Switchable oil-water phase separation of ionic liquid-based microemulsions by CO2

Article abstract of DOI:10.1039/c8gc00801a

Phase separation of microemulsions plays an important role in many applications such as oil recovery, nanomaterials synthesis, and chemical reactions. However, reversible switching from ionic liquid-based microemulsions to complete oil-water phase separation has not been reported so far. In this work, we developed a novel class of stimuli-responsive microemulsions composed of CO₂-responsive ionic liquids, n-pentanol and water. The microstructures and phase behavior of the microemulsion systems before and after the bubbling of CO₂ were investigated by electrical conductivity, dynamic light scattering, small-angle X-ray scattering, cryogenic transmission electron microscopy, and optical microscopy. It was found that these microemulsions could be reversibly switched from W/O monophase to complete oil-water phase separation upon alternate bubbling and removal of CO₂ at atmospheric pressure. Furthermore, ¹³C NMR spectroscopy was used to understand the CO₂-driven reversible phase separation of the microemulsions. The results suggest that the mechanism behind the reversible phase separation involved the reversible formation of bicarbonate and carbamate from the reaction between CO₂ and the anions of the ionic liquids in the presence of water, which resulted in the increase of ionic strength (or vice versa) in the mixture. Using the microemulsions as microreactors, the phase separation protocol was applied in the Knoevenagel reaction for an efficient coupling of a chemical reaction, product separation, and recycling of the microemulsions.

Full text of DOI:10.1039/c8gc00801a

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ARTICLE
Switchable oil-water phase separation of ionic liquids based
microemulsions by CO2
Received 00th January 20xx,
Accepted 00th January 20xx
Xiaoyan Pei, Dazhen Xiong, Yuanchao Pei, Huiyong Wang and Jianji Wang*
DOI: 10.1039/x0xx00000x
Phase separation of microemulsions plays an important role in many applications such as oil recovery, nanomaterials
synthesis, and chemical reactions. However, reversible switching from ionic liquids based microemulsions to complete oil -
water phase separation has not been reported so far. In this work, we developed a novel class of stimuli-responsive
www.rsc.org/
microemulsions composed of CO
2
-responsive ionic liquid, n-pentanol and water. The microstructures and phase behavior
were investigated by electrical conductivity, dynamic light
of the microemulsion systems before and after bubbling of CO
2
scattering, small-angle X-ray scattering, cryogenic transmission electron microscopy, and optical microscope. It was found
that these microemulsions could be reversibly switched from W/O monophase to complete oil - water phase separation
1
3
upon alternatively bubbling and removal of CO
understand the CO -driven reversible phase separation of the microemulsions. The results suggest that the mechanism
behind the reversible phase separation involved the reversible formation of bicarbonate and carbamate from the reaction
between CO and anions of the ionic liquids in the presence of water, which resulted in the increase of ionic strength (or
2
at atmospheric pressure. Furthermore, C NMR spectroscopy was used to
2
2
vice versa) in the mixture. By using the microemulsions as microreactors, the phase separation protocol was applied in
Knoevenagel reaction for efficient coupling of chemical reaction, product separation, and recycling of the microemulsions.
1
0
For example, Tabor et al. reported a light-responsive
reversible phase separation of water-in-decane microemulsion
stabilized by sodium bis(2-ethyl-hexyl)sulfosuccinate (AOT)
through doping sodium 4-butylphenyldiazenylphenoxybutane-
1-sulfonate (AZOT-S4) into the system. Upon exposure to UV
light of different wavelengths, the AZOT-S4 exhibited
reversible trans−cis isomerization, resulting in reversible phase
Introduction
Microemulsions are thermodynamically stable, optically clear
1
and isotropic mixtures of oil, water and surfactant. They have
2
a broad range of applications in pharmaceuticals, organic
3
catalysis, oil recovery, drug delivery, and nanoparticle
4
5
6
synthesis. In these applications, microemulsions often need to
1
1
separation of the microemulsion. Zhang et al. found a CO2-
switchable n-heptane-in-water microemulsion stabilized with
sodium dodecyl sulphate (SDS) by introducing N,N-dimethyl-N-
dodecyl amine (C A) into the O/W system. Upon alternatively
be temporarily stabilized for the reactions and subsequently
demulsified for products separation. Thus de-emulsification of
microemulsions is also critical in practical applications.
Traditional destabilization methods of microemulsions include
1
2
7
bubbling and removal of CO
, disruption and re-formation of
2
pervaporation, temperature-induced instability, and addition
8
,9
the microemulsion could be achieved based on the response
of electrolytes or solvents, which are usually less sustainable
due to the high energy input and irreversible change in system
composition. Therefore, it is of significance to develop efficient
strategies for reversible formation and disruption of
microemulsions for green and sustainable processes.
1
of C A to CO . Recently, Liu et al. demonstrated a CO2-
2
1
2
2
switchable bicontinuous microemulsion by using N,N,N’,N’-
tetramethyl-1,3-propanediamine (TMPDA), SDS, n-hexane, n-
butanol and water, and the reversible switching from
monophasic bicontinuous microemulsion to complete phase
separation could be achieved for several cycles owing to the
One of the important strategies is to develop stimuli-
responsive microemulsions. For this purpose, a number of
surfactants together with additives have been used to
formulate stimuli-responsive microemulsions which can be
reversibly destabilized and regenerated with different triggers.
CO -responsive performance of TMPDA. However, in these
2
switchable microemulsions, stimuli-responsive additives have
to be used, which complicated the microemulsions
composition and the components recovery.
Another viable option is ionic liquids (ILs) based stimuli-
responsive microemulsions, where ILs are nonvolatile and their
13-
Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine
Chemicals, Henan Key Laboratory of Green Chemistry, Key Laboratory of Green
Chemical Media and Reactions, Ministry of Education, School of Chemistry and
Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, P. R.
China. E-mail: Jwang@htu.cn
structures and properties are tunable for a specific purpose.
1
5
16
In this context, Brown et al. reported a CO -responsive
2
nonaqueous microemulsion containing cyclohexane and two
different ionic liquids: 1-hexadecyl-3-methylimidazolium
Electronic
DOI: 10.1039/x0xx00000x
Supplementary
Information
(ESI)
available.
See
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bromotetradecane (C14
H29Br, 98 wt%), 1-bromohexadecane
(C
16
H
33Br, 98 wt%), and anion exchange resin (Amersep 900
OH) were purchased from Alfa Aesar. N, N-dimethylethylamine
DMEA), imidazole (Im), trizole (Triz), and pyrazole (Pyr) were
(
acquired from Aladdin. n-Pentanol was purchased from
Sinopharm Chemical Reagent Co., Ltd. Double distilled water
was used throughout the experiments. CO
2
(Praxair, SFC grade,
9
9.998 vol%) and N (Praxair, 99.9993 vol%) were purchased
2
from Beijing Praxair Application Gas Corp. All the chemicals
were used as received unless otherwise stated.
Preparation of the ILs
[C
8
DMEA][Br], [C10DMEA][Br], [C12DMEA][Br], [C14DMEA][Br]
and [C16DMEA][Br] were synthesized and purified by the
Scheme 1 Chemical structures of anions and cations of the studied ILs.
1
8,19
following procedures.
alkyl bromide (C
33Br) were mixed at a molar ratio of 1.1:1.0 (DMEA: alkyl
bromide) in absolute ethanol, and the mixture was refluxed at
Briefly, N, N-dimethylethylamine and
8 10 12 14
H17Br, C H21Br, C H25Br, C H29Br or
chloride ([C16mim][Cl]) and 1-butyl-3-methylimidazolium
16
C H
4
triazolide ([C mim][tria123]). The former acts as a surfactant
and the latter acts as a polar domain, but a cosurfactant (1-
decanol) has to be added in the system. They investigated the
phase transition of the microemulsion from a monophasic
o
7
0
C for 72 h under magnetic stirring. Then, ethanol was
removed by rotary evaporation, and the products
([C DMEA][Br], [C DMEA][Br], [C DMEA][Br], [C DMEA][Br]
1
7
8
10
12
14
system to a turbid emulsion driven by CO
proposed a temperature-induced reversible switching of
amphiphilic self-assemblies between micelles and
microemulsions by using a temperature-sensitive ionic liquid
tetrabutylphosphonium trifluoroacetate ([P4444][CF COO]) in
Triton X-100-H O system. To our knowledge, reversible
2
. Dong et al.
and [C16DMEA][Br]) were obtained by extraction with
petroleum ether. The neat products were dried under vacuum
for 12 h at 50 °C. Then, a solution of [C
8
DMEA][OH],
[C14DMEA][OH] or
16DMEA][OH] in the mixed solvent of ethanol and water (1:1
[C10DMEA][OH],
[C12DMEA][OH],
3
[C
2
volume ratio) was prepared by means of the anion-exchange
method. Equimolar Im (Triz or Pyr) was added and the mixture
was then stirred for 12 h at room temperature. After that, the
switching from ionic liquids based microemulsions to complete
oil-water phase separation has not been reported so far.
Herein, we developed
a
2
class of CO -responsive
o
solvent was distilled off at 45 C by rotary evaporation.
microemulsions comprising IL, n-pentanol and water, where
The obtained products were dried for at least 24 h under
reduced pressure. The chemical structures of these ILs were
the ILs composed of N-alkyl-N,N-dimethylethylamine cation
+
DMEA] (n = 8, 10, 12, 14, 16) and azole-based anion,
[
C
n
1
13
-
including imidazolium ([Im] ), trizolium ([Triz] ) and pyrazolium
-
confirmed by H NMR, C NMR and IR spectroscopy (see
supporting information). The residual bromide content in
these azole-based ILs was determined by using the procedures
-
[Pyr] ) (Scheme 1), were employed as the surfactants in the
(
formulation of our microemulsion systems. From phase
diagram, electrical conductivity, dynamic light scattering,
small-angle X-ray scattering, cryogenic transmission electron
2
0,21
reported previously.
It was found that the bromide content
1
-
was less than 0.0090 mol kg (Table 1). The purity (expressed
in mole percent) of these ILs was determined by quantitative
1
3
microscopy, optical microscope, and C NMR spectrum
investigations, it was found that these microemulsion systems
underwent a reversible switching from an apparent water-in-
n-pentanol monophase microemulsion to complete water-oil
1
H NMR measurements according to the method described by
2
2
Xu et al.,
and the following results were obtained:
DMEA][Im] (99.5%), [C10DMEA][Im] (99.1%), [C12DMEA][Im]
99.5%), [C14DMEA][Im] (99.3%), [C16DMEA][Im] (99.5%),
[
(
[
[
[
[
[
C
8
phase separation by alternatively bubbling and removal of CO
at atmospheric pressure. Such a CO -driven reversible phase
2
C
C
C
C
C
8
DMEA][Pyr]
(99.0%),
(99.0%),
(99.5%),
(99.0%),
[C10DMEA][Pyr]
[C14DMEA][Pyr]
(98.8%),
(99.5%),
(99.5%),
(98.7%),
2
12DMEA][Pyr]
16DMEA][Pyr]
10DMEA][Triz]
separation was studied in detail and the results have been
explained with the reversible formation of bicarbonate and
8
[C DMEA][Triz]
[C12DMEA][Triz]
2
carbamate from the reaction between CO and anions of the
14DMEA][Triz] (99.2%) and [C16DMEA][Triz] (99.5%).
ionic liquids in the systems. Based on this unique property of
the microemulsions, the phase separation protocol was
applied in Knoevenagel reaction for efficient coupling of
chemical reaction, product separation, and recycling of the
microemulsions.
Measurements of NMR, IR spectrum, melting point and
thermal decomposition temperature
1
13
H NMR and C NMR spectra of these ILs were recorded on a
Bruker spectrometer (400 MHz). FTIR spectrum was measured
by using a PerkinElmer FTIR-400 spectrometer with universal
ATR sampling accessory. Glass transition temperature (T
melting point (T ) were determined by a Netzsch 204F1
differential scanning calorimeter (DSC). Thermal
decomposition temperature (T ) was acquired with a Netzsch
g
) and
Experimental section
m
Materials
d
1
-Bromooctane (C
8
H17Br, 98 wt%), 1-bromodecane (C10
H21Br,
Sta 449 C thermal analyzer. Each experiment was carried out in
triplicate, and average values were calculated.
98
wt%), 1-bromododecane (C12
H
25Br, 98 wt%), 1-
2
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Determination of Phase diagram
the plot of scattering intensity vs the scattering vector, q =
Phase diagram was constructed by adding a given IL into the (4π/λ) sinθ (2θ=scattering angle). Then pair distance
mixtures of H O and alcohol at 25.0 ± 0.1 °C. For this purpose, distribution function (PDDF) was calculated by using the
2
a mixture with the desired mass ratio of the alcohol to water generalized indirect Fourier transform program package.
was prepared in a dry test tube. Then, an appropriate mass of
the IL was added and the mass fraction of the IL at which
Results and discussion
turbidity-to-transparency occurred was calculated. Based on
the critical values obtained at different mass ratios of alcohol
to water, the phase boundary was established. The whole
procedure was repeated three times, and an average value
was acquired.
Thermal properties of the ILs
Table 1 shows glass transition temperature (T
g
), melting point
) of these ILs.
DMEA][Im], [C DMEA][Triz],
8
[C DMEA][Pyr] and [C10DMEA][Pyr] exhibited only glass
(
T
m
) and thermal decomposition temperature (T
d
It was found that [C
8
8
Electrical conductivity, optical microscopy and droplet size
measurements of the microemulsions
transition temperature, and most of the other compounds had
a melting point below 50 °C. This indicates that all these
compounds are really ionic liquids. Additionally, thermal
decomposition temperature of these ILs ranges from 140 to
Electrical conductivity measurements were carried out by
using a DDSJ-308A conductivity meter (Shanghai Leici Precision
Instrument Co., Ltd.) with an accuracy of ± 0.5%. The
temperature was controlled by a precision DC-2006 bath
circulator (Tianheng Precision Instrument Co., Ltd.) to be 25.0
1
88 °C, and the effect of alkyl chain length of the cations and
type of anions is not significant. These results suggest that the
ILs investigated here have good liquid temperature range and
reasonable thermal stability.
±
0.1 °C. The micrographs of microemulsion and emulsion
droplets were recorded by using a DYP-990 microscope system
Shanghai Dianying Optical Instrument Co., Ltd.). For droplet
(
Phase diagrams and microstructures of the microemulsion
systems
size measurements by dynamic light scattering (DLS), all
sample solutions were filtered through a 0.45 μm Millipore
filter to remove any dust or contaminants and then kept at the
measuring temperature for 12 h before determination. DLS
measurements were performed by using a Malvern Zetasizer
Nano-90 light scattering instrument (Malvern, United
Kingdom) equipped with a solid-state He−Ne (4.0 mW) laser
operating at λ = 633 nm. All the samples were measured at a
scattering angle of 90° and 25.0 ± 0.1 °C.
To prove the formation of microemulsions in the
homogeneous systems, the equilibrium phase diagrams of the
n-pentanol/ILs/H
5.0 °C, and the results were shown in Fig. 1 for n-pentanol/
DMEA][Im] (n = 8, 10, 12, 14, 16)/H O system. It is obvious
2
O ternary systems were determined at
2
[C
n
2
that a wide monophasic region and a narrow biphasic region
were obtained in all the phase diagrams. The monophasic
region of the systems was only slightly changed as carbon
atom number in the alkyl chain of the ILs increased from 8 to
Cryogenic transmission electron microscopy observation of
the microemulsions
1
6. Similar results were observed for n-pentanol/[C
8, 10, 12, 14, 16)/H (Fig. S1) and n-
pentanol/[C DMEA][Triz] (n = 8, 10, 12, 14, 16)/H O systems
Fig. S2). Furthermore, the effect of anions of the ILs on the
monophasic region of the n-pentanol/ILs/H O systems was
n
DMEA][Pyr]
Cryogenic transmission electron microscopy (Cryo-TEM)
samples were prepared by using a custom-built chamber. After
the Cryo-TEM grid was fixed in the preparation chamber, a 5μL
of sample solution was dropped on a carbon-coated holey film
which was supported by a copper grid at room temperature.
After the excess solution was blotted away to form a thin
liquid film, the grid was quenched rapidly into liquid ethane
cooled by liquid nitrogen (-175 °C), and then the specimen
stored in liquid nitrogen was transferred into a JEOL JEM1400
cryomicroscope. The acceleration voltage was 200 kV, and the
working temperature was kept approximately at -175 °C. The
images were recorded digitally with a high speed charge
coupled device (CCD) camera.
(n
=
2
O
n
2
(
2
Table 1. Glass transition temperature (T
g
)/melting point (T
m
) and
-
thermal decomposition temperature (T
d
) of the ILs as well as Br
content in the ILs.
-
[Br ]/mol kg
-1
IL
T
g
(°C)
T
m
(°C)
T
d
(°C)
[C
8
DMEA][Im]
-65
153
174
175
188
183
140
149
143
141
159
164
177
187
168
165
0.0090
0.0084
0.0051
0.0074
0.0023
0.0079
0.0074
0.0070
0.0074
0.0025
0.0075
0.0066
0.0061
0.0062
0.0059
[
C
C
C
C
10DMEA][Im]
12DMEA][Im]
14DMEA][Im]
16DMEA][Im]
30
[
[
[
46
54
79
Small-angle X-ray scattering experiments
Small-angle X-ray scattering (SAXS) experiments were
performed by using a sealed-tube Cu target (40kV, 40mA)
2
instrument named SAXSess mc (Anton Paar, Austria) which
[C
8
DMEA][Triz]
-72
[
C
C
C
C
10DMEA][Triz]
12DMEA][Triz]
14DMEA][Triz]
16DMEA][Triz]
11
34
41
49
[
[
[
was equipped with a line-collimation Kratky block system and
an image plate detector. Both parts were placed in vacuum
chamber to avoid air scattering. The samples were held in a
quartz capillary fixed in a stainless steel tank at 25 °C.
Scattering data of sample and background were obtained in a
[
C
8
DMEA][Pyr]
-68
-60
[
C
10DMEA][Pyr]
12DMEA][Pyr]
14DMEA][Pyr]
C16DMEA][Pyr]
[C
28
31
44
30 min exposure. Then the background scattering from empty
[C
sample holder was subtracted from sample curves after
absorption correction. The scattering curves were output as
[
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and n-pentanol-in-H
2
O
(O/W), were identified for the
microemulsion (Fig. 2b). In addition, the microemulsion type
1,26
1
was also identified by using drop test approach.
By placing
a drop of each microemulsion on both water and oil phases,
respectively, the microemulsion type could be determined
through observing which phase the drop was easily dispersed
into. It was found that the results were in good agreement
with that determined from electrical conductivity
measurements. Similar results were obtained for the n-
pentanol/[C
n
DMEA][Pyr] (n = 8, 10, 12, 14, 16)/H
2
O (Figs. S5-S6)
and n-pentanol/[C
n
DMEA][Triz] (n = 8, 10, 12, 14, 16)/H O (Figs.
2
Fig. 1 Phase diagrams of the n-pentanol/[C
n
DMEA][Im] (n = 8, 10, 12, S7-S8) systems.
1
4, 16)/H
2
O microemulsions (in mass fraction) at 25.0 °C.
The microstructure of H O-in-n-pentanol microemulsions
2
was studied by using DLS and SAXS. Taking n-
also small (Fig. S3). These features are beneficial to the pentanol/[C DMEA][Im]/H O system as
composition optimization of the microemulsion systems.
a
representative
1
2
2
example, samples with different compositions were chosen, as
Next, electrical conductivity was measured to determine the marked in Fig. 2b. The hydrodynamic diameter of the H O-in-n-
2
microstructure of microemulsions at 25 °C. Taking n- pentanol microemulsion droplets were shown in Fig. 3a. It was
pentanol/[C12DMEA][Im]/H
conductivity was determined as a function of water mass increased (from 2.0 nm, 5.9 nm, 9.1 nm, 10.7 nm to 16.2 nm)
fraction (W ) at any given mass ratio of n-pentanol to the IL (I). with increasing molar ratio (R) of H O to [C DMEA][Im] from
2
O system as an example, electrical shown that the size of the W/O microemulsion droplets
0
2
12
Fig. 2a shows the conductivity at I = 1.0 and different water 9.28 to 57.01 at I = 9.0. The polydispersity index (PDI) values
contents. It can be seen that with the increase of water were found to be smaller than 0.23 (Table S1), which indicated
content (W
Then, it quickly increased almost linearly with the increase of the aggregates in the microemulsions. In addition, a linear
water content (W < 0.68) and reached the maximum of correlation was observed between the size of the
0
< 0.32), conductivity initially increased gradually. a narrow size distribution and a homogeneous distribution of
25
0
conductivity. Finally, the conductivity decreased with microemulsion droplets and the content of water (inset of Fig.
increasing water content. The initial increase in conductivity 3a), which suggests the existence of spherical droplets in the
2
5
may be owing to successive increase of conductivity of W/O IL-based microemulsions. To confirm the effect of solution
2
3
microemulsion droplets with the addition of water. The color on the size measurements of the microemulsion droplets,
followed linear increase reveals that the system underwent a SAXS tests were performed for the n-pentanol/
structural transition and became
2
a bicontinuous (BC) [C DMEA][Im]/H O microemulsion at different water contents.
The final decrease in conductivity can be Representative SAXS and PDDF curves were shown in Fig. S9
12 2
3,24
microstructure.
interpreted from the consequence of the formation of O/W and Fig. 3b, respectively. The SAXS curves further proved the
microemulsion resulted from the partial fusion of clustered
2
5
inverse micro-droplets. Similar conductivity change in the n-
pentanol/[C12DMEA][Im]/H O system with water content was
2
also found at other I values, as shown in Fig. S4 (supporting
information). Therefore, three different micro-regions,
2
including H O-in-n-pentanol (W/O), bicontinuous region (BC),
Fig. 2 Electrical conductivity of the n-pentanol/[C12DMEA][Im]/H
microemulsion as a function of water content (W ) at I = 1.0 and 25 °C pentanol/[C12DMEA][Im]/H
a); phase diagram of the n-pentanol/[C12DMEA][Im]/H
pentanol/[C12DMEA][Im], 9:1, w/w) and different R values (molar ratio
to [C12DMEA][Im]); Linear correlation of size of the
DMEA][Im] (n= 8, 10, 12, 14, 16) microemulsion droplets with R
2
O
Fig.
3
Size distributions (a) and PDDF curves (b) of the n-
0
2
O microemulsion droplets at I = 9.0 (n-
(
2
O
microemulsion with different types of microstructure at 25.0 °C (b); of
circles (pink and blue) were results from DLS measurements; the red [C
2
H O
n
star represents the composition of the samples investigated in the values at 25 °C (c). Inset of (a) is the linear change of mean size with R
next sections. The composition is expressed in mass fraction.
values.
4
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existence of droplets in the microemulsions. It was noted that
each PDDF curve ascended and descended steeply before and
after the maximum, and then decayed to zero at a certain r
value, which indicates the droplet size of the
2
7
microemulsions. Thus, it could be seen that the droplet size
derived from SAXS was about 2.0 nm, 6.0 nm, 8.8 nm, 12.0 nm
and 16.0 nm at the given water contents. Obviously, the
droplet size determined from SAXS was consistent with that
from DLS measurements. Therefore, the effect of the solution
color on the size measurements of the microemulsion droplets
was negligible in this work.
Fig. 4 Reversible switching of the n-pentanol/[C12DMEA][Im]/H
microemulsion: monophasic W/O microemulsion before bubbling CO
a), opaque emulsion after bubbling of CO for less than 1 min (b), a
clear phase separation of oil and water after bubbling CO for 30 min
c).
2
O
2
(
2
The swelling behavior of the microemulsion with different
contents of [C12DMEA][Im] was also investigated by DLS. For
2
(
this purpose, n-pentanol/H
the molar ratio of H O to [C12DMEA][Im] (R) was changed to
reflect the variation of the [C12DMEA][Im] content. It was
shown that the droplet size of the n-
pentanol/[C12DMEA][Im]/H O microemulsion increased from
2
O mass ratio was fixed at 4:1, but
2
that the concentration of the residul [C DMEA][Im] in the
12
-1
aqueous solution is 74.5 mmol·L (2.3 %). However, when CO
2
o
was removed by N bubbling at 50 C, the system returned to
2
2
transparency again (Fig. 4a). Therefore, the ternary systems
could be reversibly transformed upon alternatively bubbling
1
.1 to 21.1 nm with the increase of R value from 17.96 to
40.40 (Fig. S10). This regular swelling behavior was in
and removal of CO . Similar reversible phase separation
2
agreement with the volume of dispersed nanodomains being
directly proportional to the amount of the added inner phase,
process was also observed for the other microemulsion
systems investigated in this work.
1
7
which is common in many traditional microemulsions, and
suggests the formation of H O-in-n-pentanol microemulsions.
Microstructures of the microemulsions after bubbling of CO2
2
were investigated by DLS, Cryo-TEM and optical microscopy.
Furthermore, microemulsions containing different alkyl chain
lengths of ILs were also studied by using DLS. As shown in Fig.
For example, DLS result showed that the droplet size of the
initial n-pentanol/[C DMEA][Im]/H O microemulsion was
3c, the size of the H
2
O-in-n-pentanol microemulsions increased
12
2
about 22.3 nm (Fig. 5). Upon bubbling of CO , it was increased
2
with increasing alkyl chain length of the ILs at a given R value.
Therefore, the droplet size of the longer chain ILs-based
microemulsions was much bigger than that of the shorter
chain ILs-based microemulsions under the same conditions,
to 3100 nm due to the disruption of the W/O microemulsion.
When the W/O microemulsion was formed once again by
bubbling of N at 50 °C, the droplet size was decreased to 22.0
2
2
5
nm which was very close to its original value. Therefore, the n-
pentanol/[C DMEA][Im]/H O microemulsion could be
which is consistent with the result reported in literature.
Similar trends were obtained for the n-pentanol/[C DMEA][Pyr]
8, 10, 12, 14, 16)/H O (Figs. S11-S13) and n-
pentanol/[C DMEA][Triz] (n = 8, 10, 12, 14, 16/H O (Figs. S14-
S16) systems.
CO -driven reversible oil-water phase separation of the
microemulsions
CO -responsive characteristics of the n-pentanol/ILs/H
microemulsions were investigated at 25.0 °C. Here, the n-
pentanol/[C12DMEA][Im]/H microemulsion with the
12
2
n
reversibly disrupted and re-formed with alternatively bubbling
(n
=
2
and removal of CO . Cryo-TEM images further showed that
2
n
2
many well-dispersed spherical-like microstructures existed in
the initial and recycled n-pentanol/[C DMEA][Im]/H O
12
2
2
microemulsion (Figs. 6a and 6c). Furthermore, the size of the
spherical-like structures from Cryo-TEM images was consistent
with that from the DLS measurements (Figs. 6b and 6d).
In addition, optical microscope was applied to study the
2
2
O
2
O
phase separation of microemulsions after bubbling of CO . As
2
composition (marked by a red star in Fig. 2b) fixed at 54.2 wt%
of n-pentanol, 13.3 wt% of [C12DMEA][Im], and 32.5 wt% of
H
2
O was studied as an example. It was shown that in the
absence of CO , the microemulsion system was homogeneous
Fig. 4a). When CO was bubbled through the system at
2
(
2
approximately 2-3 bubbles per second under stirring at room
temperature, it turned to be opaque emulsion in less than one
minute (Fig. 4b). Further bubbling of CO
microemulsion was separated into two transparent phases (Fig.
c). It can be seen from Fig. 4c that the lower aqueous phase
2
for about 30 min, the
4
was almost colorless while the upper organic phase was pale
yellow, indicating transfer of the IL into the organic phase.
Furthermore, the amount of the residual [C12DMEA][Im] in the
aqueous solution after bubbling CO
1
2
was determined by Fig. 5 Sizes of the n-pentanol/[C DMEA][Im]/H O microemulsion
12 2
1,28
spectrophotometric method.
It was obtained from Fig. S17
before bubbling CO2 (black), after bubbling CO2 (red), and after
removal of CO by bubbling N at 50 °C (blue).
2
2
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1
3
C
Fig.
8
NMR spectra of [C12DMEA][Im] in the n-pentanol/
2 2
[C12DMEA][Im]/H O microemulsion before bubbling of CO and after
bubbling of N
separation.
2
as well as in the aqueous solution after phase
bubbling of N
2
as well as in the aqueous solution after phase
2
Fig. 6 Cryo-TEM images and droplet size distribution of the CO -
separation (the lower phase in Fig. 4c). Since the
concentrations of the ILs in aqueous solutions after phase
separation were small, these solutions were concentrated
switchable microemulsion: Cryo-TEM micrograph (a) and droplet size
distribution (b) of the initial n-pentanol/[C12DMEA][Im]/H
microemulsion; Cryo-TEM micrograph (c) and droplet size distribution
d) of the n-pentanol/[C12DMEA][Im]/H O microemulsion after one
cycle.
2
O
1
3
before measurements. As an example, C NMR spectrum of
12DMEA][Im] in the systems mentioned above was
(
2
[C
o
determined at 25 C, and the results were shown in Fig. 8. It is
clear that after phase separation, two new signals appeared at
an example, the optical micrographs of n-pentanol/
12DMEA][Im]/H O microemulsion were exhibited in Fig. 7. It
is clear that no drops were found in initial n-pentanol/
12DMEA][Im]/H O microemulsion because of their small size
Fig. 7a). Once CO was bubbled into the system, numerous
larger emulsion drops ( m in diameter) appeared in the
1
3
1
59.0 and 160.0 ppm in the
12DMEA][Im] in aqueous solution. The signal at 160.0 ppm
could be ascribed to the carbonyl carbon of bicarbonate
C NMR spectrum of
[C
2
[C
[C
2
-
generated from the reaction of [Im] anion with CO
2
in the
Additionally, the signal at 159.0 ppm
was identified to be the carbonyl group of the carbamate
(
2
1
8,29
presence of water.
∼3 µ
system (Fig. 7b). These emulsion drops were highly unstable
-
-
[ImCO
2 2
] formed from the reaction between CO and [Im]
due to their high interfacial energy. Thus by further bubbling of
anion, and the chemical shift and signal intensity were very
2
CO , these unstable drops gradually combined to form large
1
5,30
close to those reported previously.
The strong signal at
20.9 ppm was mainly assigned to the Im molecule, and the
small shoulder at 121.0 ppm indicated the formation of the
drops that were more unstable. Finally, the emulsion was
completely disrupted, and the drops were no longer visible (Fig.
1
7c).
1
8
unsymmetrical monocarbamate. The former indicates that
Im molecules were formed after bubbling of CO . On the other
hand, the spectrum of the IL after removal of CO by bubbling
of N was found to be the same with that before bubbling of
CO (that is, in the initial microemulsion). Similar results were
obtained for the interactions of CO with anions of
DMEA][Pyr] (n = 8, 10, 12, 14, 16) (Fig. S18) and
DMEA][Triz] (n = 8, 10, 12, 14, 16) (Fig. S19). Therefore, it
Possible mechanism for CO
microemulsions
2
-driven phase separation of the
2
2
Considering the fact that the microemulsions studied in this
-
work have N-containing heterocyclic anion such as [Im] , [Pyr]
-
2
-
15
2
and [Triz] which can efficiently absorb CO
of the microemulsions driven by CO
reaction of these anions with CO . To verify this speculation,
C NMR spectroscopy was used to probe the interactions
between CO and anions of the ILs in the n-pentanol/ILs/H
microemulsion systems before bubbling of CO and after
2
,
phase separation
2
2
may be ascribed to the
[
C
n
2
1
3
[C
n
can be deduced that the mechanism behind the reversible
switching of the microemulsions may involve the reversible
formation of ammonium salts (bicarbonate and carbamate)
2
2
O
2
2
from the reaction between the anions and CO . This results in
the increase of ionic strength (or vice versa) in the mixtures,
and the salting-out effect makes the microemulsions to be
3
1
disrupted. The reversible change in ionic strength gives rise
to the reversible switching from a single phase microemulsion
to a clear oil-water phase separation.
In order to confirm that the phase separation of the
microemulsions was caused by the increase of ionic strength in
Fig.
pentanol/[C12DMEA][Im]/H
CO
exposure for 10 min at room temperature (b), and the upper oil microemulsions. As an example, Fig. S20 shows the effect of
phase after CO
7
The
optical
micrographs
of
3 2 3
the initial n- the systems, NaCl, NaHCO , and Na CO were chosen to
2
O microemulsion (a), the emulsion after examine the effect of ionic strength on the stability of
2
2
bubbling for 30 min at room temperature (c).
NaHCO
3
content on the droplet size of n-pentanol/
6
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ARTICLE
[
C
12DMEA][Im]/H
2
O microemulsion determined by DLS. It was malononitrile under the same conditions and excellent yields
content in the system was increased were obtained (Table S2 and Figs. S23-S27). Comparing with
shown that when NaHCO
3
-1
36,37
from 0 to 83.32 mmol
from 22.0 nm (without salt) to 2305 nm, and obvious phase exhibited superior catalytic activity for the Knoevenagel
separation was observed at NaHCO concentration of 35.71 reaction. The resulting products could be easily separated
L . Based on the above results, it can be concluded that from the reaction mixture by filtration, and the
·
L , the droplet size changed remarkably other catalytic systems,
the microemulsions reported here
3
-1
mmol
·
the oil-water phase separation of microemulsions was ascribed microemulsions could be readily recovered and recycled.
to the increase of ionic strength in the mixture.
2
Application of the CO -responsive microemulsions as
Conclusions
microreactors for organic reaction
Knoevenagel condensation reaction is a general and important
reaction for the formation of carbon-carbon bond, and has
been widely applied to synthesize a number of important fine
We have demonstrated a class of simple stimuli-responsive
2
microemulsions composed of CO -responsive ILs, n-pentanol
and water. It was found that these systems underwent a
reversible switching from a monophasic W/O microemulsion
to complete oil-water phase separation by alternatively
32-34
chemicals and heterocyclic compounds.
As a proof of
concept, our microemulsions were used as microreactors for
Knoevenagel reaction to demonstrate the coupling of chemical
reaction, product separation and recycling of the
microemulsions. As shown in Fig. 9, the reaction for the
preparation of 2-benzylidenemalononitrile proceeded well in
2
bubbling and removal of CO at atmospheric pressure. The
reversible phase separation of the microemulsion was ascribed
to the reversible change (increase and decrease) of ionic
strength in the systems due to the reversible formation of
2
the n-pentanol/[C12DMEA][Im]/H O microemulsions at 25 °C.
bicarbonate and carbamate from the reaction between CO
2
Upon bubbling of CO for less than 10 min, the microemulsion
2
and anions of the ILs in the presence of water. Therefore,
these microemulsions can be formed or disrupted on demand
was disrupted, and the product was precipitated from the
reaction system and suspended in the water phase. Then, the
product was filtered out from the water phase. On the other
by using CO
2 2
/ N as a cheap and green trigger. By utilizing the
CO -responsive property of these microemulsions, a highly
2
hand, when CO
2
was removed by
N
2
bubbling, the
efficient Knoevenagel reaction in microemulsion and simple
product separation in aqueous phase was achieved.
Additionally, the microemulsions could be reused for at least
three runs without any significant impact on the yield of the
products. These findings are significant for the development of
new strategies for sustainable chemical processes.
microemulsion was regenerated, and once new reactants were
added in the system, the same reaction happened again.
Then, the product 2-benzylidenemalononitrile was identified
1
by H NMR spectroscopy (see Fig. S21), and the yield was up to
97.2%. After three recycles, its yield still reached to 94.5% (Fig.
S22). Besides, it was found that the yield for the same reaction
in different microemulsions decreased in the order:
[C
12DMEA][Im] based microemulsion (97.2%) > [C12DMEA][Triz] Conflicts of interest
based microemulsion (81.1%) [C12DMEA][Pyr] based There are no conflicts to declare.
>
microemulsion (75.3%), indicating that the microemulsions
with stronger basicity showed higher catalytic activity for
1
8,35
Knoevenagel reactions.
To extend the substrate scope, Acknowledgements
several benzaldehyde derivatives were chosen to react with
This work is supported by the National Natural Science
Foundation of China (No. 21603063, U1704251 and
2
1733011), the National Key Research and Development
Program of China (2017YFA0403101), and the 111 Project (No.
D17007).
Notes and references
1
2
3
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