Multifunctional amphiphilic ionic liquid pathway to create water-based magnetic fluids and magnetically-driven mesoporous silica

Article abstract of DOI:10.1039/c8ra10065a

Amphiphilic ionic liquids, 1-alkyl-3-methylimidazolium chloride (C_nmimCl with n = 10, 12, 14, 16) were firstly used as modifiers to construct a self-assembly bilayer on the surface of iron oxide nanoparticles for generation of highly stable, water-based magnetic fluids. Subsequently, a magnet-driven mesoporous silica was synthesized by in situ self-assembly in the bilayer C_nmimCl-stabilized magnetic fluid using the C₁₆mimCl as template and tetraethylorthosilicate (TEOS) as silicon source via a hydrothermal synthesis and following calcination procedure. A systematic study was carried out addressing the influence of the alkyl chain length of C_nmimCl in the primary and secondary layers on the stability of magnetic fluids. The characterization of TEM, XRD, VSM, electrophoresis experiments, TGA and DTA showed that stable water-based magnetic fluids can be synthesized based on the assembly of the well-defined bilayer-C_nmimCl structure with long-chain C₁₆mimCl as secondary layer on the magnetite (Fe₃O₄) nanoparticles. The results of small and wide-angle XRD, TEM, VSM, and N₂ absorption experiments indicated that the nano-scale magnetic Fe₃O₄ particles were inlayed into hexagonal p6mm mesoporous silica (MCM-41 type) framework. Importantly, it was found that the obtained Fe₃O₄/MCM-41 was an appropriate adsorbent for the adsorption of rhodamine B and methylene blue from their aqueous solution. In addition, the adsorbent could be separated and reclaimed fleetly from the solution under external magnetic field.

Full text of DOI:10.1039/c8ra10065a

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Multifunctional amphiphilic ionic liquid pathway to
create water-based magnetic fluids and
magnetically-driven mesoporous silica†
Cite this: RSC Adv., 2019, 9, 3504
a
b
b
Jing Shen, * Wen He and Tongwen Wang
Amphiphilic ionic liquids, 1-alkyl-3-methylimidazolium chloride (C
n
mimCl with n ¼ 10, 12, 14, 16) were
firstly used as modifiers to construct a self-assembly bilayer on the surface of iron oxide nanoparticles
for generation of highly stable, water-based magnetic fluids. Subsequently, a magnet-driven mesoporous
silica was synthesized by in situ self-assembly in the bilayer C
16mimCl as template and tetraethylorthosilicate (TEOS) as silicon source via a hydrothermal synthesis
and following calcination procedure. A systematic study was carried out addressing the influence of the
alkyl chain length of C mimCl in the primary and secondary layers on the stability of magnetic fluids. The
n
mimCl-stabilized magnetic fluid using the
C
n
characterization of TEM, XRD, VSM, electrophoresis experiments, TGA and DTA showed that stable
water-based magnetic fluids can be synthesized based on the assembly of the well-defined bilayer-
C
n
mimCl structure with long-chain
nanoparticles. The results of small and wide-angle XRD, TEM, VSM, and N
indicated that the nano-scale magnetic Fe particles were inlayed into hexagonal p6mm mesoporous
silica (MCM-41 type) framework. Importantly, it was found that the obtained Fe /MCM-41 was an
C
16mimCl as secondary layer on the magnetite (Fe
3 4
O )
2
absorption experiments
3
O
4
Received 7th December 2018
Accepted 18th January 2019
3 4
O
appropriate adsorbent for the adsorption of rhodamine B and methylene blue from their aqueous
solution. In addition, the adsorbent could be separated and reclaimed fleetly from the solution under
external magnetic field.
DOI: 10.1039/c8ra10065a
rsc.li/rsc-advances
slightly superior to that of the typical cationic surfactants. These
self-organized advantages of amphiphilic ionic liquids have
Introduction
The use of ionic liquids has opened new avenues in the been applied to construct ordered mesoporous materials. For
synthesis and fabrication of various new materials with instance, ordered mesoporous silica with 2D hexagonal struc-
1
–3
advanced properties. Particularly, amphiphilic ionic liquids, ture was synthesized by using amphiphilic ionic liquids, 1-alkyl-
composed of a charged hydrophilic imidazolium head group 3-methylimidazolium chloride (C mimCl, n is number of
n
10
and a hydrophobic ‘long tail’ domain, can provide not only the carbon atoms in alkyl chains), as templates. Zhou et al. re-
amphiphilic properties of conventional surfactants, but also the ported a preparation of supermicroporous lamellar silica
specicity of typical short-chain room temperature ionic liquids through 1-hexadecyl-3-methylimidazolium chloride (C16mimCl)
11
such as low melting point, strong polarity and high resolvability as template. Our group has also contributed much effort to
in aqueous solution etc. Much work has been made to explore successfully synthesize mesoporous silica with high-quality
self-organized behaviour of amphiphilic ionic liquids in cubic gyroid and 2D hexagonal mesoporous structures by
4
–6
12
aqueous and other solvents. For example, Zheng and co- using the C16mimCl as template. More recently, we have
workers measured the micelle formation and the micelle employed a C16mimCl-assisted synthesis method to prepare
aggregation numbers of amphiphilic ionic liquids in aqueous polymer/Pd microspheres and a bimodal porous Pd-decorated
13
solution by surface tension, electrical conductivity and uo- silica. These research results conrmed that C
mimCl
They found that the surface possess a specic templating performance in the synthesis of
activity of amphiphilic ionic liquids in aqueous solution is ordered porous structural materials. However, their potential as
modifying agent, especially for the formation of stable C
mimCl-functionalized magnetic Fe nanoparticles (NPs) or
n
7
–9
rescence measurement etc.
n
-
3 4
O
a
Department of Applied Chemistry, College of Vocational Education, Yunnan Normal
their multifunctional combination for the fabrication of hybrid
porous materials is less commonly known.
The aim of this work is twofold: rstly, it is intended to
demonstrate that 1-alkyl-3-methylimidazolium chloride (Cn-
mimCl) can serve as a surface functional molecule to construct
University, Kunming 650092, China. E-mail: shenjingbox0225@hotmail.com
b
College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming
6
†
1
50092, China
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ra10065a
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a bilayer of C
n
mimCl in the surface of magnetic Fe
3
O
4
NPs, product was further puried by recrystallization in tetrahydro-
which can effectively produce a stable water-based magnetic furan (THF). Aer being washed several times with THF, the
uid. Secondly, based on the C mimCl-based magnetic uid, white crystalline C mimCl powder was collected by ltration,

n
16
a more facile access to prepare magnetically-driven mesoporous and dried in air at room temperature. The preparation of the
silica with magnetic Fe NPs inlayed randomly into ordered other C mimCl was achieved by repeating the above procedure
hexagonal mesoporous silica framework by using C16mimCl as with substitution of the corresponding 1-alkyl chloride for 1-
template and their adsorption properties is presented. hexadecyl chloride, i.e., 1-decyl chloride for C10mimCl, 1-
The C mimCl-bilayer structure onto Fe NPs is similar to dodecyl chloride for C12mimCl and 1-tetradecyl chloride for
the surfactant-bilayer modied on the surface of iron oxide 14mimCl. The structures of the obtained C mimCl were
3
O
4
n
n
3 4
O
C
n
particles, which has been conrmed to form stable water-based identied by IR spectrum (see Fig. S1†).
magnetic uid by effectively overcoming the agglomeration of
Fe O NPs. The combinations of various surfactants, such as
3
4
14,15
16
Synthesis of bilayer C mimCl-stabilized magnetic uids
n
fatty acid/fatty acid,
oleate sodium/PEG-4000, oleic acid/
17
18
succinimide, and oleic acid/sodium dodecyl sulfate etc., have The bilayer C mimCl-stabilized magnetic uids were synthe-
n
2
+
3+
been used as modied bilayer to stabilize Fe
solution. In our design, the rst research goal is to build a C
mimCl-bilayer on the surface of Fe NPs. We expect that the number of C
C mimCl should exhibit the strong self-assembly ability on the primary and secondary C mimCl, respectively. In a typical
3
O
4
NPs in aqueous sized by the chemical coprecipitation of Fe and Fe salts in
1 : 2 M ratio from a basic aqueous solution containing a small
mimCl, followed by the modifying process of the
n
-
3
O
4
n
n
n
surface of Fe O NPs to form a high charge density and strong preparation of primary C mimCl-coated iron oxide particles,
3
4
10
hydrophilic surface, which can form stable water-based 0.86 g of FeCl $4H O, 2.35 g of FeCl $6H O, and 0.1 g of
2
2
3
2
ꢀ
magnetic uid.
C mimCl were dissolved in 40 mL of distilled water at 60 C
10
Recently, signicant advances have been achieved in the under mechanical paddle stirring and N purge. Aer 10 min,
2
ꢀ
synthesis of magnetic mesoporous silica materials because of the temperature was raised to 80 C, followed by 0.5 mL of 33%
1
9
their versatile application such as adsorption and separation,
4
(w/w) NH OH to adjust the pH value of the mixture to 9. Aer
20
21
catalysis and biological technology. In the synthesis of these reaction for 10 min, the pH value of the mixture was adjusted
magnetic mesoporous silica materials, the commonly used again to 13 by adding tetramethylammonium hydroxide drop-
approach is based on the construction of a core–shell structure, wise, and then, C mimCl was further added to the suspension
1
0
in which magnetite particle services as inner core and a layer in ve 0.2 g amounts over 5 min. The reaction was allowed to
21
ꢀ
mesoporous silica acts as outer shell. This synthesis procedure proceed for 30 min at 80 C under constant stirring. Subse-
is complicated, and it is difficult to accurately control the quently, the suspension was cooled slowly to room temperature
experimental conditions. It is also impossible to exclude the and the precipitates were isolated from the solution by
formation of some irregular pore structures. Thus, the exploi- magnetic decantation. The precipitates were further puried
tation of a convenient way of homogeneous incorporation of with the magnetic decantation and redispersion in water and
magnetic particles into the framework of mesoporous silica by ethanol for three cycles. The resulted product was coded as
optimized various methods is still a valuable research issue. Fe O /C mimCl. The other primary C mimCl-coated iron oxide
3
4
10
n
Another objective of our research is to demonstrate the supra- particles were prepared by repeating above procedure with the
molecular templating function of C16mimCl for the preparation corresponding C mimCl as primary layer modier and coded as
n
of highly ordered magnetic mesoporous silica in the prepared Fe O /C mimCl, Fe O /C mimCl and Fe O /C mimCl,
3
4
12
3
4
14
3
4
16
Fe
3
O
4
/C16mimCl/C16mimCl magnetic uid. We expect that C
n
-
respectively. For comparison, bare Fe O nanoparticles were
3 4
mimCl can be manipulated in tandem as bilayer modifying prepared by above similar procedure without addition of any
agent and templating agent in one system, should provide
important guidelines for the multifunctional applications of
C
n
mimCl.
To prepare stable water-based magnetic uids, a series of
amphiphilic ionic liquids in the synthesis of various nano- secondary C mimCl with similar chemical structures to the
n
structured materials.
n n
primary C mimCl were coated on the primary C mimCl-coated
iron oxide particles. In a typical preparation of Fe
3 4
O /
C mimCl/C mimCl magnetic uid, 1 mL of C mimCl solu-
1
0
16
ꢁ
16
Experimental section
1
tion (40 g L ) was rstly added to 20 mL of distilled water. 0.5 g
of Fe /C10mimCl obtained above was added to the solution
Amphiphilic ionic liquids, 1-alkyl-3-methylimidazolium chlo- under ultrasound at room temperature. Aer 10 min, C16mimCl
Synthesis of C
n
mimCl (n ¼ 10, 12, 14, 16)
3 4
O
ꢁ
1
ride (abbreviated as C
n
mimCl, n is the number of carbon atoms solution (40 g L ) was further added dropwise to the suspen-
in alkyl chains, n ¼ 10, 12, 14, 16, respectively), were prepared sion in 1 mL amounts over 5 min under vigorous mechanical
13,22
according to a route reported in the literatures.
All chemicals paddle stirring until no visible settling and phase separation
were purchased from Acr ¨o s and used as received. As a typical was observed when the solution was placed over a permanent
synthesis of C mimCl, an excess of 1-hexadecyl chloride magnet for a period of 5 min. The sample was coded as Fe O /
1
6
3 4
(
33.41 g, 0.128 mol) was mixed with 1-methylimidazole (10.26 g, C mimCl/C mimCl. The preparations of the other bilayer
10 16
0
9
.125 mol). The mixture was put into a 250 mL ask, reuxed at structures were achieved by repeating the above procedure with
ꢀ
0
n
C for 24 h, and then cooled to room temperature. The corresponding bilayer combination of C mimCl as primary and
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secondary layers. In order to make subsequent various analyses,
a small portion of dried magnetic nanoparticles can be sepa-
rated from the magnetic uids by a decantation procedure where C
q ¼ (C ꢁ C )V/m
(2)
t
0
t
ꢁ1
0
t
and C (mg L ) are the concentrations of the dye at
using a permanent magnet placed next to the bottle to provide initial time and time t, respectively. V is the volume of solution
a magnetic eld of about 0.3 T over 3–4 weeks.
3 4
(L), and m is the amount of Fe O /MCM-41 (g). The experi-
mental data were treated according to Freundlich isotherm
equation eqn (3)
Synthesis of magnetic mesoporous silica
1
/n
The magnetic mesoporous silica was synthesized by a hydro-
thermal synthesis procedure in the Fe O /C mimCl/C mimCl
q ¼ K C
(3)
e
F e
3
4
16
16
ꢁ
1
1

uid using tetraethylorthosilicate (TEOS) as silicon source and where q
e
is the equilibrium adsorption capacity (mg g ); C
16mimCl as template. In a typical synthesis procedure, the equilibrium concentration of dye solution (mg L ); K
g ) and n are empirical constants which reecting the
16mimCl uid under mechanical paddle stirring. Aer adsorption capacity and heterogeneity factor, respectively. The
e
is
ꢁ
C
C
C
F
(mg
ꢁ1
16mimCl and NaOH were dissolved in the Fe
3
O
4
/C16mimCl/
homogenization of the mixture, TEOS was added dropwise at linearized form of Freundlich isotherm can be expressed as
room temperature. The molar compositions of the starting eqn (4)
mixtures
were
1.0
TEOS : x
Fe O /C mimCl/C -
3 4 16 16
log q
e
¼ log K
F
+ (1/n)log C
e
(4)
mimCl : 0.1C mimCl : 0.45 NaOH : 140H O (x ¼ 0.025, 0.05,
1
6
2
0.10, respectively). The resulted mixtures were stirred at room
Moreover, aer the adsorption process, the precipitates were
temperature for 60 min, and then transferred into a PTFE-lined
steel autoclave and heated at 100 C for 3 days. Aer hydro-
thermal treatment, the mixtures were ltered, washed with
deionized water, dried under atmosphere at room temperature,
ꢀ
calcined at 550 C for 5 h to remove the adsorbed dyes. The
ꢀ
3 4
reusability of the recovered Fe O /MCM-41 adsorbents was also
studied by tracking the changes in UV-vis absorption spectro-
graph of the adsorption system under different cycles.
ꢀ
and nally calcined at 550 C for 5 h with a temperature ramp of
ꢀ
ꢁ1
2
C min under static air conditions to remove the template.
Characterization
The nal product, coded as Fe O /MCM-41, was ground into
3
4
powder for further characterization.
Small- and wide-angle X-ray diffraction (XRD) patterns were
measured on a TTR III powder X-ray diffractometer using Cu Ka
ꢀ
ꢁ1
radiation (wavelength 0.154 nm) at a rate of 0.05 2q s and
operated at 40 kV and 30 mA. Transmission electron micros-
copy (TEM) images were taken on a JEM-2100 electron micro-
scope at an acceleration voltage of 200 kV. The specimens for
TEM were prepared by dropping a small drop of the solutions
onto a carbon-coated copper grid. Electrophoresis experiments
were performed on a Nanjing Sangli DYL-3 electrophoresis
apparatus with 15 V of external voltage. The magnetization of
the samples was measured by a vibrating sample magnetometer
Adsorption experiment
The adsorption experiments were carried out by the adsorption
of two dyes, rhodamine B (RhB) and methylene blue (MB), using
the prepared Fe O /MCM-41 as adsorbent at room temperature.
3
4
For comparison, the adsorption experiments of two dyes on the
pure mesoporous silica MCM-41 (without Fe O NPs) prepared
3
4
1
2
using C16mimCl template method which we reported earlier,
and on the bare Fe particles synthesized using the same
3 4
O
chemical coprecipitation method without adding ionic liquid
were also studied at room temperature. In a typical adsorption
(
(
VSM 7407) at room temperature. Thermogravimetric analysis
TGA) and differential thermal analysis (DTA) were performed
procedure, Fe
3 4
O /MCM-41 (0.1 g) was added to the aqueous
with a ZRY-1P thermal analysis system. Nitrogen sorption
experiments were conducted using a Micromeritics Tristar 3000
automated gas adsorption analyzer. A TENSOR27 Fourier
transform infrared (FT-IR) spectrometer was employed for
recording IR spectra.
suspensions (50 mL) of the dye solutions with concentration of
ꢁ1
ꢁ1
x mg L (where x ¼ 10, 20, 50, 100, 200, and 400 mg L
,
respectively) under mechanical stirring. At xed intervals, the
precipitates were isolated from the suspensions by magnetic
decantation, and the absorbance of the solutions was measured
by UV-vis spectrophotometer (SHIMADZU UV-1780) at 554 and
6
64 nm for RhB and MB solutions, respectively. The decolor- Results and discussion
ization rates of the solutions were calculated according to eqn
1)
Synthesis and characterization of C
onto Fe particles
As schematically depicted in Scheme 1, the strategy to prepare
the hydrophilic magnetic Fe NPs with ionic liquid C mimCl
are absorbance values of the solution measured bilayer as shell involves the rst synthesis of Fe NPs, fol-
before and aer the adsorption, respectively. The concentra- lowed by coating magnetite NPs with C
mimCl (n ¼ 10, 12, 14,
tions of the solutions were also determined by the standard 16) as inner layer and outer layers, respectively. The Fe O NPs
n
mimCl bilayer assembled
(
3 4
O
D% ¼ (A
0
ꢁ A
t
) ꢂ 100%/A
0
(1)
3
O
4
n
where A
0
and A
t
3 4
O
n
3
4
working curves of two corresponding standard dye solutions. were synthesized based on the well-established coprecipitation
ꢁ
1
2+
3+
The adsorption capacity of Fe
time t was determined using eqn (2)
3 4 t
O /MCM-41, q (mg g ), at any method using Fe and Fe salts from a basic aqueous solu-
23
tion. We found that during the precipitation process, the size
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C
16mimCl molecules toward the surrounding solution, and
their hydrophobic long alkyl chain extending or inserting into
the fence of alkyl chain of Fe O /C mimCl by means of
3
4
10
a hydrophobic interaction. The exposure of imidazole head
groups in the surface of Fe /C10mimCl/C16mimCl provided
3 4
O
high charge density and strong hydrophilic surfaces, resulting
in the formation of stable and water-based magnetic uid. We
observed the formation of similar stable water-based magnetic

uids for Fe
3 4 3 4
O /C12mimCl/C16mimCl, Fe O /C14mimCl/
C mimCl and Fe O /C mimCl/C mimCl.
1
6
3
4
16
16
Fig. 1A and B show the typical TEM images of ionic liquid
bilayer-stabilized Fe O /C mimCl/C mimCl and Fe O /
3
4
10
16
3 4
C
16mimCl/C16mimCl particles, respectively. It is clear that both
Scheme 1 Schematic diagram of the synthesis procedure of Fe
C mimCl/C mimCl particle using C mimCl as bilayer modifier.
n n n
O
3 4
/
magnetic oxides formed a well-dispersed quasi spherical
morphology with clear boundary and mean diameter of
approximately 10.5 and 10.1 nm. Electron diffractions (the
3 4
of Fe O NPs can be easily controlled with the initial presence of insets in Fig. 1A and B) measured from a large zone present
ꢁ
1
a small quantity of C mimCl (2.5 g L ). Before coating primary a series of rings that can be indexed to the magnetite structure,
n
C mimCl, the pH value of the suspension was adjusted to 13 by which is consistent with the results obtained from wide-angle
n
adding TMAOH, which led to negatively charged iron oxide XRD patterns described in Fig. S3.†
24
particles. The precipitated Fe
a primary layer of C mimCl adsorbed to the iron oxide surfaces Fe O /C mimCl/C mimCl (Fig. 2A) and bare Fe O , Fe O /
through an electrostatic attraction between the negatively C mimCl, Fe O /C mimCl/C mimCl (Fig. 2B) displayed
3
O
4
NPs were rst coated with
3 4 3 4 10
The magnetization curves of bare Fe O , Fe O /C mimCl,
n
3
4
10
16
3
4
3 4
1
6
3
4
16
16
charged Fe
of C mimCl. For example, the particles coated with monolayer and coercivity were observed, revealing the superparamagnetic
C mimCl were unstable and settled from the aqueous solution nature of these magnetic particles. The saturation magnetiza-
3 4
O NPs and the positively charged imidazole heads a typical magnetization “S” curve, where no reduced remanence
n
1
0
ꢁ
1
in a few minutes due to its hydrocarbon chains extending from tion (M ) of 74.2, 65.1 and 55.1 emu g was determined for bare
s
the particle surface (Fig. S2a†). These particles were attracted by Fe O
Fe O /C mimCl and Fe O /C mimCl/C mimCl,
3 4 10 3 4 10 16
3
4,
placing a magnet near the glass bottle, demonstrating that the
particles possess magnetic properties (Fig. S2b†). Compared
with the frequently-used surfactants such as fatty acids that are
sparingly soluble in water and their solubility was enhanced by
15
the presence of acetone,
n
C mimCl is highly soluble in water
because of its special hydrophilic imidazole head. The excellent
solubility of C mimCl in water provide the possibility of direct
n
interactions between individual molecules of C mimCl and
n
Fe
growth of Fe
the particles. Moreover, it was found that a drop by drop and
slow adding way for the C mimCl aqueous solution is especially
important for preventing the micelle formation of C mimCl in
the dispersion. For example, the cmc of C mimCl at 298 K was
3
O
4
particle, which is especially important for limiting the
3 4
O
particle and preventing the agglomeration of
n
n
1
6
25
reported to be 1.21 mM, and the concentration of one drop of
the C mimCl aqueous solution in the suspension is about
1
6
0
.043 mM, which is well below the cmc of C16mimCl. Hence,
this adding way is favourable to coat the individual C16mimCl
molecule onto Fe particles efficiently.
To prepare stable water-based magnetic uids, C
secondly coated on the Fe /C mimCl particles to form Fe
C mimCl/C mimCl particles. As shown in Fig. S2c,† Fe O /
3 4
O
n
mimCl was
3
O
4
n
3 4
O /
n
n
3 4
C mimCl/C mimCl colloidal suspension exhibited a remark-
1
0
16
able stability as evidenced by a lack of observable precipitation
over periods of more than 12 months. The zeta potential (z) of
the Fe O /C10mimCl/C16mimCl colloidal suspension was found
3 4
to be approximately +40 mV measured by electrophoresis
experiments. These results imply a hypothetical structure of the
Fig.
1
TEM images of (A) ionic liquid bilayer-stabilized Fe
3 4
O /
C
10mimCl/C16mimCl and (B) Fe O
3 4
/C16mimCl/C16mimCl particles.
exposure of imidazole head groups of the secondary layer The insets show the corresponding electron diffraction images.
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Fig. 2 (A) Magnetization curves of (a) bare Fe
O
3 4
, (b) Fe
3
O
4
/C10mimCl, (c) Fe
O
3 4
/C10mimCl/C16mimCl, and (B) magnetization curves of (a) bare
3 4 3 4 3 4
Fe O , (b) Fe O /C16mimCl, (c) Fe O /C16mimCl/C16mimCl.
respectively. The similar results of the gradual reducing M from typical TGA and DTA curves of Fe O /C mimCl and Fe O /
s
3
4
10
3 4
ꢁ
1
7
4.2 to 61.8 and 55.0 emu g can be observed for bare Fe O
Fe O /C mimCl and Fe O /C mimCl/C mimCl (see Fig. 2B). (TGA curves), at the temperature below 150 C, both samples
C mimCl/C mimCl, respectively. As can be seen in Fig. 3A
10 16
3
4,
ꢀ
3
4
16
3
4
16
16
Obviously, the coating of primary C10mimCl (or C16mimCl) exhibited a small decrease step with a percentage weight loss of
layer onto Fe NPs led to a signicant decrease in the values about 4.0% and 3.6% owing to the removal of adsorbed water in
of M , and the coating of secondary C16mimCl layer onto Fe the samples. For the monolayer C10mimCl-coated particles
10mimCl (or Fe /C16mimCl) particles showed lower value of (Fe /C10mimCl), a well-dened percentage weight loss of
, i.e. M (Fe ) > M
3 4
O
s
3 4
O /
C
3
O
4
3 4
O
M
s
s
3
O
4
s
(Fe
3
O
4
/C10mimCl) > M
s
(Fe
3
O
4
/C10mimCl/ about 4.5% was detected over a temperature range of 170–
ꢀ
C mimCl). These phenomena can be attributed to the 400 C. The weight loss should be assigned to desorption and
1
6
detachment from dipole coupling due to the introduction of subsequent combustion of the monolayer C mimCl on the
1
0
nonmagnetic species (C mimCl) on the surface of Fe O
surface of particles. For the bilayer C mimCl/C mimCl-coated
10 16
n
3
4
26,27
particles.
To demonstrate the existence of C
particles (Fe
3
O
4
/C10mimCl/C16mimCl),
a
more signicant
n
mimCl bilayer on the percentage weight loss of about 12.7% was observed between
ꢀ
surface of iron oxide NPs, thermogravimetric analysis (TGA) and 170 and 450 C. The difference of weight loss between Fe
3
O
4
/
differential thermal analysis (DTA) were performed for the
3 4
C10mimCl and Fe O /C10mimCl/C16mimCl can be attributed to
samples dried at room temperature. Fig. 3A and B present the the removal of the secondary C16mimCl shell layer. The DTA
Fig. 3 (A) TGA and (B) DTA curves of Fe
Fe /C16mimCl/C16mimCl.
3 4 3 4 3 4
O /C10mimCl and Fe O /C10mimCl/C16mimCl. (C) TGA and (D) DTA curves of Fe O /C16mimCl and
3 4
O
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Table 1 Coverage parameters of the monolayer and secondary layer C
n
mimCl on Fe
3
O
4
particle surface
c
N
out (number of out layer
a
a
b
wt %
wt % (C
n
mimCl wt% (out
wtout (weight of out layer
n
C mimCl molecules
Sample
(adsorbed water) onto Fe
3
O
4
n
NPs) layer C mimCl)
C
n
mimCl on per g Fe
3
O
4
) (g) on each particle)
Fe
Fe
Fe
Fe
3
O
3
O
3
O
3
O
4
4
4
4
/C10mimCl
/C10mimCl/C16mimCl 3.6
/C16mimCl 2.8
/C16mimCl/C16mimCl 2.2
4.0
4.5
12.7
9.2
4.5
8.2
9.2
10
0.05
316
469
501
608
0.098
0.105
0.127
19.2
a
b
The mass percent content of component was determined on the basis of the results of TGA. wtout ¼ wt% (out)/wt% Fe
was calculated by a formula that subtracts the mass percent of C mimCl and corresponding adsorbed water.
¼ wtout ꢂ N /Mout is the number of C mimCl molecules per g Fe , with N being the Avogadro constant and Mout being the
molar mass of out layer C mimCl, and N /V is the number of Fe particles per g Fe
¼ V
) being the volume per g Fe and the volume per particle, respectively, where r is the density of Fe
+ l, with l being the length of primary layer C mimCl.
3 4
O , where the mass
c
percent content of Fe
/N , where N
3
O
4
n
N
out
¼
N
1
2
1
A
n
3
O
4
A
3
3
n
2
1
2
3
O
4
3
O
4
, with V
1
¼ 1/r and V
2
¼ 4pR
1
/3 (or V
1
2
¼ 4pR
2
/
ꢁ3
3
Fe
3
O
4
3
O
4
(5.18 g cm ) and R is the radius of
3
O
4
particle, and R
2
¼ R
1
n
curve (Fig. 3B) of Fe
3
O
4
/C10mimCl exhibited one well-dened particle surface. It is clear that the increase in the number of
ꢀ
exothermic peak in the temperature range of 200–350 C, cor- carbon atoms of the monolayer layer ionic liquid molecule from
responding to decomposition of the monolayer C10mimCl on 10 to 16 increased the weight percent of the ionic liquid from
the surface of particles, while The DTA curve of Fe O / 4.5 to 9.2, and the number of the ionic liquid molecules on each
3
4
C mimCl/C mimCl (Fig. 3B) displayed two distinguishable particle increased from 316 to 501. Meanwhile, the number of
1
0
16
ꢀ
exothermic peaks between 250 and 450 C, which might be due the C16mimCl molecules in secondary layer on each particle
to the decomposition of the outer layer (the secondary layer) increased from 469 to 608. These results demonstrated that the
C
16mimCl and the inner layer (the primary layer) C10mimCl, long-chain C16mimCl has stronger anchoring capacity on the
ꢀ
respectively. A small exothermic peak at around 500–600 C is iron oxide surface. The molecule number of 608 on each particle
possibly due to the structural transformation from magnetite to is close to that of the saturated fatty acids (e.g. myristic acid)
14
hematite when the temperature of magnetite NPs was exceeded adhered to iron oxide particle by a densely packed pattern. We
ꢀ
00 C. These observations can be easily explained in terms of observed that the long-chain C mimCl exhibited a stronger
16
28
4
the different molecular weights and boiling points of C mimCl assembly capability than that of the short-chain C mimCl for
1
0
10
and C16mimCl molecules. Moreover, the two exothermic peaks the secondary layer construction. The phenomenon can be
taken place at different temperature can be assigned to the explained on the basis of a competition between affinity of the
difference of interactions, such as electrostatic attraction imidazole head groups of C
between C10mimCl and magnetite NPs, and hydrophobic interaction between hydrocarbon tails of the primary and
interaction between the alkyl chain of 10mimCl and secondary C mimCl. It was known that the positively charged
16mimCl. The similar results can be observed in Fig. 3C for imidazole head groups possess intense hydrophilic properties.
Fe O /C mimCl (weight loss of about 9.2%) and Fe O / The shorter the hydrocarbon tail of C mimCl is, the stronger the
n
mimCl for water and hydrophobic
C
n
2
C
3
4
16
3
4
n
C mimCl/C mimCl (weight loss of about 19.2%), and Fig. 3D hydrophilic ability of C mimCl has. Hence, as the short-chain
1
6
16
n
for Fe
3
O
4
/C16mimCl with one exothermic peak and for Fe
3
O
4
/
C
10mimCl was used as the secondary layer stabilizer, a highly
C
16mimCl/C16mimCl with two exothermic peaks. These results affinity of water might make its molecules to have a tendency to
revealed the existence of two different molecular layers coated remain in aqueous solution, resulting in a loose array of
on the iron oxide surface. Analogous to our results, Shen et al. molecules on the particle surface. We found that the construc-
observed the signicant difference in the weight loss for tion /C10mimCl/C10mimCl,
of
Fe
3
O
4
Fe
3
O
O
4
4
/C12mimCl/
/C16mimCl/
monolayer and bilayer fatty acid surfactant-coated magnetite 10mimCl, Fe O /C14mimCl/C10mimCl and Fe
C
3 4
3
15
particles.
We calculated further the coverage parameters of the particles settled from the aqueous solution in a few minutes.
monolayer C mimCl and the secondary layer C mimCl on the When the carbon atom number of the secondary layer C mimCl
Fe particle surface, respectively, in terms of the above TGA increases to 16, the hydrophobic interaction is evidently
results. In these calculations, Fe particle was regard as enhanced, leading to a signicant increase in the assembly
spherical shape with mean diameter D of 10 nm on the basis of capability on the particle coated with the primary layer C
the results of TEM (Fig. 1). The surface areas of the spheres were mimCl. The bilayer C mimCl-coated Fe NPs, such as Fe
determined with diameters D and D + 2l for the monolayer and 16mimCl/C16mimCl, having close-packed imidazole head-
C mimCl cannot produce stable magnetic uids, and their
10
n
n
n
3 4
O
3 4
O
n
-
n
3
O
4
3 4
O /
C
15
secondary coating of surfactants, respectively, with l being the groups exposing to the surrounding solution would provide
chain length of the primary surfactant. For example, the a highly charge-density and strongly hydrophilic surface. The
surfactant with chain length of C10 was estimated to be ca. special structure surface can prevent the particles not only from
.415 nm based on the method proposed by Shaw. Table 1 lists aggregating but also from oxidizing through the electrostatic
the coverage parameters of the monolayer and secondary layer and steric repulsions between the particles, leading to the
mimCl assembled with C10mimCl and C16mimCl on Fe highly stable, water-based magnetic uids.
29
1
C
n
3 4
O
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Characterization of C16mimCl-based magnetic mesoporous
silica
As shown in the Scheme 2, in the above synthesized Fe
3 4
O /
C
16mim/C16mim magnetic uid, a magnetic mesoporous silica
was synthesized by a hydrothermal synthesis using C16mimCl
as template and TEOS as silicon source. Fig. 4 shows small-
angle XRD patterns of the calcined samples prepared from
reaction mixtures with an initial molar ratio n(Fe O /C mimCl/
3
4
16
C mimCl)/n(TEOS) of 0.025, 0.05 and 0.10, respectively. It is
1
6
seen that very similar diffraction patterns with three well-
resolved characteristic peaks attributed to (100), (110) and
(200) planes of p6mm hexagonal structure (with reciprocal
spacing, 1/dhkl, ratios of 31/2, 2, 71/2) were detected for two
samples (Fig. 4a and b). These well-dened reections peaks are
in good agreement with the peaks of patterns from the siliceous
Fig. 4 Small-angle XRD patterns of calcined samples prepared using
C
3 4
16mimCl as template in the Fe O /C16mimCl/C16mimCl magnetic
MCM-41 prepared using quaternary ammonium ion surfac- fluid with an initial molar ratio n(Fe O /C16mimCl/C16mimCl)/n(TEOS)
3
4
30
tants, which suggests that the presence of magnetic particles of (a) 0.025, (b) 0.05 and (c) 0.10.
with n(Fe O /C mimCl/C mimCl)/n(TEOS) of 0.025 and 0.05
3
4
16
16
did not destroy the ordered array of mesoporous cannels.
However, when the molar ratio was increased to 0.10 (Fig. 4c),
only one well-resolved diffraction peak (100) in the 2q range
between 2 and 8 was detected, indicating that the ordered
degree of mesopores is reduced. This is probably owing to the
overfull addition of Fe O /C mimCl/C mimCl particles, dis-
that can be assigned to magnetite crystalline phase (JCPDS card
no. 28-0491), which further conrms the existence of magnetite
Fe
3 4
O NPs in the sample. A gradual protuberance at low angle
32
can be attributed to amorphous silica of the pore walls.
Magnetization curve of this sample shown in Fig. S4B† exhibi-
ted also a typical magnetization “S” curve without obvious
remanence and coercivity, revealing the superparamagnetic
feature of the magnetic mesoporous silica.
3
4
16
16
rupting the charge density matching between the silicon species
and C mimCl liquid crystal phase, nally, leading to the partial
1
6
breakdown of long-range ordered mesoporous silica structure.
Generally, when guest species are loaded in the mesopores, the
31
intensity of their XRD lines decreases.
TEM images recorded along [100] (Fig. 5A) and [001] (Fig. 5B)
directions for the calcined sample with n(Fe /C16mimCl/
C mimCl)/n(TEOS) of 0.05 showed ordered hexagonal
3 4
O
1
6
arrangement of mesopore channels as well as dark quasi
spherical Fe O NPs. These Fe O NPs were enwrapped by the
3
4
3 4
ordered mesoporous silica channels and distributed randomly
throughout the mesoporous silica matrix. The TEM images
provided a direct and conclusive evidence of the coexistence of
3 4
Fe O NPs and well-dened ordered mesoporous silica. Wide-
angle XRD pattern (Fig. S4A†) of this sample measured at 2q
ꢀ
of 20–80 presented some distinguishable weak Bragg peaks
Scheme 2 Schematic diagram of prepared route of magnetic meso- Fig. 5 TEM images recorded along the (A) [100] and (B) [001] direc-
porous silica using C16mimCl as template and TEOS as silicon source in tions of calcined sample prepared using C16mimCl as template in the
the Fe O
3 4
/C16mimCl/C16mimCl magnetic fluid.
3 4
Fe O /C16mimCl/C16mimCl magnetic fluid.
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Fig. 6 (A) N
2
adsorption–desorption isotherm of calcined sample prepared using C16mimCl as template in the Fe
O
3 4
/C16mimCl/C16mimCl
magnetic fluid, and (B) its BJH pore size distribution.
¼ 196 mg gꢁ ) of the
1
Nitrogen physisorption isotherm (Fig. 6A) of this sample the maximum adsorption capacity (q
m
displayed a type IV adsorption isotherm with an obvious Fe O /MCM-41, the pure mesoporous silica MCM-41 had higher
3
4
ꢁ
1
hysteresis loop at a relatively high p/p values according to adsorption capacity (qm ¼ 393 mg g ), which is exactly two
0
33
IUPAC, indicating the presence of open pores. A steep times that of the Fe O /MCM-41. The reason is probably that the
3
4
2
ꢁ1 12
increasing occurs at a relative pressure 0.30 < p/p
0
< 0.43, which specic surface area (1200 m
g
)
of the pure MCM-41 is
is due to the lling of mesoporous walls by capillary conden-
12
sation. In the previous publication, the hysteresis phenom-
enon was observed and attributable to the lling of a secondary
pore structure, which resulted from grain boundaries, creating
small cavities between adjacent ordered regions. In this sample,
the Fe O NPs can be regarded as grain, and its boundaries
3
4
might create small cavities, therefore, resulting in the visible
hysteresis loop. Pore size distribution calculated by BJH model
(Fig. 6B) showed a narrow pore size distribution with the mean
size of about 2.5 nm, which is in agreement with the value from
TEM images. The BET surface area and total pore volume are
2
ꢁ1
3
ꢁ1
9
15 m g and 0.78 cm g , respectively.
Adsorption of magnetic mesoporous silica
To evaluate the adsorption properties of the prepared Fe O /
3
4
MCM-41, we employed rhodamine B (RhB) and methylene blue
MB) solutions as model systems at room temperature. Fig. 7
(
shows the time-dependent decolorization rates of two dye
solutions measured at different initial concentrations aer
adsorption using the Fe O /MCM-41 as adsorbent. It is clear
3 4
that the decolorization rates of both solutions increased fast
and reached a nearly plateau slope (about 90%) within 10 min
ꢁ1
except for MB with initial concentration of 200 mg L . Fig. 8
shows the effect of contact time on the adsorption capacity of
the Fe
3 4
O /MCM-41 in RhB (Fig. 8A) and MB (Fig. 8B) solutions
with different initial concentrations of 20, 50, 100, 200 and
ꢁ1
4
00 mg L , respectively. The two dyes were adsorbed rapidly in
the initial 5 min. Aer that, the adsorption capacity did not
change with increasing of the contact time, indicating the
adsorption equilibrium was approached. For adsorption of
RhB, the adsorption capacity of the Fe O /MCM-41 in RhB
Fig. 7 Decolorization rates of (A) rhodamine B and (B) methylene blue
solutions after adsorption using Fe /MCM-41 as adsorbent and
magnetic separation at different time interval. The initial concentra-
3
O
4
3
4
solutions with different initial concentrations of 20, 50, 100, 200
tions of solutions for rhodamine B: (a) 20, (b) 50, (c) 100, (d) 200 (e)
ꢁ
1
ꢁ
ꢁ1
1
and 400 mg L at equilibrium (30 min) can reach 9.74, 24.4,
4
00 mg L , and for methylene blue: (a) 10, (b) 20, (c) 50, (d) 100 (e)
ꢁ1
4
8.8, 98.0 and 196 mg g , respectively (Fig. 8A). Compared with
200 mg L
.
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Table 2 Recycling of the adsorbents
Dye
Cycle
Decolorization rates (%)
Rhodamine B
1
2
3
4
1
2
3
4
95
88.5
82.1
72.2
93
87.6
80.5
73.7
Methylene blue
found that for the adsorption of MB, the q of 97.5, 88.0 and
m
ꢁ
1
5
.50 mg g was determined for pure MCM-41, Fe O /MCM-41
3 4
3 4
and bare Fe O particles, respectively, which showed a similar
trend with that of the adsorption of RhB.
The well-dened linear relations (see Fig. S5†), tted by
Freundlich isotherm equation were observed for RhB solution
ꢁ
1
in the range of 20–400 mg L and for MB solution in the range
ꢁ
1
of 10–100 mg L adsorbed on the Fe O /MCM-41. The obtained
3
4
ꢁ1
values of K and 1/n were found to be 20 mg g and 1.074 for
RhB, and 11 mg g and 0.957 for MB, respectively. Obviously,
F
ꢁ1
the adsorption capacity of the Fe
than that of MB.
3 4
O /MCM-41 to RhB is higher
3 4
These results of adsorption analyses imply that the Fe O /
MCM-41 has luxuriant mesoporous channels and high surface
area, and can be potentially used as an adsorbent in liquid-
phase processes. It is well-known that silicon-based MCM-41
mesopores have abundant and negatively charged surface
silicon hydroxyl groups, which can adsorb the positively
charged cationic dye, such as RhB or MB, by means of an
electrostatic adsorption.
3 4
Fig. 8 Adsorbed capacity of Fe O /MCM-41 as a function of contact
time for (A) RhB and (B) MB at different initial concentrations of (a) 20,
ꢁ1
(
b) 50, (c) 100, (d) 200 (e) 400 mg L at room temperature.
2
ꢁ1
3 4
higher than that of the Fe O /MCM-41 (915 m g ). However,
the pure mesoporous silica MCM-41 can only be recovered from
the solution by centrifugation, while the Fe O /MCM-41 can be
3
4
The magnetic separability of the dye-adsorbed Fe
1 adsorbents was tested by placing a conventional laboratory
magnet near the small beaker containing the mixtures of two
dye solutions (100 mg L ) and the adsorbent powder. As shown
in Fig. S6,† aer a certain time interval, the powder was
attracted by the magnet (on the right), and the colour of the
powder changed from grey to orange-red (Fig. S6A†) and grey to
3 4
O /MCM-
easily recovered by magnetic separation technology. The qm of
the bare Fe particles prepared using the same chemical
coprecipitation method without adding ionic liquid was found
4
3 4
O
ꢁ
1
ꢁ1
to be 7.5 mg g , which is lower than that of the Fe
3
O
4
/MCM-41.
This may be attributed to the fact that the Fe
adequate mesoporous channels.
3 4
O /MCM-41 has
Similarly, for adsorption of MB, the adsorption capacity of
the Fe O /MCM-41 in MB solutions with different initial
concentrations of 20, 50, 100, 200 and 400 mg L at equilib-
rium (30 min) was found to be 9.62, 24.1, 47.5, 76.9 and 88.0 mg
3 4
blue (Fig. S6B†) for RhB-adsorbed and MB-adsorbed Fe O /
MCM-41, respectively. The clear solution could be decanted off
or removed by pipette. These simple experiments conrmed
3
4
ꢁ
1
ꢁ
1
that the Fe
a magnetic adsorbent to remove dyes in liquid-phase.
The regeneration of the Fe O /MCM-41 was conducted by
3 4
O /MCM-41 is magnetic and can be used as
g
, respectively (Fig. 8B). It is clear that when the initial
ꢁ
1
concentration of dyes is less than 100 mg L , the adsorption
capacity of the Fe /MCM-41 for both dye solutions on is close.
However, when the initial concentration of dyes was increased
from 200 to 400 mg L , the adsorption capacity of the Fe O /
MCM-41 increased from 98.0 to 196 mg g for RhB solution,
while for MB solution, the value increased slowly from 76.9 to
O
4
3 4
3
calcination of the dye-adsorbed Fe O /MCM-41 powder at
3
4
ꢁ
1
ꢀ
5
50 C for 5 h to remove the dyes from the pores. The reclaimed
3
4
ꢁ1
powder can be used as an adsorbent again. Table 2 shows the
ꢁ
1
decolorization rates of RhB and MB solutions (100 mg L ) aer
four cycles. Aer the three cycles, the decolorization rates of
both dye solutions can be maintained at more than 80%.
However, aer the fourth cycle, the decolorization rates were
reduced to about 70%. The phenomenon may be caused by
partial collapse of the mesoporous structure of the adsorbent
aer several cycles.
ꢁ
1
88.0 mg g . It can be seen that for both dye solutions with high
ꢁ1
concentration ($200 mg g ), the Fe
benecial for adsorption of RhB than MB. This phenomenon
may be attributed to the fact the Fe /MCM-41 is more likely to
3 4
O /MCM-41 is more
3 4
O
adsorb RhB with larger molecular structure because of the
special mesoporous structure of the Fe O /MCM-41. It was also
3
4
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9
B. Dong, X. Zhao, L. Zheng, J. Zhang, N. Li and T. Inoue,
Colloids Surf., A, 2008, 317, 666–672.
Conclusions
In summary, amphiphilic 1-alkyl-3-methylimidazolium chlo- 10 C. J. Adams, A. E. Bradley and K. R. Seddon, Aust. J. Chem.,
ride (C mimCl) can serve as a surface functional molecule to
2001, 54, 679–681.
construct bilayer C
mimCl structure in the surface of magnetic 11 Y. Zhou and M. Antonietti, Adv. Mater., 2003, 15, 1452–1455.
Fe nanoparticles. It was found that C
mimCl (n ¼ 10, 12, 14, 12 T. Wang, H. Kaper, M. Antonietti and B. Smarsly, Langmuir,
6) can be used to assemble the primer layer, and long chain
2007, 23, 1489–1495.
C mimCl is favourable for the construction of the secondary 13 T. Wang, T. Fu, Y. Meng, J. Shen and T. Wang, RSC Adv.,
n
n
3
O
4
n
1
1
6
layer. The bilayer C mimCl-based Fe O particles provide
2018, 8, 25141–25149.
a highly charge-density and strongly hydrophilic surface, 14 A. Wooding, M. Kilner and D. B. Lambrick, J. Colloid
leading to the highly stable, water-based magnetic uids.
Interface Sci., 1991, 144, 236–242.
Moreover, in the C
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cally-driven mesoporous silica with magnetic Fe NPs
15, 447–453.
inlayed randomly into ordered hexagonal mesoporous silica 16 R. Y. Hong, S. Z. Zhang, Y. P. Han, H. Z. Li, J. Ding and
MCM-41 type) framework can be prepared by using C16mimCl
Y. Zheng, Powder Technol., 2006, 170, 1–11.
as template. Furthermore, the obtained Fe O /MCM-41 with 17 B. Bateer, Y. Qu, X. Meng, C. Tian, S. Du, R. Wang, K. Pan
n
3 4
n
3 4
O
(
3
4
2
ꢁ1
high surface area of up to 915 m
decolorization rates (about 95%) within 5 min for the rhoda- 18 M. Soleymani and M. Edrissi, J. Dispersion Sci. Technol., 2016,
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37, 693–698.
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and H. Fu, J. Magn. Magn. Mater., 2013, 332, 151–156.
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(
development of multifunctional amphiphilic ionic liquid in the
practical nanomaterial applications.
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20 Q. Y. Li, K. R. Ma, Z. J. Ma, Q. Wei, J. G. Liu, S. P. Cui and
Z. R. Nie, Microporous Mesoporous Mater., 2018, 265, 18–25.
2
1 N. Z. Knezevic, E. Ruiz-Hernandez, W. E. Hennink and
M. Vallet-Regi, RSC Adv., 2013, 3, 9584–9593.
Conflicts of interest
There are no conicts to declare.
22 Y. Zhou and M. Antonietti, Chem. Mater., 2004, 16, 544–550.
2
2
3 R. Massart and V. Cabuil, J. Chem. Phys., 1987, 84, 967–973.
4 E. Dubois, J. Chevalet and R. Massart, J. Mol. Liq., 1999, 83,
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We gratefully acknowledge the nancial support from National 25 C. Jungnickel, J. Łuczak, J. Ranke, J. F. Fern ´a ndez, A. M u¨ ller
Natural Science Foundation of China (21564018, 21063017 and
and J. Thming, Colloids Surf., A, 2008, 316, 278–284.
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