Synthesis and surface functionalization of multi-walled carbon nanotubes with imidazolium and pyridinium-based ionic liquids: Thermal stability, dispersibility and hydrophobicity characteristics

Article abstract of DOI:10.1016/j.molliq.2018.07.028

Functionalized multi-walled carbon nanotubes (MWCNTs) were synthesized by simple chemical method, and dispersed using imidazolium and pyridinium-based ionic liquids (ILs). The as-synthesized ILs-MWCNT composites were studied using FTIR spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), thermogravimetric analysis (TGA), and solubility in different polar and non-polar solvents. Spectroscopic and microscopy analyses confirmed the formation of the ILs-MWCNT composites with new functionalities. Spectra studies showed graphitic and carboxylic groups in the pure MWCNTs. MWCNT SEM images showed entangled bundles, while ILs-MWCNTs showed debundled composites with increased diameter and unaltered MWCNTs morphology. TGA indicates that the MWCNTs are thermally stable which could be ascribed to van der Waals and non-covalent interactions within the composites matrices. Solubility studies indicate the ILs-MWCNT composites are hydrophobic, insoluble in water and other polar solvents.

Full text of DOI:10.1016/j.molliq.2018.07.028

Accepted Manuscript
Synthesis and surface functionalization of multi-walled carbon
nanotubes with imidazolium and pyridinium-based ionic liquids:
Thermal
stability,
dispersibility
and
hydrophobicity
characteristics
Mzukisi Matandabuzo, Peter A. Ajibade
PII:
S0167-7322(18)32558-3
doi:10.1016/j.molliq.2018.07.028
MOLLIQ 9349
DOI:
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
18 May 2018
6 July 2018
7 July 2018
Please cite this article as: Mzukisi Matandabuzo, Peter A. Ajibade , Synthesis and surface
functionalization of multi-walled carbon nanotubes with imidazolium and pyridinium-
based ionic liquids: Thermal stability, dispersibility and hydrophobicity characteristics.
Molliq (2018), doi:10.1016/j.molliq.2018.07.028
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ACCEPTED MANUSCRIPT
Synthesis and surface functionalization of multi-walled carbon nanotubes with
imidazolium and pyridinium–based Ionic Liquids: Thermal stability, dispersibility and
hydrophobicity characteristics
*
Mzukisi Matandabuzo, Peter A. Ajibade
School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X01, Scottsville,
Pietermaritzburg 3209, South Africa
Abstract
Functionalized multi-walled carbon nanotubes (MWCNTs) were synthesized by simple
chemical method, and dispersed using imidazolium and pyridinium-based ionic liquids (ILs).
The as-synthesized ILs-MWCNT composites were studied using FTIR spectroscopy,
scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS),
thermogravimetric analysis (TGA), and solubility in different polar and non-polar solvents.
Spectroscopic and microscopy analyses confirmed the formation of the ILs-MWCNT
composites with new functionalities. Spectra studies showed graphitic and carboxylic groups
in the pure MWCNTs. MWCNTs SEM images showed entangled bundles, while ILs-
MWCNTs showed debundled composites with increased diameter and unaltered MWCNTs
morphology. TGA indicates that the MWCNTs are thermally stable which could be ascribed
to Van der Waals and non-covalent interactions within the composites matrices. Solubility
studies indicates the ILs-MWCNT composites are hydrophobic behaviour, insoluble in water
and other polar solvents.
KEYWORDS: Carbon nanotubes; ionic liquids; imidazolium; pyridinium; functionalization.
*
Corresponding author Email: ajibadep@ukzn.ac.za
1

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1
. Introduction
Ionic liquids (ILs) (also known as electrolytes), are organic/inorganic salts made up of large
and asymmetric ions with delocalized charges and different chemical functional groups [1-3].
ILs exists in liquid form at room temperature with melting point lower than 100 °C [4-5]. ILs
possesses special properties such as non-volatility, good thermal stability, low melting point,
and relatively good electrical conductivity [5]. ILs has been used as solvents and potential
stabilizers for various nanoparticles used in catalysis and material science [6-7]. ILs are being
used in different applications such as electrochemical energy and electrochemical double-
layer capacitors (EDCLs) [4, 8-12].
The incorporation and/or coating of carbon nanotubes
with different salts, nanoparticles, polymers, and ionic liquids produces composites with
enhanced physical, chemical and mechanical stability that can be easily process and
hydrophobic [13]. The formation of bucky gels is known as the first discovery of ionic
liquids/carbon nanotubes composites [14-17]. However, due to the presence of highly
polarizable π electrons in ionic liquids, the interactions between the carbon-based π-systems
and ionic liquids are very complex [15]. Espejo et al., [18] reported the dispersion of multi-
walled carbon nanotubes (MWCNTs) in imidazolium-based ILs to produce stable and
homogeneous dispersions with relatively new and unique properties. It was also reported that
dispersion of CNTs in ILs produces composites with enhanced surface area [18]. Yang et al.
[19] reported the functionalization of MWCNTs with 2,2’-(ethylendioxy)-diethylamine, 1,8-
diaminooctane, and pristine. They observed that MWCNTs functionalized with 2,2’-
2

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(
ethylendioxy)-diethylamine were individually integrated into the epoxy matrix, whereas
MWCNTs with 1,8-diaminooctane were poorly dispersed with notable weak interface
adhesion. Salam & Burk [20] reported the synthesis and functionalization of multi-walled
carbon nanotubes by octadecylamine (ODA) and polyethylene glycol (PEO). Their results
revealed that only 16% (wt) of MWCNTs was covered by PEG, while 39% (wt) was covered
by ODA. The dispersion of MWCNTs in polymer composites was reported by Pereira et al.,
[21], in their preliminary study of MWCNTs in poly(vinilydene fluoride),
MWCNT/PVDF/ZrO2. Spectra studies of the synthesized nanocomposites confirmed the
incorporation of the MWCNTs into poly(vinilydene fluoride) matrix [21].
Ohba and Chaban [22] reported the structure and dynamics of imidazolium-based ILs
confined inside carbon nanotubes. França [15] reported that CNTs covalently modified with
-
-
imidazolium-based ILs are dispersible only in water when they contain Cl or Br counter
-
-
-
ions. Further studies indicates that fluorine-based anions such as BF4 , PF6 , and Tf2N permit
the dispersion of imidazolium-modified CNTs only in organic solvents (CHCl3), forming
black homogeneous solutions. In their study, Chaban and Prezhdo [23] suggested that highly
viscous liquid composed of asymmetrical cations and small anions can penetrate inside the
apolar CNTs at ambient pressure and high temperatures. Recently, Taherkhani and Minofar
[24] studied the effect of impurity and radius of CNT on glass transition and electrical
conductivity
of
1-ethyl-3-methylimidazolium
hexafluorophosphate
[EMIM][PF6]
3

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encapsulated in CNT, using molecular dynamic simulations (MDS). Their study discovered
that the electrical conductivity of [EMIM][PF6]/CNT increases with increasing radius of
CNT, whereas the electrical conductivity of [EMIM][PF6] decreases significantly when
encapsulated in zigzag CNT. Chen et al., [25] reported the encapsulation of 1-butyl-3-
methylimidazolium hexafluorophosphate in MWCNTs, and they observed that ILs coated in
the hollow interior of MWCNTs produces ILs/MWCNT composites with high thermal
stability due to the presence of van der Waals and H-bonding interactions. Ohba et al., [26]
reported that cations adhere weakly to the sidewalls of CNTs, while anions move freely
inside the interior of CNTs.
Studies on the mechanisms of interactions between carbon nanotubes (CNTs) and ionic
liquids suggested that ILs interact with CNTs via π-cation and/or π-π interactions [25, 27-28].
In other studies, weak van der Waals or electrostatic forces inferred as basic interactions
behind the formation of CNTs-ILs composites [29-31]. For instance, Fileti and Chaban [32]
conducted a study where they concluded that fullerene-ionic liquid binding forces were not
exclusively of the van der Waals interactions. In this study, we report the synthesis and
surface functionalization of MWCNTs with some new hydrophobic and hydrophilic ionic
liquids (ILs). The ILs-MWCNT composites were studied by spectroscopic techniques,
electron microscopy (SEM), and thermal studies. The solubility and dispersibility of
MWCNTs were also investigated.
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2
. Experimental
.1 Reagents
2
All chemicals were obtained from Sigma-Aldrich and BDH chemicals Ltd, and were used as
received without further purification unless stated otherwise. Graphite powder (<µm,
synthetic, Switzerland), sulphuric acid (95-99%, A.R), nitric acid (70%, A.R), sodium nitrate
(>98%, ex NO3), dimethyl sulfoxide (ACS, ≥99.9% for analysis), dichloromethane (ACS
reagent, ISO, ≥99.9%, GC), acetonitrile (≥99.9%, gradient analysis), methanol (ACS, ISO,
reag. Ph Eur for analysis), ethanol (≥99.5%, A.R), potassium hexafluorophosphate (≥99%,
solid), bromopropane (99%), 2-bromopropane (99%), pyridine (ACS reagent, ≥99.0%), and
1
-methylimidazole (≥99%, purified by redistillation).
2
.2 Synthesis of carbon nanotubes (CNTs)
Carbon nanotubes were synthesized using a simple chemical method according to Leo and
Seo [33]. Solution of graphite powder (5 g) in water (5 mL), and solution of nitric acid (25
mL) and sulphuric acid (50 mL) were prepared separately at low temperature (ice-bath) under
constant stirring. The two solutions were mixed followed by the addition of sodium nitrate
(25 g) at 0°C. The mixture was stirred for 36h at ambient temperature, and further refluxed
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for 12h at 90°C. Thereafter, the mixture was centrifuged, neutralized with sodium hydroxide
solution, and filtered to obtain the CNTs.
2
.3 Synthesis of MWCNTs
MWCNTs were synthesized from as-synthesized CNTs, where 0.317g of CNTs was
ultrasonicated for 5 minutes in a mixture of concentrated sulphuric acid (15 mL H2SO4,
6
6
7
5%)/5 mL nitric acid (HNO3, 98%), 3:1 by volume. The solution was refluxed for 5h at
0°C, and then washed with distilled water to neutralize acid residue, dried in air vacuum at
0°C to obtain the MWCNTs.
2
.4 Synthesis of imidazolium-based ILs [3]
Scheme 1 Synthetic route of imidazolium-based ILs.
6

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+
-
2
.4.1 3-methyl-1-propylimidazolium bromide [MPIm ][Br ](Scheme 1A): Into a two-neck
round bottom flask fitted with reflux condenser, 5 mL of 1-methylimidazole, 2 mL of 1-
bromopropane in 10 mL of acetonitrile were added. The mixture was refluxed for 12 hours at
8
0-100°C with stirring until the formation of two organic phases. The top layer which
contained unreacted materials was discarded, and the colourless viscous lower layer was
dissolved in two portions of 20 mL dichloromethane. Thereafter, the solvents were removed
under vacuum. Colourless viscous liquid was obtained and dried in an oven at 70°C.
1
6
H NMR (400 MHz, ppm, DMSO-d , δ): 7.185 (1H, s, Im-Hf), 7.061 (1H, s, Im-Hg), 4.239-
4
0
3
.204 (2Hc, t, CH2), 3.963 (3Hd, s, CH3), 3.775 (1H, s, Im-He), 1.992-1.902 (2Hb, m, CH2),
1
3
6
.999-0.962 (3Ha, t, CH3). C NMR (400 MHz, ppm, D2O-d , δ): 10.05 (C1), 22.94 (C2),
9.00 (C5), 51.34 (C3), 122.26 (C4), 123.55 (C7), 127.56 (C6).
-
1
2
3
FT-IR (υ/cm ): 3289-3110 (=C-H, & C-Hsp , m), 2854 (C-Hsp , m), 1648 (C=C, m), 1132 &
9
89 (C-N, s).
+
-
2
.4.2 1-isopropyl-3-methylimidazolium bromide [IsopropylMIm ][Br ](Scheme 1A): Into a
two-neck round bottom flask fitted with reflux condenser, 5 mL of 1-methylimidazole, 2 mL
of 2-bromopropane in 10 mL of acetonitrile were added. The mixture was refluxed for 12
hours at 80-100°C while stirring until the formation of two organic phases. The top layer
which contained unreacted materials was discarded, and the golden viscous lower layer was
7

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dissolved in two portions of 20 mL dichloromethane. Thereafter, the solvents were removed
by vacuum. Golden viscous oily liquid was obtained and dried in an oven at 70°C.
1
6
H NMR (400 MHz, ppm, DMSO-d , δ): 9.4798 (1H, s, Im-H), 7.9426-7.6867 (2H, t, 2Im-
H), 4.3490-4.3131 (2H, t, CH2), 4.0512 (3H, s, CH3), 1.9333-1.8787 (2H, m, CH2), 0.9101-
1
3
6
0
3
.8731 (3H, t, CH3). C NMR (400 MHz, ppm, D2O-d , δ): 22.06 (C1, 2), 33.32 (C5),
5.92 (C3), 52.98 (C4), 123.46 (C7), 127.55 (C6).
-
1
2
3
FT-IR (υ/cm ): 3391-3150 (=C-H, & C-Hsp ), 2891 (C-Hsp , m), 1561 (C=C, s), 1101-998
(C-N,s).
+
-
2
.4.3 1-isopropyl-3-methylimidazolium hexafluorophosphate [IsopropylMIm ][PF6
+
-
]
(Scheme 1B): 3.0 mL of [IsopropylMIm ][Br ] and 3.0 g of potassium hexafluorophosphate
were mixed in a 50 mL round bottom flask with 10 mL of water. The reaction mixture was
stirred for 12 hours at room temperature. Then the resultant product was rinsed with several
portions of water and dissolved in 20 mL dichloromethane, and dried in anhydrous
magnesium sulphate (MgSO4). The solvent was then removed by vacuum. The resultant
golden liquid was dried for 12 hours under vacuum at 70°C.
1
6
H NMR (400 MHz, ppm, DMSO-d , δ): 9.1536 (1H, s, Im-H), 7.8594-7.8603 (1H, t, Im-
H), 7.7003-7.6916 (1H, t, Im-H), 4.6473-4.58603 (1H, m, CH), 3.8344 (3H, s, CH3), 1.4761-
1
3
6
1
.4554 (6H, d, CH3X2). C NMR (400 MHz, ppm, DMSO-d , δ): 22.79 (C1, 2), 36.19
(C5), 40.04 (C3), 52.62 (C4), 120.92 (C7), 124.15 (C6).
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3
1
6
-
1
19
P NMR (DMSO-d , δ): -130.94 to -159.12 ppm (m, PF6 , Jp-f = 715.09 Hz). F NMR
6
-
(
DMSO-d , δ): -69.35 to -71.01 ppm (d, PF6 ).
-
1
2
3
FT-IR (υ/cm ): 3361 (=C-H, m), 3091 (C-Hsp , s), 2872 (C-Hsp , m), 1591 (C=C, m), 1150-
1
110 (C-N, s).
2
.5 Synthesis of pyridinium-based ILs [3]
Scheme 2 Synthetic route for pyridinium-based ILs.
+
-
2
.5.1 N-propyl pyridinium bromide [N-propylPyr ][Br ](Scheme 2A): Into a vigorously
stirred solution of pyridine (4.21 mL, 52.10 mmol) and 20 mL Toluene at 0°C, 1-
bromopropane (4.09 mL, 45.0 mmol) was added slowly in three-neck 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
removed under vacuum and the product was dried in an oven at 70°C for 10 hours to further
9

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remove any solvent residue. The product obtained was N-propyl pyridinium bromide [N-
+
propylPyr] [Br].
1
6
H NMR (400 MHz, ppm, DMSO-d , δ): 9.27-9.26 (2H, d, pyr-CHo), 8.19-8.16 (2H, t, pyr-
CHm), 8.66-8.62 (1H, t, pyr-CHp), 4.69-4.66 (2H, d, CH2), 1.95-1.87 (2H, m, CH2), 0.83-0.78
3H, t, CH3).
(
-
1
3
FT-IR (υ/cm ): 3348 (=C-H, w), 2870 (C-Hsp , s), 1591 (C=Cpyr, s), 1417 (C=Npyr, m), 1145
C-N, m).
(
+
-
2
.5.2 N-isopropyl pyridinium hexafluorophosphate [N-isopropylPyr ][PF -](Scheme 2B):
6
+
-
Viscous liquid of [N-isopropylPyr] [Br] (1.0 mL), and 2.02 g (11 mmol) of potassium
hexafluorophosphate salt were mixed in a 100 mL round bottom 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 solid products were obtained and further dried at 70°C for 2 hours in an oven. It was
observed that the products melt and become liquids in an oven, while it tends to crystallized
in fume hood at room temperature.
1
6
H NMR (400 MHz, ppm, DMSO-d , δ): 9.17- 9.15 (2H, d, pyr-CHo), 8.61 -8.57 (1H, m,
pyr-CHp), 8.17- 8.13 (2H, t, pyr-CHm), 5.03- 5.01 (1H, m, CH), 1.62 – 1.61 (6H, d, CH3x2).
3
1
6
-
1
19
P NMR (DMSO-d , δ): -131.54 to -157.32 ppm (m, PF6 , Jp-f = 713.09 Hz). F NMR
6
-
(DMSO-d , δ): -69.25 to -71.13 ppm (d, PF6 ).
-
1
2
3
FT-IR (υ/cm ): 3336 (=C-Hpyr, m), 3150-3109 (C-Hsp , m), 2907 (C-Hsp , m), 1635 (C=Cpyr,
s), 1486 (C=Npyr, s), 1156 (C-N).
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2
.6 Synthesis of ILs-MWCNT composites
ILs-MWCNT composites were synthesized according using reported methods with minor
modifications [17, 25, 34]. Briefly, a solution of 5 mg of MWCNTs and 10 mg of each ILs in
1
0 mL of N,N-dimethylformamide (DMF) was ultrasonicated for 20 mins. The mixture was
then vigorously stirred for 24h at 50°C. The unreacted MWCNTs residues were removed by
centrifugation. Thereafter, ILs-MWCNT composites were filtered, thoroughly washed with
DMF, ethanol, and water, respectively.
2
.7 Characterization techniques
Fourier transform infrared spectroscopic measurements were carried out using Perkin- Elmer,
1
13
31
19
Universal ATR sampling Accessory. H, C, P, and F-nuclear magnetic resonance (NMR)
6
spectra were obtained in D2O and DMSO-d on a Bruker Avance III 400 at frequencies 500
1
MHz or 400 MHz ( H). Thermal stability of MWCNTs, ILs, and ILs-functionalized
composites were determined by Perkin-Elmer TGA 4000 using 10-30mg of sample heated in
the temperature range of 50-900°C at a heating rate of 40°C/min under nitrogen flow.
Scanning electron microscopy (SEM) micrographs were taken using Zeiss Evols 15 Scanning
electron microscopy combined with energy dispersive X-ray spectroscopy (EDXS).
11

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3
. Results and Discussion
3
.1 Nuclear magnetic resonance studies
1
The H-NMR spectra of imidazolium-based ionic liquids showed large frequency downshift
and overlapping of three proton peaks around the imidazole ring. For instance, the protons in
positions 4 and 5 were found to resonate or de-shielded (downfield signal) around low field
due to the presence of bromide ions for halide-containing ILs shows the proton NMR of [N-
+
-
+
-
propylPyr] [Br] , and [N-isopropylPyr] [PF6] . As it is observed, the signals between 9.162-
8
.147 ppm corresponds to the pyridinium ring protons at ortho-, meta-, and para-positions.
+
-
For [N-propylPyr] [Br] on the hand, the triplet, multiplet, and triplet due to the –CH2
attached to the pyridyl nitrogen, -CH2 and –CH3 of propyl moiety can be observed around
+
-
upfield region, respectively. Proton NMR for [N-isopropylPyr] [PF6] further shows multiplet
around 5.051-4.984 ppm corresponding to the single proton of the central carbon of the
isopropyl group, and a doublet around 1.625-1.609 ppm corresponding to the six protons of (-
1
3
CH3x2) of the isopropyl group. The C-NMR spectra of the ILs showed direct information
about the carbon skeleton and the number of equivalents/non-equivalents carbons of the ILs.
Generally, the presence of electronegative atoms and π-bonds causes downfield chemical
shifts. For example, C4, C6, and C7 of the imidazolium rings are deshielded (shifted
downfield) due to the presence of the π-electron system in the imidazolium ring and the
12

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influence of the athached electronegative bromide counter-ion. As expected, C1 and C2 of all
isopropyl-containing ILs are chemically equivalent and are assigned to one signal. However,
the appearance of the peak at 40.041 ppm corresponds to the overlap of the C3 carbon of the
3
1
isopropyl group and the DMSO solvent. P NMR spectra of both hydrophobic ILs were
found to have multiplet (m) between -130 to -159 ppm confirming the presence of the
1
9
phosphorous atom coupled with six fluorine atoms. On the other hand, the F NMR spectra
were found to contain doublets (d) between -69 to -71.0 ppm that confirmed the coupling of
1
13
19
six fluorine atoms with one phosphorous atom. To further complement the, H, C, F, and
3
1
P-NMR results, single mass analysis data of the synthesized compounds based on C, H, and
N established that the experimental accurate mass values (125.1075, 122.0966, and 122.0973
m/z for methyl-propyl imidazolium, isopropyl-methyl imidazolium, isopropyl pyridinium and
propyl pyridinium, respectively) are in agreement with their calculated mass from respective
molecular formulas. This thus confirmed synthesis of the compounds [3].
3
.2 Infrared spectral studies
FTIR spectra of MWCNTs and ILs-MWCNT composites provide information about the
functional groups present in the MWCNTs with ILs. Figs. 1 & 2 shows the FTIR spectra of
pure ILs (Fig. 1), and FTIR spectra of MWCNTs and ILs-modified MWCNT hybrids (Fig.
+
-
2
). As shown in Fig. 1, the FTIR spectrum of [N-propylPyr ][Br ] shows a very weak band at
13

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-
1
3
348 cm which can be attributed to =C—H stretch in ortho, meta, and para positions of the
-
1
pyridinium cation. The strong stretching vibrations found at 2870 and 1579 cm are ascribed
3
to C-Hsp of propyl group, and C=C of pyridine, respectively. Weak to medium peaks
-
1
observed at 1417 and 1145 cm are due to C=N of pyridinium cation, and C-N between the
nitrogen of pyridinium ring and first carbon of propyl, respectively.
+
-
The FTIR spectrum of [N-isopropylPyr ][PF6 ] appeared similar but less shifted vibrational
bands compared to bromide containing ILs-counterparts. Another important peak found
-
1
3
around 2976 cm assigned to C-Hsp of the isopropyl group attached to pyridinium cation.
+
-
-1
[
MPIm ][Br ] FTIR spectrum shows well pronounced peaks in the range 3390-3150 cm
2
assigned to the =C-H stretch and C-Hsp stretching vibrations. The shifts in the peaks could be
-
1
attributed to the presence of halide counter ions. The peaks at 2891 and 1561 cm are
3
assigned to the C-Hsp of propyl and C=C of the imidazolium ring, respectively. The strong
-
1
vibrational bands in the range 1101-998 cm were ascribed to the presence of the C-N
functional group between the imidazolium ring and alkyl groups. The FTIR spectrum of [N-
+
-
+
-
isopropylMIm ][PF6 ] shows similar vibrational bands compared to that of [MPIm ][Br ] but
3
shifted slightly. Stretching vibrations of C-Hsp of propyl and isopropyl were observed in
relatively similar position [25].
14

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-
As shown in Fig. 2, the FTIR spectrum of MWCNTs shows stretching vibrations at 1734 cm
1
corresponding to the carboxylic and graphite groups on the surface of pure nanotubes. A
-
-1
weak OH stretching vibration around 3598 cm is due to the moisture content absorbed by
the carbon materials and was also confirmed by the thermogravimetric analysis [26, 34].
-
1
Other important vibrational bands found at 2323 and 1239 cm are attributed to CO2, and C-
O, respectively. After functionalization, the skeletal vibrational bands of graphitic and
-
1
carboxylic groups at 1734 cm corresponding to C=O disappeared in ILs-MWCNT
-
1
composites spectral. The shift of the weak OH stretching vibration from 3619 cm to 3789
-
1
cm in the ILs-MWCNT composites with bromide ions indicates successful modification.
-
1
2
The sharp peak at 3170 cm is ascribed to the stretching vibration of C-Hsp of the ILs
heterocyclic rings in the ILs-MWCNTs composites. The introduction of ionic liquids on the
surface of the nanotubes does change the functional groups or morphology of the nanotubes.
It can also be stated that the disappearance or less intense of former pure ILs stretching
vibration bands signify the total incorporation of ILs with carbon nanotubes and the
formation of new functional composites with completely different properties . The vibrational
-
1
bands around 798 and 788 cm are ascribed to P-F bond stretching in hydrophobic ILs. The
results obtained from the FTIR spectra studies indicates that the interactions between the ILs
with MWCNTs occur via non-covalent interactions after functionalization.
15

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Figure 1 FTIR spectra of ILs.
16

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Figure 2 FTIR spectra of MWCNTs and IL-MWCNT composites, IL1= [IsopMIm][PF6],
IL2= [N-IsopPyr][PF6], IL3= [MPIm][Br], IL4= [N-PropylPyr][Br].
3
.3 Thermogravimetric analysis of ILs and ILs-MWCNT composites
Thermogravimetric analysis (TGA) was used to gather information about the thermal
degradation of ILs and ILs-MWCNT composites synthesized. TGA results obtained for
functionalized materials were compared with those of pure ILs and MWCNTs, respectively.
+
-
As shown in Fig. 3, the ILs exhibit weight loss at different temperatures. The [MPIm ][Br ]
showed a weight loss of Td, < 30% with onset decomposition at 250°C and complete second
o
+
-
decomposition around 400 C. Another imidazolium-based ILs, [isopropylMIm ][PF6 ] shows
a considerable stability with weight degradation recorded to be less than 15% at 400 °C.
+
-
However, at 450 °C, a complete decomposition of [isopropylMIm ][PF6 ] ILs was observed.
+
-
In the thermal analysis of the pyridinium-based ILs, [N-propylPyr ][Br ] weight decreased
steadily with increasing temperature and reached 99% weight loss around 350°C, whereas
+
-
[
N-isopropylPyr ][PF6 ] showed multi-step degradation profile with highest thermal stability
up to 400°C and less than 35% weight loss was recorded above 450 °C. It was also observed
that hydrophobic ILs exhibits high thermal stability than their hydrophilic counterparts and
both of them were thermally stable up to 400 °C.
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Fig. 4 shows the thermograms of MWCNTs and ILs-MWCNT composites. Pure MWCNTs
thermogram showed multi-steps degradations, though the weight loss is insignificant (initial
step at Td, 5% around 250°C, and final degradation step at Td, 10% around 750°C). The TGA
results obtained for the MWCNTs are in agreement with the findings from other studies [35-
3
6]. Comparison of the TGA curves of pure ILs, MWCNTs, and their ILs-functionalized
composites clearly shows that ILs-MWCNT composites were highly stable with two major
weight losses. The first small weight loss could be attributed to the degradation of ILs on the
surface of nanotubes and the second decomposition corresponds to CNTs fragments [37]. It
can thus be concluded that composites of ILs-MWCNT are effectively thermally stable than
pure ILs and/or MWCNTs [25]. The level of stability of ILs-MWCNT hybrids can be
associated with non-covalent (ionic-interactions or exchange), and strong van der Waals
interactions between the ILs and MWCNTs. Another potential contributing factor to the
stability of ILs-MWCNT hybrids could be the differences in polarity of both the dispersing
molecule (ILs) and the solute (MWCNTs). However, other studies suggested that the thermal
stability of CNT-ILs depends on the type of the ILs counter-ions [37].
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Figure 3 TGA curves of ILs, IL1= [IsopMIm][PF6], IL2= [N-IsopPyr][PF6], IL3=
[MPIm][Br], IL4= [N-PropylPyr][Br].
Figure 4 TGA curves of MWCNTs and IL-MWCNT composites, IL1= [IsopMIm][PF6],
IL2= [N-IsopPyr][PF6], IL3= [MPIm][Br], IL4= [N-PropylPyr][Br].
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3
.4 Solubility studies of the MWCNTs and ILs-MWCNT composites in different solvents
Solubility studies of the MWCNT’s and ILs-MWCNT was carried out in nine solvents, polar
to non-polar (from left to right) in Figs. 5 & 6: water (H2O), ethanol (EtOH), acetonitrile
(ACN), dimethyl sulfoxide (DMSO), acetone (Ace), tetrahydrofuran (THF), diethyl ether
(DEE), toluene (TOL), and hexane (HEX). Consequently, some MWCNTs and/or their ILs-
MWCNT composites were not soluble (N.S), some were partially soluble (P.S), while some
were completely soluble (S) in different solvents depending on their polarities. Pure
MWCNTs were insoluble or not dispersible in polar solvents such as water and ethanol, yet
suspendable in other polar solvents (acetonitrile, dimethyl sulfoxide, acetone, and
tetrahydrofuran), (Fig. 5). On the other hand, MWCNTs were insoluble in non-polar solvents
(diethyl ether, toluene, and hexane). As it was once suggested by Salam & Burk [20], a
solvent or solute will only dissolve or be dissolved in a substance of similar polarity,
respectively. Therefore, the latter solubility characteristics of MWCNTs can be attributed to
its hydrophobicity character (non-polar substance). Another important observation is the
relative insolubility of MWCNTs in more non-polar solvents such as toluene and hexane.
This indicates that differences in polarity between MWCNTs and other non-polar solvents
played a significant role.
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The solubility of different ILs-MWCNT hybrids in different solvents is presented in Fig. 6.
+
-
Fig. 6(A) shows the solubility of [N-isopropylPyr ][PF6 ]/MWCNT which indicates the
composite is insoluble in all solvents. On the other hand, the composite of MWCNTs with
+
-
hydrophilic pyridinium-based ILs such as [N-propylPyr ][Br ]/MWCNT provided different
+
-
solubility trends. As shown in Fig. 6(D), [N-propylPyr ][Br ]/MWCNT showed varied
solubility profiles by being insoluble and slightly soluble in polar solvents with varied
polarity index, while it showed better solubility in most non-polar solvents. This suggests
non-polar and hydrophobicity character of the solute were maintained after functionalization.
Solubility studies of the MWCNT composites with imidazolium-based ILs shown in Fig.
6
(B) indicates that the solute was insoluble in water (a strong polar solvent), but soluble in
ethanol (another strong polar solvent). Secondly, the same solute was soluble or partially
suspended in non-polar solvents except hexane. The insolubility of this composite in water
and hexane brings the discussion of polarity level between the solute and the solvent. For
instance in this case, strong polar solvent (water) molecules cannot be detached by relatively
non-polar substance, and also strong non-polar solvent (hexane) cannot detach MWCNT
(non-polar) molecules. This observation is in agreement with the study of Salam and Burk
[20].
+
-
In the solubility study of [isopropylPyr ][PF6 ]/ MWCNT shown in Fig. 6(C), the composite
was soluble in hexane and not in water. This latter confirmed the hydrophobicity of the
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synthesized material. Based on the solubility study conducted, it can be concluded that when
the polarity of both the solvent and the solute are similar, then the solute and the solvent
definitely separate each other leading to increase in solubility.
Figure 5 Solubility of MWCNTs in different solvents.
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+
+
Figure 6 Solubility of four ILs-MWCNT composites; (A) [N-isopropylPyr ][PF6 ]/MWCNT,
+
-
+
-
(
B) [MPIm ][Br ]/MWCNT, (C) [isopropylMIm ][PF6 ]/ MWCNT, and (D) [N-
+
-
propylPyr ][Br ]/MWCNT: Where (S, N.S, and P.S) stands for soluble, non-soluble, and
partial soluble, respectively.
3
.5 Scanning Electron microscopy and EDS spectra of MWCNTs and MWCNT-ILs composites
The surface morphologies of MWCNTs, and the ILs-functionalized hybrids presented in Fig.
7
shows highly twisted or tangled tubes of MWCNTs. Figs. 7 (A) and (B), shows carbon
nanotubes with clustered or fused morphologies with limited surface area. The ILs-
functionalized MWCNT hybrids shows different morphological orientations. They are well
dispersed with uniformly pronounced tubing and increased surface area, while in some areas
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the MWCNTS are restacked together inside ionic liquids producing flake-layers orientations
[34]. It is also worth noting that the morphology of the MWCNTs is unaltered but shows
improved dispersion (Fig. 8). Energy-dispersive X-ray spectroscopy (EDXS) results
confirmed the presence of constituents’ elements in MWCNTs and ILs-MWCNT composites
(Figs. 7 C-D and 9). The intensity of the carbon peak (Fig. 7C-D) shows the amount of
carbon material produced [26]. The presence of phosphorous (P) and fluorine in Fig. 9 (A &
C) confirms the successfully dispersion of MWCNTs in hydrophobic ILs [25], whereas in
Fig. 9 (B & D) the presence of bromide ions signify the combination of halide-containing ILs
and MWCNTs.
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Figure 7 SEM and EDS spectra of MWCNTs.
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+
-
Figure 8 SEM micrographs of MWCNT-IL composites: (A) [N-propylPyr ][Br ]/MWCNT,
+
-
+
-
(B)
[N-isopropylPyr ][PF6 ]/MWCNT,
(C)
[MPIm ][Br ]/MWCNT,
and
(D)
+
-
[
isopropylMIm ][PF6 ]/MWCNT.
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+
-
Figure 9 Elemental content in samples shown from EDS spectra; (A) [isopropylMIm ][PF6
+
-
+
-
]
/MWCNT, (B) [MPIm ][Br ]/MWCNT, (C) [N-isopropylPyr ][PF6 ]/MWCNT, and (D) [N-
+
-
propylPyr ][Br ]/MWCNT.
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4
Conclusions
MWCNTs, ILs, and ILs-MWCNT composites were synthesised and characterized using
FTIR spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray
spectroscopy (EDS), thermogravimetric analysis (TGA), and solubility in different polar and
non-polar solvents. NMR spectra studies confirmed the synthesis of the ILs. FTIR
spectroscopic studies revealed graphitic and carboxylic functional group in the MWCNTs.
Thermal studies shows that the hydrophobic ILs are more thermally stable than hydrophilic
counterparts. However, ILs-MWCNT hybrids are also more thermally stable and versatile
than individual ILs and/or MWCNT components. The observed high thermal stability in
MWCNT composites are ascribed to strong van der Waals and non-covalent interactions.
Finally, the ILs-MWCNT hybrids were observed to be soluble and/or insoluble in different
solvents. The solubility of these composites depends on the polarity of both the solvents and
the composites. Therefore, it can be concluded that ILs-MWCNT compounds in this study
are hydrophobic (non-polar), immiscible with water and other polar solvents. The as-
synthesized thermally stable ILs-MWCNT composites with increased surface area can be
used in a number of applications such as metal ions removal (adsorption).
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Acknowledgements
The authors gratefully acknowledge National Research Foundation (NRF) of South Africa
and Sasol for financial support. M. Matandabuzo gratefully acknowledges NRF for PhD
scholarship.
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