Synthesis, Characterization, and Physicochemical Properties of Hydrophobic Pyridinium-based Ionic Liquids with N-Propyl and N-Isopropyl

Article abstract of DOI:10.1002/zaac.201800006

Synthesis and physicochemical properties of four pyridinium-based ionic liquids (ILs), N-propylpyridinium bromide [N-propylPyr]⁺[Br]^–, N-isopropylpyridinium bromide [N-isopropylPyr]⁺[Br]^–, N-propylpyridinium hexafluorophosphate [N-propylPyr]⁺[PF₆]^–, and N-isopropylpyridinium hexafluorophosphate [N-isopropylPyr]⁺[PF₆]^– are reported. The molecular structures of these compounds were characterized by FT-IR, ¹H, ¹⁹F, and ³¹P NMR, spectroscopy. The thermal properties, conductivity, and solubility of these ionic liquids were also investigated. The effects of propyl and isopropyl alkyl lateral chain at the N-position of pyridinium cation on the thermal stability, conductivity, and solubility of ionic liquids are discussed. The results obtained confirmed that the ionic liquids based on pyridinium cations exhibit higher decomposition temperature, low melting points, immiscible with water, and their conductivities are mainly influenced by mobility of ions.

Full text of DOI:10.1002/zaac.201800006

Title: Synthesis, characterization, and physicochemical properties of
hydrophobic pyridinium-based ionic liquids with N-propyl and N-
isopropyl
Authors: Mzukisi Matandabuzo and Peter A. Ajibade
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To be cited as: Z. anorg. allg. Chem. 10.1002/zaac.201800006
Link to VoR: http://dx.doi.org/10.1002/zaac.201800006

Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.201800006
Synthesis, characterization, and physicochemical properties of hydrophobic
pyridinium-based ionic liquids with N-propyl and N-isopropyl
Mzukisi Matandabuzo, Peter A. Ajibade*
School of Chemistry & Physics, College of Agriculture, Engineering & Science, University of
KwaZulu- Natal, P/Bag X01, Scottsville 3209, Pietermaritzburg, South Africa
Abstract
In this study, synthesis and physicochemical properties of four pyridinium-based ionic liquids
+
-
(
ILs): N-propylpyridinium bromide [N-propylPyr] [Br] , N-isopropylpyridinium bromide [N-
+
-
+
]-, and N-
isopropylPyr] [Br] , N-propylpyridinium hexafluorophosphate [N-propylPyr] [PF
6
+
-
isopropylpyridinium hexafluorophosphate [N-isopropylPyr] [PF
6
] are reported. The molecular
1
19
31
structures of these compounds were characterized by FT-IR, H, F, and P-NMR, spectroscopy.
The thermal properties, conductivity, and solubility of these ionic liquids were also investigated.
The effects of propyl and isopropyl alkyl lateral chain at the N-position of pyridinium cation on
the thermal stability, conductivity, and solubility of ionic liquids are discussed. The results
obtained confirmed that the ionic liquids based on pyridinium cations exhibit higher
decomposition temperature, low melting points, immiscible with water, and their conductivities
are mainly influenced by mobility of ions.
Keywords: Ionic liquids; pyridinium cation; hexafluorophosphate; thermal stability; conductivity;
solubility in water.
*Corresponding author email: ajibadep@ukzn.ac.za
1
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1
Introduction
Ionic liquids (ILs) are salts (organic and/or inorganic), with melting point relatively lower than
<100°C) and are liquid at room temperature. [1-2] They were first discovered in 1914, when
Weldon reported the physical properties of ethyl ammonium nitrate (C NH )NO with melting
(
2
H
5
3
3
point of 12°C. [2] Ionic liquids consist of an organic cation with delocalised charge, and organic or
inorganic anion (known as counter-ion). [3-4] The tunability of ionic liquids to give specific
properties for a particular application is the reason for being given nickname “designer solvents”.
[
1-5]
Ionic liquid salts possess special properties such as relatively non-volatile [6], good thermally
stability [7], low melting point [6, 8], and high density. [9] Some of the properties of ionic liquids
largely depend on the chosen cation and/or alkyl chain, and anion. [10-11] For instance, Kubisa [1]
reported that both the cation and anion contribute to the melting point of an ionic liquid. An ionic
liquid salt has no measurable vapour pressure, which makes them good replacements for volatile
organic solvents. [12] Literature has also reported that organic cation(s) with low symmetry or
branched chain relatively produce low melting point salts. [13] Hydrophobicity, viscosity, density,
and solvation of ionic liquids can be changed or manipulated by changing the anion. [14-15]
The common procedure used to synthesize water-immiscible ILs involves the preparation of
aqueous solution of halide salt with the desired cation (i.e. 1-ethly-3-methylimmidazolium halide),
2
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followed by the anion-exchange process. [16, 18] The water solubility or miscibility of ionic liquids
usually depends on the type of counter-ion used. [15, 17] A typical example is that of 1-butyl-3-
-
methylimidazolium, whereby the ionic liquid of 1-butyl-3-methylimidazolium cation and PF
6
-
anion is immiscible in water, whereas that of 1-butyl-3-methlyimmidazolium cation and BF
4
is
soluble. [15] The functionality of ionic liquids is tunable by adjusting or functionalising the
molecular structure of both the organic cation and anion. [16, 19] Originally, ionic liquids were
associated with green chemistry as substitutes of volatile and flammable solvents. However, these
types of salt have recently drawn a significant interest and application in organic and polymer
chemistry, catalysis, electrochemistry, analytical chemistry, nanotechnology, energy,
micellization, and biotechnology. [20-23] They are also used as plasticizers, additives, components
of polymer electrolytes, and porogenic agents to polymers [23]. Besides, there is very limited report
on the application of ionic liquids for wastewater treatment and for acid mine drainage
remediation.
There are some reports of ionic liquids containing pyridinium cation in the literature. [24-27]
Papaiconomou et al. [24] synthesized and studied the physicochemical properties of four
pyridinium-based ionic liquids namely: 1-butyl-4-methylpyridnium bis(trifluoromethylsulfonyl)
+
-
+
-
imide [MBPYR] [Tf
2
N] , 1-butyl-4-methylpyridnium trifluoromethyl sulfonate [MBPYR] [TfO]
3
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+
-
,
1-butyl-4-methylpyridnium nonafluorobutyl sulfonate [MBPYR] [NfO] , and 1-butyl-4-
+
-
methylpyridnium dicyanamide [MBPYR] [Br] , respectively. They discovered that two of the four
+
-
+
-
pyridinium-based ionic liquids, [MBPYR] [Br] and [MBPYR] [NfO] were solid at room
+
-
+
-
temperature, while [MBPYR] [Tf
2
N] and [MBPYR] [TfO] were found to have densities higher
-1
than that of water at ambient temperature, which were 1.35 and 1.17 g∙mL , respectively. Again,
+
-
+
-
[
MBPYR] [NfO] and [MBPYR] [Tf
2
N] were found to be insoluble in water at 25°C. The melting
+
-
points obtained for these four ionic liquids followed this increasing order: [MBPYR] [Br] >
+
-
+
-
+
-
[
MBPYR] [NfO] > [MBPYR] [TfO] >> [MBPYR] [Tf N] . They concluded that ionic liquids
2
containing 1-butyl-4-methylpyridinium cations have considerable high melting points, high
densities, but lower solubility’s in water than their imidazolium cation counterparts. [24-25]
Dzyuba [26] reported the synthesis of N-substituted pyridinium bromide ionic liquids. Although the
research and synthesis of pyridinium-based ionic liquids is gaining momentum and recognition
lately, the wide applicability of imidazolium–based ionic liquids makes the pyridinium-containing
ionic liquids less attractive. N-substituted pyridinium halides are known to be solids, which is in
agreement with the findings of Papaiconomou and co-workers [24]. Dyzuba [26] also reported the
novel synthesis of dicationic salts containing both pyridinium and ammonium cations. Herein, we
report the synthesis, characterization, and physicochemical properties of four hydrophobic
4
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10.1002/zaac.201800006
pyridinium–based ionic liquids with hexafluorophosphate inorganic counter anion and three-
carbon isomeric alkyl chain lateral attached to the N-position of the pyridinium cation. To the best
of our knowledge, no one has reported the synthesis, characterization, and physicochemical
properties of pyridinium-based ionic liquids based on comparison of three carbons isomeric alkyl
groups (propyl or isopropyl) at the N-position of pyridinium cation.
2
Experimental Sections
2
.1 Materials and Chemicals
All chemicals were obtained from Sigma-Aldrich or Merck and were used as received without
further purification unless stated otherwise. 1-bromopropane (99%), 2-bromopropane (99%),
pyridine (≥99.0%), dimethyl sulfoxide (≥99.9%), dimethyl formamide (≥99.8%), Toluene (≤99%),
dichloromethane (≥99.9%), and potassium hexafluorophosphate (≥99%). The ionic liquids: N-
+
-
propylpyridinium bromide [N-propylPyr] [Br] , N-isopropylpyridinium bromide [N-
isopropylPyr] [Br] , N-propylpyridinium hexafluorophosphate [N-propylPyr] [PF
]-, N-
+
-
+
6
+
]- were synthesized based on
isopropylpyridinium hexafluorophosphate [N-isopropylPyr] [PF
6
literature method. [10, 24, 26-27]
2
.2 Synthesis of ionic liquids (ILs)
5
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Four ionic liquids were synthesized as shown in Scheme 1. The synthesized ionic liquids contain
pyridinium cation and either halide or hexafluorophosphate as counter anion. These ionic liquids
were synthesized via two-step metathesis method that involves the quaternization of pyridine with
three-carbon alkyl bromide, followed by ion exchange with potassium hexafluorophosphate
(
KPF ), as detailed below.
6
Scheme 1 General synthetic route of ionic liquids.
Synthesis of N-alkylpyridinium bromide: Into a vigorously stirred solution of pyridine (4.21 mL,
5
2.10 mmol) and 20 mL toluene at 0°C, 1-bromopropane (4.09 mL, 45.0 mmol) or 2-
bromopropane (4.91 mL, 45.0 mmol) was added slowly in three-neck round bottom flask. The
mixture was heated to reflux at 100°C for 24 hours. Toluene was decanted and the remaining
brown to gold viscous liquid was re-crystallized in dichloromethane twice. Dichloromethane was
evaporated using vacuum rotary evaporator and the product was dried in an oven at 70°C for 10
6
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hours to further remove any solvent residue. The product obtained was either N-propylpyridinium
+
+
-
bromide [N-propylPyr] [Br], or N-isopropyl pyridinium bromide [N-isopropylPyr] [Br] .
+
-
1
6
[
N-propylPyr] [Br] : H NMR (400 MHz, ppm, DMSO-d , δ): 9.27-9.26 (2H, d, pyr-CH
o
), 8.19-
8
0
.16 (2H, t, pyr-CH
.83-0.78 (3H, t, CH
m
), 8.66-8.62 (1H, t, pyr-CH
p
), 4.69-4.66 (2H, d, CH
2
), 1.95-1.87 (2H, m, CH ),
2
3
).
FT-IR (ʋ/cm ): 3348 (=C-H, w), 2870 (C-Hsp3, s), 1591 (C=Cpyr, s), 1417 (C=Npyr, m), 1145 (C-
-1
N, m).
+
-
1
6
[
-
N-isopropylPyr] [Br] : H NMR (400 MHz, ppm, DMSO-d ): 9.21- 9.22 (2H, d, pyr-CH
8.58 (1H, m, pyr-CH ), 8.17- 8.13 (2H, t, pyr-CH ), 5.08- 5.04 (1H, m, CH), 1. 62- 1.59 (6H, d,
x2).
o
), 8.62
p
m
CH
3
FT-IR (ʋ/cm ): 3374 (=C-H, s), 3150-3109 (C-Hsp2, s), 3091-2978 (C-Hsp3, s), 1635 (C=Cpyr, m),
-1
1
456 (C=Npyr, m), 1157-1110 (C-N, m).
Synthesis of N-alkylpyridinium hexafluorophosphate: Viscous liquid of either [N-
+
-
+
-
propylPyr] [Br] or [N-isopropylPyr] [Br] (1.0 mL), and 2.02 g (11 mmol) of potassium
hexafluorophosphate salt were mixed in a 100-mL round-bottomed flask containing 10 mL of
water and stirred for 12 hours at 50°C. The resultant product was re-crystallized from
dichloromethane (2 x 10 mL) and dried with anhydrous magnesium sulphate to remove water
residue. The solvent was evaporated by vacuum rotary evaporator at 40°C. Light brown to gold
and white solid products were obtained and further dried at 70°C for 2 hours in an oven. It was
observed that the obtained products have a tendency to melt and become liquids when stored in an
7
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oven, although when stored in a fume hood at room temperature they tend to crystallized
immediately.
+
-
1
6
[
8
N-propylPyr] [PF
.62- 8.58 (1H, t, pyr-CH
m, CH ), 0.90- 0.86 (3H, t, CH
6
] : H NMR (400 MHz, ppm, DMSO-d , δ): 9.07- 9.06 (2H, d, pyr-CHo),
p
), 8.17- 8.13 (2H, t, pyr-CH
m
), 4.58- 4.55 (2H, t, CH
2
), 1.99-1.90 (2H,
2
3
).
3
1
6
-
1
19
6
P NMR (DMSO-d , δ): -144.179 ppm (m, PF
6
, Jp-f = 709.24 Hz). F NMR (DMSO-d , δ): -
-
6
9.240 to -71.129 ppm (d, PF
6
).
FT-IR (ʋ/cm ): 3348 (=C-Hpyr, m), 3104 (C-Hsp2, s), 2978-2885 (C-Hsp3, s), 1640 (C=Cpyr, m),
-1
1
566-1409 (C=Npyr, s), 1179 (C-N, s).
+
-
1
6
[
8
(
N-isopropylPyr] [PF
.61 -8.57 (1H, m, pyr-CH
6H, d, CH x2).
6
] : H NMR (400 MHz, ppm, DMSO-d , δ): 9.17- 9.15 (2H, d, pyr-CH
o
),
p
), 8.17- 8.13 (2H, t, pyr-CH ), 5.03- 5.01 (1H, m, CH), 1.62 – 1.61
m
3
3
1
6
-
1
19
P NMR (DMSO-d , δ): -131.54 to -157.32 ppm (m, PF , Jp-f = 713.09 Hz). F NMR (DMSO-
6
6
-
d , δ): -69.25 to -71.13 ppm (d, PF
6
).
FT-IR (ʋ/cm ): 3336 (=C-Hpyr, m), 3150-3109 (C-Hsp2, m), 2907 (C-Hsp3, m), 1635 (C=Cpyr, s),
-1
1
486 (C=Npyr, s), 1156 (C-N).
2
.3 Characterization techniques
FTIR spectral studies were carried out on (Perkin-Elmer, Universal ATR Sampling Accessory).
1
31
19
6
H, P, and F-NMR spectra were obtained in D
2
O and DMSO-d on a Bruker Avance III 400 at
1
frequencies 500 MHz or 400 MHz ( H). Thermal stability and glass transition temperature (T
g
) of
ionic liquids were determined by Perkin-Elmer thermo-gravimetric analyser (TGA 4000) and
differential scanning calorimeter (DSC 4000). Samples (10-30 mg) for TGA were placed in a
8
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ceramic sample pan and heated above a temperature range of 50-900°C at a heating rate of
0°C/min. For the total decomposition of carbon materials from the samples, gas was switched
from nitrogen to air at 700°C. Samples in a DSC were heated within a temperature range of 25-
50 °C at a heating rate of 20 °C/min for pyridinium ionic liquids with halide anion and 25- 400°C
4
2
for pyridinium ionic liquids with hexafluorophosphate anion. DSC runs were obtained under
nitrogen gas flow. Conductivity of the ionic liquids was measured by JENWAY 4510 Conductivity
meter in temperature range of 22-24°C. To test the solubility of the ionic liquids, the samples were
dissolved in different aqueous and organic solvents. Again, they were dissolved in the mixture of
methanol/water (80/20%), vigorously stirred and passed through solid phase extraction tube (SPE).
The solutions collected from the SPE tank were analysed by liquid chromatography (Shimandu,
LC-20AB, Prominance Liquid chromatography), using mixture of methanol/water (80/20) at a
flow rate 1.0 mL/min.
3
Results and discussion
3
.1 FTIR spectra studies
Four ionic liquids have been synthesized in this study, and their molecular structures, composition,
1
31
19
and functional groups were determined by FTIR, H, P, and F-NMR analyses. The FTIR spectra
obtained for the ionic liquids in this study are presented as supplementary material SI. 01. In [N-
9
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+
-
-1
propylPyr] [Br] , the presence of a weak band around 3348 cm can be attributed to =C-H stretch
for pyridinium in ortho, meta, and para positions. Strong vibrational bands at 2870 and 1579 cm-
1
can be categorically assigned to C-Hsp3 for propyl group, and C=C of pyridine, respectively.
-1
Medium peaks at 1417 and 1145 cm are due to the C=N of pyridinium cation and C-N between
the nitrogen group of pyridinium and first carbon of propyl, respectively. For [N-
+
-
isopropylPyr] [Br] , although slightly shifted, but similar bands were observed. For ionic liquids
containing PF
6
-, vibrational bands were found to be similar to those obtained for ionic liquids
containing bromide. These closely related observations confirmed that ion-exchange was carried
out successfully and the cationic components of the ionic liquids were unaltered.
3
.2 NMR spectra studies
1H-NMR spectra of all the as-synthesized ionic liquids were obtained successfully and confirmed
1
+
-
the purity of the compounds synthesized. For example, H-NMR spectrum of [N-propylPyr] [Br]
SI.04A) shows signals at 9.27-9.26, 8.19-8.16, and 8.66-8.62 ppm due to the protons resonance
at ortho-, meta-, and para-positions of pyridinium ring. Signals at 4.94-4.66 and 1.95-1.87 ppm
can be correctly assigned to the two -CH consecutive groups of the propyl lateral chain, while the
(
2
signals at 0.83-0.78 ppm are due to the methyl (-CH
3
) group of propyl. As depicted in (SI.04B),
1
+
]-. For
relatively similar but less deshielded H-NMR signals were obtained for [N-propylPyr] [PF
6
1
0
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+
-
1
isopropyl-featuring ionic liquids, [N-isopropylPyr] [Br] (SI.02), H-NMR signals responsible for
proton resonances at ortho-, para-, and meta-positions were observed at 9.22-9.21, 8.62-8.58, and
8
.17-8.13 ppm, respectively. The multiplet at 5.08-5.04 ppm is due to the single-proton of the
isopropyl carbon close to pyridinium-nitrogen atom. The doublet at 1.62-1.59 ppm is due to the
six-protons of the two methyl groups (CH
3
x2) of isopropyl. Again, the hydrophobic moiety of the
+
-
+
]- shows similar but comparable less
[
N-isopropylPyr] [Br] , that is, [N-isopropylPyr] [PF
6
deshielded proton signals. The presence of bromide ion in hydrophilic ionic liquids has resulted in
some proton resonances being deshielded. It has been reported that the electron density of
1
substituents in a compound usually affects peak/signal position in the H-NMR. However, due to
high electronegative behaviour of halogens, protons close or associated with halides are normally
deshielded and appear at downfield.
Furthermore, 31P and
19
F-NMR for both hydrophobic [N-propylPyr] [PF
counteranion are presented (supplementary SI.
] , P was found to have a multiplet (m) around -144.179 ppm,
+
]- and [N-
6
+
]- ionic liquids containing PF -
isopropylPyr] [PF
6
6
+
- 31
0
2-05). For [N-propylPyr] [PF
6
which confirmed the presence of phosphorous atom coupled with six fluorine atoms. 19F was
found to have a doublet (d) around -169.25 to -71.13 ppm, confirming the coupling of six fluorine
31
19
atoms with one phosphorous atom. The coupling distances between the peaks in both P and F
1
1
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NMR spectra were found to be approximately the same and constituted the coupling constant of
1Jp-f = 709.24 Hz. For [N-isopropylPyr] [PF
+
] , P was found to have multiplet peaks around -
- 31
6
1
9
1
31.54 ppm, while F was found to have doublet around -69.25 to -71.13 ppm and with coupling
constant of 1Jp-f = 713.09 Hz. However, both P and F NMR confirmed the presence of fluorine
31
19
-
and phosphorous atoms coupled together in the form of PF
6
in the as-synthesized ionic liquids.
The close range of the obtained coupling constants for both hydrophobic ionic liquids is the clear
indication that the counter anion was successfully attached to the pyridinium counter cation in both
1
19
ionic liquids and the synthetic method was successful. To further complement the FTIR, H, F,
and 31P-NMR results, single mass analysis data was collected. The accurate mass of the
synthesized compounds based on C, H, and N is presented in (S1.06 to S1.07). Comparatively, it
is established that the experimental accurate mass values (122.0973 and 122.0966 m/z for
isopropyl pyridinium and propyl pyridinium, respectively) are in agreement with the calculated
+
1
19
mass (122.10 m/z C
8
H
12N ). This therefore in combination with the results obtained from H, F,
3
1
and P-NMR confirmed the successful synthesis and absolute purity of the compounds.
3
.3 Physicochemical Properties
3
.3.1 Thermogravimetric analyses
1
2
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Thermogravimetric analysis (TGA) is usually used to obtain information about the thermal
stability character of the synthesized ionic ILs. As shown in Fig. 1, pyridinium-based ionic liquids
+
-
+
-
with halide counter anion, [N-propylPyr] [Br] and [N-isopropylPyr] [Br] displayed minor weight
losses, less than 5% within 100°C which is due to the loss of moisture from the samples.
Considerable weight loss between 18-35% was observed at temperatures above 290°C for both
pyridinium ionic liquids with halide due to the decomposition of the ionic liquid salts. [N-
+
-
+
-
propylPyr] [Br] and [N-isopropylPyr] [Br] were seen to be thermally stable below 400°C and
their total decompositions were recorded at 356°C and 342°C, respectively, showing unimodal
decomposition character.
+
] and [N-isopropylPyr] [PF
]-, that is, hydrophobic pyridinium-based ILs with
-
+
[
N-propylPyr] [PF
6
6
-
PF
6
counter anion were observed to be thermally stable up to 400°C. Minor weight loss of about
8
-17% responsible for moisture weight loss was seen at onset temperatures 301°C and 322°C for
the two hydrophobic ionic liquids. Significantly decomposition between 400-550°C was observed
for both hydrophobic ionic liquids. Contrarily, hydrophobic ionic liquids showed multimodal
+
]- and [N-
decomposition characteristics. Carbon loss for both [N-propylPyr] [PF
6
+
]- was also observed at 721°C and 789°C, respectively.
isopropylPyr] [PF
6
1
3
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Figure 1 TGA for pyridinium-based ionic liquids
-
It can therefore be concluded that ionic liquids with hydrophobic counter anion such as PF
6
are
more stable than those with halide counter anion (hydrophilic). Papaiconomou et al. [24] also
discovered that thermal stability of ionic liquids is related to the nature of the counteranion
involved. For the ionic liquids investigated in this study, it was also evident that ILs with
pyridinium cation containing isopropyl group on the N-position of pyridinium were more stable
+
-
than those with propyl counterpart in the following order: [N-IsopropylPyr] [Br] >[N-
+
-
+
-
+
]- in terms of thermal
propylPyr] [Br] , while [N-isopropylPyr] [PF
6
] >[N-propylPyr] [PF
6
stability. Thus, the thermal stability of ionic liquids in this study is in accordance with counter
1
4
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anions involved and the isomeric alkyl groups on the N-position of pyridinium in the following
trend: [N-isopropylPyr][PF ] > [N-propylPyr][PF ] > [N-IsopropylPyr][Br] > [N-propylPyr][Br].
6
6
3
.3.2 Melting point and Glass transition temperatures
Melting point and glass transition temperatures were obtained using differential scanning
calorimetry (DSC). DSC runs were conducted under nitrogen gas flow and sample weight range
5
-20 mg. Table 1 show the melting point (Tm), glass transition (T
temperatures in degrees Celsius, and ionic conductivity (σ, mS) at given temperatures. Figs. 2 and
show the DSC thermograms of the ILs synthesized in this study. For instance, Fig. 2 show the
transition T of [N-propylPyr]Br] at 28°C and that of [N-isopropylPyr][Br] at 25°C, that is, the
inflation point (endothermic direction) between the first peak onset temperature and offset
temperature, respectively. Glass transitions of hydrophobic [N-propylPyr][PF ] and [N-
isopropylPyr][PF ] ILs were observed to be slightly smaller in comparison to other corresponding
pyridinium-ionic liquids with halide. Eftakhari and Saito [31] once concluded that larger counter
anions lower the T due to their bulkiness and distributed charges. As shown in Figs. 2 and 3, and
g
), decomposition (T )
d
3
g
6
6
g
Table 1, pyridinium ionic liquids with hexafluorophosphate counter anion exhibit high melting
temperature than their halide counterparts in this study. These results are agreement with the results
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obtained from the thermogravimetric analyses, which suggested that pyridinium ILs with
hydrophobic counter anion are more stable than those with halide counter anion counterpart.
Figure 2 DSC overlay thermograms of pyridinium ionic liquids with halide
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Figure 3 DSC thermograms of pyridinium ionic liquids with PF -
, [N-propylPyr] [PF
+
]- (A), and
6
6
+
]- (B).
[
N-isopropylPyr] [PF
6
3
.3.3 Conductivity
The conductivity measurements of pyridinium ionic liquids with halide anion were obained in
aqueous solution (water), while those of hydrophobic pyridinium ionic liquids with
hexafluorophosphate anioin were obtained in DMSO. Table 1 show conducitivities obtained for
the synthesized ionic liquids and temperatures at which the measurements were recorded. It is
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evident that pyridinium ILs with halide have higher conductivities compared to those with
hexafluorophosphate anion. In terms of isomeric alkyl chain length at the N-position of pyridinium
cation, pyridinium ionic liquids with propyl group were found to have higher conductivities than
those with isopropyl group. Dharaskar et al. [30] once reported a small ionic conductivity of 1-
butyl-3-methylimidazolium tatrafluoroborate and they concluded that the ionic liquid conductivity
is mainly influenced by the mobility of its own cation because of the diffusion coefficients
differences between the cation (higher) and anion (lower). Therefore, it can be concluded that an
increase in alkyl chain length also increases the conductivity of that particular ionic liquid.
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3
.3.4 Solubility of synthesized ionic liquids
Figure 4 Solubility studies of the synthesized ILs in a range of aqueous and organic solvents. Polar
protic: water (H O) and methanol (MeOH), Non-polar: hexane (HEX) and chloroform (CHCl ),
2
3
and Polar aprotic: dimethyl sulfoxide (DMSO) and dimethy formamide (DMF). Soluble (S) and
Non-soluble (N.S).
The solubility of the syntthesized ILs was determined by dissolving equal amount of ILs in
different aqueous and organic solvents. Owing to the type of counter anion, and/or isomeric carbon
lateral chain involved, these ILs showed variable solubility characteristics as shown in Fig. 4. It
has been established that a solvent or solute will only dissolve or be dissolved in a substance of
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similar polarity, respectively (“like dissolves likes”). However, as shown in Fig. 4 (A), [N-
+
-
propylPyr] [Br] was soluble in polar protic and non-polar solvents, and slightly insoluble in polar
aprotic solvents. This behaviour can be correctly linked to the formation of hydrogen bonding
(intermolecular forces) between the polar protic or non-polar solvents and ILs. On the other hand,
+
-
[
N-IsopropylPyr] [Br] behaved comparable different and showed significant miscibility only in
non-polar solvents. The hydrophilic character and the formation of hydrogen bonding are
responsible for this latter-IL behaviour in aqueous and organic medium. Another thing worthy to
be mentioned which comparable affect the solubility of ILs is the alkyl lateral chain. It is evident
that pyridinium-ILs with straight-lateral alkyl chain attached on pyridinium-nitrogen atom are
more soluble in aqueous and organic medium than their corresponding isomeric counterparts.
+
] and [N-isopropylPyr] [PF
]- ionic liquids were observed to be
-
+
Hydrophobic [N-propylPyr] [PF
6
6
insoluble in all polar protic and non-polar solvents, yet soluble in DMSO. The hydrophobicity of
these ILs and large dielectric constant of DMSO played significantly role in the insolubility of the
ILs in other solvents except DMSO.
2
0
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+
] and [N-isopropylPyr] [PF
]- from liquid
-
+
The results obtained for [N-propylPyr] [PF
6
6
chromatography showed that the ionic liquids could not be identified and there were no retentions
times recorded for these ionic liquids. It was observed that these two ionic liquids could not go
through the SPE packing because they were insoluble in methanol and water. The absence of these
ionic liquids from the solution collected by SPE tank is a direct indication that these ionic liquids
are immiscible in both methanol/water used for SPE, and also mobile phase used in the Liquid
chromatography.
2
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4
. Conclusions
Four ionic liquids were successfully synthesized and their purity was confrimed by NMR and FTIR
spectroscopic studies. Pyridinium based ionic liquids with halide counter anion were observed to
be stable below 400 °C and were less stable than their hexafluorophosphate counterparts. Carbon
loss was identified at temperatures above 700 °C. In terms of the alkyl chain isomerism,
pyridinium-ionic liquids with isopropyl alkyl group on N-position of pyridinium cation were
significantly more stable than ionic liquids with propyl group. However, pyridinium-ionic liquids
with propyl alkyl group were found to have high conductivity than isopropyl containing ionic
liquids. Studies on solubilities of these ionic liquids showed that hexafluorphosphate containing
ionic liquids were insoluble in water (hydrophobic).
Acknowldgements
The authors gratefully acknolwedge National Research Foundation (NRF) of South Africa and
Sasol for financial support. M. Matandabuzo thanks NRF for the award of PhD scholarship.
Supporting information (SI): FT-IR spectra of four as-synthesized ionic liquids (SI. 01), all NMR
spectra of ionic liquids studied in this current work (SI. 02-05) and accurate mass analysis (SI.06-
0
7).
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