Effect of Cation Modification on the Physicochemical Properties and

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

pubs.acs.org/jced

Effect of Cation Modification on the Physicochemical Properties and CO2 Solubility: Nonfluorinated Phosphonium-Based Ionic Liquids Incorporating a Dioctylsulfosuccinate Anion Abobakr Khidir Ziyada*,† and Cecilia Devi Wilfred‡ †

Department of General Studies, Jubail Industrial College, Jubail Industrial City 31961, Saudi Arabia Fundamental and Applied Sciences Department, Universiti Teknologi PETRONAS, Tronoh 31750, Perak, Malaysia

‡

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S Supporting Information *

ABSTRACT: In the current work, a series of phosphonium-based monocationic and dicationic ionic liquids (trihexyltetradecylphosphonium dioctylsulfosuccinate, [P6,6,6,14]DOSS; trioctyltetradecylphosphonium dioctylsulfosuccinate, [P8,8,8,14]DOSS; 1,6-bis(trioctylphosphonium)hexane dioctylsulfosuccinate, [P8,8,8C6P8,8,8] DOSS2; and 1,10-bis(trioctylphosphonium)decane dioctylsulfosuccinate, [P8,8,8C10P8,8,8]DOSS2) were prepared, and their characterization was done using NMR and elemental analysis. Their physiochemical properties (thermal stability, viscosity, density, and refractive index) and CO2 solubility (at pressures 1, 5, 10, 15, and 20 bar) were investigated. The cation and alkyl chain effects on the properties and CO2 capturing efficiency were discussed. The thermodynamic functions of solvation were calculated using the variation of Henry’s law constants with temperature.

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capture, and it is not necessary to have fluorination to achieve higher CO2 solubility.4,5 In comparison to other ILs, the notable characteristics of phosphonium ILs are their thermal, chemical, and electrochemical stabilities. In addition, some phosphonium ionic liquids reveal lower viscosities and lower melting points which are practical advantages for various applications.6 Related to the common imidazolium-based ILs, phosphonium ILs are known to possess more interesting and useful properties such as lower density and lack of an acidic proton.7,8 Data and consideration of the IL properties are crucial for selecting appropriate liquids for envisaged applications, process equipment’s design and justification, development of the property estimation methods, molecular simulation, and progress of the commercial process.9 Even though properties and CO2 solubility of many phosphonium-based ILs have been studied, 8,10−16 the physiochemical properties and CO 2 solubility of the trihexyl and trioctyl phosphonium-based monocationic and dicationic ILs incorporating dioctylsulfosuccinate anions have not been studied. Based on previous studies of the preparation, characterization, physicochemical properties, and CO2 solubility of new ILs,17−20 trihexyltetradecylphosphonium dioctylsulfosuccinate [P6,6,6,14]DOSS, trioctyltetradecylphosphonium dioctylsulfosuccinate

INTRODUCTION

The economic and political issues coupled with growing social concerns and desire for clean, sustainable technologies induce the researchers to find or develop new materials superior to ordinary amines for CO2 separation. Ionic liquids (ILs) are a promising alternative for CO2 capture which presents many chances to re-evaluate and enhance current technologies and processes for CO2 capture.1 They have been widely studied as prospective substitute solutions of aqueous amine because of their unique characteristics and their potential to provide improvements in CO2 absorption capacity and selectivity by making use of their designing character and choosing applicable aggregation of anions and cations.2 The attractiveness of ILs could be referred to their noticeable properties such as relatively high CO2 absorption capacity, high thermal stability, negligible volatility, nonflammability, large liquidus range, and solvation properties. In order to achieve an IL that has high CO2-capturing capacity, it is essential to synthesize ILs that encompass CO2philic groups (on the cation/anion) that have proven to increase CO2 capture.3 It was reported that fluorination of the cation/anion could increase the solubility of CO2 in ILs. Conversely, the accompanying limitations are poor degradability, costly, and undesirable environmental impact.3 Therefore, it is important to synthesize ILs with enhanced CO2 solubility and without fluorination. Recently, phosphoniumbased ILs were reported as suitable alternatives for CO2 © XXXX American Chemical Society

Received: February 4, 2018 Accepted: August 23, 2018

A

DOI: 10.1021/acs.jced.8b00109 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Figure 1. Synthesis route of trialkyltetradecylphosphonium dioctylsulfosuccinate.

Figure 2. Synthesis route of phosphonium-based dicationic ILs incorporating a dioctylsulfosuccinate anion.

ohexane (2163-00-0, Aldrich, 98%), 1,10-dichlorodecane (2162-98-3, Aldrich, 99%), sodium dioctylsulfosuccinate (209-406-4, Aldrich 98%), ethyl acetate (141-78-6, SigmaAldrich, 99.8%), acetone (67-64-1, Sigma-Aldrich, ≥99.5%), diethyl ether (60-29-7, Sigma-Aldrich, 99%), trihexyltetradecylphosphonium chloride (258864-54-9, Aldrich, ≥95%), and trioctylphosphine (4731-53-7, Aldrich, ≥99%). All the chemicals were used without more purification. Purified gases (provided by MOX-Linde Gases Sdn Bhd, Malaysia) were used for the CO2 solubility measurement. The purity of

[P8,8,8,14]DOSS, and trioctylphosphonium-based dicationic ILs ([P8,8,8CnP8,8,8]DOSS2, where n = 6, 10) were synthesized, and their properties such as viscosity, density, and refractive index were measured at atmospheric pressure and a temperature range of (293.15 to 353.15) K. The efficiency for capturing CO2 was studied at a pressure range of (1 to 20) bar.

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MATERIALS The ILs were synthesized using chemicals of analytical grade. The CAS number, source, and grades of the chemicals used are 1-chlorotetradecane (2425-54-9, Aldrich, 98%), 1,6-dichlorB



Article

4.0) mm guard column (Metrosep A Supp 4/5) and (150 × 4.0) mm analytical column (Metrosep A Supp 5−150). The results were analyzed using Metrodata IC Net 2.3 software.9 A thermogravimetric analyzer (TGA, PerkinElmer, Pyris V3.81) was used to determine the thermal stability of the synthesized ILs at a temperature range (50−500) °C, heating rate of 5 °C min−1 (temperature accuracy better than 2 °C), and under a nitrogen atmosphere. The values for thermal decomposition of all samples are reported in terms of Tstart and Tonset. Density and Viscosity. An Anton Paar viscometer (model SVM3000) was used to measure the density and viscosity of the synthesized ILs over a temperature range T = (293.15 to 353.15) K at atmospheric pressure. Before each series of measurements, the measuring device was calibrated. The instrument uncertainty of the density, viscosity, and temperature (instrument) was ±0.0006 g·cm−3, ±0.7%, and 0.05 K, respectively. The reproducibility of the measurements were ±5 × 10−4 g·cm−3 and 0.35% for density and viscosity, respectively. The temperature was controlled to within ±0.01 °C. Refractive Index. An ATAGO programmable digital refractometer (RX-5000 alpha) with 5 × 10−4 and 0.2 K uncertainty of the refractive index and temperature, respectively, was used to determine the refractive index of the synthesized ILs in the temperature range T = (298.15 to 333.15) K. The samples were dried and retained in desiccators until directly placed into the measuring cell. Millipore quality water and pure organic solvents with known refractive indices were used to calibrate the instrument. To ensure the measurement accuracy, three independent measurements were conducted for each sample at each temperature. CO2 Solubility in ILs. The CO2 solubility in ILs was studied using a magnetic suspension balance (MSB) from Rubotherm, Präzisionsmesstechnik GmbH, Bochum, Germany. The MSB employs a magnetic suspension coupling comprised of a suspension magnet and an electromagnet, which sustain freely suspended contactless balance connections. The MSB was controlled by the MessPro software which also was used to record the data. The temperature was controlled using an overlapping controller involving a direct temperature controller (JUMO IMAGO 5000 and/or JULABO F-25ME) and a software controlled on the computer. The explanation of the measuring method, equipment, and sample preparation has been reported,22 and the details of the measurement were presented in the Supporting Information. Measurement Uncertainties. There are several sources of uncertainty when measuring the physicochemical properties and CO2 solubility. The measurement uncertainties of the physicochemical properties and CO2 solubility were calculated, and the IL purities were considered in the calculations. The combined standard uncertainties of the density, viscosity, refractive index, and CO2 solubility for the ILs [P6,6,6,14]DOSS, [P8,8,8,14]DOSS, [P8,8,8C6P8,8,8]DOSS2, and [P8,8,8C10P8,8,8]DOSS2 were (0.015, 0.016, 0.012, and 0.011), (2.3, 2.4, 2.0, and 1.8), (0.016, 0.017, 0.013, and 0.011), and (0.016, 0.017, 0.013, and 0.011), respectively. The measurement uncertainties of densities, viscosities, refractive indices, and CO2 solubilities are mainly due to the impurities in the ILs.

gases used (carbon dioxide (CO2), methane (CH4), nitrogen (N2), and helium (He)) is greater than 99.9%. Synthesis of Trialkyltetradecylphosphonium Dioctylsulfosuccinate. Trihexyltetradecylphosphonium chloride [P6,6,6,14]Cl was purchased from Aldrich. Trioctyltetradecylphosphonium chloride [P8,8,8,14]Cl was prepared using a roundbottomed flask. The flask is attached to a reflux condenser and flushed with dry nitrogen. Then, 1-chlorotetradecane (0.041 mol) and trioctylphosphine (0.04 mol) were added. The mixture was heated with stirring for 48 h at 80 °C. The product was washed with ethyl acetate and dried in vacuum oven at 80 °C for 48 h to produce a viscous residue [P8,8,8,14]Cl. Trialkyltetradecylphosphonium-based ILs were prepared by following the method as shown in Figure 1. Trialkyltetradecylphosphonium dioctylsulfosuccinate [Pn,n,n,14]DOSS ILs (where n = 6, 8) were synthesized by mixing stoichiometric amounts of trialkyltetradecylphosphonium chloride [Pn,n,n,14]Cl and sodium dioctylsulfosuccinate with diethyl ether and stirred for 48 h, and then the solid was separated. The products were washed with acetone and dried in a vacuum oven at 80 °C for 48 h to produce the clear viscous gel products trihexyltetradecylphosphonium dioctylsulfosuccinate [P6,6,6,14]DOSS and trioctyltetradecylphosphonium dioctylsulfosuccinate [P8,8,8,14]DOSS. Synthesis of Phosphonium-Based Dicationic ILs (DCILs). Phosphonium-based dicationic ILs [P8,8,8CnP8,8,8]DOSS2 (where n = 6, 10) were synthesized by following the method shown in Figure 2. They were synthesized in two steps using a round-bottomed flask equipped with a nitrogen inlet adapter, magnetic stirrer, reflux condenser, and heating oil bath. In the first step, trioctylphosphine (0.041 mol) and 0.2 mol of 1,n-dichloroalkane (where n = 6, 10) were mixed, stirred, and heated at 100 °C for 24 h under nitrogen atmosphere in the round-bottom flask. The products were cooled and washed with acetone, and the mixture was heated at 80 °C under vacuo to remove volatile components. The product was dried in a vacuum oven for 48 h to produce a clear viscous gel product 1,n-bis(trioctylphosphonium)hexane chloride [P8,8,8CnP8,8,8]Cl2. In the second step, stoichiometric amounts of [P8,8,8CnP8,8,8]Cl2 and sodium dioctylsulfosuccinate were mixed with diethyl ether and stirred for 48 h. The product was washed again with acetone and dried in a vacuum oven at 80 °C for 48 h to produce a clear viscous gel product [P8,8,8CnP8,8,8]DOSS2. Characterization. 1H and 13C NMR spectra of the prepared ILs were obtained using DCl3 solvent and A Burcher Avance 300-JEOL JNM-ECA400 nuclear magnetic resonance spectrophotometers. A CHNS-932 (LECO instruments) elemental analyzer was used to measure the percentage of carbon, hydrogen, nitrogen, and sulfur of the synthesized ILs. The instrument was standardized before each measurement, using a standard calibration sample with determined chemical composition provided by supplier. Water content of the present ILs was measured using a coulometric Karl Fischer titrator, DL 39 (Mettler Toledo), with CombiCoulomat fritless Karl Fischer reagent (Merck) and Hydranal coulomat AG reagent (Riedel-de Haen). Triplicate measurements were carried out, and the average value was reported.21 Ion chromatography (Metrohm model 761) was used to determine the chloride content of the prepared ILs. The measurement was conducted by Compact IC with a (5.0 × C



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Table 1. Molecular Weight (Mw), Mass Fractiona of Water (w), Mass Fractiona of Bromide (wCl), Start Temperatures (Ts), Decomposition Temperatures (Td), and Purity (Mass Fraction) [P6,6,6,14] Mw/g mol−1 w/106w wCl/106w Ts/K Td/K b purity %

[P8,8,8,14]

[P8,8,8C6P8,8,8]

[P8,8,8C10P8,8,8]

DOSS

Cl

DOSS

Cl

DOSS2

Cl

DOSS2

905.4 127 69 575 598 96.5

603.5 214 99.2

989.6 121 61 579 592 96.2

896.3 245 98.7

1668.6 142 78 603 629 97.2

952.4 268 98.1

1724.7 136 74 597 615 97.6

a Mass fraction which is also known as weight fraction is ratio of the mass of one component of a solution to the total mass of the solution. bPurity of the present ILs was determined using the methods reported by Stark et al.28

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ammonium ILs. This is an important finding to suggest the chemical stabilities of phosphonium ILs.25 In addition, most of the phosphonium-based ILs are recognized to be thermally more stable than many nitrogen-based ILs.8 As reported by Tsunashima et al., the mechanism details of the thermal decomposition for phosphonium ILs remain unclear. In the decomposition processes, less volatile phosphorus-containing species (such as organophosphates, polyphosphates derivatives, and organophosphine oxides) appear most expected to be present.25 The decomposition temperatures of the prepared phosphonium ILs are similar to that reported for other monocationic phosphonium-based ILs: the Td of [P6,6,6,14][3-Triz], [P6,6,6,14]tetrazolide, [P6,6,6,14][4,5-climide], and [P6,6,6,14][4-NO2pyra] is 601, 602, 628, and 580 K, respectively.26 The present phosphonium-based ILs showed lower decomposition temperatures compared to other monocationic phosphonium ILs with shorter alkyl chain length: the Td of [P2,2,2,8]NTf2 and [P2,2,2,12]NTf2 is 673 and 653 K, respectively.25 In addition, the synthesized ILs showed lower decomposition temperatures compared to the [P6,6,6,14] incorporated with other anions: the T d of [P 6,6,6,14 ]NfT 2 , [P 6,6,6,14 ]TfO, [P 6,6,6,14 ]BF 4 , and [P6,6,6,14]SAC is 653, 678, 643, and 638 K, respectively. Moreover, the present phosphonium-based dicationic ILs showed higher decomposition temperatures compared to C10(P3,3,3)2Br2, C12(P3,3,3)2Br2, and PEG3(P3,3,3)2Br2 (the Td of these ILs is in the range 513−573 K) and lower decomposition temperatures compared to the same ILs that were incorporated with the anion NfT2 (the Td of these ILs is in the range 683−698 K).8 The decomposition temperature of the present ILs is higher than that of the imidazolium-based dicationic ILs incorporating glycine and proline anions (for [bis(mim)C2][Gly]2, [bis(mim)C2][Pro]2, [bis(mim)C4][Gly]2, and [bis(mim)C4][Pro]2 the decomposition temperatures are (433, 433, 473, and 479) K, respectively), while it is lower than the imidazolium-based dicationic ILs incorporating the bis(trifluoromethylsulfonyl)imide anion (for [bis(mim)C6][Tf2N]2 and [N111-C6-(mim)][Tf2N]2 the decomposition temperatures are 685 and 699 K, respectively).27 The decomposition temperatures of the DCILs are slightly affected by the spacer alkyl chain length; the decomposition temperature decreases, whereas the spacer alkyl chain length increases as reported by Jared et al.7 for DCILs. This is likely to be due to an increase of reactivity with increase of alkyl chain length. The purity of the prepared ILs was estimated using the method reported by Stark et al.28 and reported in Table 1. Density. The experimental density values of the present phosphonium-based ILs at atmospheric pressure and temper-

RESULTS AND DISCUSSION The structures of the present synthesized ILs were confirmed using elemental analysis (CHNS) and NMR. The results were reported in the Supporting Information, and they confirmed the desired structures. The influence of impurities (such as water and halide) on the physiochemical properties of ILs was well recognized. The water existence in ILs is due to either ineffective drying after preparation or absorption from the atmospheric air. The impurities such as water and halide may significantly impact the physicochemical properties of ILs. The existence of water could have an intense effect on the properties such as thermal stability, density, viscosity, and refractive index (the chloride existence, for example, increases the viscosity of the ionic liquids, while the water existence, or other cosolvents, decreases the viscosity). Thus, cautious attention is to be given to the selected synthetic methods to ionic liquids, particularly when physicochemical properties are to be determined. Purity measurements must always accompany reported physicochemical data. The mass fraction values of chloride content and water content of the present ILs are reported in Table 1 and are similar to other phosphoniumbased ILs.23,24 The start and decomposition temperatures for weight loss (Ts and Td) of the prepared ILs were reported in Table 1, and the thermograms (obtained using TGA) were reported in the Supporting Information (Figure S1). The decomposition temperature for the present ILs follows the order [P 8 , 8 , 8 C 6 P 8 , 8 , 8 ]DOSS 2 > [P 8 , 8 , 8 C 1 0 P 8 , 8 , 8 ]DOSS 2 > [P6,6,6,14]DOSS > [P8,8,8,14]DOSS. Therefore, the decomposition temperature of these ILs relies on the alkyl chain length. The decomposition temperature for the present phosphoniumbased ILs is generally higher than that of the imidazoliumbased ones (Td of [C2CNBim]DOSS and [C2CNDim]DOSS is 580 and 534 K, respectively).21 The absence of acidic protons in the tetraalkyl phosphonium ILs made them thermally more stable than their equivalent imidazolium.10 Tsunashima and Sugiya studied the chemical stabilities of phosphonium cations and corresponding ammonium cations by comparing their acidities using 1H NMR chemical shifts of methylene groups (CH2) adjacent to the cationic centers of cations. The results showed that the chemical shifts of the protons in phosphonium ILs significantly shifted to a high magnetic field compared to that for the corresponding ammonium ILs. The shifts detected are ascribed to comparatively high electron density of the protons in the phosphonium cations, which is an indication of the lower acidity of phosphonium ILs compared to that of the D



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Table 2. Experimental Density Values (ρ) for [P6,6,6,14]DOSS, [P8,8,8,14]DOSS, and [P8,8,8CnP8,8,8]DOSS2 at Atmospheric Pressure As a Function of Temperature (T)a,b ρ/(g cm−3) T/K

[P6,6,6,14]DOSS

[P8,8,8,14]DOSS

[P8,8,8C6P8,8,8]DOSS2

[P8,8,8C10P8,8,8]DOSS2

293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15

0.991 0.988 0.985 0.981 0.978 0.975 0.972 0.968 0.965 0.962 0.959 0.955 0.952

0.975 0.972 0.969 0.965 0.962 0.959 0.956 0.952 0.949 0.946 0.943 0.940 0.936

0.966 0.963 0.960 0.957 0.953 0.950 0.947 0.944 0.940 0.937 0.934 0.931 0.929

0.958 0.954 0.951 0.948 0.944 0.941 0.938 0.934 0.931 0.928 0.924 0.921 0.918

a

Pressure = 101.7 kPa. bStandard uncertainties u of temperature and pressure were 0.05 K and 1 kPa, respectively. The combined standard uncertainties of the density for the ILs [P6,6,6,14]DOSS, [P8,8,8,14]DOSS, [P8,8,8C6P8,8,8]DOSS2, and [P8,8,8C10P8,8,8]DOSS2 were 0.015, 0.016, 0.012, and 0.011, respectively.

Table 3. Experimental Viscosity Values (η) for [P6,6,6,14]DOSS, [P8,8,8,14]DOSS, and [P8,8,8CnP8,8,8]DOSS2 ILs at Atmospheric Pressure As a Function of Temperature (T)a,b η/(mPa s) T/K

[P6,6,6,14]DOSS

[P8,8,8,14]DOSS

[P8,8,8C6P8,8,8]DOSS2

[P8,8,8C10P8,8,8]DOSS2

293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15

1795 1272 912 668 496 374 286 222 174 138 111 91 75

2051 1447 1029 742 553 417 316 245 193 154 123 101 83

10138 6637 4489 3120 2203 1587 1134 798 578 427 326 243 202

18767 11761 7690 5213 3625 2565 1844 1355 1012 718 537 403 324

a

Pressure = 101.7 kPa. bStandard uncertainties u of temperature and pressure were 0.05 K and 1 kPa, respectively. The combined standard uncertainties of the viscosity for the ILs [P6,6,6,14]DOSS, [P8,8,8,14]DOSS, [P8,8,8C6P8,8,8]DOSS2, and [P8,8,8C10P8,8,8]DOSS2 were 2.3, 2.4, 2.0, and 1.8, respectively.

ature range T = (298.15 to 353.15) K are reported in Table 2. IL density is primarily determined by two important factors: molecular packing and cation−anion. Typically, the IL anion strongly affects the IL density; a relatively long alkyl chain can reduce the IL density. The influence of DOSS anion on density is lower compared to other sulfonate-based anions such as sulfobenzoic acid, benzene sulfonate, dodecylsulfate, and trifluoromethanesulfonate as reported previously by our groups.29 The lower density of the DOSS anion is likely to be due to the long alkyl chains compared to the other anions which prohibit the formation of tight molecular assemblies leading to a lower density as reported by Benjamin et al.30 Moreover, the lower density of the DOSS anion can also be explained by the large free volume and the weak localized charge which decreases the possibility of a strong ion pairing with the imidazolium cation causing a lower density. Moreover, adding −CH2− groups to the alkyl chain of cation reduces the density, whereas the larger hydrophilic anions increase the density of the IL. The effect

of hydrophilic anions on density may be due to strong hydrogen bonding and molecular attraction which increases the molecular agglomeration.31 The density for the prepared phosphonium-based ILs is in comparison with that reported for [P8,8,8,14]Br, [P6,6,6,14]OTf, [P 6 , 6 , 6 , 1 4 ][4-NO 2 imid], [P 6 , 6 , 6 , 1 4 ][4,5-Climid], and [P6,6,6,14]NTf2, lower compared to [P6,6,6,14][3-Triz], [P6,6,6,14][Tetrazolide], [P2,2,2,12]NTf2, and [P2,2,2,8][4-NO2pyra], and higher compared to [P6,6,6,14]DCA and [P6,6,6,14]Cl.10,15,24,26 The density for the present dicationic ILs is lower than the 39 imidazolium and pyrrolidinium-based ILs reported by Jared et al.,7 and the density of these DCILs was in the range (1.27 to 1.61) g cm−3. Moreover, the present phosphonium-based dicationic ILs showed higher density related to the phosphonium-based dicationic ILs C12(P3,3,3)2NTf2 and PEG3(P3,3,3)2NTf2 (the densities of these ILs are 1.4514 and 1.4469 g cm−3, respectively).8 The dicationic phosphonium ILs showed density which is similar to ammonium-based ILs but still somewhat lower than dicationic imidazolium-based ILs.8 E



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Table 4. Experimental Refractive Index Values (nD) for [P6,6,6,14]DOSS, [P8,8,8,14]DOSS, [P8,8,8,14]Cl, [P8,8,8CnP8,8,8]Cl2, and [P8,8,8CnP8,8,8]DOSS2 ILs at Atmospheric Pressure As a Function of Temperature (T)a,b nD [P6,6,6,14]

[P8,8,8,14]

[P8,8,8C6P8,8,8]

[P8,8,8C10P8,8,8]

T/K

DOSS

Cl

DOSS

Cl2

DOSS2

Cl2

DOSS2

298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15

1.474 1.472 1.470 1.469 1.467 1.465 1.464 1.462

1.478 1.477 1.475 1.474 1.472 1.470 1.469 1.467

1.469 1.468 1.466 1.464 1.463 1.461 1.459 1.458

1.491 1.490 1.488 1.487 1.486 1.484 1.483 1.481

1.475 1.474 1.472 1.471 1.469 1.468 1.466 1.465

1.487 1.486 1.484 1.483 1.481 1.480 1.478 1.477

1.475 1.473 1.472 1.470 1.469 1.467 1.465 1.464

a

Pressure = 101.7 kPa. bStandard uncertainties u of temperature and pressure were 0.2 K and 1 kPa, respectively. The combined standard uncertainties of the refractive index for the ILs [P6,6,6,14]DOSS, [P8,8,8,14]DOSS, [P8,8,8C6 P8,8,8]DOSS2, and [P8,8,8C10P8,8,8]DOSS2 were 0.016, 0.017, 0.013, and 0.011, respectively.

the van der Waals interactions. The increase in viscosity with increasing alkyl chain length, hydrocarbon linkage chain length, and molecular weight was noted for a large series of ILs.36 A recent experimental study showed that the viscosity is dependent on the interactions (includes not only Coulomb interaction but also van der Waals and H-bonding interactions which lead to a complex structural/nanostructural organization) between the cations and anions (the anions that are capable of forming stronger H-bonding network with the cations will lead to a higher viscosity of ILs) and the ion stacking (which result from the H-bonds) between ion pairs fo IL (simplest repeat unit in IL).37,38 An increase in temperature caused a substantial decrease in the viscosity of the studied ILs. Variation of viscosity in relation to temperature reflects information about the structure of ILs.39 The effect of temperature (293.15−353.15) K on viscosity of the present ILs was studied, and the plots were fitted with the logarithmic form of the Arrhenius equation (the ILs with asymmetric cations obey Arrhenius law) as in eq 1.34

The density of the phosphonium-based ILs increased with decreasing length of the hydrocarbon linkage chain and also with alkyl chain length (same trend was reported for imidazolium-based dicationic ILs), and the density of [P8,8,8C10P8,8,8]DOSS2 ILs is lower compared to that of [P8,8,8C6 P8,8,8]DOSS2. It was reported for a large series of dicationic ILs that the decrease of the linkage alkyl chain length will increase the density.7,32 Viscosity. The values of viscosity for the present ILs are presented in Table 3. [P8,8,8C10P8,8,8]DOSS2 has the highest value, and [P6,6,6,14]DOSS has the lowest viscosity value among the prepared ILs. The effect of incorporation of the DOSS anion in viscosity was reported previously. As the results show, the higher viscosity of the DOSS anion is a result of its larger size, which is consistent with the literature.29 In addition, low anionic basicity is one of the factors that determines the viscosity; increasing anion basicity leads to tighter cation− anion pairing, which also increases the intermolecular forces such as hydrogen bonding.30 The DOSS anion incorporates with donating groups (alkyl chains) which increase the negative charge of the SO3− group and hence increase the basicity. The present ILs showed a wide range of high viscosity which is a characteristic of most ILs.33 However, the high thermal stability exhibited by ILs allows for applications at higher temperatures where the viscosity is reduced. The viscosity values for the prepared ILs are higher in comparison to other phosphonium-based ILs; the viscosity of [P8,8,8,,8]NTf2, [P8,8,8,8] dithiomalonitrile, [P6,6,6,14][3-Triz], [P6,6,6,14][4NO2pyra], and [P6,6,6,14][4-NO2imid] is 418, 5590, 438, 716, and 735 mPa s, respectively.13,26 Moreover, the present phosphonium-based dicationic ILs showed higher viscosity compared to the phosphonium-based dicationic ILs C12(P3,3,3)2 NTf2 and PEG3(P3,3,3)2NTf2 (the viscosity of these ILs at 303 K is 1265.83 and 460.85, respectively).8 The viscosity increases with increasing alkyl chain length and molecular weight. Similar results were noticed for the ILs with alkylammonium cations.34 The high viscosities of the present ILs are a result of the long alkyl chain of both the cation and DOSS anion (the increased volume of the anion and cation here lead to higher viscosity through reduction in ion mobility).35 The large volume of the present ILs causes low ion mobility and hence high viscosity. Further, the higher viscosity of the present phosphoniumbased ILs is attributed to the long alkyl chains which increase

ln η = ln η∞ +

Eη RT

(1)

where η, η∞, Eη, R, and T are the viscosity, viscosity at infinite temperature, activation energy for viscous flow, universal gas constant, and temperature in Kelvin. The activation energies for viscous flow (Eη) and viscosity at infinite temperature (η∞) were calculated from the intercept and slope (respectively) of the Arrhenius plots. Table S1 shows the Arrhenius parameters obtained from the Arrhenius plots. The Eη value could be linked to structural information about the ILs to estimate the level of energy required by the ions to move freely inside the IL. Moreover, the viscosity at infinite temperature (η∞) is representative of a structural contribution of the ions on the viscosity;39,40 at infinite temperature, (η∞) is governed only by the geometric structure of the ions in the IL.40 The prepared ILs showed lower activation energies in comparison to that of imidazolium-based ILs described by Sánchez et al.39 (activation energies for [Bmim]BF4 and [Amim]BF4 are (33.53 and 49.18)·103 KJ mol−1) and are comparable to [Bmmim]BF4, [Bmim]PF6, and [Bmim]CF3SO3 ILs that have activation energies within the range of 21−26.17 KJ mol−1.40 The viscosity at infinite temperatures (η∞) for the prepared ILs is higher in comparison to nitrilefunctionalized ILs and dual-functionalized imidazolium-based F



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Table 5. Experimental Solubility Data for CO2 in Phosphonium-Based ILs at Temperature 298 K CO2 (mol fraction) pressure (kPa)

[P6,6,6,14]DOSS

[P8,8,8,14]DOSS

[P8,8,8C6P8,8,8]DOSS2

[P8,8,8C10P8,8,8]DOSS2

100 500 1000 1500 2000

0.049 0.225 0.384 0.511 0.600

0.057 0.263 0.438 0.573 0.685

0.046 0.211 0.341 0.458 0.556

0.043 0.182 0.320 0.421 0.511

a

Standard uncertainties u of temperature, pressure, and MSB (mass reading) are 0.05 K, 0.25 kPa, and 0.00002 g, respectively. The combined standard uncertainties of the CO2 solubility for the ILs [P6,6,6,14]DOSS, [P8,8,8,14]DOSS, [P8,8,8C6 P8,8,8] DOSS2, and [P8,8,8C10 P8,8,8] DOSS2 were 0.016, 0.017, 0.013, and 0.011, respectively.

ILs. These results showed that the structural contribution of the phosphonium-based IL ions on the viscosity is high, which might be due to the large number of alkyl chains accomplished by increasing the van der Waal’s interactions between the alkyl chains. Refractive Index. The measured data of the refractive index of the present ILs within the temperature range from (298.15 to 333.15) K are reported in Table 4. The values of refractive index for the prepared ILs are in complete agreement with that noticed for the nitrile-functionalized ILs; for [C3CN Mim]NTf2, [C3CN Mim]BF4, [C2CN Bim]Br, and [C2CN Oim]Br the refractive index is in the range of (1.4349− 1.54540).18,41 The refractive index values for the studied ILs are similar to that noticed for [P4,4,4,1][CH3SO4], [P4,4,4,2][(C2H5O)2PO2], and [P6,6,6,14]OAc (1.47632, 1.46618, and 1.48176, respectively) and lower compared to [P4,4,4,1][Tos] (1.52030).10,24 The results showed that the refractive index for the ILs incorporated with the chloride anion is greater than the ILs incorporated with the DOSS anion. This result is a reflection of the large free volume of the DOSS anion as reported by Brocos et al.;42,43 the unfilled part of the molar volume of a substance has smaller refractive index. In addition, the higher refractive index for the ILs incorporating with the chloride anion is attributed to the relatively large polarizability of the chloride anion.10 These results indicate that the cation type and the alkyl chain of the cation and anion have a pronounced effect on the refractive index. Moreover, the refractive indices of the prepared phosphonium-based ILs increase with decreasing the alkyl chain spacer. The molar free volume for the synthesized ILs was estimated using the molar volume and molar refraction values as shown in eq 2.24

Vf = Vm − RM

Vm =

ρ /(g·cm−3) = A 0 + A1T

(5)

nD = A 2 + A3T

(6)

where ρ, T, and nD are density, temperature in Kelvin, and refractive index respectively; A0, A1, A2, and A3 are correlation coefficients. The coefficients of correlation were determined using linear regression analysis and reported together with the standard deviations (SD) in the Supporting Information together with their standard deviations (Table S3 and Table S4). The standard deviations were estimated by applying the expression given in eq 7.45 n

SD =

∑ j DAT (Zexptl + Zcalcd)2 nDAT

(7)

where nDAT is the number of experimental points and Zexptl and Zcalcd are the experimental and calculated values, respectively. CO2 Solubility. CO 2 solubility in the synthesized phosphonium-based ILs incorporating DOSS anion at temperature 298.15 K and pressures 1, 5, 10, 15, and 20 bar is measured and reported as mole fraction in Table 5. Moreover, the time dependence of CO2 uptake by the studied ILs is reported in Tables S5−S8 in the Supporting Information. In general, the monocationic-based phosphonium ILs ([P6,6,6,14]DOSS and [P8,8,8,14]DOSS) show higher CO2 solubility compared to the dicationic-based phosphonium ILs ([P8,8,8C10P8,8,8]DOSS2 and [P8,8,8C6P8,8,8]DOSS2). For the monocationic-based phosphonium ILs, the CO2 solubility was higher in [P8,8,8,14]DOSS compared to [P6,6,6,14]DOSS, as a result of the increase of the alkyl chain length. As expected, larger free volume (the method used to calculate the free volume was presented in the Supporting Information) originating from the longer alkyl chain of the cation makes the CO2 more soluble in IL with long alkyl chain. This trend is comparable to that of the imidazolium-based ILs.20 However, the dicationic-based phosphonium ILs have a greater free volume compared to the monocationic-based

(2)

1 yzz zz 2 z{

(4)

where M and ρ are the molar mass (g mol−1) and density (g cm−3), respectively. The results showed that the molar free volumes for the present phosphonium-based ILs [P6,6,6,14]DOSS, [P8,8,8,14]DOSS, [P8,8,8C6P8,8,8] DOSS2, and [P8,8,8C10P8,8,8]DOSS2 are 964.1, 1060.9, 1734.9, and 1822.1 cm3 mol−1, respectively. The experimental values of density and refractive index could be represented by the empirical eqs 5 and 6, respectively, as a function of temperature.19,45

where Vf, Vm, and RM are the molar free volume, molar volume (cm3 mol−1), and molar refraction (cm3 mol−1), respectively. The molar refraction which is usually considered as a measure of hard-core molecular volume was estimated using the following Lorentz−Lorenz24,44 relationship ij n 2 − RM = Vmjjj D2 jn + k D

M ρ

(3)

where nD is the refractive index. The molar volume was estimated using the following equations G



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to the longer alkyl side chain of the polycation (of the polymerized ILs) which may pose a steric hindrance that prohibits CO2 sorption as a result of the decrease of microvoid volume with fewer interactions between the cation and CO2 as reported by Privalova et al.52 Moreover, the present ILs showed higher CO2 solubility compared to the polymerized ILs incorporating amino acids as anions (for poly[VBTMA][Arg] and poly[VBTMA][pro] the solubilities are 0.530 and 0.380 mole fraction, respectively).20 Thermodynamic Parameters. Knowledge and understanding of the thermodynamic properties and equilibrium solubility behavior for new ILs allow for the development and optimization of the separation processes.53,54 The gas solubility in liquids is often reported using the Henry constant which correlates the substance equilibrium mole fraction and its partial pressure in the gaseous phase.55 Henry’s constant value is an indication for the gas solubility in a solvent; the decrease of the value is an indication of the increases of gas solubility in the solvent. In addition, Henry’s constants are also used to classify the gas uptake into physical and chemical absorption. Usually, when Henry’s constant is less than 3 MPa at 298 K, this indicates the chemical absorption of CO2 into ILs.56 There are several forms of Henry’s law, each of which requires different dimensional units. One of these forms is the expression of the solute concentration as mole fraction or molality. Henry’s law constant is usually calculated by using the fugacity coefficient. However, the ionic liquids have negligible vapor pressure due to its nonvolatility; hence, the gas phase can be considered to be purely component. In addition, if the gas phase behaves ideally, the fugacity is equivalent to the gas pressure above the IL sample (the fugacity coefficient is close to 1). The experimental solubility data can be used to estimate Henry’s law constant by taking the slope of the tangent line at zero concentration from the plot of the experimental equilibrium pressure in terms of the mole fraction of gas in the ionic liquid phase.57−59 Due to their relatively low vapor pressure, the fugacity of the ILs was approximately simplified to be equal to the pressure of the ILs. Thus, Henry’s law constant on the basis of mole fraction was calculated. Equation 8 is generally used to estimate the Henry’s law constant (kH) from the solubility data at equilibrium conditions and infinite dilution.60

phosphonium ILs studied, and the CO2 solubility in these ILs was lower. This result is a consequence of the extremely high viscosity of the dication in comparison to the monocationbased phosphonium ILs (at 298.15 K, the viscosity of [P8,8,8,14]DOSS and [P6,6,6,14]DOSS is 1272 and 1447 mPa s and for [P8,8,8C6P8,8,8]DOSS2 and [P8,8,8C10P8,8,8]DOSS2 is 6637 and 117617 mPa s, respectively). The high viscosity of the DCILs was ascribed to an increase of hydrogen bonding, van der Waals forces, and the symmetry of the anion and cation,46 and the increases of the viscosities of ILs (up to 2 orders of magnitude) during the solubility process of CO2 which results in extremely slow absorption kinetics and the absorption capacity remain poor.47 Moreover, the dicationic ILs incorporating a long alkyl chain are more likely to have a steric hindrance which restrains CO2 sorption.20 In addition, the CO2 solubility in these ILs is inversely proportional to the spacer alkyl chain length. It was noticed that not only interaction strength and free volume are responsible for the solubility of CO2 in ionic liquids as reported in some studies.48−50 In addition, at ambient pressure, the molar volume is directly proportional to the chain length, and this effect is reduced with large amounts of dissolved CO2. The molar volumes of each IL become very comparable.51 The CO2 solubility in the present ILs increased speedily (0.3 to 0.4 mole fraction) with increasing pressure up to 10 bar. When the pressure exceeded 10 bar, the increasing rates of solubility decreased, and ultimately the solubility leveled off. The high rate of solubility at low pressure is likely to be due to the Henry sorption in the interion space. The large volume of cation and anion expands the interion spaces where CO2 can squeeze in. On expansion of the interion spaces between cation and anion, CO2 molecules can easily penetrate into that space. However, with increasing pressure these spaces are incessantly occupied by CO2 molecules, which prevent the CO2 molecules from residing. It is required to expand the interion spaces in order to let more CO2 molecules get in. Since energy is required for the expansion, small amounts of CO2 can penetrate into the IL. Thus, ILs show particular leveling of solubility behavior. Seki et al.50 in a recent work showed that the interactions alone are not sufficient to provide a full description for the CO2 sorption. They recognized that “the CO2 solubility is not affected only by the strong Lewis acid−base interactions between the ILs and the dissolved CO2”.48 The solubility of CO2 in the ILs studied in this work is among the highest reported values. The CO2 solubility of the present ILs is higher than that reported for imidazolium-based ILs incorporating fluorinated anions such as bis(trifluoromethylsulfonyl)imide (NTf2).11 The results showed that the present ILs have higher affinity for CO2 with 0.551 to 0.685 mole fraction compared to the imidazolium-based ILs incorporating the same anion (for [CNC2Bim]DOSS, [CNC2Him]DOSS, [CNC2Oim]DOSS, and [CNC2Dim]DOSS the solubilities are (0.412, 0.443, 0.502, and 0.548) mole fraction, respectively).20 This result could be due to the long alkyl chains of the phosphoniumbased ILs which increase the dispersion force of the cation for better interaction with CO2.51 The CO2 solubility in the present ILs is higher compared to the polymerized ILs (for poly[VBTEA][NO 3 ], poly[VBTMA][NO 3 ], and poly[METMA][NO3] the solubilities are 0.133, 0.177, and 0.066 mole fraction, respectively).20 The high CO2 solubility in the present ILs compared to these polymerized ILs could be due

i PCO y kH = lim jjjj 2 zzzz x → 0k x {

(8)

where P and kH(T) are the partial pressure of the gas and Henry’s law constant (have units of pressure), respectively. Generally, kH is inversely related to the mole fraction of a gas, and the solubility of the gases with nearly ideal behavior is linearly associated with the pressure. Therefore, Henry’s constant was determined by using the linear slope of the data. CO2 gas shows a nonlinear tendency with increasing CO2 pressure for the studied ILs as shown in Figure 3 (the flattening out of the curve is an indication that the IL is starting to reach its maximum, pressure-independent capacity for CO2).61 Hence, Henry’s constant can be calculated by fitting a second-order polynomial to the solubility values and estimating the limiting slope when the absorption of CO2 approaches zero.54,62 Accordingly, eq 9 was used to model the H



Article

The results of temperature effect in solubility of CO2 in [P8,8,8,14]DOSS IL were reported in Table 6. The solubility of CO2 is inversely proportional to temperature, which is in agreement with the results obtained in most of the reports of gas dissolution into liquid.56 Enthalpy, Entropy, and Gibbs Free Energy. Standard enthalpy (ΔH°), Gibbs free energy (ΔG°), and entropy (ΔS°) of gas dissolution can be estimated by virtue of the temperature effect on the gas solubility.53,67 The dependence of the measured solubility on temperature is related to the solvation thermodynamic properties. At infinite dilution and low pressure, Henry’s constant (KH) can be used to describe the thermodynamic solution properties.61 The enthalpy of solvation provides a hint of the strength of interactions between the gas and the IL, while the entropy explains the extent of organization existing in the IL/gas mixture.11 The standard enthalpy (ΔH°), entropy (ΔS°), and Gibbs free energy (ΔG°) of the gas solubility were estimated from the calculated Henry’s constants. The standard Gibbs free energy was calculated using eq 11, where P0 is a standard pressure, the value of which is taken to be 1.01325 bar.67

Figure 3. Solubility of CO2 in phosphonium-based ILs as a function of pressure for (○) [P6,6,6,14]DOSS, (●) [P8,8,8,14]DOSS, (⧫) [P8,8,8C6P8,8,8]DOSS2, and (□) [P8,8,8C10P8,8,8]DOSS2.

experimental values63 with a correlation coefficient (R2) greater than 0.996. kH = ax 2 + bx + c

ij P 0 yz ΔG° = −RT lnjjj zzz jk z k H{

(9)

Henry’s law constant at infinite dilution (kH) was calculated by using eq 10. The results along with the correlation coefficients of the polynomial equation were presented in Table 6.

(11)

The standard heat of the gas solubility (ΔH°) is correlated with Henry’s law constant at infinite dilution using eq 12.67

i PCO y kH = lim jjjj 2 zzzz = b x → 0k x { (10) The behaviors of solubility are noticeably explained in terms of thermodynamic excess Gibbs enthalpy and entropy energies functions. More negative values in excess Gibbs generally show formation of some chemical complexes. Excess Gibbs energy around 50 mol % and 33 mol % of the CO2 + IL system indicates 1:1 and 1:2 complex formation, respectively.64 The DOSS-based IL shows lower kH value compared to other anions which may be due to the high affinity of the DOSS anion for CO2 compared to the ILs incorporating other sulfonated anions such as dodecylsulfate, trifluoromethylsulfonate, sulfobenzoic acid, and benzenesulfonate. The DOSS anion has several features that are known to improve the attraction between the IL molecule and CO2 which lead to good solubility of CO2 (DOSS anion composed of sulfonyl, carbonyl, ether, long branched, and unbranched alkyl chains which are known to have strong affinity to CO2 compared to the other nonfunctionalized anions).11,65 In addition, another major factor that affects the capacity of CO2 solubility is the ability of the IL to form large free volume. In general, the molecular size of the anion is directly proportional to the free volume in which CO2 can occupy.66

ΔH ° RT

ln kH = ln k 0 +

(12) 67

i ΔH ° − ΔG° yz zz ΔS° = jjj T { k

The standard entropy was estimated using eq 13.

(13)

The standard enthalpy (ΔH°), entropy (ΔS°), and Gibbs free energy (ΔG°) for CO2 dissolution in the [P8,8,8,14]DOSS ILs at different studied temperatures were reported in Table 7. Table 7. Thermodynamic Properties for the Solution of CO2 in [P8,8,8,14]DOSS IL temperature (K) thermodynamic property −1

ΔG° (kJ mol ) ΔH° (kJ mol−1) ΔS° (J mol−1 K−1)

298

313

343

5.54 −14.45 −73.23

8.98 −7.27 −51.90

11.00 −2.98 −40.77

As indicated above, the increase in kH with increasing T is inversely proportional to the mole fraction (x) of CO2 solubility in the ILs. ΔG° positive value is an indication of a nonspontaneous process of the CO2 solubility in the [P8,8,8,14]DOSS IL at temperature range 298−343 K,67 whereas

Table 6. Henry’s Law Constant and Correlation Coefficients for the Studied ILs 298 K

313 K

IL

kH

R2

[P6,6,6,14]DOSS [P8,8,8,14]DOSS [P8,8,8C6P8,8,8]DOSS2 [P8,8,8C10P8,8,8]DOSS2

13.33 11.62 17.47 19.17

0.999 0.999 0.998 0.999 I

343 K

kH

R2

kH

R2

31.88

0.998

48.02

0.999



Article

ΔH° negative value shows more solubility of CO2 in the ILs in the stated temperature range. ΔH° showed very small decreases from moderately strong acid−base bonds at 298 K to weak acid−base bonds at 343 K. The direct proportion of ΔG° with T shows that the CO2 solubility process will require more energy as the temperature increases.67 The solubility of CO2 in the [P8,8,8,14]DOSS IL is associated with an unfavorable change in entropy (ΔS°) between 298 and 343 K.67 The negative values noticed for the solvation entropy are most likely due to the effect of the structure produced by the interactions between the IL charged centers and the solute IL. It was stated (using molecular simulation) that the negative values for the entropy of solvation are referred to the interactions between the solute and charged centers of ionic liquid.68 The entropy of solvation is related to the solvent organization surrounding the solute; the more negative entropy indicates a higher degree of ordering when CO2 dissolves in these ILs.69 It is well-known that ILs have a tendency to form phase-separated self-assembled structures as the alkyl chain length increases, which indicates that the entropy is inversely proportional to the alkyl chain length.70 Moreover, the basicity of the anions (DOSS) produces more tightened cations−anions, which also increase the intermolecular forces such as hydrogen bonding. Also, increased volumes of the cation and anion (here leading to a reduction in ion mobility) affect the disorder of the system when CO2 dissolves in these ILs.30 The entropy of CO2 in [P8,8,8,14]DOSS is higher than that in [THTDP][NTf2] and [THTDP][Cl] which are −32.31 and −53.86 J mol−1 K−1, respectively.11 The result indicated a stronger structural solvation interaction for the CO2− [P8,8,8,14]DOSS system. The enthalpy of solvation of the CO2 in [P8,8,8,14]DOSS is higher than in [THTDP][NTf2] (10.54 kJ mol−1) and lower than in [THTDP][Cl] (17.86 kJ mol−1). The result shows that the interaction between the CO2 and [P8,8,8,14]DOSS is stronger than the interaction between the CO2 and the [THTDP][NTf2] and weaker than that between the CO2 and the [THTDP][Cl].11 The present ILs showed enthalpies of solvation which are comparable to those reported for the particular imidazolium-based ionic liquids [C5mim][NTf 2 ], [C 4 mim][trifluoroacetate], and [C 4 mim][dicyanamide].11,71,72

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AUTHOR INFORMATION

Corresponding Author

*E mail: [email protected]; [email protected]; [email protected]. ORCID

Abobakr Khidir Ziyada: 0000-0003-3529-2755 Funding

The authors would like to acknowledge the Center of Research in Ionic Liquids (CORIL) and Universiti Teknologi PETRONAS for their lab facilities and sponsorship for Abobakr K. Ziyada. Notes

The authors declare no competing financial interest.

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CONCLUSIONS New experimental results for the density, viscosity, refractive index, thermal stability, and solubility of the CO2 in the synthesized ionic liquids [P6,6,6,14]DOSS, [P8,8,8,14]DOSS, [P8,8,8C6P8,8,8]DOSS2, and [P8,8,8C10P8,8,8]DOSS2 are measured and reported for a temperature range of (293.15 to 353.15) K and at atmospheric pressure (CO2 measured at a pressure range of (1 to 20 bar)). The solubility data and the effect of the temperature on the gas solubility were used to calculate the Henry’s law constant, Gibbs free energy (ΔG°), standard enthalpy (ΔH°), and entropy (ΔS°) of gas dissolution.

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CO2 solubility in ILs, Arrhenius parameters obtained from plotting of ln η against 1/T, fitting parameters of density (ρ), refractive indices (nD) and standard deviations (SDs), method used to estimate the free volume, and the experimental solubility data for CO2 in the synthesized ILs (PDF)

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00109. Abbreviations of ILs presented, NMR and elemental analysis results, TGA thermograms, measurement of J



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