

≥0.99 ≥0.99 ≥0.99 ≥0.995 ≥0.98 ≥0.98 ≥0.98

All chemicals were dried for 48 h and used without further purification.

Table 2. Chemicals Used for DES Synthesis, Abbreviation for the DESs, Molecular Weight, Water Content, and Decomposition Temperature, Td, of the Synthesized DESs S. No. 1 2 3 4 5 6 7 8 a

hydrogen bond acceptor (HBA)

hydrogen bond donor (HBD)

abbreviation

benzyltrimethylammonium chloride

ethylene glycol

BTMEG

benzyltributylammonium chloride

diethylene glycol triethylene glycol glycerol ethylene glycol diethylene glycol triethylene glycol glycerol

BTMDEG BTMTEG BTMGLY BTBEG BTBDEG BTBTEG BTBGLY

mol. wt.a (g·mol−1)

water content (%)

decomposition temperature (Td/K)

92.98

0.55

370.9

126.02 159.06 115.49 124.54 157.58 190.62 147.05

0.40 0.18 0.57 0.41 0.30 0.22 0.31

378.1 404.8 444.2 360.0 378.9 400.8 427.3

At a mole ratio of HBA:HBD of 1:3 and standard uncertainty u in compositions (x) in moles of DES mixtures of u(x) = 0.005.

DESs based on benzyltrialkylammonium salts and with common HBDs. The present work is an extension of our previous work27 on the similar HBA series with a variety of HBDs. A new series of DESs have been synthesized and characterized through Fourier-transform infrared (FT-IR) and NMR techniques. Further, thermal properties of synthesized DESs have been determined using TGA and DSC techniques. Principle physicochemical properties such as density, speed of sound, viscosity, electrical conductivity, and refractive index were measured in a wide range of temperature (283.15−343.15 K). This work also sheds some light on the internal organization of DESs with the help of rheological, FT-IR, and NMR studies. The effect of the chain length of HBAs as well as HBDs has also been analyzed. Additionally, the efficiency of the synthesized DESs has been scrutinized for the formation of aqueous biphasic systems (ABSs). Overall, the current study could contribute toward enhancing our current understanding of DES systems and may find some suitable applications in potential fields.

to other HBAs like quaternary ammonium and phosphonium based salts9−11 and a variety of HBDs like ethylene glycols, glycerol, phenols, diols, carboxylic acids, etc.,7,12−14 thus formulating newer combinations. Several natural deep eutectic solvents (NADESs) have also been proposed based on betaine, menthol, sugars, and other components which are of natural origin and, therefore, environmentally sustainable.15−17 Considering the wide choice of HBAs and HBDs available, an abundant number of DES combinations is possible. Therefore, it is of great importance to have a ready database of the basic physicochemical properties of DESs from the perspective of both finding suitable new applications as well as process designing18 and/or optimizing predictive modeling for prospective DES formulations.19 The immense development in the DES related fields could be observed from the significant rise in publications, and special issues have also been published with respect to DESs.20 Further, applications of DESs have been extending to numerous fields ranging from their usage as solvents, as catalysts, as aggregation media, in the electrochemical field, for capturing gases like CO2 and SO2, in biomass processing, in desulfurization of fuels, and as separation and extraction media for natural products.21−23 In recent years, in addition to pure DESs, aqueous solutions of DESs have also found interesting biological applications like solubilizing biomass, extraction of bioactive compounds, etc.24−26 The wide scope of applications shows that DESs have potential to be fit in both fields of scientific quest and industrial applications; however, analyzing their thermophysical properties remains a primary requisite. In this context, development of newer DES systems and systematic investigation of their physicochemical properties are essential for enriching the currently limited database as well as in enabling development of predictive models. Physicochemical property study also helps in opening up new possibilities to solve practical industrial problems with better and low cost solvents. Therefore, in this work, we have tried to develop new

2. EXPERIMENTAL SECTION 2.1. Materials. In the present work, two series of DESs were synthesized by using benzyltrimethylammonium chloride, [BTMA]Cl, and benzyltributylammonium chloride, [BTBA]Cl, as HBAs and four different HBDs, namely, ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG), and glycerol (GLY), and all details about the chemicals used in present study are given in Table 1. Further, the information on HBAs and HBDs used in the synthesis of DESs, the corresponding abbreviations, molecular weights, water content, and decomposition temperatures are provided in Table 2. All starting components were thoroughly dried for 48 h prior to their use in synthesis and were further stored under a nitrogen atmosphere. The structures of the HBAs and HBDs used in the current study are given in Figure 1. B



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pan maintained in a nitrogen atmosphere. The constant heating rate of 10 K·min−1 was maintained throughout the experiment. The weighing precision of the instrument is ±0.01%. The instrument is provided with ultrasensitive thermobalance and an efficient horizontal purge gas system with mass flow control. Two thermocouples are placed adjacent to the sample in order to measure sample temperature and heating rate control with accuracy and precision. Spectral Characterization and Water Content Determination. FT-IR spectra were recorded on a FT-IR spectrophotometer (JASCO FT-IR 4100; wavenumber range 7800−400 cm−1; wavelength range 1282−25,000 nm using KBr disk). The instrument has a 22,000/1 signal-to-noise ratio with a maximum resolution of 0.9 cm−1. Proton NMR was recorded on a Bruker Avance 500 MHz spectrometer using D2O as a solvent. Water content in synthesized DESs was determined using a Metrohm (870 KF Titrino Plus) Karl Fischer Titrator. It works on the volumetric titration principle. The instrument was calibrated as per the instructions given in the supplier’s manual. The water content of DES samples is given in Table 2. Density and Speed of Sound. The density and speed of sound of the synthesized DESs were simultaneously measured using an Anton Paar DSA 5000 M instrument. All measurements were performed in the temperature range from 283.15 to 343.15 K with 5 K intervals and at atmospheric pressure. The precise temperature was maintained by a Peltier device with an accuracy of ±0.01 K. Calibration of the instrument was verified prior to measurements with Millipore water and dry air. Dynamic Viscosity. Viscosity measurements of samples were performed using an Anton Paar Lovis 2000ME instrument attached to a DSA 5000M master instrument. The Lovis 2000ME works on rolling ball in capillary method, and the viscosities of DES samples were measured in the temperature range from 283.15 to 343.15 K with 5 K intervals. The temperature is maintained through a built-in Peltier device with an accuracy of ±0.02 K. All measurements were performed for a minimum of three times, and the average of these measurements is considered. Rheology. The rheological behavior of DES samples was studied using an Anton Paar MCR 102 (Modular Compact Rheometer), with a CP 40 cone and plate measuring geometry. The cone inclination was 1°, and the gap between the cone and plate was maintained as 0.1 mm. The temperature of the system was maintained by a P-PTD 200/AIR Peltier temperature device. Steady shear measurements were performed at 298 K and at 0.1−1000 s−1 shear rate. Electrical Conductivity. A Eutech (PC 700) instrument was used to measure the electrical conductivity of the synthesized DESs at variable temperatures. It consists of an electrode with a cell constant of K = 1 and a built-in temperature sensor. The temperature of the system was maintained using a double walled glass jacketed vessel connected to a Julabo refrigerated/ heating circulator bath (CORIO CD-300F). The instrument measures the conductivity in the range from 0.0 μS to 200 mS. The conductivity of the cell was calibrated with an aqueous solution of 0.01 N KCl. Refractive Index. An Anton Paar Abbemat 500 Refractometer was used to measure the refractive indices of the synthesized DES samples at 589 nm and at atmospheric pressure. The sample cell was thoroughly cleaned, and prior to measurements, the instrument was calibrated with freshly degassed Millipore water. Refractive indices were measured in the temperature range from 283.15 to 343.15 K with 5 K

Figure 1. Structures of hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs) used in the synthesis of DESs.

2.2. Preparation of the DESs. The DESs of the current study were synthesized by taking accurate weights of HBAs and HBDs in the appropriate mole ratio. Starting components were thoroughly mixed by stirring them at 333.15 K and atmospheric pressure. The stirring and heating of components was ensured by using a magnetic stirrer with temperature control. The procedure was continued for a minimum of 2 h to obtain clear and homogeneous liquids. Thus, obtained DESs were dried under a vacuum and stored under a nitrogen atmosphere for measuring their physicochemical properties. Different mole ratios of HBAs and HBDs (2:1, 1:1, 1:2, 1:3, 1:4, and 1:5) were tried for the formation of DESs. Some of the combinations turned solid upon keeping them overnight at room temperature or cooling up to 263 K by using a Julabo refrigerated/heating circulator bath (CORIO CD-300F). Upon careful observation, one common mole ratio (HBA:HBD is 1:3) at which all combinations of HBAs and HBDs remained liquid was selected for current study. The representative phase diagram for BTBGLY (for different mole ratios of HBA:HBD) is given in Figure S1 and corresponding experimental data presented in Table S1. For 1:3 mole ratio of HBA:HBD, BTBGLY presented the lowest glass transition temperature (Tg) of 191.3 K, and no other phase transition was observed (from Tg to 343.15 K). Phase transitions for various combinations of HBA and HBDs have been determined using Q200 MDSC with Refrigerated Cooling Systems (RCS90) from TA Instruments, and the lowest temperature possible with this instrument is 183.15 K (−90 °C). Except for BTBGLY, the phase diagram for all other studied DESs could not be determined, since the phase transition temperatures for one or more ratios of HBAs to HBDs were beyond the detection limit of the instrument (below 183.15 K). Therefore, a common ratio of HBA to HBD (i.e., 1:3) which remained as stable liquid in the temperature range of 283.15−343.15 K has been selected for all DESs in the present work. Further, a uniform composition would be favorable to compare the effect of HBDs on the thermophysical properties of studied DESs. 2.3. Apparatus and Procedure. Thermal Stability Determination. Thermal stability of DES samples was determined using the thermogravimetric analysis technique, and the instrument TGA Q500 Hi-Res from TA Instruments was employed for these measurements. The experiment is usually carried out by taking the sample in an open platinum C



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extent of hydrogen bonding.30 Significant types of vibrations like stretching, bending, and other types of vibrations observed for various DESs are presented in Table S2, and FT-IR spectra obtained for DESs based on the [BTMA]Cl series are given in Figure 2. Further, the FT-IR spectra for DESs based on the [BTBA]Cl series are given in Figure S3.

intervals. The deviation associated with temperature is 0.01 K. The sample kept on the measuring prism is irradiated with the light emitting diode at different angles. The refracted light is used to measure the refractive index, and the maximum uncertainty associated is 5 × 10−4. Liquid−Liquid Equilibrium Studies. The phase behavior of the synthesized DESs was analyzed by determining phase diagrams of pseudo ternary systems composed of DESs, water, and an inorganic salt, K3PO4. Phase diagrams were determined at 298.15 K and atmospheric pressure. A well-known cloud point titration method usually employed for binodal curve determination in the case of IL based ABS was used here also.28 Aqueous solutions of DESs and K3PO4 were prepared at a weight percentage composition of 60 and 50%, respectively. An aqueous solution of K3PO4 was added dropwise to the DES aqueous solution until turbidity which is an indication of biphase formation was observed. Later, Millipore quality water was added dropwise until the turbid solution turned clear and implied monophase formation. The process was repeated several times to obtain sufficient binodal curve data. The experiment was carried out under constant stirring provided by a magnetic stirrer, and further, the temperature was maintained by using a Julabo circulator bath.

3. RESULTS AND DISCUSSION 3.1. Thermal Stability Determination. Thermogravimetric analysis was performed to determine the thermal stability of the synthesized DESs in terms of decomposition temperature, Td. The percentage loss in weight of DES samples as a function of temperature is graphically presented in Figure S2, and consequently obtained Td values are given in Table 2. Td values were determined at 10% weight loss and were found to be in the range from 373.15 to 443.15 K for studied systems. Among various HBDs, systems with EG exhibited the lowest thermal stability, while that of the system with GLY was found to be highly stable. The order of thermal stability with respect to studied HBDs with [BTMA]Cl as HBA was observed as BTMGLY > BTMTEG > BTMDEG > BTMEG. Similar order was detected in the case of systems with [BTBA]Cl as HBA also. A similar range of Td values was observed in the case of NDES formed by ChCl and sugars.29 As in the case of ILs, thermal stability depends on various factors like the type of HBD and HBA, the alkyl chain length of the HBD, the type of interactions, etc. With an increase in the alkyl chain length of HBDs, the viscosity of DESs was found to increase and DESs of higher viscosity showed higher thermal stability, as shown by BTMGLY and BTBGLY. On the basis of the structures of the HBDs studied, it can also be inferred that the presence of ether groups adds to the thermal stability of DESs and the currently obtained order of thermal stability also confirms this observation. Similar behavior was exhibited by DESs formed by allyltriphenylphosphonium bromide with DEG and TEG.11 3.2. FT-IR and NMR Studies. FT-IR is an effective analytical technique for determining the functional groups present in synthesized compounds. The presence of hydrogen bonding is clearly evident in the IR spectra of DESs especially with respect to stretching bond vibrations of the hydroxyl group (OH) group. In general, the presence of a hydroxyl group usually corresponds to the stretching bands present in the region 3700−3100 cm−1. However, these bands are significantly affected by the strength of hydrogen bonding between the halide anion of the HBA and HBD moiety and may shift to higher or lower wavenumbers depending on the

Figure 2. FT-IR spectra for the synthesized DESs based on [BTMA]Cl: (A) OH stretching and aromatic CH stretching; (B) aliphatic stretching; (C) CC and CC ring stretching; (D) C OH bending; (E and F) CO and COC stretching; (G) aromatic CH wagging and OH wagging. Black, orange, blue, and green lines represent BTMEG, BTMDEG, BTMTEG, and BTMGLY, respectively.

In order to characterize synthesized DESs, their 1H NMR spectra have been recorded. Literature reports have suggested that, depending on the NMR spectra obtained, the efficiency of the DES synthesis method could be analyzed as is done in the case of choline chloride and carboxylic acid based DESs.31 The representative NMR spectra obtained for two DES systems, namely, BTBEG and BTBGLY, are given in Figure 3. 3.3. Density and Related Properties. It is important to determine the effect of temperature on density and related properties, since these parameters help in the designing of industrial processes such as gas separations.32 The experimental density values of eight synthesized DESs over the complete temperature range of 283.15−343.15 K are presented in Table 3. Parts a and b of Figure 4 graphically represent the effect of HBDs on the densities of DESs composed of [BTMA]Cl and [BTBA]Cl, respectively, as HBAs. It is clear from Figure 4 that the ethylene glycol based DESs show the lowest density values for both HBAs. The additional −OH group present on the HBD leads to stronger hydrogen bonding interactions and thereby higher densities, as can be seen from DESs based on glycerol. Another noticeable trend is that density values were found to be increasing with the additional ethylene glycol moiety or in other terms with additional ether linkages. Also, the density values for all DESs are higher compared to their pure solid constituents but comparably lower than the liquid constituents. Both of these observations could possibly be explained in the light of hole theory, which proposes that the DES contains internal empty vacant sites, or holes, which determines the density of the liquid.33 On varying the HBDs, the amount of the empty spaces changes, thereby changing the density. The fact that the glycerol based DESs show the highest D



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Figure 3. 1H NMR spectra of (A) BTBEG and (B) BTBGLY.

Table 3. Experimental Values of Density, ρ, for Synthesized DESs at Temperature T = 283.15−343.15 K and Pressure p = 0.1 MPaa ρ (kg·m−3)

a

T (K)

BTMEG

BTMDEG

BTMTEG

BTMGLY

BTBEG

BTBDEG

BTBTEG

BTBGLY

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15

1110.06 1107.00 1103.95 1100.92 1097.89 1094.87 1091.85 1088.84 1085.83 1082.83 1079.82 1076.82 1073.83

1120.06 1116.91 1113.75 1110.60 1107.46 1104.33 1101.21 1098.09 1094.99 1091.89 1088.79 1085.70 1082.63

1127.45 1124.08 1120.70 1117.31 1113.93 1110.57 1107.21 1103.87 1100.54 1097.22 1093.90 1090.59 1087.29

1190.87 1187.92 1184.97 1182.02 1178.99 1176.07 1173.14 1170.19 1167.23 1164.25 1161.27 1158.29 1155.33

1045.37 1042.15 1039.00 1035.88 1032.74 1029.60 1026.44 1023.29 1020.13 1016.97 1013.82 1010.67 1007.53

1063.94 1060.76 1057.65 1054.57 1051.50 1048.41 1045.31 1042.22 1039.12 1036.03 1032.94 1029.85 1026.78

1077.97 1074.64 1071.36 1068.09 1064.81 1061.53 1058.25 1054.97 1051.70 1048.43 1045.18 1041.92 1038.69

1112.02 1108.90 1105.78 1102.66 1099.54 1096.41 1093.28 1090.07 1086.95 1083.85 1080.72 1077.58 1074.43

Standard uncertainties u are u(T) = 0.01 K and u(p) = 10 kPa, and relative standard uncertainties ur are ur(ρ) = 0.005.

density is seen in previous studies with other HBAs.10,13 Among the eight studied DES compositions, BTBEG shows the lowest density (1035.88 kg·m−3), whereas BTMGLY shows the highest (1182.02 kg·m−3) at 298.15 K. However, the current DESs under study have comparatively lower density than that of choline chloride−urea DESs1 but are comparable with other ethylene glycol or glycerol based DESs reported earlier.19

The temperature dependence of density values can be expressed by the linear eq 1 as follows

ρ = A + BT

(1)

where ρ is the density, T is the temperature, and A and B are fitting parameters. The calculated values for the fitting parameters along with their average absolute relative deviation E



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Figure 4. Variation of density, ρ, as a function of temperature, T = 283.15−343.15 K, at atmospheric pressure, p = 0.1 MPa, for synthesized DESs. Solid lines indicate linear fitting (eq 1): (a) black ■, BTMEG; orange ▲, BTMDEG; blue ●, BTMTEG; green ◆, BTMGLY. (b) Maroon ▼, BTBEG; pink ⬢, BTBDEG; red ★, BTBTEG; blue ▶, BTBGLY.

Table 4. Fitting Parameters A and B of eq 1 and Absolute Relative Deviation (ARD) Calculated from eq 2 DES

BTMEG

BTMDEG

BTMTEG

BTMGLY

BTBEG

BTBDEG

BTBTEG

BTBGLY

A·10−3 (kg·m−3) B·10 (kg·m−3·K−1) ARD × 103

1280.9 −0.604 2.5

1296.6 −0.624 3.8

1316.9 −0.670 4.5

1358.6 −0.592 1.6

1223.8 −0.630 1.4

1239.0 −0.619 2.2

1263.3 −0.655 1.9

1289.5 −0.623 1.8

Table 5. Calculated Values of Molar Volume, Vm (from eq 3), Standard Molar Entropy, S° (from eq 5), and Lattice Energy, UPOT, of Synthesized DESs DES

BTMEG

BTMDEG

BTMTEG

BTMGLY

BTBEG

BTBDEG

BTBTEG

BTBGLY

Vm (cm3·mol−1) S0 (J·K−1·mol−1) UPOT (kJ·mol−1)

84.45 204.3 555

113.47 264.3 513

142.36 324.1 483

97.71 231.7 534

120.22 278.3 505

149.42 338.7 477

178.47 398.9 456

133.36 305.5 492

∼ BTMTEG > BTBGLY > BTBEG > BTMDEG > BTMGLY > BTMEG. The isobaric expansion coefficient, α, is an important and useful parameter in industrial applications, and can be used to evaluate the free volumes or interstices of DESs as similar to the case of ILs. This parameter has also been correlated to the ability to solubilize CO2 or SO2 in the case of ILs35,36 and is calculated using eq 4

(ARD) are reported in Table 4. The ARD were calculated by the following equation34 (eq 2): ⎛ 1 ARD = ⎜⎜ ⎝n

∑

|ρexp − ρcal | ⎞ ⎟100 ⎟ ρexp ⎠

(2)

As seen from the low ARD ( BTBDEG F



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have been performed in the temperature range from 283.15 to 343.15 K and have been presented in Table 6. The variation in speed of sound for the DESs has been graphically presented in Figure 6a and b. As seen from the plots, speed of sound always decreases on going from BTM to BTB as HBA. Also, similar to most ILs and other DESs, the speed of sound decreases linearly with an increase in temperature in most cases.39 However, in the case of BTMGLY and BTBGLY, the temperature dependence of the speed of sound is nonlinear, especially at the lower temperature region. This type of behavior has also been observed for a few ILs, where the possible rearrangements in IL structures have been considered to explain this observation.40 In general, at elevated temperature, the molecules of the liquids move further apart from each other, generating more free space. As the sound wave takes a longer time to travel through the less-dense medium, its speed will be reduced at higher temperature.39 The same assumption is further supported by the empirical relation formulated by Jacobson41 which gives a measure of the intermolecular free length, Lf, as given by eq 7

Figure 5. Variation of volume expansion coefficient, α, as a function of temperature, T = 283.15−343.15 K, at atmospheric pressure, p = 0.1 MPa, for synthesized DESs: black ■, BTMEG; orange ▲, BTMDEG; blue ●, BTMTEG; green ◆, BTMGLY; maroon ▼, BTBEG; pink ⬢, BTBDEG; red ★, BTBTEG; blue ▶, BTBGLY.

Lf = K

observed. This infers that the H-bonding interactions in BTM are quite different and probably stronger than those for BTB. Two more related properties, the standard molar entropy, S°, and the lattice energy, UPOT, can also be calculated from density values38 by the following empirical equations (eq 5 and eq 6) as S° = 1246.5Vm + 29.5

(5)

⎛ ρ ⎞1/3 UPOT = 1981.2⎜ ⎟ + 103.8 ⎝M⎠

(6)

1 ρu 2

(7)

where K is a temperature dependent variable, called Jacobson’s constant, ρ is the density in g·cm−3, and u is the speed of sound in m·s−1. From the observed increase in Lf values reported in Table S4 with temperature, it can be inferred that there is more free space at higher temperature. The isentropic compressibility, βs, an important thermodynamic parameter useful in industrial applications, can be derived from the well-known Newton−Laplace equation (eq 8) as follows

The obtained values are reported in Table 5 for 298.15 K. Due to the lack of reported data for the DESs, the values could not be compared with DESs other than ChCl based acidic DESs, which show comparable values.38 3.4. Speed of Sound. The speed of sound data of DESs is scarcer as compared to their ionic liquid analogues,39 and considering the importance of the speed of sound data in optimization of industrial processes, we have measured the speed of sound of all eight synthesized DESs. All measurements

βs =

1 ρu 2

(8)

where ρ is the density of the IL and u is the speed of sound. The calculated values of βs have been reported in Table S5, showing the range of distribution from 211 to 414 TPa−1. 3.5. Viscosity. The experimental viscosity data of DESs formed by a combination of various HBAs and HBDs is crucial for planning applications of DESs in gas solubility, separation

Table 6. Experimental Values of Speed of Sound, u, for Synthesized DESs at Temperature T = 283.15−343.15 K and Pressure p = 0.1 MPaa u (m·s−1)

a

T (K)

BTMEG

BTMDEG

BTMTEG

BTMGLY

BTBEG

BTBDEG

BTBTEG

BTBGLY

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15

1856.70 1843.64 1830.76 1817.97 1805.30 1792.73 1780.23 1767.78 1755.40 1743.00 1730.71 1718.48 1706.56

1782.61 1768.95 1755.79 1742.88 1730.19 1717.67 1705.30 1692.97 1680.79 1668.65 1656.59 1644.66 1633.09

1776.24 1761.01 1746.18 1731.42 1716.93 1702.79 1688.52 1674.64 1660.85 1647.16 1633.60 1620.06 1606.61

1992.60 1970.80 1953.00 1937.57 1923.40 1910.07 1897.23 1884.63 1872.28 1860.01 1847.86 1835.78 1824.15

1734.93 1716.28 1698.92 1682.24 1666.20 1650.53 1635.22 1620.11 1605.22 1590.53 1575.97 1561.66 1547.90

1706.83 1689.50 1673.25 1658.14 1643.55 1629.47 1615.60 1602.02 1588.61 1575.27 1562.14 1549.17 1536.74

1710.12 1692.43 1675.75 1659.77 1644.25 1629.18 1614.34 1599.78 1585.40 1571.17 1557.13 1543.26 1529.96

1908.00 1866.41 1832.98 1805.29 1782.00 1761.67 1743.52 1726.80 1711.15 1696.03 1681.52 1667.34 1653.87

Standard uncertainties u are u(T) = 0.01 K and u(p) = 10 kPa, and relative standard uncertainties ur are ur(u) = 0.001. G



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Figure 6. Variation of the speed of sound, u, as a function of temperature, T = 283.15−343.15 K, at atmospheric pressure, p = 0.1 MPa, for synthesized DESs. Solid lines are presented for visual indication only. (a) Black ■, BTMEG; orange ▲, BTMDEG; blue ●, BTMTEG; green ◆, BTMGLY. (b) Maroon ▼, BTBEG; pink ⬢, BTBDEG; red ★, BTBTEG; blue ▶, BTBGLY.

Table 7. Experimental Values of Viscosity, η, for Synthesized DESs at Temperature T = 283.15−343.15 K and Pressure p = 0.1 MPaa η (mPa·s)

a

T (K)

BTMEG

BTMDEG

BTMTEG

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15

101.9 76.1 58.2 45.3 35.9 29.0 23.9 19.9 16.8 14.3 12.3 10.7 9.5

263.4 185.4 134.1 98.6 74.8 57.9 44.4 34.8 27.8 23.2 18.7 16.0 14.2

620.1 470.8 370.5 308.8 214.3 155.7 115.6 87.8 68.5 54.9 43.5 34.3 27.4

BTMGLY

BTBEG

BTBDEG

BTBTEG

BTBGLY

1874.9 1170.9 753.5 501.5 342.7 238.3 171.8 126.7 95.4 73.0 58.2 46.3

901.4 598.4 415.2 286.0 211.0 155.5 117.1 89.9 70.7 55.8 43.6 34.9 28.1

961.86 644.25 433.54 310.37 224.90 168.37 125.21 96.78 76.23 62.63 50.43 40.68 32.76

738.7 506.0 356.9 256.5 188.5 142.4 109.1 85.2 67.7 54.4 43.3 34.9 28.3

2328.1 1709.2 1112.1 740.9 497.7 346.1 254.6 185.3 138.1 105.0

Standard uncertainties u are u(T) = 0.01 K and u(p) = 10 kPa, and relative standard uncertainties ur are ur(η) = 0.01.

Figure 7. Variation of viscosity, η, as a function of temperature, T = 283.15−343.15 K, at atmospheric pressure, p = 0.1 MPa, for synthesized DESs. Solid lines represent VTF equation fitting (eq 9). (a) Black ■, BTMEG; orange ▲, BTMDEG; blue ●, BTMTEG; green ◆, BTMGLY. (b) Maroon ▼, BTBEG; pink ⬢, BTBDEG; red ★, BTBTEG; blue ▶, BTBGLY.

H



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Table 8. Fitting Parameters η0, B, and T0 of the VTF Equation (eq 9) and Regression Values R2 for Viscosity Values of the Studied DESs DES

BTMEG

BTMDEG

BTMTEG

BTMGLY

BTBEG

BTBDEG

BTBTEG

BTBGLY

η0 (mPa·s) B (K) T0 (K) R2

0.1155 748.6 172.8 0.999

0.0537 967.9 169.3 0.999

0.0353 1133.2 173.2 0.999

0.0101 1490.0 165.3 0.999

0.0515 1111. 5 169.4 0.999

0.0577 1105.0 169.5 0.999

0.0777 1044.0 169.2 0.999

0.0472 1339.1 174.6 0.992

Figure 8. Arrhenius plot (eq 10) representing the variation of viscosity, ln η, as a function of temperature, T = 283.15−343.15 K, at atmospheric pressure, p = 0.1 MPa, for synthesized DESs. (a) Black ■, BTMEG; orange ▲, BTMDEG; blue ●, BTMTEG; green ◆, BTMGLY. (b) Maroon ▼, BTBEG; pink ⬢, BTBDEG; red ★, BTBTEG; blue ▶, BTBGLY.

processes, lubrication, high-pressure operations, etc.42 In this view, dynamic viscosities, η, for all eight synthesized DESs have been measured in the present work at constant shear to determine the effect of structure and temperature on the DESs. The results are tabulated in Table 7 and graphically presented in Figure 7. As observed, the ethylene glycol based DESs show the lowest viscosity and are in fact comparable to ChCl:EG based DESs7 for BTMEG. However, changing the HBA from BTM to bulkier BTB leads to an instant almost 6-fold increase in viscosity value (from 45 to 286 mPa·s at 298.15 K). The addition of −OH in terms of functionalization leads to a regular increase in viscosity vales for BTM as HBA. However, in the case of BTB as HBA, the increase is irregular, as observed in density, and follows the trend BTBDEG > BTBTEG > BTBEG. The decrease in viscosity as a function of temperature followed a nonlinear behavior. The effect of temperature on the viscosity of DESs is more significant at higher temperatures in comparison to lower temperatures.7 Viscosity decreases rapidly with temperature and at higher temperature (above 313 K) approaches almost similar values for DESs consisting of the same HBA with different HBDs. On the other hand, glycerol based DESs show the highest viscosity values similar to earlier reports of high viscosity glycerol based DESs,12,13 though compared to the ChCl:urea DES, BTMGLY is almost 2 times more viscous (450 mPa·s vs 753 mPa·s at 298.15 K). On the other hand, the very high viscosity observed in the case of BTBGLY may be indicative of an extra degree of favorable H-bonding. However, all of the studied DESs are far less viscous than the benzyltrimethylammonium based DESs with acid based HBDs.42,43 From the plot of the temperature dependence of the DESs, it is clear that it follows the usual exponential decrease in viscosity

with increase in temperature. It is also evident that the effect of temperature on viscosity is the most prominent compared to other physical properties like density or refractive index. This may be due to the weakening of interactions between the molecules of the HBAs and HBDs.35 This temperature dependence can be represented best by the Vogel−Tamman−Fulcher (VTF) equation (eq 9), as suggested by Seddon, specifically where symmetrical moieties are involved.8 The equation is expressed as ⎛ B ⎞ η = η0 exp⎜ ⎟ ⎝ T − T0 ⎠

(9)

where η0 is the limiting value of viscosity at high temperature; B is a fitting parameter controlling the curvature, also called the pseudo activation energy; and T0 is the ideal glass transition temperature in K. The parameter values along with regression coefficients are given in Table 8. The lines in Figure 7 show the fitting according to the VTF equation, and it is clear that it gives a good fit for the measured data. However, the temperature dependence of viscosity for the DESs on temperature can also be expressed in the logarithmic form of the Arrhenius equation (eq 10) as follows8 ln η = ln η∞ +

Eη RT

(10)

where η is the dynamic viscosity, T is the temperature in K, R is the universal gas constant, η∞ is the viscosity at infinite temperature, and Eη is the activation energy. Values of the fitting parameters are summarized in Table S6 and shown in the Arrhenius plot in Figure 8. The Arrhenius fitting gives a measure of the activation energies, Eη, of the liquid, which is I



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Figure 9. Variation of viscosity, η, as a function of shear rate, γ, at 298.15 K and atmospheric pressure, p = 0.1 MPa, for synthesized DESs. (a) Black ■, BTMEG; orange ▲, BTMDEG; blue ●, BTMTEG; green ◆, BTMGLY. (b) Maroon ▼, BTBEG; pink ⬢, BTBDEG; red ★, BTBTEG; blue ▶, BTBGLY.

Table 9. Experimental Values of Electrical Conductivity, σ, for the Synthesized DESs at Temperature T = 283.15−343.15 K and Pressure p = 0.1 MPaa σ (mS·cm−1)

a

T (K)

BTMEG

BTMDEG

BTMTEG

BTMGLY

BTBEG

BTBDEG

BTBTEG

BTBGLY

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15

2.63 3.20 3.91 4.61 5.52 6.43 7.50 8.72 9.97 11.17 12.52 14.12 15.65

0.88 1.09 1.32 1.65 2.01 2.54 3.11 3.56 3.98 4.74 5.39 6.11 6.81

0.48 0.63 0.81 1.01 1.24 1.52 1.87 2.26 2.78 3.25 3.77 4.40 5.06

0.12 0.19 0.27 0.39 0.55 0.77 1.04 1.38 1.81 2.31 2.83 3.40 4.13

0.30 0.39 0.48 0.64 0.81 0.99 1.22 1.48 1.81 2.19 2.62 3.15 3.65

0.19 0.25 0.32 0.40 0.52 0.66 0.81 0.99 1.15 1.30 1.54 1.78 2.07

0.13 0.17 0.21 0.27 0.35 0.43 0.54 0.67 0.82 0.98 1.14 1.45 1.74

0.03 0.05 0.07 0.09 0.13 0.19 0.25 0.38 0.47 0.65 0.86 1.09 1.38

Standard uncertainties u are u(T) = 0.01 K and u(p) = 10 kPa, and relative standard uncertainties ur are ur(κ) = 0.02.

viscosity of DES samples at continuously increasing shear rate in the range from 0.01 to 1000 s−1. The results obtained are presented in Figure 9, and they indicated that viscosity remains constant throughout the applied shear rate range and thus exhibits Newtonian liquid behavior. This also denotes the absence of molecular structuring and entanglements.44 In other terms, shear stress varies proportionally with the applied shear rate. Understanding the behavior of DESs in the presence of variable stress and shear rate is useful in analyzing their prospective applications in fields like CO2 capture.45 From the obtained results, it can be concluded that DESs behave similar to liquid solvents as well as some IL samples.46 3.7. Ionic Conductivity. Ionic conductivity, σ, data is crucial for finding possible applications of DESs in electrochemical fields, as it constitutes the primary characterization parameter. In this study, the electrical conductivity values of eight DESs in the temperature range from 283.15 to 343.15 K are reported in Table 9. From Table 9, it is clear that the conductivity values are comparatively lower than those of their choline chloride or phosphonium based analogues.10,13 This difference in conductivity can be attributed to their relatively high viscosity values. The same assumption explains the lower

described as the minimum energy barrier needed to be crossed in order for the liquid to flow. The higher the value of Eη, the more difficulty for the particles to move over each other, or in other words the more interactions between the molecules, which in the case of DESs are mostly extent of H-bonding and van der Waals interactions. The obtained values of Eη for the studied DESs are comparable with already reported values for other classes of DESs.13 Additionally, viscosity at infinite temperature is expressed by η∞, when the molecules become so distant from each other that the intermolecular interaction contribution to the viscosity becomes almost insignificant, and mostly determined by the structural geometry of the constituent molecules.43 The comparatively better fit given by the VTF equation (R2 > 0.999) than the Arrhenius equation (R2 > 0.994−0.997) indicates that the viscoelastic properties of DESs are different than common solvents. This non-Arrhenius temperature dependence was reported earlier to have been attributed to the formation of glass-like structures by association or suffer from hindered rotation at lower temperatures.6 3.6. Rheological Studies. The rheological behavior of the studied DESs was determined by measuring the apparent J



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Figure 10. Variation of experimental conductivity, σ, as a function of temperature, T = 283.15−343.15 K, at atmospheric pressure, p = 0.1 MPa, for synthesized DESs. Solid lines indicate VTF fitting (eq 11). (a) Black ■, BTMEG; orange ▲, BTMDEG; blue ●, BTMTEG; green ◆, BTMGLY. (b) Maroon ▼, BTBEG; pink ⬢, BTBDEG; red ★, BTBTEG; blue ▶, BTBGLY.

Table 10. Fitting Parameters σ0 and Eσ of the Arrhenius Equation (eq 11) and Regression Values R2 for Conductivity Values of the Studied DESs DES −1

σ0 × 10 (mS·cm ) Eσ (J·mol−1) R2 5

BTMEG

BTMDEG

BTMTEG

BTMGLY

BTBEG

BTBDEG

BTBTEG

BTBGLY

0.74 −347.7 0.998

1.35 −405.5 0.995

3.54 −458.4 0.999

862.14 −689.8 0.995

5.06 −487.7 0.999

1.82 −466.9 0.995

3.74 −507.0 0.999

1167.09 −750.9 0.999

Table 11. Experimental Values of Refractive Index, nD, for the Synthesized DESs at Temperature T = 283.15−343.15 K and Pressure p = 0.1 MPaa nD

a

T (K)

BTMEG

BTMDEG

BTMTEG

BTMGLY

BTBEG

BTBDEG

BTBTEG

BTBGLY

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15

1.4995 1.4980 1.4966 1.4951 1.4935 1.4921 1.4907 1.4893 1.4878 1.4864 1.4849 1.4835 1.4820

1.4954 1.4940 1.4925 1.4911 1.4897 1.4884 1.4869 1.4855 1.4841 1.4826 1.4813 1.4798 1.4784

1.4923 1.4908 1.4894 1.4880 1.4865 1.4849 1.4836 1.4821 1.4807 1.4792 1.4778 1.4763 1.4749

1.5126 1.5114 1.5102 1.5089 1.5078 1.5064 1.5052 1.5039 1.5027 1.5014 1.5002 1.4989 1.4977

1.4993 1.4979 1.4963 1.4947 1.4932 1.4917 1.4901 1.4886 1.4871 1.4855 1.4840 1.4825 1.4809

1.4957 1.4945 1.4929 1.4909 1.4893 1.4878 1.4860 1.4849 1.4836 1.4815 1.4804 1.4786 1.4776

1.4936 1.4921 1.4906 1.4889 1.4874 1.4857 1.4842 1.4827 1.4811 1.4795 1.4779 1.4764 1.4748

1.5095 1.5079 1.5065 1.5051 1.5036 1.5021 1.5006 1.4992 1.4977 1.4962 1.4948 1.4933 1.4917

Standard uncertainties u are u(nD) = 0.0005, u(T) = 0.01 K, and u(p) = 10 kPa.

conductivity of BTB based DESs compared to their BTM counterparts with comparatively less bulky structure. As expected, the ethylene glycol based DESs have the highest conductivity values among the studied DESs (4.605 and 0.643 mS·cm−1 at 298.15 K), which may be due to the small size of the HBDs. However, the values are comparable with the TEG based DESs (0.127−5.047 mS·cm−1) at all temperatures compared to the 0.212−8.77 mS·cm−1 reported values.47 The variation of conductivity at different temperatures is plotted in Figure 10 and shows that conductivity follows the opposite trend to that of viscosity. As the temperature increases, the increased kinetic energy of the molecules makes it easier for the charged molecules to move, thereby increasing the conductivity with temperature.

Similar to viscosity, most of the studies have assumed that the DESs are more likely to follow the Arrhenius equation (eq 11) and the temperature dependence is generally expressed as10,13 ln σ = ln σ0 +

Eσ RT

(11)

where σ is the ionic conductivity in mS·cm−1, σ0 is the limiting ionic conductivity in mS·cm−1, Eσ is the activation energy for the ionic conductivity in J·mol−1, T is the temperature in K, and R is the universal gas constant in J·mol−1·K−1. The calculated values of these parameters along with their regression values are summarized in Table 10. It is clear from the table that the Arrhenius equation can well describe the temperature dependK



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Figure 11. Variation of refractive index, nD, as a function of temperature, T = 283.15−343.15 K, at atmospheric pressure, p = 0.1 MPa, for synthesized DES. The solid line indicates linear fitting. (a) Black ■, BTMEG; orange ▲, BTMDEG; blue ●, BTMTEG; green ◆, BTMGLY. (b) Maroon ▼, BTBEG; pink ⬢, BTBDEG; red ★, BTBTEG; blue ▶, BTBGLY.

Table 12. Fitting Parameters d1 and d2 (eq 12) and Regression Values R2 for Refractive Index Values of the Studied DESs DES

BTMEG

BTMDEG

BTMTEG

BTMGLY

BTBEG

BTBDEG

BTBTEG

BTBGLY

d1 d2 R2

1.5818 −2.91 0.999

1.5755 −2.83 0.999

1.5745 −2.90 0.999

1.5833 −2.50 0.999

1.5863 −3.07 0.999

1.5830 −3.08 0.998

1.5826 −3.14 0.999

1.5929 −2.95 0.999

ence of conductivity values. The magnitude and sign of activation energies for viscosity and conductivity are opposite to each other for all studied DESs (see Tables 10 and S6). A higher activation energy for the viscosity of DESs indicates the high energy barrier to be overcome by mass transport and thus the corresponding activation energy for conductivity is lower.7,9 3.8. Refractive Index. Refractive index, nD, is a physical property characteristic of a liquid that arises from the electronic polarizability of a molecule.48 In this study, the nD values for all eight DES systems were experimentally measured in the temperature range from 283.15 to 343.15 K, reported in Table 11. The variation in nD with temperature is also graphically presented in Figure 11. It is clear from Figure 11 that the TEG based DES has the lowest values of nD in both HBAs. The refractive index of DESs decreases at all temperatures with the decrease in molar mass of DESs, similar to those of the density and molar volume trends. Also, the order followed by DESs is not the same as density, though refractive index also decreases with temperature increase similar to density.48 This decrease is generally attributed to the thermal expansion leading to a decrease in density and therefore allowing greater freedom of light rays to pass through the medium.43 This temperature dependence has been expressed by the linear equation as expressed in eq 1248 nD = d1 + d 2T

(1-ethyl-3-methylimidazolium hydrogen sulfate/methanesulfonate) and choline/benzyltrimethylammonium chloride based DESs.36,43 Therefore, these DESs could be potentially used as immersion fluid in mineralogical studies and related fields50 with the possibility of tuning the values to the desired level by varying the amount of HBDs or their ratios. 3.9. Formation of Aqueous Biphasic Systems. The binodal curves were obtained for synthesized DESs composed of [BTMA]Cl as the HBA and EG, DEG, TEG, and GLY as the HBDs. The salting out agent used in the current study is K3PO4. The experimentally obtained binodal curves were compared with those of pure [BTMA]Cl.51 The weight fraction data is presented in Table S7, and binodal curves are given in Figure 12. The binodal curve separates the biphasic region from the monophasic region. The closer the binodal curve is to the origin, the higher its ability to form phases. As can be observed from Figure 12, the position of binodal curves obtained for DES indicates that the phase formation ability of DES is lower in comparison to pure benzyltrialkylammonium salts. Further, it can be seen from binodal curves that, as the concentration of inorganic salt increased, binodal curves of all DESs started to merge. This suggests that, at lower concentrations of DESs, the phase behavior essentially represents that of the HBA irrespective of the HBD.26 Though initial reports have treated DES based ABS as ternary systems, recent reports have proved them to be pseudo ternary systems, since the integrity of the DES is destroyed due to disruption of hydrogen bonding.52 Components of the DES on isolation get preferentially solvated by the excess of water present. It was also shown that DES based ABS has potential extraction capabilities for dyes. Currently studied DES systems can also be further explored for their extraction capabilities by thorough analysis of their phase behavior.

(12)

where nD is the refractive index of liquids, T is the absolute temperature in K, and di are fitting parameters. The fitting parameters of the equation have been reported in Table 12. The high R2 (>0.9999) as obtained from the above equation shows that nD varies very regularly with temperature and their values at any intermediate temperature can be estimated with acceptable accuracy. The nD values observed for the studied DES systems are comparatively higher than common solvents like ethanol, acetone, chloroform, etc., but similar to some ILs49 L



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AUTHOR INFORMATION

Corresponding Author

*Phone: +91 44 2257 4248. Fax: +91 44 2257 4202. E-mail: [email protected]. Web: http://www.iitm.ac.in/info/fac/ gardas. ORCID

Somenath Panda: 0000-0003-3045-7370 Ramesh L. Gardas: 0000-0002-6185-5825 Author Contributions †

A.B. and S.P. have contributed equally to this work.

Funding

The authors are grateful to IIT Madras for financial support, through Institute Research and Development Award (IRDA): CHY/15-16/833/RFIR/RAME. Notes Figure 12. Phase diagrams for systems composed of DESs based on [BTMA]Cl series and K3PO4 at 298.15 K and at atmospheric pressure, p = 0.1 MPa. Black ■, BTMEG; orange ▲, BTMDEG; blue ●, BTMTEG; teal ◆, BTMGLY; green ⬢, pure [BTMA]Cl.

The authors declare no competing financial interest.

4. CONCLUSIONS In this work, eight different DESs based on benzyltrimethylammonium and benzyltributylammonium chloride as HBAs have been successfully synthesized. The experimental densities, speed of sound, refractive indices, conductivities, and viscosities of all DESs formed by HBA:HBD in the molar ratio 1:3 have been measured in the temperature range from 283.15 to 343.15 K under atmospheric pressure. The temperature dependence of the physical properties was further analyzed by fitting the experimental data to appropriate equations and compared to already reported DESs and ILs. The density, speed of sound, viscosity, and refractive index data showed a common detrimental trend with increasing temperature, whereas conductivity showed the opposite trend. Additionally, the derived isentropic compressibility, βs, and volumetric expansion, α, values were found to be in a similar range of other reported values of DESs. Transport properties such as viscosity and conductivity were found to follow the Arrhenius equation to a reasonable limit. The study on rheological behavior indicated internal associated structure, which was further supported by the H-bonding interactions as seen by FTIR spectra. Finally, the phase behavior of DES has shown promising biphasic formation abilities and these could further be explored for applications in extraction of bioactive compounds. The detailed study on the thermophysical properties will help in better understanding of the interaction in DESs as well as for finding possible applications in various fields.

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ACKNOWLEDGMENTS The authors acknowledge V. P. Priyanka for the help in a few experiments on aqueous biphasic systems. REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00213. Tables of Tg of BTMGLY, IR spectral regions, values of thermal expansivity, intermolecular free length, isentropic compressibility, fitting parameters of the Arrhenius equation for viscosity values, experimental weight fraction for pseudo ternary systems, plots of phase diagram, thermogravimetric analysis, and FT-IR spectra of [BTBA]Cl based DES series (PDF) M



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