Molar Heat Capacity of Selected Type III Deep Eutectic Solvents

Mar 7, 2016 - In this work, the molar heat capacity of type III deep eutectic solvents (DESs) for three quaternary salts was investigated in the tempe...
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Molar Heat Capacity of Selected Type III Deep Eutectic Solvents Jamil Naser, Farouq S. Mjalli,* and Zaharaddeen S. Gano Department of Petroleum and Chemical Engineering, Sultan Qaboos University, Al Khoud, Muscat 123, Oman S Supporting Information *

ABSTRACT: In this work, the molar heat capacity of type III deep eutectic solvents (DESs) for three quaternary salts was investigated in the temperature range of 298.15−353.15 K. Several hydrogen bond donors were used with the three salts to prepare the DESs. Melting and/or glass transition points of the DESs were also measured and presented. The results showed that the molar heat capacity for all systems increased almost linearly with temperature. In addition, the results showed that molar heat capacity is directly proportional to the molar mass of the DES. The molar heat capacity for all DES systems studied in this work ranged from 219.3 to 605.9 J·mol−1·K−1. The tetrabutylammonium chloride (TBAC)−urea DES system showed the highest molar heat capacity range of 590.1−605.9 J·mol−1·K−1 while the choline chloride (ChCl)−phenol system had the lowest range of 219.3−236.8 J·mol−1·K−1. The TBAC−urea DES system had the highest melting point (300.29 K) compared to all other DES systems in this work. Melting and/or glass transition points for all studied systems ranged from 224.8 to 300.29 K. This indicated that these solvents could be used in the liquid phase at room temperature for future applications.

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INTRODUCTION In recent years, deep eutectic solvents (DESs) had gained growing attention of several research groups as a promising alternative to conventional solvents for a number of chemical and industrial applications. These applications include electrodeposition, electropolishing, and electroplating of metals,1,2 drug solubilization vehicles,3 solvents for materials processing and synthesis,4−6 solvents for enzyme-based biotransformations and biocatalytic processes,7,8 biodiesel purification,9−11 carbon dioxide capture,12,13 and deep desulfurization and dearomatization of liquid fuels.14,15 This is in part because deep eutectic solvents are more environmentally friendly than conventional solvents and are considered to be green solvents. Deep eutectic solvents share some characteristics with ionic liquids (ILs) including nonvolatility, wide liquid range, high conductivity, thermal stability, and high solvation capacity.16 In addition, DESs have several advantages over ILs as they are biodegradable, easy to prepare, and less expensive.17 Further details and more in depth information about deep eutectic solvents and their applications can be found in several detailed reviews.16,18,19This work is focused on type III DESs. Type III DESs can be prepared by mixing a quaternary ammonium salt with a hydrogen bond donor (HBD) to form an eutectic mixture, which has a substantially less melting point than the salt and the HBD.20 The HBD forms a complex with the simple anion of the salt; this results in a reduction of the lattice energy in the system and substantial depression in the freezing point.21 For the potential use of these DESs in large scale applications, there is an evident need for the availability of physical and thermodynamic properties of these solvents that would be necessary for the evaluation of the DESs for these © XXXX American Chemical Society

applications. A considerable effort of several research groups has already been made on measuring physical properties including density, viscosity, conductivity, surface tension, and refractive index for several well-known as well as new emerging DES systems.22−29 However, fundamental thermodynamic properties of DESs such as molar heat capacity are rather scarce in the current literature. To the best of the authors’ knowledge, the only work available in the literature on molar heat capacity of DESs is the measurement of the molar heat capacity of two ammonium-based DESs, N,N-diethylethanolammonium chloride−glycerol and N,N-diethylethanolammonium chloride−ethylene glycol, and their aqueous solutions,30 and three choline chloride-based DESs, reline, ethaline, and glyceline, and their binary mixtures with water.31 Thus, this presented work is aimed at adding a new contribution to the database of this property for DES solvents as essential data for thermal related applications of these solvents. The molar heat capacity and melting point or glass transition point are measured and reported in this work. The heat capacity measurement for all systems was done using differential scanning calorimetry (DSC) over a temperature range of 298.15−353.15 K. Three different quaternary ammonium salts with an array of different hydrogen bond donors were used in these systems. The three used salts are choline chloride, tetrabutylammonium chloride (TBAC), and methyltriphenylphosphonium bromide (MTPB). The hydrogen bond donors used with these salts are triethylene glycol (TEG), fructose, Received: November 21, 2015 Accepted: February 18, 2016

A

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baseline calibration and cell constant/temperature calibrations using indium metal were performed on the calorimeter. The calibration yielded a cell constant of 1.014. In determining the heat capacity of the DES the modulated DSC (MDSC) technique was used. In this technique, the difference in heat flow between a sample and the reference as a function of time and temperature is measured similar to the standard procedure. However, in the MDSC a sinusoidal heating ramp is used for the heating profile. This method of measurement eliminates the need to repeat the measurement as in the standard method. In order to achieve reliable DSC thermograms, the experimental parameters should be carefully selected and tested. For example, the period of modulation, which is a crucial measurement parameter, needs to be carefully selected. It is calculated as

glucose, malonic acid, citric acid, oxalic acid, and phenol; they were used with choline chloride salt. Glycerol, urea, ethylene glycol, malonic acid, and triethylene glycol were used with TBAC. Malonic acid, glycerol, and ethylene glycol were used with methyltriphenylphosphonium bromide. For each of these DES systems, a screening process was conducted to determine the most practical ratio at which the DES exists as a liquid at room temperature conditions. The salt:HBD ratio of the system was selected for further measurements of the molar heat capacity.

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EXPERIMENTAL METHODS Chemicals Used. All chemicals used in this work were purchased from Merck Co. (Merck Chemicals, Darmstadt Germany) with at least 99% purity and used as supplied by the manufacturer without further purification. Table 1 shows a

⎛ P ⎞ ⎟ Tamp = Hr ⎜ ⎝ 120π ⎠

Table 1. Chemical Samples Descriptions chemical namea TEG D-fructose D-glucose malonic acid citric acid oxalic acid phenol glycerol urea EG CHCl TBAC MTPB

source Merck Merck Merck Merck Merck Merck Merck Merck Merck Merck Merck Merck Merck

Chemicals Chemicals Chemicals Chemicals Chemicals Chemicals Chemicals Chemicals Chemicals Chemicals Chemicals Chemicals Chemicals

initial mole fraction purity

purification method

0.99 0.98 0.98 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.98 0.98 0.98

none none none none none none none none none none none none none

where Tamp = maximum temperature amplitude for heating (°C), Hr = average heating rate (°C/min), and P = period (s). Hence at any certain heating rate, period (P) needs to be selected in a way to achieve a useful temperature amplitude. A reference table between the heating rate and the modulation period is provided by the calorimeter manufacturer to aid in the proper selection process. The mass of all samples was measured using a SHIMADZU balance (model AUW220D), which is equipped with an internal calibration procedure and has an accuracy of 0.01 mg. The average standard uncertainty in the molar heat capacity measurements based on repeated measurement of the standard indium sample was estimated to be around 1%. For the DSC temperature measurements repeatability was estimated to be 0.2 °C. In a typical heat capacity measurement procedure, an accurately measured mass of 15−30 mg of the DES is encapsulated in an airtight Tzero aluminum hermatic lid pan to prevent any losses from the sample during the test. The sample pan and a blank (empty pan) were placed on separate raised platforms within the DSC chamber. A temperaturecontrolling program was used to perform a controlled cooling and heating process. The sample and reference pans were first cooled to −70 °C and held there for 5 min to allow for temperature equilibration with the chamber. After the 5 min equilibrium stage, a controlled heating process at a fixed heating rate of 2 °C/min up to 85 °C was performed using a modulation of ±0.318 °C for every 60 s in order to correspond with the selected heating rate.

a

TEG, triethylene glycol; EG, ethylene glycol; CHCl, choline chloride; TBAC, tetrabutyle ammonium chloride; MTPB, methyltriphynele phosphonium bromide.

chemical sample description table. Hygroscopic chemicals were dried overnight in a vacuum oven operated at 80 °C before use to minimize contamination with atmospheric moisture. DES Preparation. DESs samples were prepared by mixing various combinations of salts and hydrogen bond donors at different molar ratios in an incubator shaker (Brunswick Scientific Model INNOVA 40R) operated at 270 rpm and 353.15 K for 2 h. The studied DES components were dried in a vacuum oven overnight in order to get rid of any possible traces of moisture in the samples. The mixtures were shaken until a homogeneous clear solution was obtained with no visible precipitate or suspended solids. All DES samples were prepared at atmospheric pressure and under tight control of moisture content and also kept in airtight vials for storage in a moisture controlled desiccator. The water content of samples of the prepared DESs was measured by Karl Fisher titration method. The average measured moisture content was less than 0.05 wt %. Molar Heat Capacity Measurement. Molar heat capacity measurements were conducted using a differential scanning calorimeter from TA Instruments (model TA-Q20) equipped with an autosampler and a refrigeration cooling system (RSC90) capable of cooling to as low as −90 °C. Nitrogen gas with a purity of 99.999% was used to purge the system at a flow rate of 50 mL/min. Prior to conducting experiments,

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RESULTS AND DISCUSSION Table 2 shows the set of DES systems studied in this work, the ratio of salt:HBD used, and the melting and/or glass transition points. Melting and/or glass transition points for the investigated DES systems were measured using the DSC instrument as discussed in Experimental Methods. All melting and/or glass transition points for studied systems were measured in this work, with the exception of three systems with already reported melting points. The melting points of these three systems were listed in the table as reported. The melting points of all of the systems ranged from 241.28 to 300.14 K; however, some of the systems did not show a melting and/or a glass transition point at the used salt:HBD ratio. The aforementioned values of the measured melting points indicate that these DES systems will exist in the liquid phase at B

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malonic acid-based system which showed very close molar heat capacity to that of the phenol-based system. The citric acidbased system had the highest values of molar heat capacity at 422−449.4 J· mol−1·K−1 which is almost double the values for the phenol system. The molar heat capacity for the rest of the choline chloride-based systems studied here was located in between these two systems and ranged from 219.3 to 356.3 J· mol−1·K−1. The molar heat capacity for some systems was more sensitive to temperature than others. The molar heat capacity of glucose was the most sensitive to temperature (with an average rate of heat capacity change of 0.52/K) while the molar heat capacity of oxalic acid was the least sensitive among the choline chloride-based systems studied here (with an average rate of heat capacity change of 0.21/K). The values of the molar heat capacity for all of the choline chloride-based systems ranged from 219.3 to 449.4 J·mol−1·K−1. These values are higher than those reported by ref 23 for commercial grade DESs of the same salt but different HBDs. The reported values for the reline DES is 181.4−190.8 J·mol−1·K−1, for ethaline DES is 190.8− 205.6 J·mol−1·K−1, and for glyceline is 237.7−254.3 J·mol−1· K−1. Figure 2 shows the variation of molar heat capacity with temperature for the TBAC-based DES system over the studied

Table 2. DES Systems Studied in This Work and Their Melting and/or Glass Transition Points at Pressure p = 0.1 MPaa salt

HBD

molar ratio (salt:HBD)

molar mass

Tm (K)

Tg (K)

ChCl

TEG fructose glucose malonic acid citric acid oxalic acid phenol glycerol urea EG malonic acid TEG malonic acid glycerol EG

1:2 2:1 2:1 1:1 1:2 1:2 1:3 1:5 4:1 1:3 1:3 1:1 2:3 1:3 1:4

231.82 153.13 153.13 121.84 174.62 130.59 105.48 123.06 234.34 105.24 147.52 214.04 205.32 198.14 121.10

no Tm [283.15]21 [288.15]22 no Tm no Tm no Tm [253.10]23 no Tm 300.29 241.28 no Tm 287.24 no Tm no Tm no Tm

243.94 no Tg no Tg no Tg 249.16 230.12 no Tg 249.12 no Tg no Tg 247.74 no Tg 238.77 255.58 224.80

TBAC

MTPB

a Standard uncertainty, u, in temperature measurement is u(T) = 0.2 K and standard uncertainty in pressure is u(p) = 1 kPa.

room temperature, which is one of the great advantages for these solvents for future applications. Molar Heat Capacity. The molar heat capacity for the DES systems was measured using a DSC instrument as explained in Experimental Methods. The values of molar heat capacity were measured in the temperature range of 298.15−353.15 K, during which the DES is in the liquid phase. This is the range at which these solvents are expected to be used in potential future applications. Figure 1 shows variation of the molar heat capacity

Figure 2. Molar heat capacity variation with temperature for tetrabutylammonium chloride-based DESs.

temperature range of 298.15−353.15 K. As shown in Figure 2, urea had the highest molar heat capacity values among the TBAC-based DES systems of 590.1−605.9 J·mol−1·K−1 while glycerol had the least values of molar heat capacity of 281.2− 310.9 J·mol−1·K−1 followed by ethylene glycol with 288.3− 312.6 J·mol−1·K−1 which had very close molar heat capacity to the glycerol system. The malonic acid data showed some curvature, which indicated that the rate of change of molar heat capacity with temperature decreased as the temperature increased for this system. Among these systems, the malonic acid- and the TEG-based DES molar heat capacities were the most sensitive to temperature while that of the urea-based DES was the least. The molar heat capacities for all of the TBACbased systems ranged from 281.2 to 605.9 J·mol−1·K−1. The TEG system showed an intermediate range of molar heat capacity values with clear gap from the highest and lowest systems of this group. Figure 3 shows the variation of molar heat capacity with temperature for the MTPB-based DES systems using malonic

Figure 1. Molar heat capacity variation with temperature for choline chloride-based DESs.

with temperature for the choline chloride-based DESs with different hydrogen bond donors. As shown in Figure 1, the molar heat capacity of all systems increased almost linearly with temperature in the studied temperature range of 298.15− 353.15 K. This behavior is expected from a thermodynamic point of view and had been reported for other DES systems investigated by Leron et al.29,30 The phenol-based DES system had the least molar heat capacity of 219.3−236.8 J·mol−1·K−1 among the choline chloride-based systems, followed by the C

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temperature decreased with temperature increase. This behavior was not observed for the other DESs, which showed more linear change of molar heat capacity with temperature. The molar heat capacity for all of the MTPB-based systems ranged from 237.6 to 354.2 J·mol−1·K−1. Measured values of molar heat capacity for the choline chloride, MTPB, and tetrabutylammonium chloride DES systems are shown in Tables 3−5 respectively. The molar heat capacity values were measured at 2.5 K intervals from 298.15 to 353.15 K. Figure 4 depicts a visual comparison of the reported Cp values at 303.15 K with the studied DESs in this work. The average molar heat capacity value for all DESs shown in the figure is 288.07 J·mol−1·K−1. The values of the molar heat capacity of the last five reported DESs fall in the same range of the newly reported DES data in this work. However, it is clear that the DESs (ChCl:citric acid) and (TBAC−TEG) have noticeably higher molar heat capacity values. Figure 5 shows the molar heat capacity data of some ionic liquids reported by another research group.33 The average Cp value for these ILs is 684.56 J·mol−1·K−1. This level of Cp values is more than double thos of the DESs average values reported in this work. The molar heat capacity is an intrinsic property of the substance and its value depends solely on the number of motion degrees of freedom that are available to the particles in the substance. Translational, vibrational, and rotational are examples of such degrees freedom. Bonds between the different atomic species in the molecules as well as the newly formed hydrogen bonds between DES components may have different capabilities for storing potential energy without increasing the kinetic energy of the atoms and consequently affecting the overall capacity of the DES structure to store energy. Bonds that do not allow storage of thermal energy cause loss of some

Figure 3. Molar heat capacity variation with temperature for methyltriphenylphosphonium bromide-based DESs.

acid, glycerol, and ethylene glycol as HBDs. Over the studied temperature range of 298.15−353.15 K, all DESs showed a steady increase in molar heat capacity with temperature. The malonic acid and glycerol DES systems had a high range of molar heat capacity of 328.5−354.2 J·mol−1·K−1 while the ethylene glycol system had a significantly less range of molar heat capacity of 237.6−256.7 J·mol−1·K−1. The molar heat capacity of the three DESs in this group were of comparable sensitivity to temperature. The malonic acid system here again showed similar curvature as observed in the tetrabutylammonium chloride-based DES system discussed earlier, which indicated that the rate of change of molar heat capacity with

Table 3. Experimentally Measured Molar Heat Capacities (J·mol−1·K−1) for the Choline Chloride-Based DESs for Different HBDs at Pressure p = 0.1 MPaa T (K)

TEG

fructose

glucose

malonic acid

citric acid

oxalic acid

phenol

298.15 300.65 303.15 305.65 308.15 310.65 313.15 315.65 318.15 320.65 323.15 325.65 328.15 330.65 333.15 335.65 338.15 340.65 343.15 345.65 348.15 350.65 353.15

299.0 300.3 301.5 302.8 303.7 304.5 304.7 304.6 304.6 304.3 304.3 304.9 305.8 306.7 307.4 308.6 309.4 309.9 310.0 311.8 312.2 312.4 314.6

311.5 313.8 316.7 319.4 322.0 324.2 326.2 328.3 330.5 332.6 334.1 334.9 335.8 336.1 335.8 335.7 335.2 334.7 335.2 335.8 336.6 337.7 340.1

327.5 329.1 330.7 332.4 334.5 336.7 338.6 340.6 342.5 344.5 346.0 347.2 348.4 349.5 350.6 350.9 351.4 352.0 352.6 353.8 354.2 354.4 356.3

226.9 227.6 228.3 228.8 229.4 230.2 230.6 231.1 231.7 232.3 232.7 233.2 233.8 234.3 234.9 235.8 237.0 238.0 238.4 238.9 239.5 239.9 240.8

422.0 422.9 424.0 425.2 426.1 427.2 428.7 429.8 430.6 431.9 433.0 433.5 435.4 436.7 437.5 438.6 439.7 440.6 442.1 446.1 447.2 446.8 449.4

270.8 271.8 272.1 272.7 273.6 274.6 275.4 276.3 276.8 277.0 277.1 277.4 277.8 278.3 278.7 279.6 280.0 280.6 281.2 281.7 281.9 282.2 282.9

219.3 220.3 221.2 222.3 223.3 224.2 225.1 225.8 226.6 227.1 227.9 228.6 229.3 230.2 231.1 231.9 232.6 233.1 233.5 234.3 234.5 235.0 236.8

a Standard uncertainty, u, in temperature measurement is u(T) = 0.2 K, standard uncertainty in pressure is u(p) = 1 kPa, and relative standard uncertainty in molar heat capacity measurement is ur(Cp) = 1%.

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Table 4. Experimentally Measured Molar Heat Capacities (J· mol−1·K−1) for the TBAC-Based DESs for Different HBDs at Pressure p = 0.1 MPaa T (K)

glycerol

298.15 300.65 303.15 305.65 308.15 310.65 313.15 315.65 318.15 320.65 323.15 325.65 328.15 330.65 333.15 335.65 338.15 340.65 343.15 345.65 348.15 350.65 353.15

281.2 282.6 283.9 285.4 286.9 288.1 289.4 290.7 292.0 293.4 294.5 296.1 297.4 298.9 300.4 301.9 303.2 304.6 305.7 306.9 307.8 308.6 310.9

urea

EG

malonic acid

TEG

590.1 593.1 598.0 601.0 602.6 600.6 601.5 602.9 601.7 603.0 604.9 605.1 603.2 604.0 604.4 605.2 604.8 605.4 605.9

288.3 289.6 291.0 292.2 293.2 294.5 295.8 296.8 297.9 299.5 300.5 301.2 302.4 303.5 304.8 305.9 306.9 307.7 308.5 309.1 309.6 310.6 312.6

299.8 302.6 305.7 309.4 313.3 316.9 320.6 323.7 324.9 328.8 333.3 335.9 337.5 339.2 339.5 340.0 340.2 340.6 341.1 341.2 341.7 342.0 342.8

445.0 447.8 448.9 450.6 452.3 454.2 456.1 457.4 458.7 460.2 461.9 463.0 464.5 466.2 467.7 469.2 471.1 472.6 473.7 475.2 475.6 476.0 479.7

Figure 4. Molar heat capacity of the studied DESs at 303.15 K as compared to some reported systems.

a Standard uncertainty, u, in temperature measurement is u(T) = 0.2 K, standard uncertainty in pressure is u(p) = 1 kPa, and relative standard uncertainty in molar heat capacity measurement is ur(Cp) = 1%.

Table 5. Experimentally Measured Molar Heat Capacities (J· mol−1·K−1) for the MTPB-based DESs for Different HBDs at Pressure p = 0.1 MPaa T (K)

malonic acid

glycerol

EG

298.15 300.65 303.15 305.65 308.15 310.65 313.15 315.65 318.15 320.65 323.15 325.65 328.15 330.65 333.15 335.65 338.15 340.65 343.15 345.65 348.15 350.65 353.15

336.9 338.0 339.2 340.0 341.1 342.1 343.1 343.9 345.2 346.0 346.6 347.4 348.4 350.1 350.3 350.7 351.3 351.5 351.9 352.1 352.5 354.0 354.2

328.5 329.6 330.5 331.6 332.7 333.7 334.8 335.8 336.5 338.1 339.4 339.6 340.8 341.8 342.6 343.5 344.6 345.7 346.7 347.8 348.0 348.3 350.2

237.6 238.6 239.4 240.8 241.6 242.4 243.4 244.5 245.2 246.1 247.2 247.9 248.9 249.8 250.8 251.8 252.7 253.5 254.3 255.0 255.5 255.0 256.7

Figure 5. Reported molar heat capacity of some ILs at 303.15 K.33

of the degrees of freedom and therefore a decrease in molar heat capacity.32 Hence, to explain the variation of molar heat capacity among different DESs as well as compared to ILs, a thorough quantum mechanics study is suggested as a future work. Figure 6 shows the molar heat capacities of the studied DESs as well as those for reported ILs33 as a function of molar mass. The molar heat capacity for both DESs and ILs exhibited an approximately linear relationship with their corresponding molar mass. This behavior is expected since the number of translational, vibrational, and rotational energy storage modes depends on the molar mass of the molecules. The experimental molar heat capacity of the DESs was correlated with a general temperature-dependent four parameter empirical equation of the following form: a Cp = a1 + a 2T + a3T 2 + 42 (1) T −1 −1 where Cp (J·mol ·K ) is the molar heat capacity, T is temperature (K), and ai are the model parameters. This form of empirical equation is commonly used to report incompressible liquids heat capacity data and is suitable for implementation in well-known process simulator packages. The changes in enthalpy and entropy relative to the reference temperature

a Standard uncertainty, u, in temperature measurement is u(T) = 0.2 K, standard uncertainty in pressure is u(p) = 1 kPa, and relative standard uncertainty in molar heat capacity measurement is ur(C,p) = 1%.

E

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where n is the number of experimental measurements and m is the number of model parameters. Table 6 shows the heat capacity model parameters, the average absolute relative deviation (ARD), R2, and standard deviation for the studied DESs. The ARDs between the experimental and models calculated values were expressed as ARD =

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(Tf) can then be calculated using the same heat capacity parameters as

= a1(T − Tf ) +

∫T

Cp dT f

a a2 2 (T − Tf 2) + 3 (T 3 − Tf 3) − a4(T −1 − Tf −1) 2 3

(2) ΔST = ST − STf =

∫T

Cp

f

T

dT

⎛T ⎞ a a = a1 ln⎜⎜ ⎟⎟ + a 2(T − Tf ) + 3 (T 2 − Tf 2) − 4 (T −2 − Tf −2) T 2 2 ⎝ f⎠

(3)

The heat capacity model of eq 1 was used to regress the experimental heat capacity data. A total of 13 experimental values was used for each DES under consideration. The exp regression of experimental heat capacity data (Cp,I ) was performed using an optimization algorithm based on the genetic algorithms technique with an objective function of the following form: n pred 2 1/2 σ = [∑ (C pexp , i − Cp , i ) /(n − m)]

(4)

i=1

∑

C pexp ,i

i=1

(5)

gave very efficient values of less than close to unity. In spread around the by the very small

CONCLUSION

Molar heat capacity for type III deep eutectic solvents of three quaternary ammonium salts and several hydrogen bond donors was studied and presented in this work. The molar heat capacity for 15 combinations of these DESs in the range of 298.15−353.15 K was measured using DSC and presented. In addition, the melting point and/or glass transition point was measured and presented for most of the systems. The molar heat capacities for all of the studied systems increased with temperature in the range of (298.15−353.15 K. In addition, the molar heat capacities for the DESs increased with increasing molar mass. This behavior is in agreement with ionic liquids investigated by other researchers. Citric acid had the highest molar heat capacity for the choline chloride system, while phenol had the lowest. Urea and malonic acid had the highest values of molar heat capacity for TBAC and MTPB systems, respectively, while glycerol and ethylene glycol had the least values for the TBAC and MTPB systems, respectively. The melting and/or glass transition points for the systems ranged from 224.8 to 300.29 K. This indicated that all these solvents could be used in the liquid phase at room temperature for potential applications.

T

T

pred C pexp , i − Cp , i

n

In general, eq 1 for the heat capacity predictions. This is indicated by the ARD 0.5% and the regression coefficient (R2) addition, the experimental data are evenly regressed model predictions as indicated values of the standard deviation (SD).

Figure 6. DESs molar heat capacity (circles) as well as some reported ILs data (triangles) at 303 K as a function of molar mass.

ΔHT = HT − HTf =

100 n

Table 6. Molar Heat Capacities (J·mol−1·K−1) Model Parameters for the Studied DESs a1 ChCl: TEG ChCl: fructose ChCl: glucose ChCl: malonic acid ChCl: citric acid ChCl: oxalic acid ChCl: phenol TBAC: glycerol TBAC: urea TBAC: EG TBAC: malonic acid TBAC: TEG MTPB: malonic acid MTPB: glycerol MTPB: EG

8.552 1.214 −8.906 8.295 3.869 2.930 1.274 −1.387 3.226 −6.868 −6.344 8.192 −1.214 −5.446 −2.300

× × × × × × × × × × × × × × ×

a2 1003 1004 1002 1002 1003 1003 1003 1003 1004 1002 1003 1002 1003 1002 1003

−3.427 −4.583 6.562 −2.695 −1.475 −1.070 −4.090 6.827 −1.257 4.296 3.121 −1.293 6.965 3.708 1.064

× × × × × × × × × × × × × × ×

a3 1001 1001 1000 1000 1001 1001 1000 1000 1002 1000 1001 1000 1000 1000 1001

4.017 5.012 −8.879 3.584 1.811 1.227 4.717 −7.366 1.403 −4.899 −3.976 1.757 −8.427 −4.064 −1.220 F

× × × × × × × × × × × × × × ×

a4 10−02 10−02 10−03 10−03 10−02 10−02 10−03 10−03 10−01 10−03 10−02 10−03 10−03 10−03 10−02

−1.427 −2.332 4.448 −1.045 −5.877 −4.984 −2.259 2.555 −5.950 1.154 7.735 −1.285 1.988 1.146 4.003

× × × × × × × × × × × × × × ×

1008 1008 1006 1007 1007 1007 1007 1007 1008 1007 1007 1007 1007 1007 1007

ARD

R2

SD

0.179 0.285 0.144 0.091 0.089 0.096 0.080 0.060 0.157 0.066 0.280 0.076 0.068 0.056 0.079

0.986 0.991 0.998 0.998 0.998 0.996 0.999 0.999 0.962 0.999 0.997 0.999 0.998 0.999 0.999

0.673 1.125 0.577 0.265 0.565 0.314 0.247 0.234 1.152 0.280 1.137 0.484 0.333 0.275 0.268

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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.5b00989. DSC Thermograms of the results reported in Table 2 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

The authors appreciate the financial support of The Research Council and Sultan Qaboos University, Muscat, Oman, under Project RC/ENG/PCED/12/02. Notes

The authors declare no competing financial interest.

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NOMENCLATURE ARD average absolute relative deviation Cp molar heat capacity (J·mol−1·K−1) Cexp experimental molar heat capacity p,I Cpred model predicted molar heat capacity p,I HT enthalpy at any temperature HTf enthalpy at reference temperature m number of model parameters N number of experimental measurements T temperature (K) Tf reference temperature (K) ai model parameters ΔHT changes in enthalpy ΔST changes in entropy

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DOI: 10.1021/acs.jced.5b00989 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jced.5b00989 J. Chem. Eng. Data XXXX, XXX, XXX−XXX