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Viscosities and Carbon Dioxide Solubilities of Guanidine Carbonate and Malic Acid-Based Eutectic Solvents Nouman R. Mirza, Nathan J. Nicholas, Yue Wu, Kathryn H. Smith, Sandra E. Kentish, and Geoffrey W. Stevens* Department of Chemical and Biomolecular Engineering and Peter Cook Centre for CCS Research, The University of Melbourne, Parkville, Victoria, Australia 3010 S Supporting Information *

ABSTRACT: Carbon capture and storage (CCS) is one of the technologies needed to reduce anthropogenic emissions of CO2 in the atmosphere. Protic ionic liquids (PILs) are potential nonaqueous solvents that can combine the benefits of ionic liquids (ILs) and deep eutectic solvents (DESs) to make the carbon capture process more sustainable. In the present study, the viscosities of six eutectic-based solvents have been measured within a temperature range of 303.2−330.2 K and have been modeled using a Vogel−Fulcher− Tammann (VFT)-type equation. The results showed that guanidium malate-based eutectic solvents have significantly lower viscosities than most other deep eutectic solvents. Additionally, CO2 solubilities in three different guanidium-based eutectic solvents have been measured within a temperature range of 313.2−333.2 K and pressures of up to 200 kPa. Henry’s law constants of CO2 in pure solvents were obtained by modeling the solubility in mixed (aqueous) solvents. The values obtained for Henry’s law constants of CO2 in the studied guanidium-based eutectic solvents were found to lie within a range of 1.3−24 MPa. The values of the Gibbs free energy and the dissolution enthalpy and entropy showed that the CO2 absorption is exothermic, and after CO2 absorption, the entropy of the systems decreased.

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INTRODUCTION Global anthropogenic emissions of CO2 are increasing exponentially, causing the world to heat up.1−3 According to the U.S. Inventory of Greenhouse Gas Emissions and Sinks, power plants contribute to more than 40% of the total CO2 emissions in the U.S.4 To reduce global warming, these anthropogenic emissions have to be reduced.5 Carbon capture and storage (CCS) consists of a set of technologies that can significantly reduce the CO2 emissions from power plants.6 The most mature technology among a range of different capture technologies is the absorption of CO2 in a solvent. Some of the solvents used to absorb CO2 include physical solvents such as rectisol and selexol as well as chemical solvents including monoethanolamine (MEA) and methyldiethylamine (MDEA). Amines (MEA and MDEA) are the most widely used solvents to absorb CO2 from the flue gas emitted by power plants. Although these solvents have a high CO2 capture capacity and good kinetics, they are volatile and can degrade at high temperatures. Because of these inherent major drawbacks, a lot of research has been focused on finding alternative solvents to capture CO2 from flue gas. In recent years, much of the research focus has been on eutectic-based solvents. Such solvents include ionic liquids (ILs), deep eutectic solvents (DESs), and their corresponding variants. ILs can be task-specific, tailor-made chemicals that consist of cations and anions. By altering the charge balance between the cations and anions, the physicochemical properties of ILs can be adjusted. Besides their good CO2 solubility and © XXXX American Chemical Society

kinetics, the commercial application of these solvents is hindered by high viscosity, toxicity, poor biodegradability, vulnerability to contamination, cost, and complex manufacturing processes.7−12 Deep eutectic solvents (DESs) have emerged as potential solvent media that could help to address some of these issues in ILs. A DES is a combination of a salt and one or more organic compounds, which when mixed in a certain ratio form a eutectic. Most of the DESs are based on a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA), which not only can affect the physicochemical properties of the solvents but also can have a significant influence on the way these interact with other molecules. Although most of the DESs are green in nature and have physicochemical properties similar to those of ILs, their ability to absorb CO2 is poorer than most of the ILs used for the same purpose.13,14 Previously, researchers have combined choline chloride with urea, malic acid, ethylene glycol, 1,4-butanediol, 2,3-butanediol, 1,2propanediol, phenol, triethylene glycol, lactic acid, glycerol, and levulinic acid (or furfuryl alcohol) in various molar ratios and have studied CO2 solubility in the resulting DESs at moderate temperature (up to 80 °C) and high pressure ranges (up to 6 MPa).13,15−23,43 Later studies24,25,44 were conducted to study the effect of the addition of water on CO2 solubility in Received: July 29, 2016 Accepted: November 10, 2016

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DOI: 10.1021/acs.jced.6b00680 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. Names, Sources, and Purities of Chemicals Used in the Current Study chemical

physical state

source

assay

CAS registry number

guanidine carbonate DL-malic acid choline chloride ethylene glycol urea L-arginine glycerol water (Millipore Milli-Q) carbon dioxide nitrogen

solid solid solid liquid solid solid liquid liquid gas gas

Sigma-Aldrich Pty. Ltd., Australia Sigma-Aldrich Pty. Ltd., Australia Sigma-Aldrich Pty. Ltd., Australia Sigma-Aldrich Pty. Ltd., Australia Sigma-Aldrich Pty. Ltd., Australia Sigma-Aldrich Pty. Ltd., Australia Sigma-Aldrich Pty. Ltd., Australia Merck KGaA, Germany Coregas Pty. Ltd., Australia Coregas Pty. Ltd., Australia

99% 99% ≥98% 99.8% 99% ≥98.5% ≥99% resistivity = 18.2 MΩ·cm 99.999% 99.999%

593-85-1 6915-15-7 67-48-1 107-21-1 57-13-6 74-79-3 56-81-5 7732-18-5 124-38-9 7727-37-9

ethaline, and malinine.14 The Vogel−Fulcher−Tammann (VFT) model has been used to describe the temperature dependence of the measured viscosities.

choline chloride-based DESs with ethylene glycol, glycerol, malonic acid, and urea. The results showed that water acts as an antisolvent and decreases the CO2 solubility in these solvents. To combine the benefits of ILs and DESs and find a solution to their drawbacks, a possible alternative is to use protic ionic liquids (PILs) to absorb CO2.26 PILs are a subset of ionic liquids in which the cation includes an available proton. Their physicochemical properties can be tuned by adjusting both the charge balance and the molar ratios of cations and anions.27 Like ILs, these solvents can have a higher CO2 solubility, are similar to most of the DESs, can be green, and are manufactured easily in the laboratory with high purity. Viscosity is an important physical property that determines the energy requirements during the production and transportation of materials. It can significantly affect the masstransfer process, lowering the CO2 absorption kinetics and decreasing the solvent’s CO2 capture capacity. Researchers have determined the viscosities of various eutectic-based solvents and have found that these have order of magnitude higher viscosities than water.28 The viscosities of ILs at room temperature have a lower bound value of around 10 mPa·s and continue to values higher than 500 mPa·s.28−31 PILs follow a similar trend and exhibit a viscosity range of around 11−1000 mPa·s at room temperature.27 Similarly, under comparable thermal conditions, DESs are found to have viscosities in the range of 37−85 000 mPa·s.32 In the present study, two parent PILs (PPILs) have been prepared by mixing guanidine carbonate, malic acid, and ethylene glycol in 2:1:9.5 (PPIL-1) and 2:1:16.3 (PPIL-2) molar ratios, respectively. Guanidine carbonate contains abundant amine groups that have significant affinity for CO2. A 2:1 molar ratio between guanidine carbonate and malic acid is needed to balance the charges and hence form a eutectic solvent, whereas the amount of ethylene glycol is adjustable according to the desired viscosity of the solvent. Three variants of each PPIL were prepared by adding quantities of water. The water content was varied by the consideration that these solvents would eventually face wet flue gas to capture CO2 and therefore would contain some moisture within a CO2 capture facility. Experiments have been performed to determine the CO2 solubility in these PILs within a temperature range of 313.2 to 333.2 K and a pressure of up to 200 kPa. After determining the CO2 solubility in these water-containing solvents, the Henry’s law constants in pure PILs were calculated on the basis of the mixture rules.33 The Gibbs free energy, enthalpy of dissolution, and entropy of dissolution were also calculated. The viscosities of nine PILs were determined within the temperature range of 303.2−333.2 K and compared to that of three other DESs previously reported, namely, reline,

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EXPERIMENTAL SECTION Materials. Nitrogen and carbon dioxide gas, having purities of greater than 99.99 wt %, were supplied by Coregas Pty. Ltd., Australia. Analytical-grade guanidine carbonate, malic acid, choline chloride, urea, glycerol, and ethylene glycol were obtained from Sigma-Aldrich Pty. Ltd., Australia and were used without any further purification. Table 1 lists the names, sources, and purities of chemicals used in the current study, whereas the chemical structure of L-arginine used is shown in Figure S1. Each eutectic solvent was prepared by mixing the appropriate mass ratios of the precursors and then heating the mixture mildly (at ca. 70 °C) overnight or until it formed a clear, singlephase solution. All prepared solvents were stored in a dry, thermally controlled oven at 40 °C. Initially, two parent protic ionic liquids (PPIL) were prepared by mixing guanidine carbonate, malic acid, and ethylene glycol in 2:1:9.5 (PPIL-1) and 2:1:16.3 (PPIL-2) molar ratios, respectively. Later, two more variants of PPIL-1 and PPIL-2 were prepared by taking 90 wt % of each and separately mixing with 10 wt % arginine (hereafter called PPIL3 and PPIL-4, respectively). Reline was prepared by mixing choline chloride and urea in a 1:2 molar ratio, respectively, and ethaline was prepared by mixing choline chloride and ethylene glycol in a 1:2 molar ratio, respectively. Malinine was prepared by mixing choline chloride, malic acid, and ethylene glycol in a 1.3:1:2.2 molar ratios, respectively. Postcombustion flue gas from coal-fired power stations will contain moisture.34 Therefore, it is important to assume that these solvents, if used to capture carbon dioxide from a wet flue gas, would eventually contain some water during their operation. Considering this aspect of carbon capture with the studied solvents, three variations of each PPIL were prepared by varying the water content in them. These variants are identified by using a nomenclature that contains the notation of the parent PPIL, followed by a number that represents the moisture content (wt %) in the variant. For example, PPIL-10.1 represents a variant of PPIL-1 containing 0.1 wt % water (with the balance being the PPIL-1). DES solvents, such as reline, were prepared and given similar nomenclature. The water content was varied from 0.1 wt % to around 20 wt % and was measured using a Metrohm Karl Fischer titrator (915 KF Ti-Touch). The names, designations, physical appearance, and corresponding water contents in various solvent variants are given in Table S1 (Supporting Information). B



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Methodology. Viscosity Experiments. The dynamic viscosity of the studied solvents was measured using a Brookfield DV-II+ Pro viscometer. The maximum relative error in the measured viscosity values is ±5.4%. The temperature was controlled by an external water bath having an accuracy of ±0.05 K. The viscometer works by driving a spindle (immersed in the fluid) through a calibrated spring. The spring deflection, which is measured with a rotary transducer, is proportional to the viscous drag of fluid on the spindle. On the basis of the settings, the viscometer directly gives the viscosity value for the fluid under observation. CO2 Solubility Experiments. The experimental rig for measuring CO2 solubility consists of a stainless steel equilibrium vessel, a buffer tank, a temperature-controlled incubator, and gas supplies (Figure 1). The buffer tank is used

in temperature, pressure, and molar ratios were 0.05 K, 2.5 kPa, and 0.0015 respectively.

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RESULTS AND DISCUSSION Temperature and Composition Dependence of Viscosity. The calibration of the viscometer was checked by measuring the viscosity of glycerol within the temperature range of 293.2−353.2 K. As shown in Figure 2, the values obtained are highly consistent with those published in a previous study,35 giving a value of 0.966 for the coefficient of determination.

Figure 1. Simplified schematic of the static VLE rig. Figure 2. Comparison of published values for the viscosity of glycerol35 with those obtained in the current work. (■) Values from the literature.35 (○) Current work.

to pressurize the equilibrium vessel to a certain level. A more detailed description of the experimental setup has been given in one of our previous studies.14 A known amount of a PIL is weighed using an A&D digital balance (model FX-400, accuracy ±0.001 g) and placed in the equilibrium vessel. The vessel is purged with N2 gas and then incubated at the desired temperature for 2 h, with a magnetic stirrer used to ensure uniform temperature inside the vessel. The vessel was then pressurized to the desired level by delivering pure CO2 from the buffer tank. The contents of the equilibrium vessel were stirred continuously at the experimental temperature for at least 20 h. Equilibrium was determined to be reached once the pressure of the vessel remained constant for 2 to 3 h. Because of the very low vapor pressure of the PILs, the gas phase was considered to be pure CO2. On the basis of this assumption, by using the initial pressure, temperature, and volume of the vessel, the moles of CO2 present in the system were calculated using the ideal gas law. Later, on the basis of the final (equilibrium) pressure reading, the volume of the vessel, and the temperature of the system, the amount of CO2 remaining in the system was calculated using the ideal gas law. The difference in the initial and final amounts of CO2 was the amount absorbed in the solvent and was calculated by using eq 1

ΔnCO2 =

ΔPV RT

In all eutectic solvents studied, the viscosity value decreased with an increase in temperature. Such trends for different variants of PPIL-1 and Reline (a DES) are shown in Figures 3 and 4, respectively, and for all other solvents, these have been included in Figures S2−S6 and Table S3.

(1)

where ΔnCO2 is the number of moles of CO2 absorbed in the solvent (mole), ΔP is the change in pressure from the initial value to equilibrium (kPa), V is the volume of the equilibrium vessel (m3), T is the experimental temperature (K), and R is the universal gas constant (8.314 J/mol·K). Standard uncertainties

Figure 3. Change in viscosity of PPIL-1 with temperature for different solvent concentrations. (■) 99.9, (●) 94, and (▲) 86 wt %. C



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Table 2. Values of Various Parameters Estimated for the Viscosity of the Studied Solvents

Figure 4. Change in viscosity of reline with temperature for different solvent concentrations. (■) 99.9, (●) 97.5, and (▲) 91.3 wt %.

Figures 3 and 4 also show that the viscosity of reline for a certain composition and temperature is higher than the corresponding value for PPIL-1. This could be attributed to the presence of a higher concentration of ethylene glycol in PPIL-1, which has a lower viscosity than the resultant eutectic solvents and therefore any presence of it in a eutectic solvent,would result in a decrease in its viscosity. The temperature dependence of viscosity for all eutectic solvents has been correlated using the VFT model as described by eq 2 B ln η = A + T − T0 (2)

solvent

moisture content (wt %)

A

B

T0 (K)

E (kJ/mol)

Reline-0.1 Reline-2.5 Reline-8.7 Ethaline-0.2 Ethaline-3.4 Ethaline-8.8 Malinine-1.6 Malinine-3.2 Malinine-8.7 PPIL-1-0.1 PPIL-1-6.0 PPIL-1-14.0 PPIL-2-0.4 PPIL-2-10.0 PPIL-2-20.0 PPIL-3-0.9 PPIL-3-6.1 PPIL-3-17.3 PPIL-4-0.14 PPIL-4-7.80 PPIL-4-13.0

0.1 2.5 8.7 0.2 3.4 8.8 1.6 3.2 8.7 0.1 6.0 14.0 0.4 10.0 20.0 0.9 6.1 17.3 0.14 7.80 13.0

3.56 2.76 0.76 0.09 0.11 −2.96 2.03 2.75 1.30 0.69 1.02 −0.81 −1.93 −0.94 −0.82 2.29 2.17 9.51 2.08 −0.48 7.38

92.77 104.17 243.44 445.47 214.75 970.04 228.12 123.41 161.13 245.92 184.56 465.11 691.70 390.94 337.68 172.68 112.16 1460.76 206.93 513.34 800.82

272.03 264.50 219.31 198.76 233.63 133.11 239.81 251.54 246.95 241.38 234.53 182.70 185.00 208.72 207.13 246.21 254.10 524.11 239.90 180.80 491.42

36.70 30.46 20.97 26.30 25.29 23.84 31.28 26.75 23.41 35.12 22.21 21.33 32.84 27.47 23.06 38.08 28.98 23.01 28.44 22.90 22.44

where η is the dynamic viscosity (mPa·s) and A, B, and T0 are empirical constants, the values of which have been found by fitting eq 2 to the experimental data (Table 2). For all eutectic solvents, the correlation resulted in a coefficient of determination value of greater than 0.99, showing a very good correlation in all cases. On the basis of these parameters, the energy for viscous flow (E) was calculated at 318.2 K using eq 3 ⎛ T ⎞2 E = RB⎜ ⎟ ⎝ T − T0 ⎠

(3)

Figure 5. Variation of reline viscosity with its composition at 318.2 K. (■) Experimental values. (---) Best fit.

where R is the universal gas constant (8.314 J/mol·K) and all other terms carry the same meaning as described in eq 2. The results show that for each solvent the value of E increases as the moisture content decreases. The viscosity of water is an order of magnitude lower than the viscosity of these solvents, so its presence reduces the viscosity at any given temperature, thus reducing the corresponding viscous flow energy (E) value. Figure 5 shows the change in viscosity of Reline with composition at a temperature of 318.2 K. As the moisture content decreases, the viscosity increases nonlinearly. Similarly, Figure 6 shows the variation in the viscosity for PPIL-3 and PPIL-4 with composition at 318.2 K. Figure 6 also shows that as the moisture content increases, the viscosity of the solvent decreases. For a certain composition, the viscosity of all three variants of PPIL-3 is higher than that of the PPIL-4’s. This is due to the smaller amount of ethylene glycol in PPIL-3 than in PPIL-4. Ethylene glycol serves as a solvent thinner and reduces the viscosity of

the solvent. Similar trends have been found for other eutectic solvents studied in this work and have been reported in Figures S7−S9. CO2 Solubility in the Eutectic Solvents. The experimental procedure used here has already been validated in one of our previous studies,14 where the CO2 solubility was measured in three different choline chloride-based deep eutectic solvents at various temperatures and pressures. To model the experimental data obtained in the current study (presented in Table S2), the Henry’s law constants of CO2 in water were first calculated on the basis of Novak’s work36 and were provided in a previous study.37 The Henry’s law constants of CO2 in various PILs containing variable quantities of water were then experimentally measured. The model proposed by O’Connel and Prausnitz,33 as represented by eq 4, was finally D



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temperature. The Henry’s law constants for CO2 for the pure eutectic solvents HCO2,PIL determined from this approach have been plotted in Figure 8.

Figure 6. Variation in viscosity for PPIL-3 and PPIL-4 with composition at 318.2 K. (●, ■) Experimental values for PPIL-3 and PPIL-4, respectively. (---) Best fit.

used to calculate the Henry’s law constants of CO2 in the mixed eutectic solvents at 50 kPa.

Figure 8. Variation of Henry’s law constant for CO2 of parent protic ionic liquids PPIL-1 (■), PPIL-2 (●), and PPIL-4 (▲) with temperature.

ln HCO2,solvent = x PIL ln HCO2,PIL + x water ln HCO2,water − a13x PILx water

(4)

The results show that as the temperature increases, the values for Henry’s law constants for CO2 increase. At any given temperature, PPIL-4 has the smallest Henry’s law constant for CO2, and PPIL-2 has the highest value. PPIL-4 contains arginine, which has significantly increased the CO2 solubility in the solvent. However, PPIL-2, which contains more ethylene glycol than PPIL-1, exhibits a higher Henry’s law constant for CO2 at any temperature. Table 3 compares the values of Henry’s law constants of various similar solvents to those obtained from the eutectic solvents studied here. Table 3 shows that the Henry’s law constant for PPIL-1 is lower than that for PIL m-2-HEAF,40 highlighting an improved performance for CO2 solubility by PPIL-1. Although the CO2 capture performance of these solvents can be increased further

where HCO2,solvent is the Henry’s law coefficient of CO2 in the water-containing solvent, xPIL is the concentration of PIL in the mixed solvent, HCO2,PIL is the Henry’s law constant of CO2 in PIL, xwater is the water concentration in the mixed solvent, HCO2,water is the Henry’s law constant for CO2 in water, and a13 is a temperature-dependent interaction parameter that has been estimated using the one-component solubility parameter method as described in previous studies.38,39 The Henry’s law constant of CO2 (HCO2,PIL) is based on the mole fraction and has units of MPa. The variation of a13 with temperature is shown in Figure 7. The results showed that the CO2 solubility in all of the eutectic solvent mixtures studied decreased with an increase in

Table 3. Comparison of Henry’s Law Constants of CO2 for Various Solvents at 313.2 K system (CO2−solvent)

type of solvent

Henry’s law constants (MPa)

CO2−m-2-HEAF CO2−[bmim][PF6]c CO2−[emim][Tf2N]c CO2−[bmmim][BF4]c CO2−furfuryl alcohol CO2−γ-valerolactone CO2−dibutyl succinate CO2−furfurylinea CO2−levulinineb CO2−reline CO2−ethaline CO2−malinine CO2−PPIL-1 CO2−PPIL-2 CO2−PPIL-4

PIL IL IL IL organic organic organic DES DES DES DES DES PIL PIL PIL

17.0640 8.2442 5.2242 9.3442 20.3241 7.8241 4.2641 33.1119 21.9119 43.3814 42.1014 45.6014 15.96 (current work) 23.57 (current work) 1.33 (current work)

a Furfuryl alcohol and choline chloride in a 3:1 molar ratio. bLevulinic acid and choline chloride in 3:1 molar ratio. cAt 323.2 K.

Figure 7. Variation of interaction parameters for PPIL-1 (■), PPIL-2 (●), and PPIL-4 (▲) as a function of temperature. E



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CONCLUSIONS In the present work, guanidium malate-based eutectic solvents have been studied for their viscosity and CO2 solubility as a function of temperature. The viscosities of three previously reported DESs, namely, reline, ethaline, and malinine, have also been studied within a temperature range of 303.2−330.2 K. A VFT-type equation has been used to correlate the viscosity values for all solvents. The results showed that guanidium malate-based eutectic solvents (PPILs) have significantly lower viscosities than the previously studied DESs (reline, ethaline, and malinine). The CO2 solubility in nine different aqueous PILs (variants of PPIL-1, PPIL-2, and PPIL-4) has been determined experimentally in the temperature range of 313.2 to 333.2 K and at pressures of up to 200 kPa. Henry’s law constants for CO2 in the mixed PPILs have been calculated using the model of O’Connel and Prausnitz33 for gas solubilities in mixed solvents. The results show that the Henry’s law constants for these eutectic solvents are lower than the corresponding values obtained for similar CO2-solvent systems studied previously under similar operating conditions. Calculations of the Gibbs free energy, dissolution enthalpy, and entropy values showed that the CO2 dissolution is exothermic in nature and occurs as a nonspontaneous process.

by decreasing the ethylene glycol content, this would increase the viscosity, resulting in higher pumping costs and reduced mass transfer in a CO2 capture process. So currently there is a trade-off between the CO2 solubility and the corresponding viscosity of the solvent at a certain temperature. Knowledge of thermodynamic properties such as the Gibbs free energy, enthalpy of dissolution, and entropy of dissolution is vital in order to understand the dissolution of CO2 from solvents and effectively design the carbon capture process. On the basis of the Henry’s law constants of CO2 determined here, eqs 5−7 have been used to determine the Gibbs energy, enthalpy, and entropy of the CO2−solvent systems. ΔdisGo = RT ln

HCO2

PIL

Po

(5)

HCO2 ⎡ PIL ⎢ ∂ ln P0 o ΔdisH = R ⎢ 1 ⎢ ∂ T ⎢⎣

{

()

} ⎤⎥⎥

⎥ ⎥⎦ P

(6)

ΔdisH − ΔdisG (7) T o o o where ΔdisG , ΔdisH , and ΔdisS are the Gibbs free energy, enthalpy, and entropy changes of CO2 solution at a standard pressure of P0 = 0.1 MPa. The enthalpy of dissolution for all three eutectic solvents (PPIL-1, PPIL-2, and PPIL-4) has a negative value, indicating that the CO2 absorption is an exothermic process. However, as compared to other similar solvent systems (under similar conditions), the enthalpy values are much smaller. This shows that the interactions between CO2 and the solvent molecules are not as strong as those in other solvent systems presented in Table 4; therefore, the solvent regeneration process will ΔdisS o =

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CO2−m-2-HEAF40 CO2−[bmim][PF6]42 CO2−[emim][Tf2N]42 CO2−[bmmim][BF4]42 CO2−furfuryl alcohol41 CO2−γ-valerolactone41 CO2−dibutyl succinate41 CO2−relineb14 CO2−ethalineb14 CO2−malinineb14 CO2−PPIL-1a CO2−PPIL-2a CO2−PPIL-4a a

ΔdisGo (kJ/mol)

ΔdisHo (kJ/mol)

ΔdisSo (J/mol·K)

13.38

−17.33 −16.1 −14.2 −14.5 −7.75 −14.24 −10.95 −12.59 −11.09 −12.65 −4.21 −5.89 −5.51

−98.05 −53.2 −46.9 −47.7 −68.93 −82.93 −67.19 −91.04 −86.07 −91.70 −55.61 −64.23 −39.11

13.84 11.73 10.09 16.46 16.38 16.62 13.21 14.22 6.74

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b00680. Compositions of the eutectic solvents, relationship of their viscosities with temperature, variation of viscosity with the composition of eutectic solvents, and experimental CO2 solubilities in eutectic solvents (PDF)

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Table 4. Comparison of the Calculated Gibbs Free Energy (ΔdisGo), Enthalpy (ΔdisHo), and Entropy (ΔdisSo) of Various CO2−Solvent Systems at 0.1 MPa and 313.2 K system

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AUTHOR INFORMATION

Corresponding Author

*Tel: +61 3 83446621. Fax: +61 3 83448824. E-mail: [email protected]. ORCID

Kathryn H. Smith: 0000-0003-2060-9369 Sandra E. Kentish: 0000-0002-4250-7489 Geoffrey W. Stevens: 0000-0002-5788-4682 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support given by the Peter Cook Centre for CCS Research at the University of Melbourne for providing research facilities to accomplish this work.

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REFERENCES

(1) Reddy, P. P. Causes of Climate Change. In Climate Resilient Agriculture for Ensuring Food Security; Springer: New Delhi, 2015. (2) Metz, B., Davidson, O., De Coninck, H., Loos, M., Meyer, L., Eds.; Carbon Dioxide Capture and Storage; Cambridge University Press: Cambridge, 2005. (3) Weart, S. R. The Discovery of Global Warming; Harvard University Press: Cambridge, 2008. (4) Draft Inventory of US Greenhouse Gas Emissions and Sinks: 1990−2014; U.S. Environmental Protection Agency: Washington, DC, 2016.

Current work. bAt 319.5 K.

become energetically more favorable. Negative values of the change in entropy indicate that the change in free energy would be positive for CO2 dissolution in these PPILs and hence the CO2 dissolution in these solvents would be a nonspontaneous process. F



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