Experimental Investigation on Thermophysical Properties of Ammonium-Based Protic Ionic Liquids and Their Potential Ability towards CO2 Capture

Ionic liquids, which are extensively known as low-melting-point salts, have received significant attention as the promising solvent for CO2 capture. This work presents the synthesis, thermophysical properties and the CO2 absorption of a series of ammonium cations coupled with carboxylate anions producing ammonium-based protic ionic liquids (PILs), namely 2-ethylhexylammonium pentanoate ([EHA][C5]), 2-ethylhexylammonium hexanoate ([EHA][C6]), 2-ethylhexylammonium heptanoate ([EHA][C7]), bis-(2-ethylhexyl)ammonium pentanoate ([BEHA][C5]), bis-(2-ethylhexyl)ammonium hexanoate ([BEHA][C6]) and bis-(2-ethylhexyl)ammonium heptanoate ([BEHA][C7]). The chemical structures of the PILs were confirmed by using Nuclear Magnetic Resonance (NMR) spectroscopy while the density (ρ) and the dynamic viscosity (η) of the PILs were determined and analyzed in a range from 293.15K up to 363.15K. The refractive index (nD) was also measured at T = (293.15 to 333.15) K. Thermal analyses conducted via a thermogravimetric analyzer (TGA) and differential scanning calorimeter (DSC) indicated that all PILs have the thermal decomposition temperature, Td of greater than 416K and the presence of glass transition, Tg was detected in each PIL. The CO2 absorption of the PILs was studied up to 29 bar at 298.15 K and the experimental results showed that [BEHA][C7] had the highest CO2 absorption with 0.78 mol at 29 bar. The CO2 absorption values increase in the order of [C5] < [C6] < [C7] anion regardless of the nature of the cation.


Introduction
Natural gas is a naturally occurring hydrocarbon that consists of methane gas primarily followed by other mixtures of higher alkanes such as ethane, propane and butane. Generally, natural gas is widely used as a fuel and a raw material in the petrochemical industry [1,2]. Despite its mixture of combustible hydrocarbons content, trace quantities of argon (Ar), hydrogen (H), helium (He), nitrogen (N 2 ) as well as carbon dioxide (CO 2 ) and hydrogen sulfide (H 2 S) are also present in natural gas [3]. Sour gas, such as CO 2 , is undesirable due to its acidic property that causes corrosion in the gas pipeline [4]. Apart from that, the existence of CO 2 also reduces the fuel value of natural gas due to its non-combustible nature. Therefore, CO 2 removal in the refining process is crucial to improving the value of natural gas and the utilization of amine-based solvents, namely monoethanolamine (MEA), which had been widely practiced on industrial scales to capture CO 2 in natural gas. This chemical absorption of CO 2 by MEA is considered to be the most reliable and ionic liquids. This work serves as a continuation from our previous work on CO 2 absorption utilizing ammonium-based protic ionic liquids (PILs) [37]. Previously, the CO 2 absorption of ammonium-based PILs utilizing bis (2-ethylhexyl) ammonium, tributylammonium and ethanolammonium cations coupled with acetate and butyrate anions have been reported. The motivation to further investigate this type of ionic liquid for CO 2 capture has risen after we discovered that the PILs could be prepared via a simple synthesis procedure and their capability to absorb CO 2 under experimental conditions. To further study the binary system of PILs-CO 2

Characterization of Synthesized PILs
All six ammonium-based PILs in this work exist as liquids at room temperature. The ammonium cation, [ [C7], are presented in this sub section while all NMR spectra of PILs (Figures S1-S12) are available in the Supplementary Materials. The reported water content for all PILs is between 1.04% and 8.70%. Based on reported data by Chen et al., PILs are highly hygroscopic and own higher hydrophilicity in comparison to aprotic ionic liquids [38]. Meanwhile, the presence of water molecules lowers the electrostatic attractions between the ions and consequently reduces the viscosity of ILs [39]. Nonetheless, the thermophysical properties of our PILs are solely reported by using these water content values.

TGA and DSC Analysis
Thermogravimetric analyzer (TGA) was used to study the thermal stability of the PILs. Table 1 shows the thermal stability data while Figure 1 displays the TGA profiles of the synthesized PILs. It could be observed from the data that the thermal stability of PILs is in the range of 416 to 437 K. For a common cation, lengthening the alkyl chain branch in the anion caused an increment in the thermal decomposition (T d ) of the PIL. This could be evidenced by the relatively high T d of [ [35,44,45]. Xu and Cheng have summarized that the thermal stability of imidazolium ionic liquids was improved by increasing the degree of substitution of hydrogen by alkyl groups on the imidazolium ring [46]. The phase transitions which are glass transition temperature (Tg), and melting point (Tm) of the ammonium-based PILs were investigated by using a Differential Scanning Calorimeter (DSC) from −150 °C to 50°C and the results are tabulated in Table 1. This temperature range was chosen based on the fact that many ILs exhibit glass transition at low temperatures even beyond −100 °C [47]. Apart from providing the fundamental infor-  The phase transitions which are glass transition temperature (T g ), and melting point (T m ) of the ammonium-based PILs were investigated by using a Differential Scanning Calorimeter (DSC) from −150 • C to 50 • C and the results are tabulated in Table 1. This temperature range was chosen based on the fact that many ILs exhibit glass transition at low temperatures even beyond −100 • C [47]. Apart from providing the fundamental information, the study of phase transition of PILs at this condition is crucial due to demand in other technological areas with extreme environments. For example, in space-related applications, ILs is potentially being used as hypergolic fluids in orbiting satellites, manned spacecraft and deep-space probes [48]. Figure 2 shows the examples of DSC curves for the ammonium-based PILs synthesized in this study. Data show that all PILs possess a glass transition temperature (T g ) ranging from −98.37 • C to −90.89 • C, which indicates that all PILs experience the flow of heat from amorphous glass to liquid state [19]. As T g represents the cohesive energy of the sample, PILs that exhibit T g values have low cohesive energy that could contribute to advantageous physiochemical properties such as low viscosity and high ionic conductivity [47]. A similar trend of marginal difference in the T g values for the ammonium-based PILs was also observed and discussed by other researchers employing ammonium-based ionic liquids as well [47]. In contrast, only ammonium-based PILs with [BEHA] cation exhibited a melting temperature (T m ) in which all T m are in the range of −68.34 • C to −66.69 • C. Only a minimal increment in the T m values was observed when the alkyl chain of anion increases [C5] to [C7]. Primarily, the T m of PIL is dependent on the crystal lattice strength in the PIL. The low T m of the PIL could be related to the low crystal lattice energy due to poor packing efficiency in the crystal lattice of PIL itself [43,49]

Density (ρ), Thermal Expansion Coefficient (αp), Standard Entropy (S°) and Lattice Potential energy (Upot) Measurement
The density of ammonium-based PILs was studied at temperatures ranging from 293.15 to 363.15 K. The plots of the experimental density of the PILs are shown in Figure  3 while the experimental data and the plots with standard errors are available in Table S1 and Figure S13, respectively, in the Supplementary Material. As illustrated in Figure 3, the densities for all six ammonium-based PILs decreased linearly with temperature. Experimental data also indicates that the density of the PILs deceases as the alkyl chain of the anion increases for both [EHA] and [BEHA] PILs. The results are in accordance with published results in literature for PILs with diethylammonium and dibutylammonium cations with the density values ranging approximately from 0.82 g.cm −3 to 0.94 g.cm −3 [50]. A similar observation was also found by other researchers when the densities of their tetrabutylammonium ionic liquids were analyzed over a temperature range of 283.4 to 333.4 K [51]. As temperature increases, the volume of ionic liquids increases, and the density of  The density of ammonium-based PILs was studied at temperatures ranging from 293.15 to 363.15 K. The plots of the experimental density of the PILs are shown in Figure 3 while the experimental data and the plots with standard errors are available in Table S1 and Figure S13, respectively, in the Supplementary Material. As illustrated in Figure 3, the densities for all six ammonium-based PILs decreased linearly with temperature. Experimental data also indicates that the density of the PILs deceases as the alkyl chain of the anion increases for both [EHA] and [BEHA] PILs. The results are in accordance with published results in literature for PILs with diethylammonium and dibutylammonium cations with the density values ranging approximately from 0.82 g.cm −3 to 0.94 g.cm −3 [50]. A similar observation was also found by other researchers when the densities of their tetrabutylammonium ionic liquids were analyzed over a temperature range of 283.4 to 333.4 K [51]. As temperature increases, the volume of ionic liquids increases, and the density of the ionic liquids decreases accordingly. At higher temperatures, the intermolecular forces between the constituent ions weaken, and this increases the mobility of the ions which in turn increases the volume of these ions [37,52,53]. Further analysis also revealed that [ [37,54]. Comparable observations using PIL with ethylammonium cation were also found by several researchers, in which an increasing trending packing efficiency was proportional with the decreasing of molecular weight [51,55]. Notably, the increased alkyl chain length in both cation and anion of the PIL has promoted the steric hindrance and asymmetric nature in the PIL structure as bigger and bulkier PILs result in a lower density value for the PILs [40,41]  The thermal expansion coefficient can provide information about the intermolecular interaction in the PILs, and it can be calculated from the experimental values of density, ρ by using Equation (1). The calculated data is tabulated in Table 2. Thermal expansion coefficients, αp for the ammonium-based PILs can be defined as [37,53,56]: The calculated values in Table 2 show that the thermal expansion coefficients vary only slightly with the increase of C-numbers in the structure of the PILs. PILs with [BEHA] cation has higher αp than that of PILs with [EHA] cation. This indicates that the thermal expansion coefficient does not only depend on the cation symmetry but is also related to the length of the alkyl substituent [57]. Meanwhile, the behavior of the thermal expansion coefficient is almost similar for all PILs with common cations. Sarkar et al. have also reported a similar variation trend of the thermal expansion coefficient for diethylammonium-based PILs [19]. To conclude, the thermal expansion coefficient can be considered as temperature independent as it shows similar results over the temperature range studied.  The thermal expansion coefficient can provide information about the intermolecular interaction in the PILs, and it can be calculated from the experimental values of density, ρ by using Equation (1). The calculated data is tabulated in Table 2. Thermal expansion coefficients, α p for the ammonium-based PILs can be defined as [37,53,56]: The calculated values in Table 2 show that the thermal expansion coefficients vary only slightly with the increase of C-numbers in the structure of the PILs. PILs with [BEHA] cation has higher α p than that of PILs with [EHA] cation. This indicates that the thermal expansion coefficient does not only depend on the cation symmetry but is also related to the length of the alkyl substituent [57]. Meanwhile, the behavior of the thermal expansion coefficient is almost similar for all PILs with common cations. Sarkar et al. have also reported a similar variation trend of the thermal expansion coefficient for diethylammonium-based PILs [19]. To conclude, the thermal expansion coefficient can be considered as temperature independent as it shows similar results over the temperature range studied. The volume occupied by one mole of a compound at a given temperature and pressure is denoted as molar volume, V m . The molar volume was calculated by using an empirical equation as shown in Equation (2) and utilizing the experimental densities [41,[58][59][60][61]: where V m is the molar volume, M is the molar mass of the ammonium-based PILs, ρ is the density of PILs at 303.15 K and N A is Avogadro's number.
The calculated molar volume for all ammonium-based PILs are tabulated in Table 3. From the calculated value, the molar volume, V m , is proportional to the anion alkyl chain length as well as the size of the cation. The molar volume increases with the alkyl chain length of the anion and this behavior is caused by the addition of the CH 2 group in the anion of the PILs. Besides that, PILs with [BEHA] cation exhibit a larger molar volume value compared to PILs with [EHA] cation. This could be explained by the difference in the size of the cations. Similar findings have been observed in other studies [19,37]. Entropy is the measurement of the randomness of molecules, and generally, entropy increases with molar volume [19]. The relationship between molar volume (V m ) and standard entropy (S • ) for the ammonium-based PILs in this work can be explored by using the following standard equation that is available in the literature [62]: The results presented in Table 3 clearly indicate that the standard entropy increased with the molar volume value for all ammonium-based PILs. The increasing number of carbon atoms in the alkyl chain of carboxylate anion has resulted in the increment of the S • of the ammonium-based PILs. From the calculated values obtained, [BEHA]-based PILs depicted the highest standard entropy due to their larger size compared to [EHA]-based PILs, which causes the least interaction between cation and anion [41]. In this work, the standard entropy of [ In addition, to predict the relative stabilities of ILs, Glasser [62] has also developed a method for calculating lattice potential energies (U pot ) of ILs by using Equation (4): where γ and δ are fitting coefficients with values of 1981.9 kJ·mol −1 and 103.8 kJ·mol −1 , respectively. The lattice potential energy of the studied PILs was calculated at 303.15 K. The main factor contributing to lattice potential energy is electrostatic or columbic interaction. However, lattice potential energy is inversely related to the volume of ions [19,52,54]. As can be seen in Table 3, lattice potential energy decreases with the addition of the carbon chain length of the carboxylate groups. The addition of methylene group in the alkyl chain of both cation and anion increases the entropy, and consequently reduces packing efficiency in the PILs [63]. As a result, lattice potential energy will decrease with the increase in the alkyl chain length of the PILs.

Viscosity (η) Measurement
Viscosity is one of the important properties that governs the potential applications of any solvents, and it is largely influenced by intermolecular interactions namely hydrogen bonding, dispersive forces and columbic interactions [64]. The experimental data and the plots with standard errors for viscosity values are available in Table S2 and Figure S14 [60]. They suggested that the dynamic viscosity increases with the extension of the alkyl side chain of the cation for the three series of pyridinium-based ILs. However, in this work, PILs with [EHA] cation display a higher viscosity value than PILs with [BEHA] cation. Basically, the van der Waals attraction between the aliphatic alkyl chain affects the viscosity values of the PILs [41,53]. However, the water content of the PILs may also affected the observed viscosity results. Furthermore, PILs with [BEHA] cation displayed a marginal increment in the viscosity values as the alkyl chain length of the anion increased.

Refractive Index (n D ) Measurement
Generally, the refractive index (n D ) describes how fast light travels through material. It estimates the electronic polarizability of the molecules and shows the dielectric response to an external electric field produced by electromagnetic waves (light) [65]. Figure 5 shows the refractive index of ammonium-based PILs that were measured in a temperature range of 293.15 to 333.15 K at atmospheric pressure. The experimental data is tabulated in Table S3 while the plots with standard errors are presented in Figure S15 in the Supplementary Material. From the table, the n D values were found to be decreasing with increasing temperature. Moreover, the values of the refractive index increased with the increase in cation and anion chain length of PILs. A similar observation was also found in the literature involving PILs in which the n D values of the studied PILs were in the range of 1.45-1.41 [50]. The increment of refractive index values with increasing alkyl chain length in the cation is influenced by higher intermolecular interaction such as the van der Waals forces of the PILs [52]. 353.15) K [60]. They suggested that the dynamic viscosity increases with the extension of the alkyl side chain of the cation for the three series of pyridinium-based ILs. However, in this work, PILs with [EHA] cation display a higher viscosity value than PILs with [BEHA] cation. Basically, the van der Waals attraction between the aliphatic alkyl chain affects the viscosity values of the PILs [41,53]. However, the water content of the PILs may also affected the observed viscosity results. Furthermore, PILs with [BEHA] cation displayed a marginal increment in the viscosity values as the alkyl chain length of the anion increased.

Refractive Index (nD) Measurement
Generally, the refractive index (nD) describes how fast light travels through material. It estimates the electronic polarizability of the molecules and shows the dielectric response to an external electric field produced by electromagnetic waves (light) [65]. Figure 5 shows the refractive index of ammonium-based PILs that were measured in a temperature range of 293.15 to 333.15 K at atmospheric pressure. The experimental data is tabulated in Table  S3 while the plots with standard errors are presented in Figure S15

Thermophysical Properties Correlations
The density (ρ), dynamic viscosity (η) and refractive index (nD) experimental values were correlated by using the following equations [53,66]: where T is the temperature in K, and A1 through A6 are correlation coefficients using the least square method. Table 4 represents the estimation of values for correlation coefficients together with the standard deviations, SD which was calculated by using the Equation (8). Z expt and Z calc are experimental and calculated values, respectively, while n DAT is the number of experimental points.

Thermophysical Properties Correlations
The density (ρ), dynamic viscosity (η) and refractive index (n D ) experimental values were correlated by using the following equations [53,66]: where T is the temperature in K, and A 1 through A 6 are correlation coefficients using the least square method. Table 4 represents the estimation of values for correlation coefficients together with the standard deviations, SD which was calculated by using the Equation (8).
Z expt and Z calc are experimental and calculated values, respectively, while n DAT is the number of experimental points.

CO 2 Absorption Measurement
Carbon dioxide absorption measurements have been performed to investigate the potential ability of the ammonium-based PILs as solvents for CO 2 capture. The measurements were conducted in the pressure range of 1-29 bar at room temperature and the results are plotted in Figures 6 and 7. From the plots, the CO 2 uptake by the ammonium-based PILs shows a trend of polynomial increment with CO 2 pressure. Generally This behavior can be explained by using the data reported of density and molar volume of the ammonium-based PILs. The increment in the density value of the PIL increases the molar volume of the PILs which thus in turn causes an increase in the fractional free volume and consequently enhances the CO 2 uptake by the ammonium-based PILs [67,68]. Based on the analysis and comparison of FTIR and 13 C NMR, Xu and Oncsik et al. proposed that the mechanism of CO 2 absorption is via the interaction between gas and the basic anion [32,69]. At approximately 20 bar and 25 • C, the CO 2 uptake by the [BEHA][C7] is about 40% higher than that of bis(2-ethylhexyl)ammonium butyrate protic ionic liquid [37]. On the other hand, some researchers have performed investigations on the relationship between the viscosity of PILs and performance of CO 2 absorption by the PILs and found that PIL with low viscosity value has a high absorption capacity of CO 2 [70]. ILs with low viscosities can result in low mass transfer resistance between liquid and gas phases, and this eventually increases the CO 2  show the highest CO2 absorption capacity at 29 bar and room temperature with the values of CO2 mol fractions of 0.77 moles and 0.78 moles, respectively. These results could be considered an indication of the potential ability of the ammonium-based PILs as solvents for CO2 capture. However, more thorough studies must be conducted for further evaluation before the ammonium-based PILs can be fully used as new solvents in the field of CO2 removal.

Chemicals
To synthesize all six ammonium-based PILs, analytical grade chemicals from Merck, Darmstadt, Germany were used. The CAS numbers, abbreviations, grade percentage, density, viscosity, flash point and melting points of all chemicals are as follows

Synthesis of PILs
The synthesis of PILs was carried out by using a one-step neutralization reaction and the reaction is written as follows:

Chemicals
To synthesize all six ammonium-based PILs, analytical grade chemicals from Merck, Darmstadt, Germany were used. The CAS numbers, abbreviations, grade percentage, density, viscosity, flash point and melting points of all chemicals are as follows

Synthesis of PILs
The synthesis of PILs was carried out by using a one-step neutralization reaction and the reaction is written as follows: In a specific procedure, a 1:1 mol ratio of acid was added to the base with continuous stirring at 250 rpm for 24 h at room temperature. The resulting product was dried under vacuum at 80 • C for 6 h to remove any water traces and impurities that might be present resulting from starting reagents as well as surrounding atmosphere. The PILs which were in liquid forms without noticeable solid crystal or precipitation after the purification step were kept in sealed containers until further analysis. The proton transfer reaction had resulted in the formation of six PILs as tabulated in Table 5. Figure 8

Structural Confirmation and Water Content
Nuclear Magnetic Resonance (NMR) spectroscopy (Bruker Ascend TM 500, Billerica, MA, USA) was used to confirm the structures of the synthesized PILs. In each analysis, a 100 μL PIL sample was dissolved in a 600 μL solvent (CDCl3

Structural Confirmation and Water Content
Nuclear Magnetic Resonance (NMR) spectroscopy (Bruker Ascend TM 500, Billerica, MA, USA) was used to confirm the structures of the synthesized PILs. In each analysis, a 100 μL PIL sample was dissolved in a 600 μL solvent (CDCl3). The 1 H and 13 C spectra are reported in parts per million and the multiplicities, where applicable, are written as d (doublet), t (triplet) and m (multiplet). The water content of each PIL was determined by

Structural Confirmation and Water Content
Nuclear Magnetic Resonance (NMR) spectroscopy (Bruker Ascend TM 500, Billerica, MA, USA) was used to confirm the structures of the synthesized PILs. In each analysis, a 100 μL PIL sample was dissolved in a 600 μL solvent (CDCl3). The 1 H and 13 C spectra are reported in parts per million and the multiplicities, where applicable, are written as d (doublet), t (triplet) and m (multiplet). The water content of each PIL was determined by using Volumetric Karl Fisher V30 Mettler Toledo (Columbus, OH, USA).

Structural Confirmation and Water Content
Nuclear Magnetic Resonance (NMR) spectroscopy (Bruker Ascend TM 500, Billerica, MA, USA) was used to confirm the structures of the synthesized PILs. In each analysis, a 100 μL PIL sample was dissolved in a 600 μL solvent (CDCl3

Structural Confirmation and Water Content
Nuclear Magnetic Resonance (NMR) spectroscopy (Bruker Ascend TM 500, Billerica, MA, USA) was used to confirm the structures of the synthesized PILs. In each analysis, a 100 μL PIL sample was dissolved in a 600 μL solvent (CDCl3). The 1 H and 13 C spectra are reported in parts per million and the multiplicities, where applicable, are written as d (doublet), t (triplet) and m (multiplet

Structural Confirmation and Water Content
Nuclear Magnetic Resonance (NMR) spectroscopy (Bruker Ascend TM 500, Billerica MA, USA) was used to confirm the structures of the synthesized PILs. In each analysis, 100 μL PIL sample was dissolved in a 600 μL solvent (CDCl3). The 1 H and 13 C spectra ar reported in parts per million and the multiplicities, where applicable, are written as (doublet), t (triplet) and m (multiplet). The water content of each PIL was determined by using Volumetric Karl Fisher V30 Mettler Toledo (Columbus, OH, USA).

Structural Confirmation and Water Content
Nuclear Magnetic Resonance (NMR) spectroscopy (Bruker Ascend TM 500, Billerica, MA, USA) was used to confirm the structures of the synthesized PILs. In each analysis, a 100 µL PIL sample was dissolved in a 600 µL solvent (CDCl 3 ). The 1 H and 13 C spectra are reported in parts per million and the multiplicities, where applicable, are written as d (doublet), t (triplet) and m (multiplet). The water content of each PIL was determined by using Volumetric Karl Fisher V30 Mettler Toledo (Columbus, OH, USA).

TGA Analysis
A Simultaneous Thermal Analyzer (STA) 6000 (Perkin Elmer, Waltham, MA, USA) was used to study the thermal stability of the PILs. The reproducibility for TGA STA 6000 is <±0.5 • C with ± 2% based on metal standard. In each analysis, 10 mg of the PILs sample was placed in a crucible pan and the thermal analysis was conducted in a temperature range of 50-650 • C at a heating rate of 10 • C·min −1 under 20 mL/min of nitrogen flow.

DSC Analysis
A Differential Scanning Calorimeter (DSC) 1 Star system (Mettler Toledo, Columbus, OH, USA) was used to investigate the phase transition of the PILs. The reproducibility for DSC is ± 0.2K with <1% based on indium calibration. In total, 10 mg of samples were sealed in aluminum pans and subject to analysis in a temperature range of 50-150 • C with a heating rate of 10 • C·min −1 . The phase transition data were analyzed by using the second heating plot.

Density (p) and Viscosity (η) Measurement
An Anton Paar Stabinger Viscometer (Graz, Austria) was used to simultaneously measure the density and viscosity of the PILs in a temperature range of 293.15-363.15 K with a temperature measurement accuracy of 0.02 K. The reproducibility of the density and viscosity measurements were ±5.10 −4 g.cm −3 and 0.35%, respectively. The measurements were repeated several times and the average value was considered for further analysis. Prior to the density and viscosity measurements, the equipment was calibrated using a standard fluid provided by the supplier. A commercial imidazolium IL with known density and viscosity values was also used to validate the equipment.

Refractive Index (n D ) Measurement
An ATAGO RX-5000 Alpha Digital Refractometer (Tokyo, Japan) with a measuring accuracy of ±4.10 −5 was used to determine the refractive index values of the PILs. The measurement was done in a temperature range of 293.15 to 333.15 K. The instrument was also calibrated by using standard organic solvents provided by the supplier. In addition, a commercial imidazolium IL was also used while conducting the validation test and the result was compared with the values available from the literature [50].

CO 2 Absorption Measurement
The CO 2 absorption of the PILs was studied by using a magnetic suspension balance (MSB) from Rubotherm Präzisionsmesstechnik GmbH (Bochum, Germany). In this gravimetric method, the weight change of the PILs upon absorption of CO 2 was measured and calculated in a range of pressure from 1 to 29 bar at room temperature. The sample absorption chamber linked to the microbalance, which has a precision of ±20 µg, via an electromagnet and a suspension magnet which keeps the balance at ambient conditions during the CO 2 absorption experiments. In a typical CO 2 absorption measurement, approximately 1g of PIL sample was loaded in the sample chamber and the absorption system was evacuated at 10 −3 mbar (Pfeiffer model DUO5) to remove any impurities until the weight remained constant. Then, the sample chamber was pressurized with CO 2 at a constant temperature by means of an oil circulator (Julabo, model F25-ME, ±0.1 • C accuracy, Seelbach, Germany) and the weight change due to the absorption of the gas in the PIL was observed and recorded. Once a constant weight reading was recorded, the system was allowed to stand in the condition for an additional 3-4 h to ensure complete equilibration of the binary CO 2 -PIL system. The absorption measurement was repeated with different pressure values of CO 2 to yield a series of absorption isotherm. The weight of the CO 2 dissolved in the PILs sample was calculated using Equation (9) available from literature [72,73].
wt CO 2 = [wt − (wt Sc + wt S )] + [(V Sc + V S )(ρCO 2 )] (9) where wt (g) is the corrected weight of the balance, wt Sc + wt S (g) are the weights of sample cell and sample, respectively, V Sc + V S (cm −3 ) are the volumes of the sample cell and sample, respectively, and ρCO 2 (g.cm −3 ) is the density of CO 2 at the pressure and temperature during the CO 2 absorption. The results of CO 2 absorption are presented in terms of mole fraction of CO 2 (x) dissolved in the PIL, which was calculated using Equation (10): x = n CO2 /(n liq +n CO2 ) (10) where n CO2 is the mole of CO 2 absorbed in the PIL and n liq is the mole of the PIL.

Conclusions
In this work, six new ammonium-based PILs have been successfully synthesized through a one-step procedure. The thermophysical properties including density, viscosity, refractive index and thermal stability have been measured. The experimental results revealed the dependency of the experimental values namely the ρ, η, n D and T d on the alkyl chain of the anion, size of the cations and the temperature of measurement. The phase transition analysis of the PILs yielded the glass transition temperature (T g ) and melting point (T m ) of the PILs studied. These synthesized ammonium-based PILs have been tested for their ability towards CO 2 absorption in which [BEHA][C7] displayed the highest CO 2 uptake in the experimental conditions signifying its capability to be a potential solvent in the application of CO 2 capture. Future works should include CO 2 desorption studies of the PILs for the purpose of recyclability and sustainability of the absorbents.