Effect of Cation Modification on the Physicochemical Properties and CO2 Solubility: Nonfluorinated Phosphonium-Based Ionic Liquids Incorporating a Dioctylsulfosuccinate Anion

Article abstract of DOI:10.1021/acs.jced.8b00109

In the current work, a series of phosphonium-based monocationic and dicationic ionic liquids (trihexyltetradecylphosphonium dioctylsulfosuccinate, [P_6,6,6,14]DOSS; trioctyltetradecylphosphonium dioctylsulfosuccinate, [P_8,8,8,14]DOSS; 1,6-bis(trioctylphosphonium)hexane dioctylsulfosuccinate, [P_8,8,8C₆P_8,8,8] DOSS₂; and 1,10-bis(trioctylphosphonium)decane dioctylsulfosuccinate, [P_8,8,8C₁₀P_8,8,8]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.

Full text of DOI:10.1021/acs.jced.8b00109

Article
pubs.acs.org/jced
Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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
S
* Supporting Information
ABSTRACT: In the current work, a series of phosphonium-based
monocationic and dicationic ionic liquids (trihexyltetradecylphospho-
nium dioctylsulfosuccinate, [P6,6,6,14]DOSS; trioctyltetradecylphospho-
nium dioctylsulfosuccinate, [P8 , 8 , 8 , 1 4 ]DOSS; 1,6-bis-
(trioctylphosphonium)hexane dioctylsulfosuccinate, [P8,8,8C6P8,8,8
]
DOSS2; and 1,10-bis(trioctylphosphonium)decane dioctylsulfosucci-
nate, [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.
capture, and it is not necessary to have fluorination to achieve
INTRODUCTION
■
higher CO2 solubility.4,5
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 attractive-
ness 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 CO2-
philic 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 degrad-
ability, costly, and undesirable environmental impact.3 There-
fore, it is important to synthesize ILs with enhanced CO2
solubility and without fluorination. Recently, phosphonium-
based ILs were reported as suitable alternatives for CO2
In comparison to other ILs, the notable characteristics of
phosphonium ILs are their thermal, chemical, and electro-
chemical 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 CO2
solubility of the trihexyl and trioctyl phosphonium-based
monocationic and dicationic ILs incorporating dioctylsulfo-
succinate anions have not been studied. Based on previous
studies of the preparation, characterization, physicochemical
properties, and CO2 solubility of new ILs,17−20 trihexylte-
tradecylphosphonium dioctylsulfosuccinate [P6,6,6,14]DOSS,
trioctyltetradecylphosphonium dioctylsulfosuccinate
Received: February 4, 2018
Accepted: August 23, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.jced.8b00109
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Figure 1. Synthesis route of trialkyltetradecylphosphonium dioctylsulfosuccinate.
Figure 2. Synthesis route of phosphonium-based dicationic ILs incorporating a dioctylsulfosuccinate anion.
[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.
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, Sigma-
Aldrich, 99.8%), acetone (67-64-1, Sigma-Aldrich, ≥99.5%),
diethyl ether (60-29-7, Sigma-Aldrich, 99%), trihexyltetrade-
cylphosphonium 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
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-dichlor-
B
DOI: 10.1021/acs.jced.8b00109
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gases used (carbon dioxide (CO2), methane (CH4), nitrogen
(N2), and helium (He)) is greater than 99.9%.
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 V-
3.81) 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
Synthesis of Trialkyltetradecylphosphonium Dioctyl-
sulfosuccinate. Trihexyltetradecylphosphonium chloride
[P6,6,6,14]Cl was purchased from Aldrich. Trioctyltetradecyl-
phosphonium chloride [P8,8,8,14]Cl was prepared using a round-
bottomed 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. Trialkyltetrade-
cylphosphonium 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 trihexylte-
tradecylphosphonium 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 chlor-
ide [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.
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 temper-
ature (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, respec-
tively, 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, Prazisionsmesstechnik GmbH, Bochum, Ger-
many. The MSB employs a magnetic suspension coupling
comprised of a suspension magnet and an electromagnet,
which sustain freely suspended contactless balance connec-
tions. 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 measure-
ment uncertainties of densities, viscosities, refractive indices,
and CO2 solubilities are mainly due to the impurities in the
ILs.
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
DOI: 10.1021/acs.jced.8b00109
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a
a
Table 1. Molecular Weight (Mw), Mass Fraction of Water (w), Mass Fraction of Bromide (wCl), Start Temperatures (Ts),
Decomposition Temperatures (Td), and Purity (Mass Fraction)
[P6,6,6,14
DOSS
]
[P8,8,8,14
]
[P8,8,8C6P8,8,8
]
[P8,8,8C10P8,8,8
]
Cl
DOSS
Cl
DOSS2
Cl
DOSS2
Mw/g mol−1
w/106w
wCl/106w
Ts/K
905.4
127
69
575
598
96.5
603.5
214
-
-
989.6
121
61
579
592
96.2
896.3
245
-
-
1668.6
142
78
603
629
952.4
268
-
-
1724.7
136
74
597
615
Td/K
-
-
-
b
purity %
99.2
98.7
97.2
98.1
97.6
a
b
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. Purity
of the present ILs was determined using the methods reported by Stark et al.28
ammonium ILs. This is an important finding to suggest the
RESULTS AND DISCUSSION
■
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 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 phosphonium-
based ILs.23,24
The decomposition temperatures of the prepared phospho-
nium 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 temper-
atures 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
Td of [P6,6,6,14]NfT2, [P6,6,6,14]TfO, [P6,6,6,14]BF4, 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 temper-
atures 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 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
[P8 ,8, 8C6P8, 8, 8]DOSS2 > [P8, 8, 8C10P8, 8, 8]DOSS2
>
[P6,6,6,14]DOSS > [P8,8,8,14]DOSS. Therefore, the decomposi-
tion temperature of these ILs relies on the alkyl chain length.
The decomposition temperature for the present phosphonium-
based ILs is generally higher than that of the imidazolium-
based 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
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-
1
by comparing their acidities using H 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
a b
,
Pressure As a Function of Temperature (T)
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
b
Pressure = 101.7 kPa. Standard 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
a b
,
Pressure As a Function of Temperature (T)
η/(mPa s)
[P8,8,8C6P8,8,8]DOSS2
T/K
[P6,6,6,14]DOSS
[P8,8,8,14]DOSS
[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
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
18767
11761
7690
5213
3625
2565
1844
1355
1012
718
537
403
324
75
202
a
b
Pressure = 101.7 kPa. Standard 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.
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,
[P6,6,6,14][4-NO2imid], [P6,6,6,14][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
The influence of DOSS anion on density is lower compared
to other sulfonate-based anions such as sulfobenzoic acid,
benzene sulfonate, dodecylsulfate, and trifluoromethanesulfo-
nate 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
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
a b
,
[P8,8,8CnP8,8,8]DOSS2 ILs at Atmospheric Pressure As a Function of Temperature (T)
nD
[P6,6,6,14
DOSS
]
[P8,8,8,14
]
[P8,8,8C6P8,8,8
]
[P8,8,8C10P8,8,8]
T/K
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
1.478
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
1.462
a
b
Pressure = 101.7 kPa. Standard 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 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
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 organiza-
tion) 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
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
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
−
Eη
negative charge of the SO3 group and hence increase the
ln η = ln η∞ +
basicity.
(1)
RT
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][4-
NO2pyra], 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
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
C12(P3,3,3
) NTf2 and PEG3(P3,3,3)2NTf2 (the viscosity of
2
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
comparison to that of imidazolium-based ILs described by
39
́
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 phosphonium-
based ILs is attributed to the long alkyl chains which increase
Sanchez et al. (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 nitrile-
functionalized 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.
M
ρ
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
Vm =
(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
ρ/(g·cm−3) = A0 + A1T
(5)
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, respec-
tively) 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
nD = A2 + 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
nDAT
j
2
∑
(Zexptl + Zcalcd)
SD =
nDAT
(7)
where nDAT is the number of experimental points and Zexptl and
Zcalcd are the experimental and calculated values, respectively.
CO2 Solubility. CO2 solubility in the synthesized
phosphonium-based ILs incorporating DOSS anion at temper-
ature 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.
V = Vm − RM
(2)
f
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
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
i
j
j
y
z
z
z
z
z
n − 1
D
j
RM = Vmj
2
j
k
nD + 2
(3)
{
where nD is the refractive index.
The molar volume was estimated using the following
equations
G
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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 monocation-
based 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
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 under-
standing 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
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
P
i
y
CO2
j
j
j
j
z
z
z
z
k = lim
H
x→0
x
(8)
k
{
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
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 phosphonium-
based 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][NO3], poly[VBTMA][NO3], 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
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
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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.
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.
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
experimental values63 with a correlation coefficient (R2)
greater than 0.996.
0
i
y
P
j
j
j
z
z
z
kH = ax2 + bx + c
ΔG° = −RT ln
(9)
j
j
z
z
kH
(11)
k
{
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.
The standard heat of the gas solubility (ΔH°) is correlated
with Henry’s law constant at infinite dilution using eq 12.67
ΔH°
RT
ln kH = ln k0 +
(12)
P
i
y
CO2
j
j
j
j
z
z
z
z
k = lim
= b
H
The standard entropy was estimated using eq 13.67
x→0
x
(10)
k
{
i ΔH° − ΔG° y
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, trifluoromethylsulfo-
nate, 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
j
j
j
z
z
z
ΔS° =
(13)
T
k
{
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
ΔG° (kJ mol−1
ΔH° (kJ mol−1
ΔS° (J mol−1 K−1
298
5.54
−14.45
−73.23
313
8.98
−7.27
343
)
)
11.00
−2.98
−40.77
)
−51.90
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
343 K
IL
kH
R2
kH
R2
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
31.88
0.998
48.02
0.999
I
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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
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)
AUTHOR INFORMATION
Corresponding Author
*E mail: abubakrkhz@yahoo.com; taha_a@jic.edu.sa;
abubakrkhz@gmail.com.
■
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 intermo-
lecular 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]-
[NTf2], [C4mim][trifluoroacetate], and [C4mim]-
[dicyanamide].11,71,72
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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■
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jced.8b00109.
■
S
Abbreviations of ILs presented, NMR and elemental
analysis results, TGA thermograms, measurement of
J
DOI: 10.1021/acs.jced.8b00109
J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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