Tunable LCST-type phase behavior of [FeCl4]--based ionic liquids in water

Article abstract of DOI:10.1007/s11426-016-5577-0

In this work, 16 kinds of [FeCl₄]^--based magnetic ionic liquids (ILs) with different cation structures have been designed and synthesized, and their structures are characterized by IR and Raman spectroscopy. Then the lower critical solution temperature (LCST)-type phase behavior of these magnetic ILs in water is investigated as a function of concentration. It is shown that cation structure, alkyl chain length and molar ratio of FeCl₃/chloride IL have a significant influence on the LCST of the mixtures. The phase separation temperature can be tuned efficiently by these factors. Meanwhile, the LCST-type phase separation process is also investigated by dynamic light scattering. The results support the mechanism that the hydrogen bonds of the [FeCl₄]^- anion with water have been gradually disrupted to form ILs aggregates with increasing temperature. In addition, the stability of the ILs in water is also examined in some details. These LCST-type phase separation systems may have potential applications in extraction and separation techniques at room temperature.

Full text of DOI:10.1007/s11426-016-5577-0

SCIENCE CHINA
Chemistry
May 2016 Vol.59 No.5: 587–593
doi: 10.1007/s11426-016-5577-0
• ARTICLES •
· SPECIAL TOPIC · Ionic Liquids: Energy, Materials & Environment
Tunable LCST-type phase behavior of [FeCl4]-based
ionic liquids in water
Yuanchao Pei, Yuan Cao, Yanjie Huang, Xinxin Song, Huiyong Wang,
Yuling Zhao & Jianji Wang*
Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals; Key Laboratory of Green Chemical Media
and Reactions, Ministry of Education; School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang 453007, China
Received November 30, 2015; accepted December 22, 2015; published online March 28, 2016
In this work, 16 kinds of [FeCl4]-based magnetic ionic liquids (ILs) with different cation structures have been designed and
synthesized, and their structures are characterized by IR and Raman spectroscopy. Then the lower critical solution temperature
(LCST)-type phase behavior of these magnetic ILs in water is investigated as a function of concentration. It is shown that cat-
ion structure, alkyl chain length and molar ratio of FeCl3/chloride IL have a significant influence on the LCST of the mixtures.
The phase separation temperature can be tuned efficiently by these factors. Meanwhile, the LCST-type phase separation pro-
cess is also investigated by dynamic light scattering. The results support the mechanism that the hydrogen bonds of the
[FeCl4] anion with water have been gradually disrupted to form ILs aggregates with increasing temperature. In addition, the
stability of the ILs in water is also examined in some details. These LCST-type phase separation systems may have potential
applications in extraction and separation techniques at room temperature.
magnetic ionic liquids, IR and Raman spectroscopy, lower critical solution temperature, tunable phase separation
Citation:
Pei YC, Cao Y, Huang YJ, Song XX, Wang HY, Zhao YL, Wang JJ. Tunable LCST-type phase behavior of [FeCl4]-based ionic liquids in water.
–
Sci China Chem, 2016, 59: 587 593, doi: 10.1007/s11426-016-5577-0
properties such as polarity, viscosity and miscibility with
molecular solvents. Such component ions allow for the fine
tuning of liquid properties of the resulting functional ILs by
the chemical modification of both cation and anion.
1 Introduction
Ionic liquids (ILs) are entirely composed of ions as the
name implies, and they melt at the temperatures below
100 °C [1,2]. Their extraordinary high ion density results in
the negligible vapor pressure under ordinary conditions.
Such a remarkable property makes it possible for ILs to be
alternatives of volatile organic solvents used in many areas
of synthetic, separation and electrochemical applications.
The most important feature of ILs is their structural design-
ability. In general, cations are mainly responsible for the
tune of melting point and for the amphiphilic property of
ILs, and the selection of anions can readily tailor the liquid
In recent years, functional ILs have attracted great inter-
est especially those which are responsive to pH [3], temper-
ature [4], CO2 [5], light [6] and magnet [7]. Among these
external stimulus responsive ILs, magnetic ILs exhibit po-
tential applications in catalytic, separation, electrochemis-
try, magnetic hydraulics in engineering, gas absorption and
microemulsion owing to their paramagnetic nature [8–14].
Lower critical solution temperature (LCST) phase be-
havior is a not-so-common, entropy driven phenomenon
where the miscibility of two liquids increases upon cooling.
Usually, LCST-type phase separation is observed for some
*Corresponding author (email: jwang@htu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2016
chem.scichina.com link.springer.com

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Pei et al. Sci China Chem May (2016) Vol.59 No.5
non-ionic polymers with water [15,16]. For ILs, it is shown
that most of them exhibit upper critical solution temperature
(UCST)-type phase separation with water [17], only a few
ILs-water mixtures show LCST-type phase separation. In
this context, Ohno and co-workers [18–25] reported excel-
lent results on LCST-type phase separation of some func-
tional ILs. For example, in 2007 they synthesized a series of
functional amino acids ILs with tri-n-butyloctylphospho-
nium [P4448] and tetra-n-butylphosphonium [P4444] as cati-
ons, and N-trifluoromethanesulfonyl amino acid methyl
ester as anion. These ILs were found to exhibit LCST-type
phase separation with water. The effect of water content and
component-ion structure on the phase-separation tempera-
ture of IL/water mixtures was investigated [18]. After that,
Figure 1 Chemical structures for cations of the ILs investigated in this
work.
they also synthesized other LCST-type ILs based on [P4444
]
cation, and the LCST-type phase behavior of these ILs in
water was studied in detail [19–25]. In addition, Xie and
Taubert [26] investigated the thermomorphic behavior of
[C4mim][FeCl4] and [C12mim][FeCl4] in water, and the
LCST phase behavior for the mixtures of magnetic ILs with
water was reported for the first time.
2.2 Synthesis of the ILs
All the [FeCl4]-based ILs were synthesized according to
the procedures described in the literatures [27,28]. These
ILs are readily obtained by mixing equal molar iron trihal-
ide with corresponding cationic chloride ILs [29]. The mix-
tures were stirred at room temperature, and a two-phase
mixture consisting of a brown liquid and a water layer was
formed after a few minutes. The crude product was obtained
after removal of the top water layer, and was then washed
with an excess of deionized water to remove un-reacted
FeCl3 from the final product. Thus obtained magnetic ILs
were dried under vacuum at 70 °C for 48 h.
In this work, 16 kinds of [FeCl4]-based magnetic ILs
were prepared with different cation structures, such as im-
idazoliumcations, cholinium cations, quaternary ammonium
cations and quaternary phosphonium cations, and different
alkyl chain lengths. Chemical structures for the synthesized
magnetic ILs are shown in Figure 1. These ILs were char-
acterized by means of IR, Raman spectroscopy and differ-
ential scanning calorimetry (DSC), and then used to study
the LCST-type phase separation behavior in water. The
modulation of the phase separation temperature of the mix-
tures by cationic structure, alkyl chain length and molar
ratio of FeCl3/chloride IL was examined, and the results
were discussed from the hydrogen bond between the ILs
anion and water and the change in hydrophobicity of cations
of the ILs.
2.3 Characterization of the ILs
The final obtained ILs were characterized by IR, Raman
spectroscopy and DSC. Because of the magnetism of these
ILs, the synthesized precursors ILs (chloride ILs), instead of
these magnetic ILs, were characterized by 1H NMR
(Bruker, AV-400). FTIR and Raman spectra of the magnetic
ILs were recorded by using a Thermo Nicolet FTIR Nexus
470 spectroscopy (USA) with an instrument resolution of
2 cm1. The spectra measurements were performed at room
temperature. The glass transition temperature (Tg) and the
melting point (Tm) of the ILs were measured by means of a
differential scanning calorimeter (Netzsch DSC 204F-1,
Germany).
2 Experimental
2.1 Materials
N,N-dimethylethanolamine (DMEA) (99%), N-methyldieth-
anolamine (MDEA) (99%), methyl trioctylammonium chlo-
ride ([N8,8,8,1]Cl) (97%), (tri-n-butyl)-n-tetradecylphospho-
nium chloride ([P4,4,4,14]Cl) (97%), anhydrous FeCl3 (99%)
and FeCl3·6H2O (99%) were purchased from Aladdin
(China). 1-Methylimidazole (99%), allyl chloride (98%), 1-
chlorobutane (99%), 1-chlorohexane (95%), 1-chlorooctane
(99%), 1-chlorodecane (98%), 1-chlorododecane (99%),
1-chlorotetradecane (98%) and 1-ethyl-3-methylimidazo-
lium chloride (96%) were acquired from J&K Chemicals
(China). All chemicals were used as received without any
further purification.
2.4 Phase separation temperature measurements of the
IL/water mixture
The phase separation temperature (Tc) of the IL/water mix-
ture was determined based on the visual detection of the
turbidity of the mixture according to the reported proce-
dures [30,31]. Briefly, a given concentration of IL/water
mixture was gravimetrically prepared using an analytical
high precision balance with an uncertainty of ±0.0001 g.

Pei et al. Sci China Chem May (2016) Vol.59 No.5
589
The IL/water mixture was then placed in a glass tube. In
each separate experimental run, the glass tube was im-
mersed in a thermostatic water bath with an accuracy of
±0.05 °C. The mixture was heated or cooled at an interval
of 0.1 °C, and the mixture was maintained at least 5 min at
each temperature. The process was continued until the
phase transition was observed (turbidity-to-transparency
with cooling or transparency-to-turbidity with heating).
Such obtained phase separation temperature datum was then
confirmed by at least quintic cooling-heating cycles. The
final uncertainty in the phase separation temperature was
estimated to be smaller than 0.3 °C.
should be noted that the melting point was not revealed for
all ILs. It can be seen that these ILs are really room temper-
ature ILs, their glass transition temperature varies from
about 100 to 65 °C with the change of cation structure
and alkyl chain length. This suggests great advantages as
solvent and reaction medium at low temperature. These ILs
were further characterized by IR and Raman spectra. As an
example, the IR and Raman spectra of [C2mim][FeCl4] and
[C6DMEA][FeCl4] are shown in Figure 2(a) and (b), respec-
tively. It can be seen from Figure 2(a) that in the IR spec-
trum of [C2mim][FeCl4], the characteristic frequency bands
at ~3149 and ~3116 cm1 can be assigned to the stretching
modes of C–H bond of the imidazolium ring. The bands at
~2961 and ~2932 cm1 belong to aliphatic C–H vibrations
of the imidazolium ring. The peak at ~1590 cm1 is –C=N–
stretching vibration of the ring. Similarly, in the IR spec-
trum of [C6DMEA][FeCl4], the characteristic frequency
bands at ~3280 cm1 can be assigned to the stretching vibra-
tion of –OH group on the cation. The bands at ~2955,
~2928 and ~2865 cm1 are assigned to aliphatic C–H vibra-
tions of the cholinium cation. The above analysis indicates
that cationic structures of the prepared ILs are in good
agreement with those shown in Figure 1.
2.5 Droplet size measurements
The size and size distribution of the IL/water mixture were
measured by dynamic light scattering (DLS) technique us-
ing a Malvern Zetasizer Nano-90 light scattering instrument
(Malvern Instrument Ltd., UK). The scattering angle was
set at 90°. After the samples were equilibrated for 24 h, the
samples were filtered through a 0.45 μm Millopore filter to
remove the dust or any contaminants.
According to the previous report [32], [FeCl4] and
[Fe2Cl7] are the main anion in the Fe-based ILs. The
[FeCl4] anion is present in the ILs where the ratio of
FeCl3/chloride IL is less than or equal to 1:1, while the
[Fe2Cl7] anion is only present in the ILs when the ratio of
FeCl3/chloride IL is higher than 1:1. Here, Raman spectra
3 Results and discussion
3.1 Characterization of the ILs and their stability in
water
The Tg and Tm of the prepared ILs are listed in Table 1. It
Table 1 The glass transition temperature (Tg) and the melting point (Tm) of the ILs
ILs
Tm (°C)
Tg (°C)
100
87
65
84
82
81
77
65
ILs
Tm (°C)
Tg (°C)
86
84
76
80
74
65
82
94
a)
[Amim][FeCl4]
[bbim][FeCl4]
[C2mim][FeCl4]
[C4mim][FeCl4]
[C6mim][FeCl4]
[C8mim][FeCl4]
[C10mim][FeCl4]
[C12mim][FeCl4]
a) Not detected.
–
[C14mim][FeCl4]
[C4DMEA][FeCl4]
[C6DMEA][FeCl4]
[C8DMEA][FeCl4]
[C4MDEA][FeCl4]
[C6MDEA][FeCl4]
[P4,4,4,14][FeCl4]
[N8,8,8,1][FeCl4]
33
–
–
18
–
–
–
–
–
–
–
–
–
18
–
Figure 2 IR (a) and Raman (b) spectra of [C2mim][FeCl4] and [C6DMEA][FeCl4] ionic liquids at room temperature.

590
Pei et al. Sci China Chem May (2016) Vol.59 No.5
were used to examine what is the right anion in the prepared
ILs. As an example, Figure 2(b) shows the Raman spectra
of [C2mim][FeCl4] and [C6DMEA][FeCl4]. It can be seen
that a strong single peak appears at ~330 cm1, which could
be assigned to the symmetric stretching vibrations of Fe–Cl
bond in [FeCl4]. The symmetry vibrations of [Fe2Cl7]
anion should be located at ~360 cm1 [33]. Thus, it can be
confirmed that the synthesized ILs contain the same
[FeCl4] anions.
From the chemical structure of the ILs shown in Figure 1,
[N8881][FeCl4] and [P44414][FeCl4] have more hydrophobic
long alkyl chain than [C8DMEA][FeCl4], [C4DMEA]
[FeCl4] and [C4mim][FeCl4]. Thus, this result is in accord-
ance with the hydrophilic order of the traditional ILs.
Figure 4 shows the modulation of substitute group in
imidazolium cations on the phase behavior of some IL/
water mixture. It was also found that these ILs exhibited
LCST-type phase behavior in water. The aqueous solubility
of the ILs follows the order: [Amim][FeCl4]>[C4mim]-
[FeCl4]>[bbim][FeCl4] at a given temperature. In compari-
son with the chemical structure of [bbim]+, the methyl
group of [C4mim]+ cation was replaced by butyl group.
Thus [bbim]+ is more hydrophobic than [C4mim]+. In the
case of [C4mim][FeCl4] and [Amim][FeCl4], the butyl group
of [C4mim]+ cation was replaced by allyl group of [Amim]+.
Similarly, [C4mim]+ is more hydrophobic than [Amim]+.
This result can be used to understand the solubility order of
the ILs mentioned above. Therefore, the LCST of these
IL/water mixtures can be tuned by cationic hydrophobi-
city/hydrophilicity.
In addition, it was reported that hydrolysis of [FeCl4]
anions is inevitable when contacting with water [34]. How-
ever, in our experiment, pH value for the equilibrated
aqueous phase was around 1.0, and no obvious precipitation
was observed during the reversible circulation of heating
and cooling around the phase separation temperature. After
phase separation, the aqueous and ILs phases were also
transparent even at higher temperatures (say 70 °C). Fur-
thermore, the phase stability of IL/water mixture was also
examined at room temperature. It was shown that no precip-
itation was observed in IL/water mixture during 6 months.
This result implies that the two-phase mixture is stable and
the magnetic ILs do not hydrolyze significantly at such a
low pH value. The same conclusion has been obtained by
Xie and Taubert [26]. In order to examine the stability of
these ILs after phase separation with water, IR and Raman
spectroscopy was also determined for both the fresh ILs and
the ILs recovered from phase equilibrium measurements. It
was found that there are no differences in the IR and Raman
spectra from fingerprint area (400–1300 cm1) to character-
istic frequency area (1300– 4000 cm1) for such ILs. These
results indicate that these ILs are quite stable after phase
separation with water, and the hydrolysis of the ILs is neg-
ligible in the concentration and temperature range studied.
3.2 Tuning the phase behavior of IL/water mixture by
cationic structures of the ILs
Figure 3 The modulation of cationic structure on the phase separation
temperature of the IL/water mixtures.
In this work, four kinds of cations, including cholinium cat-
ion, imidazolium cation, quaternary ammonium cation and
quaternary phosphonium cation (Figure 1), were chosen to
study the modulation of cations on the phase behavior of the
IL/water mixtures. Figure 3 shows the plots of phase sepa-
ration temperature of these IL/water mixtures as a function
of IL concentration. It can be seen that the phase separation
temperature of all the mixtures decreased with increasing IL
concentration. In other words, the solubility of the ILs in
water increased with the decrease of temperature. The
IL/water mixture became clear below the phase separation
temperature, and turbid above the phase separation temper-
ature. This is the typical feature of LCST-type phase be-
havior. This process is reversible for the IL/water mixtures
upon cooling or heating. It is clear from Figure 3 that in the
temperature range investigated, solubility of the ILs in
water follows the order: [C8DMEA][FeCl4]>[C4DMEA]-
[FeCl4]>[C4mim][FeCl4]>[P44414][FeCl4][N8881][FeCl4].
Figure 4 The modulation of substitute group in imidazolium cations on
the phase separation temperature of the IL/water mixtures.

Pei et al. Sci China Chem May (2016) Vol.59 No.5
591
3.3 Tuning the phase behavior of IL/water mixture by
alkyl chain length of the ILs
the results are similar to what we have observed for long
alkyl chain imidazolium-based ILs. It is evident that solu-
bility of the ILs in water follows the trend: [C8DMEA]-
[FeCl4]>[C6DMEA][FeCl4]>[C4DMEA][FeCl4], and [C4M-
DEA][FeCl4]>[C4DMEA][FeCl4]. From the chemical struc-
ture of the ILs shown in Figure 1, the methyl group of
[C4DMEA]+ cation was replaced by hydroxyethyl group to
form [C4MDEA]+. Therefore, it seems appropriate to indi-
cate that introduction of an additional –OH group enhances
the hydrophilicity of the cation, and thus increasing solubil-
ity of the [C4MDEA][FeCl4] ionic liquid.
To examine the effect of alkyl chain length of cations on the
temperature-driven phase separation of the IL/water mix-
tures, imidazolium and cholinium ILs with different alkyl
chain lengths were investigated. The phase separation tem-
perature of these IL/water mixture was illustrated in Figures
5 and 6 as a function of the ILs concentration. As can be
seen from these figures, solubility of the ILs in water in-
creased with the decrease of temperature. This is also the
typical feature of LCST-type phase behavior.
For the imidazolium-based ILs, it was also found that
solubility of the ILs in water was significantly influenced
when the number of carbon atoms in the alkyl chain was
increased from 2 to 14. As shown in Figure 5(a), when the
number of carbon atoms is below 8, solubility of the ILs in
water decreased with the increase of the alkyl chain length
at a given temperature: [C2mim][FeCl4]>[C4mim][FeCl4]>
[C6mim][FeCl4], which is in agreement with hydrophobicity
of the ILs. However, solubility of the ILs in water was
found to increase with increasing alkyl chain length at the
same temperature when the number of carbon atoms was
equal to or greater than 8, as shown in Figure 5(b): [C14
mim][FeCl4]>[C12mim][FeCl4]>[C10mim][FeCl4]>[C8mim]
[FeCl4]. This order is not in agreement with hydrophilicity
of the ILs. The different phase behavior usually implies
different solution structures. Indeed, many studies have in-
dicated that long alkyl chain ILs behave like the traditional
cationic surfactants. As its concentration was greater than
the critical aggregation concentration (CAC), micelle of the
IL was formed. Therefore, it is appropriate to state that the
higher solubility of long alkyl chain ILs was ascribed to the
aggregation of these ILs in water. This result indicates that
the LCST phase behavior can be easily modulated by ad-
justing the alkyl chain length of the ILs.
3.4 Tuning the phase behavior of IL/water mixture by
molar ratio of FeCl3/chloride IL
The traditional chloride ILs are usually miscible with water
at room temperature, for example, [C6mim]Cl is completely
soluble in water. However, [C6mim][FeCl4] is only partially
miscible with water at around room temperature. This result
implies that the [FeCl4] anion plays the key role in the mis-
cibility of ILs with water. In order to investigate the effect
of molar ratio of FeCl3/chloride IL on the phase behavior of
IL/water mixtures, the ILs [C6mim][FeCl4]xCl1x (x=0.36,
0.54, 0.71, 1,0) and [C6DMEA][FeCl4]xCl1x (x=0.67 and
1.0) were prepared from different molar ratios of FeCl3/
imidazolium chloride and FeCl3/cholinium chloride by di-
rect mixing anhydrous FeCl3 and chloride ILs at respective
molar ratio. Then the LCST of these ILs mixtures with wa-
ter was determined, and the results are shown in Figure 7(a)
and (b), respectively. It can be seen that aqueous solubility
of these ILs decreased with increasing molar ratio of FeCl3
to imidazolium chloride or cholinium chloride. When the
molar ratio of FeCl3/[C6mim]Cl was 1:1, the LCST behav-
ior became strong. However, it became weaker when the
molar ratio was below 1:1 due to the fact that the interaction
of anion···H2O···anion between the ILs components and
water was increasingly weakened by the continuous addi-
tion of FeCl3. In addition, it should be pointed out that dur-
ing the experimental process, obvious precipitation was
As far as the cholinium-based ILs are concerned, solubil-
ity of the ILs in water was found to increase with increasing
alkyl chain length of the IL cations. As shown in Figure 6,
Figure 5 The modulation of alkyl chain length in imidazolium cations on the phase separation temperature of the IL/water mixtures. Carbon atom number
(nc) in the alkyl chain: (a) nc<8; (b) nc>8.

592
Pei et al. Sci China Chem May (2016) Vol.59 No.5
bonds were disrupted gradually. Then, the ILs molecules
were aggregated continuously with increasing temperature
until the new IL phase was formed above the phase separa-
tion temperature.
In order to examine this mechanism, the aggregation of
the ILs in aqueous solutions was investigated by means of
DLS from the temperature far below the LCST to around
the critical point. To this end, the hydrodynamic diameter
(dh) of the ILs in aqueous solutions was measured as a func-
tion of temperature at different concentrations. As an exam-
ple, the hydrodynamic diameters of [C6DMEA][FeCl4] and
[C2mim][FeCl4] are shown in Tables 2 and 3. It is clearly
indicated that the dh values increase with the increase of
temperature due to the aggregation of the ILs. Finally, the
ILs aggregate into large particles at the temperature above
phase separation temperature and form a turbid dispersion.
Similar results were observed in other IL/water systems.
Figure 6 The modulation of alkyl chain length in cholinium cations on
the phase separation temperature of the IL/water mixtures.
Table 2 The diameter (dh) of [C6DMEA][FeCl4] in water at different
temperatures
observed as the molar ratio of FeCl3/[C6mim]Cl was higher
than 1:1. This indicates that significant hydrolysis occurs in
the IL/water mixture under this condition.
W=47.00%
W=56.34%
W=66.19%
T (°C)
dh (nm)
20.6
T (°C)
dh (nm)
82.4
105
T (°C)
dh (nm)
254
25.0
28.0
30.0
31.5
32.0
23.0
25.0
26.0
27.0
27.3
16.0
17.0
18.0
19.0
20.0
21.5
23.5
296
3.5 The possible mechanism for the LCST phase be-
havior of IL/water mixture
26.0
124
349
29.5
156
370
The [FeCl4]-based ILs are partially soluble in water at
room temperature. The hydrophobicity of these ILs may be
ascribed to the nephelauxetic effect of [FeCl4] anion [35].
Tang et al. [36] suggested that the hydrogen bonding inter-
actions between [FeCl4] and water molecules led to the
partial solubilization of the [FeCl4]-based ILs in water. Xie
et al. [26] and Estager et al. [35] proposed a possible ex-
planation for the LCST-type phase behavior of [FeCl4]-
based ILs in water by considering cohesive energy of water
causing “solvophobic” immiscibility, long-range coulomb
forces and competitive hydrogen bonding between cati-
on···anion and anion···H2O···anion. It is suggested that with
the increase of temperature, the [FeCl4]···H2O hydrogen
31.7
179
562
938
Table 3 The diameter (dh) of [C2mim][FeCl4] in water at different tem-
peratures
W=24.89%
W=29.98%
T (°C)
25.0
28.0
30.0
35.0
40.0
45.0
50.0
dh (nm)
137
T (°C)
25.0
28.0
30.0
35.0
40.0
45.0
50.0
dh (nm)
163
139
183
251
193
290
228
354
346
809
792
988
867
Figure 7 The modulation of molar ratio of FeCl3/chloride ILs on the phase separation temperature of the IL/water mixtures. (a) FeCl3/[C6mim]Cl; (b)
FeCl3/[C6DMEA]Cl.

Pei et al. Sci China Chem May (2016) Vol.59 No.5
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4 Conclusions
In summary, a series of [FeCl4]-based magnetic ILs with
different cationic structures were synthesized and charac-
terized in the present work. The LCST-type liquid-liquid
phase separation behavior of these magnetic ILs with water
was then investigated in detail. It was shown that the
LCST-type phase behavior is essentially determined by the
disruption of the hydrogen bonds between the [FeCl4] ani-
on and water. When the anion was fixed, the LCST values
for the mixtures were affected significantly by cationic
structure, alkyl chain length and substitution of hydrophobic
and hydrophilic groups. Compared with imidazolium and
cholinium based ILs, quaternary ammonium and quaternary
phosphonium based ILs have lower solubility in water at a
given temperature. For the imidazolium based ILs, their
miscibility with water decreases with the increase of alkyl
chain length of the cations when the number of carbon at-
oms in the alkyl chain is smaller than 8, whereas it increases
with the increase of alkyl chain length when the number of
carbon atoms is equal to or greater than 8 due to the aggre-
gation of these ILs in water. For the cholinium based ILs,
solubility increases with the increase of alkyl chain length
for the ILs investigated. Meanwhile, the aqueous solubility
of these ILs was found to decrease with increasing molar
ratio of FeCl3 to imidazolium chloride or cholinium chloride
from 0.36 to 1.0. In addition, it is likely from the DLS re-
sults that hydrogen bonds between the ILs and water were
disrupted gradually with increasing temperature, and the
new phase was formed by the aggregation of the ILs. The
above findings may have impact on the design of magnetic
ILs and potential applications in extraction and separation
techniques at room temperature.
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Acknowledgments This work was supported by the National Natural
Science Foundation of China (21473050, 21403060), Program for Innova-
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Conflict of interest The authors declare that they have no conflict of
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