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Cite this: RSC Adv., 2017, 7, 16524
Synthesis and application of monodisperse
hydrophobic magnetite nanoparticles as an oil spill
collector using an ionic liquid†
Ayman M. Atta,*ab Abdelrhman O. Ezzata and Ahmed I. Hashemc
In this work, a new facile green method was proposed to prepare monodisperse magnetic nanoparticles at
room temperature with controlled shape and size. In this respect, a new ionic liquid based on 1-allyl-3methylimidazolium oleate (AMO) was prepared and applied as a capping and stabilizing agent for
magnetite at low temperature. The chemical structure, morphology, particle size, crystal structure and
monodispersity of magnetite capped with AMO were determined. The magnetic properties and porosity
Received 27th February 2017
Accepted 6th March 2017
of the prepared magnetite nanomaterials were evaluated using different analytical techniques. The
efficiency of the prepared magnetite nanomaterials as an oil spill collector confirmed their high
DOI: 10.1039/c7ra02426f
selectivity and oil collection efficiency on the surface of seawater using a low concentration of
rsc.li/rsc-advances
nanomaterials in relation to the size of the crude oil spill.
1. Introduction
Petroleum crude oil spill disasters can occur during the
production and transportation of crude oil and produce several
environmental hazards, such as toxic organic water pollutants.
Such disasters cost the world over billions of dollars due to the
loss of valuable energy resources in the crude oil and the
resources required to clean up the mess.1 The chemical treatment of oil spills has attracted great attention, among other
mechanical methods and equipment to prevent the oil spill
from spreading into a large area. There are different chemical
methods that have been widely used to control oil spill pollution, involving the application of either natural or synthetic
chemicals.2–5 The selection of chemicals to alleviate oil spill
pollution is based on the dispersion, emulsication, demulsication, adsorption and collection mechanisms.6–8 Recently,
the development of porous materials as oil spill adsorbents or
collectors has attracted much attention because, in terms of
initial cost, they are simple and easy to design for fast operation.
Such materials are effective and can feasibly be reused, among
other materials.9–13 It was also found that porous adsorbents
possessing a balance of both superhydrophobicity and superoleophilicity are promising materials.10–16 More recently, the use
a
Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi
Arabia. E-mail: [email protected]
b
Petroleum Application Department, Egyptian Petroleum Research Institute, Nasr City
11727, Cairo, Egypt
c
Chemistry Department, Faculty of Science, Ain Shams University, Abasia, 11566
Cairo, Egypt
† Electronic supplementary
10.1039/c7ra02426f
information
16524 | RSC Adv., 2017, 7, 16524–16530
(ESI)
available.
See
DOI:
of adsorbents with a combination of porosity and magnetic
properties has been proposed as the most effective magnetic
separation technique for applying nanocomposites for water
treatment and oil spill collection.17–21 Hydrophobic magnetic
nanomaterials have also been proposed to control water pollution from crude oil spills due to their excellent dispersion in
crude oil and good response to an external magnetic eld.
Different methods have been used to prepare magnetic nanomaterials based on magnetite, such as co-precipitation, microemulsion, thermal decomposition, and hydrothermal
synthesis.22–25 In all methods, the aim is to prepare highly
dispersed magnetite without agglomeration by selecting the
best capping agents, which help to increase the dispersion of
magnetite, reduce the particle size and control the magnetic
properties of the materials.
Ionic liquids, ILs, have received a signicant amount of
interest for the production of magnetic nanomaterials with
controlled shapes, dispersion and nanostructures.26–28 Moreover, ILs, as green chemicals, have been proposed to be alternative chemicals that can protect the environmental ecosystem
from pollution, instead of other chemicals used to treat and
clean-up oil spills.25 Magnetic nanomaterials, consisting of
high-quality, monodisperse and crystalline magnetite with
a narrow size distribution, have been prepared using hightemperature organic phase decomposition methods.29 More
recently, ILs have been used to prepare high-quality, monodisperse inorganic nanomaterials at room temperature.30,31 It
has also been reported that highly ordered ILs can be used to
form nanometer-size clusters at room temperature.23,32 These
ILs behave like a nanostructured liquid medium that has an
intrinsic “nanostructure” that is produced by electrostatic,
hydrogen bonding and van der Waals interactions.32 In this
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respect, magnetic nanoparticles can be anchored onto the ILs,
which are based on 3-methylimidazolium, to combine the
unique properties of both the magnetic nanomaterials and the
ILs at elevated temperature.27–31 A literature survey indicated
that uniform hydrophobic magnetic nanoparticles with excellent paramagnetic performance cannot be produced at low
reaction temperature. In this work, a facile, green, one-step
method was suggested to produce monodisperse hydrophobic
magnetite nanoparticles capped with a new IL based on 1-allyl3-imidazolium oleate at room temperature. The IL was used to
efficiently tune the growth of the magnetite particles and
prevent intraparticle aggregation. The application of the
synthesized magnetic nanomaterials for collecting heavy
petroleum crude oil from polluted water surfaces is another
goal of the present work.
2.
Experimental
2.1. Materials
All chemicals used in this work to prepare the hydrophobic ionic
liquid and magnetite nanoparticles were purchased from SigmaAldrich Chemicals Co. and used without further purication. The
reagents used to prepare the hydrophobic IL were 1-allyl-3methylimidazolium chloride (AMC), potassium hydroxide
(KOH), oleic acid (OA), methanol and ethanol. The reagents used
to prepare the magnetite nanomaterials were based on ferric
chloride hexahydrate (FeCl3$6H2O), potassium iodide (KI), and
ammonium hydroxide (25%).
Arabian heavy crude oil produced from the Ras Gara oil eld,
Ras Tannora, Saudi Arabia and seawater collected from the
western Arabian Gulf at the Saudi coast were used to simulate
an oil spill.
2.2. Preparation of magnetite capped with hydrophobic IL
AMC (3 g; 0.019 mol) dissolved in methanol (10 mL) was
mixed with KOH (0.019 mol dissolved in 10 mL of methanol)
under vigorous stirring to precipitate KCl. The solid precipitate was ltered off from the reaction mixture. OA (0.019 mol
dissolved in 5 mL of methanol) was added to the reaction
mixture under vigorous stirring at room temperature for 2 h.
Methanol was removed from the reaction mixture at the end
of the reaction time under reduced pressure to obtain
a brownish yellow oil with a reaction yield of 99.5%. The obtained product was 1-allyl-3-methylimidazolium oleate,
designated as AMO.
FeCl3$6H2O (7.21 g dissolved in 50 mL of distilled water) was
added to KI solution (3.3 g dissolved in 75 mL of distilled water)
under stirring and nitrogen atmosphere for 1 h. The solid
iodine precipitate was ltered off from the reaction mixture.
The AMO (2 g dissolved in 100 mL of ethanol) was added to the
reaction mixture ltrate at the same time as an ammonia
solution (50 mL) during a period of 1 h. The black colloid
reaction mixture was stirred for another 3 h under nitrogen
atmosphere. The black precipitate was separated by ultracentrifugation of the solution at 12 000 rpm, and washed with water
and ethanol several times.
This journal is © The Royal Society of Chemistry 2017
2.3. Characterization
All instruments used to characterize the prepared IL and
magnetite nanoparticles are described in the ESI.†
2.4. Application of magnetite AMO powder as a crude oil
spill collector
Arabian heavy crude oil (1 mL) was spread over 250 mL of
seawater. The magnetite powder was spread onto the surface of
the oil and the oil was mixed with a glass rod to disperse the
magnetite powder in the crude oil. Magnetite to oil ratios
ranging from 1 : 10 to 1 : 50 were used to study the effect of the
magnetite ratio on the oil spill collection efficiency. Slow stirring of the oil/powder mixture was continued for 1 minute. The
dispersed crude oil spill was collected aer 5 min using
a permanent magnet of Nd–Fe–B (4300 Gauss). The remaining
oil was extracted from the water surface using chloroform,
which was then evaporated using a rotary evaporator under
reduced pressure. The crude oil collection efficiency can be
calculated as a percentage using the following equation:
CE (%) ¼ (volume of removed oil/volume of original spill) 100.
The magnetite powders were reused aer isolation by an
external magnetic eld aer dilution of the crude oil/magnetite
solution with chloroform.
3.
Results and discussion
3.1. Characterization of AMO IL
The present work aims to prepare a hydrophobic IL by replacing
the chloride anion in AMC with an oleate group through
reacting AMC with sodium hydroxide followed by reaction with
oleic acid as represented in the Experimental section and
Scheme 1.
The chemical structure of the new ionic liquid, AMO, is
elucidated from 1H NMR of AMC and AMO as represented in
Fig. 1a and b. The protons of AMC are marked in Fig. 1a.32 The
replacement of the chloride anion with an oleate anion is
conrmed from the appearance of new peaks at chemical shis
of 0.7, 1.14, 2.5 and 5.2 ppm, which are attributed to CH3,
(CH2)7, CH2COO and CH]CH of the oleate group, respectively.
It was also found that the +N]CH protons were deshelled from
9.3 to 10.3 ppm due to the effect of the oleate carboxylate
group.32
The thermal stability of AMO was conrmed using TGA and
DTG analysis as represented in Fig. 2a–c. AMC loses approximately 7.3% of its weight aer heating up to 100 C. These data
conrm that AMC is linked with water. The decomposition of
AMO started at 240.3 C and ended at around 317.4 C with
a weight loss of 84%, as shown in Fig. 2a. The fact that AMO
Scheme 1
Synthesis of AMO.
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highest position at the peak of the heating cycle at a Tm of 34
and 4 C for AMC and AMO, respectively. The appearance of one
Tm conrms the purity of both AMC and AMO as ILs. The
existence of the crystallization temperature, Tc, of AMO at
54 C and the absence of this peak in the DSC thermogram
suggest that the presence of unsaturated OA increases the
planarity of AMO due to the overlapping and interaction
between the allyl group in the alkyl chain of the imidazolium
ring and OA.33
3.2. Synthesis and characterization of magnetic
nanoparticles capped with AMO
Fig. 1
1
H NMR spectra of (a) AMC and (b) AMO.
Fig. 2 TGA thermograms of (a) AMC, (b) AMO and (c) magnetite
capped with AMO.
didn't start to decompose until 255 C, without any water loss,
conrmed the low affinity of AMO for water due to the hydrophobicity of the oleate anion. The presence of long chain oleate
hydrocarbons assisted the easy decomposition of AMO from
290 C up to 650 C without any remaining residue, due to the
exibility of the chains. These data indicate that the oleate
group reduced the thermal stability of AMO in comparison to
AMC.
The chain exibility and thermal characteristics, such as
crystallization and melting temperature, of AMC and AMO were
determined from the DSC thermograms, as shown in the ESI
(Fig. S1†). It was noticed that the replacement of the chloride
anion in AMC with oleate converted AMC, which is solid at
room temperature, to AMO, which is liquid at room temperature, due to the unsaturation of the OA group. The melting
temperature, Tm, of an IL is an important indicator to conrm
the purity of the IL. The data show only one transition from the
16526 | RSC Adv., 2017, 7, 16524–16530
Magnetite nanoparticles (MNPs) have previously been prepared
in the presence of different ILs based on N-alkyl imidazole using
the co-precipitation method, but it was not possible to control
their dispersity and aggregation, although their sizes and
diameters could be modied.34–36 Moreover, monodisperse
magnetite nanoparticles have only been prepared at high
temperature through decomposition of organic precursors of
iron salts.37–39 Alternatively, a microwave technique has been
used as an alternative method to produce MNPs in the presence
of an IL; monodisperse nanoparticles, with a particle size of
6 nm, were prepared from an organic precursor of an iron salt
and 1,2-hexadecanediol in dibenzyl ether in the presence of
oleic acid.40 The present work aimed to prepare superior MNPs
using a new IL based on a co-precipitation method at low
temperature as reported in the Experimental section. In our
previous work, ferrous and ferric cations were produced from
the oxidation reaction of ferric chloride with potassium
iodide.21 The produced iron cations can be stabilized with the
prepared AMO in an ethanol/water mixture as represented in
Scheme 2.
The iron cations can be stabilized by AMO through chelation
with the oleate carboxylate anion and interaction with the
unsaturated double bond and allyl bonds of oleate.41 The
repulsion between the imidazolium cations of the prepared
AMO and the iron cations facilitates the capping of the iron
cations with the carboxylate anions of OA to control the dispersity, particle size and porosity. The proposed mechanism
suggests that monodisperse Fe3O4 nanocrystals can be rapidly
Scheme 2
Capping of magnetite with AMO.
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formed at low temperature, which can be observed using
different characterization techniques.
The formation of magnetite nanoparticles and the interaction between magnetite and AMO was conrmed using FTIR, as
shown in Fig. 3a and b. The appearance of a strong and broad
band at 579 cm1, corresponding to the Fe–O vibration of the
magnetic core, conrms the formation of magnetite without
impurities of other iron oxides. The strong absorbance band at
3423 cm1 is assigned to the formation of hydroxyl groups at the
magnetite surface. The bands at 3049, 2980 and 2850 cm1 were
observed for both magnetite capped with AMO and AMO,
resulting from the vibration of olen and the paraffin CH
stretching vibration of AMO. Moreover, the absorbance bands at
wavenumbers of 1562 and 1458 cm1 were assigned to the
stretching vibration of the imidazole ring, which conrms that
AMO is immobilized onto the surface of the magnetite. The
lowering of the intensity of the band at 1729 cm1, attributed to
the vibration stretching of C]O of the oleate carboxylate anion,
and increase in the intensity of the bands at 1630 cm1 and
1549 cm1, characteristic of asymmetric and symmetric COO–
vibration, conrm the chelating bidentate interaction between
oleic acid and Fe atoms on the surface of the particles when
magnetite is capped with AMO (Fig. 3b). All these data show that
the interaction of magnetite with the carboxylate anion of AMO
protects the magnetite from oxidation to other iron oxides.42
The purity of the magnetite nanoparticles capped with AMO
without the formation of other iron oxides can be conrmed
from XRD diffractograms, as shown in the ESI (Fig. S2†). It was
found that six characteristic peaks appeared at 2-theta values of
30.5 (2 2 0), 35.9 (3 1 1), 43.5 (4 0 0), 53.9 (4 2 2), 57.3 (5 1 1)
and 63.1 (440). These peaks show that the crystal structure of
the Fe3O4 particles was not changed aer modication with
AMO, and matched well with the standard diffractions of Fe3O4
(JCPDS 89-4319). The appearance of the peaks as broad bands
with a wide width and low intensity conrms the small particle
size of the magnetite capped with AMO; the particle size
calculated using Scherrer's equation from the (220), (311), and
(400) planes was approximately 6.83 nm.
The content of magnetite incorporated with AMO can be
determined from the TGA and DTG thermograms shown in
Fig. 2c. The remaining residue aer heating the solid at 750 C
in the presence of an oxygen atmosphere indicates that the
contents of magnetite and AMO are 83 and 17 wt%, respectively.
Fig. 3
FTIR spectra of (a) AMO and (b) magnetite capped with AMO.
This journal is © The Royal Society of Chemistry 2017
Fig. 4 HR-TEM micrograph of magnetite capped with AMO.
This result conrms that 17 wt% of AMO is sufficient to cap the
magnetic nanoparticles.
The surface morphology of the magnetite nanoparticles
capped with AMO can be observed from the TEM and SEM
micrographs shown in Fig. 4 and 5, respectively. The HR-TEM
image shown in the inset of Fig. 4 shows that each individual
Fe3O4 particle is single crystalline and ordered into atomic
lattice fringes. The interfringe distances are measured to be
0.421 and 0.461 nm, which is close to the lattice spacing of the
(021) planes (0.42 nm) and (020) planes (0.47 nm), respectively,
in the cubic spinel Fe3O4 structure.40 It was also found that there
was a so shell surrounding the lattice with a radius of 2.3 nm.
The SEM and TEM micrographs show the formation of monodisperse magnetic nanoparticles exhibiting a spherical geometry with a so shell and a size of 8 3 nm (Fig. 4 and 5).
The diameter and dispersity of the magnetite particles capped with AMO can be further evaluated in toluene using DLS
measurements, as summarized in the ESI (Fig. S3†). The polydispersity index (PDI) and average particle diameters of
magnetite are 0.005 and 9 nm. The increase in the particle
diameter of magnetite by about 2 nm relative to the TEM
measurements agrees with data reported for nanoparticles
capped with chains of oleic acid.43 Moreover, the increase in
magnetite particle diameter in toluene compared to that
detected by TEM (Fig. 4) indicates the high absorption of
toluene by the particles due to the presence of OA on the surface
of the particles.
3.3. Oil collection efficiency
The most important parameters used to select magnetic materials for application as oil spill collectors are hydrophobicity
with selectivity to collect oil without water, excellent magnetic
properties, fast rate of oil absorption due to the porosity of the
Fig. 5
SEM micrograph of magnetite capped with AMO.
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Fig. 6 Contact angles of a sessile drop: (a) advancing angle, (b)
receding angle and (c) photo of the water droplet on a compressed
magnetite disk.
materials, cost and reusability. In this respect, the hydrophobicity, magnetic properties and porosity of the prepared
magnetite were examined using contact angle, VSM and BET
measurements as described in the Experimental section. The
contact angle measurements of seawater on a compressed
magnetite disk were performed as illustrated in the photos
shown in Fig. 6a–c. The advancing and receding contact angles
(Fig. 6a and b) were measured to characterize the wetting
behaviour of the magnetite and to conrm the roughness of the
magnetite particles. The difference between the advancing
angle and the receding angle is called the hysteresis, which is
used to conrm the roughness of solid surfaces. The apparent
seawater advancing and receding contact angles and hysteresis
for the magnetite disks are 152.1 3.6 , 148.9 5.6 and 3.2,
conrming the low hysteresis and indicating the formation of
uniform magnetite particle surfaces.44,45 The high contact
angles of water indicate the hydrophobicity of the magnetite
due to capping with the hydrophobic AMO ionic liquid. Moreover, when a drop of diesel oil is placed on a magnetite disk, the
oil spreads quickly over the surface of the magnetite to form
a thin lm, indicating the low energy and superhydrophobicity
of the magnetite surface. These data suggest that highly
hydrophobic and superoleophilic magnetite nanoparticle
surfaces can be produced economically through a coprecipitation method in the presence of a hydrophobic IL.
The magnetic properties of the magnetite capped with AMO,
such as saturation magnetization (Ms), remnant magnetization
(Mr) and coercivity (Hc), were determined from VSM magnetic
hysteresis loops (Fig. 7). superoleophilicThe Ms, Mr and Hc
values are 59.89 emu g1, 0.12 emu g1, and 4.38 G, respectively.
Paper
These data conrm that the magnetite capped with AMO is
superparamagnetic due to the increase in Ms and the decrease
in Mr and Hc as compared with magnetite capped with surfactants or other ILs.20,28 Accordingly, the magnetite particles capped with AMO could be readily separated from solution using
an external magnet due to their superparamagnetism and large
saturation magnetization.
The porosity of the magnetite particles coated with AMO was
identied from pore volume and specic surface area
measurements and determined using the standard Brunauer–
Emmett–Teller (BET) method.46,47 Nitrogen adsorption–desorption isotherms were measured for the magnetite particles and
are shown in the ESI (Fig. S4†). The mesopore volume and
specic surface area evaluated from the nitrogen adsorption
isotherm are 0.4625 cm3 g1 and 737.5 m2 g1 at 0.04–0.2P/Po,
respectively. These data conrm the porosity of the prepared
magnetite when capped with AMO, which conrms that the iron
cations seem to be in the (II) and (III) states in the AMO
framework.
The previous data showed that the prepared magnetite
particles capped with AMO have superhydrophobic, superparamagnetic and porous surfaces, and thus are suitable for
application as an oil spill collector. In this respect, different
magnetite to oil ratios (MORs) were used to collect oil from the
surface of water as illustrated in the Experimental section.
Viscous crude oil based on Arabic heavy crude oil was used. The
magnetite was reused for 5 cycles aer separation with an
external magnet from crude oil diluted with diesel fuel. The oil
spill collection efficiencies for different MORs are summarized
in Table 1.
The data show a high collection efficiency at a low magnetite
content (MOR 1 : 50) for the rst time. Moreover, the prepared
magnetite capped with AMO has a higher reuse efficiency than
that reported in the literature.21 Magnetic polymer composites
based on the curing of alkyd resin with toluene diisocyanate in
the presence iron oxide have previously been used to remove oil
for the cleaning-up of oil spills.48 In the best case, one part of the
magnetic polymer composite could be used to remove more
than eight parts of oil from water with an efficiency of 90%.48
Magnetic polymer composites based on styrene divinyl benzene
have also been used to remove oil from the surface of water with
a capacity of 3.3 g of oil per g of magnetic composite.49 It was
also reported that the capping of magnetite with natural products based on cardanol–furfuraldehyde nanocomposites could
be used to achieve the removal of 10 g of petroleum from
Table 1
CE% of magnetite capped with AMO at different MORs
CE% at different MORs
Fig. 7 VSM loop of magnetite capped with AMO.
16528 | RSC Adv., 2017, 7, 16524–16530
Oil collection
sample
1 : 10
1 : 20
1 : 25
1 : 50
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
100
100
98
96
96
98
96
95
93
93
95
93
92
90
90
90
88
86
80
80
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water.50 Finally, we can conclude that the formation of porous
and uniform magnetite nanoparticles without other iron oxide
particles increased the magnetic properties of magnetite and
improved its efficiency to remove more oil when capped with
a hydrophobic IL.
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4. Conclusions
Superhydrophobic magnetite was prepared using hydrophobically modied 1-allyl-3-immidazolium oleate ionic liquid. The
prepared magnetite nanoparticles showed monodispersity and
superparamagnetic characteristics. Moreover, the produced
magnetite has a mesoporous structure and achieved high oil
collection efficiencies at low concentrations. These ndings
may provide a cheap available method to produce magnetite
capped with AMO on an industrial scale to collect large areas of
oil spills.
Acknowledgements
The authors extend their appreciation to the Deanship of
Scientic Research at King Saud University for funding this
work through research group No. (RGP-235).
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