Light-modulated aggregation behavior of some unsubstituted cinnamate-based ionic liquids in aqueous solutions

Article abstract of DOI:10.1039/c5ra20772j

Light modulation of the self-assembly behavior of ionic liquids in aqueous solution is of great importance from the viewpoint of fundamentals and technology. In this work, a new class of light-responsive ionic liquids composed of 1-alkyl-3-methylimidazolium cations, [C_nmim]⁺ (n = 4, 6, 8, 10, 12), and the trans-cinnamic acid ([CA]) anion was synthesized and characterized. These compounds were used to study the modulation action of UV light on the aggregation behavior of ionic liquids in aqueous solution. For this purpose, the critical aggregation concentration (CAC), ionization degree of the aggregates (α), standard Gibbs energy of aggregation (ΔG°m) and aggregate size of the ionic liquids were determined before and after UV light irradiation by conductivity and dynamic light scattering techniques. The percentage of cis-isomer of these ionic liquids was also reported at the photoisomerism equilibrium from HPLC measurements. It was found that the aggregation behavior of [C_nmim][CA] (n = 8, 10, 12) could be efficiently modulated by UV light in aqueous solution. After UV light irradiation, the values of CAC, α and ΔG°m increased, while the size of the aggregates decreased. These results have been rationalized from the photo-isomerization of the [CA] anion, hydrophilicity and structural features of the cis-isomer of the [CA] anion.

Full text of DOI:10.1039/c5ra20772j

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Light-Modulated Aggregation Behavior of Some DOI: 10.1039/C5RA20772J
Unsubstituted Cinnamate-Based Ionic Liquids in Aqueous
Solutions
Jie Yang,ac Huiyong Wang,ab Jianji Wang,*ab Xiaojia Guoab and Yue Zhangab
a
Henan Key Laboratory of Green Chemical Media and Reactions, School of Chemistry and
Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, P. R. China
b Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals,
Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, Henan Normal
University, Xinxiang, Henan 453007, P. R. China
c School of Basic Medicine, Xinxiang Medical University, Xinxiang, Henan 453007, P. R. China
ABSTRACT: Light modulation of the self-assembly behavior of ionic liquids in
aqueous solution is of great importance from viewpoint of fundamental and
technology. In this work, a new class of light-responsive ionic liquids composed of
1-alkyl-3-methylimidazolium cations, [Cnmim]+ (n = 4, 6, 8, 10, 12), and
trans-cinnamic acid ([CA]) anion was synthesized and characterized. These
compounds were used to study the modulation action of UV light on the aggregation
behavior of ionic liquids in aqueous solution. For this purpose, critical aggregation
concentration (CAC), ionization degree of the aggregates (α), standard Gibbs energy
of aggregation (ΔGm°) and aggregate size of the ionic liquids were determined before
and after UV light irradiation by conductivity and dynamic light scattering techniques.
The percentage of cis-isomer of these ionic liquids was also reported at
photoisomerism equilibrium from HPLC measurements. It was found that the
aggregation behavior of [Cnmim][CA] (n = 8, 10, 12) could be efficiently modulated
by UV light in aqueous solution. After UV light irradiation, the values of CAC, α and
ΔGm°increased, while the size of aggregates decreased. These results have been
rationalized from photo-isomerization of [CA] anion, hydrohpilicity and structural
feature of the cis- isomer of [CA] anion.
Introduction
In the past two decades, ionic liquids have been developed rapidly due to their

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attractive properties such as negligible volatility, low melting point, high thermal
stability, and strong solubility capacity for many materials.[1-4] These liquid materials
have drawn increasing interest as alternative media in a variety of chemical synthesis,
separation, electrochemical and catalytic processes.[5-14] Because of the amphiphilic
property, ionic liquids are able to form aggregates in solutions.[15-19] In fact, many
applications of ionic liquids, such as environmental pollution control,
chromatographic application, material preparation, and separation, are often closely
dependent on their aggregation behavior. Therefore, the study of aggregation behavior
in solutions is highly necessary for understanding the role of ionic liquids playing in
practical applications.
At present, many investigations have been focused on the modulation of
aggregation behavior of ionic liquids in aqueous solutions by altering the length of
alkyl chain in cation[20,21], the structure of cation, the type of anions[21,22], pH
value[23,24] and temperature[25] of the system, and by adding inorganic or organic
additive in solutions as well. [26-28] As an external stimuli, light trigger is of great
importance and superiority, since no “invasion” can be introduced into the system
during the stimuli-responsive process, and the optical signal is easy to obtain with
stability and reliability. Furthermore, light can be directed at a precise spot with a
resolution of a few micrometers, which is of particularly value in nanoscience and
nanotechnology applications. Thus photo-responsive ionic liquids have been created
in recent years. To this end, Asaka and co-workers[29] synthesized a photo-responsive
ionic liquid, 3-butyl-1-methyl-2-phenylazoimidazolium bis(trifluoro-methanesulfonyl)
amide, and investigated its isomerization mechanism and solvent viscosity effect in
[30]
E-Z photoisomerization. Branco et al
prepared some methyl orange based
photochromic ionic liquids and determined rate constants of their thermal recovery.
[31]
Tamura et al
reported some photo-responsive stilbene ionic liquids and their
emissive properties and melting points. Some azobenzene-based photo-responsive
ionic liquids were prepared by Zhang et al,[32] and reversible modulation of ionic
conductivity in some organic solvents was examined under UV/visible light
irradiation. In a previous work,[33] we reported another series of cinnamate-based ionic

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+
DOI: 10.1039/C5RA20772J
liquids composed of 1-alkyl-3-methylimidazolium cations [Cnmim]
and
trans-ortho-methoxycinnamic acid anion ([OMCA]). Highly efficient conductivity
modulation of these light-responsive ionic liquids was studied in aqueous solutions.
However, to the best of our knowledge, there was only one report [34] concerning with
light modulation of the self-assembly behavior of ionic liquids in aqueous solution up
to now. In that work, Shen et al,[34] synthesized a new surface active ionic liquid
4-butylazobenzene-4’-hexyloxy-trimethyl-ammoniumtrifluoroacetate. It was shown
that reversible micelle-vesicle transformation was achieved in aqueous solution by
alternative UV/vis light irradiation.
Considering the fact that physical and chemical properties of ionic liquids can be
finely tuned by a judicious design of the cations and anions structures, we are
interested in the creation of novel ionic liquids whose aggregation behavior in
solution can be modulated by light irradiation. In this report, we designed and
synthesized five kinds of light responsive ionic liquids composed of
1-alkyl-3-methylimidazolium cations, [Cnmim]+ (n = 4, 6, 8, 10, 12), and trans-
cinnamic acid ([CA]) anion which can be photoisomerized from trans- to cis-isomer
upon exposure to UV light. Because its photoisomerization is not reversible,[35] the
irradiated samples will not undergo any changes when stored under ambient
conditions for a long time, and it is easy to conduct subsequent tests using appropriate
methods. The modulation of aggregation behavior of these ionic liquids in water by
UV light irradiation has been investigated by conductivity and dynamic light
scattering techniques. The results have been used to understand the effect of
light-induced transformation of [CA] anion from trans- to cis-isomers on the
self-assembly of [Cnmim][CA] ionic liquids in aqueous solutions.
Materials and methods
Chemicals
1-Methylimidazole (99%), 1,1,1-trichloroethane (AR), ethyl acetate (AR) and
o
petroleum ether (bp 60-90 C) was acquired from Shanghai Chem. Co.;
1-bromobutane (99%), 1-bromohexane (99%), 1-bromooctane (99%), 1-bromodecane

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(99%), 1-bromododecane (99%), trans-cinnamic acid (98%) and Ambersep 900(OH)
anion exchange resin were purchased from Alfa Aeser. These chemicals were used
without further purification.
Synthesis of the ionic liquids
[Cnmim]Br was prepared and purified by using the procedures described in
literature.[36] Briefly, the reaction of 1-methylimidazole (0.10 mol) with an excess of
1-brominated alkanes (0.13 mol) was performed in 1,1,1-trichloroethane (100 mL) at
70oC for 48 h to obtain [Cnmim]Br. Among the products prepared, [C4mim]Br,
[C6mim]Br, [C8mim]Br and [C10mim]Br were washed with 1,1,1-trichloroethane, and
o
the residual solvents were removed by heating at 70 C under vacuum, while
[C12mim]Br was recrystallized three times from ethyl acetate and ethyl
acetate/acetonitrile (3:2 by volume), respectively.
An aqueous solution of [Cnmim]Br was allowed to pass through a column filled
with Ambersep 900(OH) anion exchange resin to obtain [Cnmim][OH]. The aqueous
[Cnmim][OH] solution was then neutralized with equal molar trans-cinnamic acid
(H[CA]). After removing water by evaporation under reduced pressure, [C4mim][CA]
was thoroughly washed with diethyl ether or petroleum ether, and the product was
obtained as slightly brown viscous liquid. The other ionic liquids were recrystallized
three times from ethyl acetate, and finally dried under vacuum for 72 h at 60 oC to
obtain products as white solids (faint yellow solid for [C6mim][CA]).
1
The chemical structures of these ionic liquids were confirmed by H NMR, 13C
NMR (Bruker Avance-400 spectroscopy) and HR-mass spectroscopy (Bruker
MicroTOF Ⅱ spectrometer). The detailed data were given in ESI. Their glass
transition temperature (Tg), melting temperature (Tm) and thermal decomposition
temperature (Td) were determined by differential scanning calorimetry (Mettler
Toledo 822e) and thermogravimetric analysis (NETZSCH STA 449 C), respectively.
Bromide content in these ionic liquids was determined by means of a Br- selective
electrode (Shanghai Precision & Scientific Instrument Co. Ltd) coupled with a
saturated calomel electrode (Shanghai Precision & Scientific Instrument Co. Ltd).

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UV-light irradiation and UV-vis spectroscopy measurements
Aqueous [Cnmim][CA] solutions were irradiated with UV light from a 300W mercury
pressure short arc lamp with a filter (<400 nm). Each sample (30 mL) was placed in a
quartz tube with a cover and then irradiated for specific duration under stirring. In
order to avoid overheating from the UV irradiation, the lamp was put in a cell with a
water circulating jacket, and the temperature was kept to be 298.2 ± 0.5 K.
UV-vis spectra for the solutions of ionic liquids were determined before and after
UV-irradiation by a UV-4100 spectrophotometer at room temperature. Deionized
water was utilized as blank in the experiments.
Conductivity measurements
Conductivity measurements were performed with a Shanghai DJS-1 electrode at
298.15 K by using a Wayne-Kerr 6430B Auto Balance Bridge with a resolution of
1×10-5 μS·cm-1. The conductance cell was equipped with a water circulating jacket,
and the temperature was controlled within ± 0.01oC with a HAAKE V26 thermostat
(Thermo Electron, Germany). The cell was calibrated with aqueous KCl solutions at
different concentrations, and a cell constant of 1.0127 cm-1 was determined.
Dynamic Light Scattering (DLS) measurements
Dynamic light scattering (DLS) measurements were carried out at 298.2 K by using a
laser light scattering photometer (Nano-ZS90, Malvern, U. K). Light of λ= 633 nm
from a solid-state He-Ne laser (4.0 mW) was used as the incident beam. All sample
solutions were filtered through a 0.22 μm hydrophilic PVDF membrane filter. All
measurements were made at 90° scattering angle. At least three measurements were
taken for each solution, and the reproducibility of aggregate sizes from DLS data was
found to be within ±3%.
High Performance Liquid Chromatography (HPLC) measurements
To determine the percentage of the cis-isomer of [Cnmim][CA], the irradiated
solutions were analyzed by using HPLC (Aglient Eclipse XDB-C18 column, 5μm, 4.6
mm×150 mm) at a flow rate of 0.8 mL/min. The eluting solvent was an aqueous
methanol solution (45:55 by volume) containing 1.0% acetic acid, and the column

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temperature was at 30 ℃. The peaks were detected using UV absorption at 254 nm,
and the cis-trans ratio was estimated by their integrated peak area.
Results and discussion
Table 1 shows the melting temperature (Tm), glass transition temperature (Tg), and
thermal decomposition temperature (Td) for the investigated ionic liquids. It can be
o
seen that [C4mim][CA] showed only glass transition temperature at about -30 C, and
the other compounds possess a melting point below 51 oC. This suggests that all these
compounds are ionic liquids. In addition, all these ionic liquids have good thermal
stability (234–248 oC) under an inert atmosphere as estimated from their Td values.
Table 1. Thermal properties of [Cnmim][CA] ( n = 4, 6, 8, 10, 12 )
IL
Tm/ oC
Tg/ oC
Td/ oC
[C4mim][CA]
[C6mim][CA]
[C8mim][CA]
[C10mim][CA]
[C12mim][CA]
-
-30
248
242
246
243
234
50
46
37
39
-
-
-
-
It has been reported that impurity in ionic liquids has an important effect on their
physico-chemical properties.[37] Considering the fact that in the synthesis process of
our ionic liquids, the main impurity was bromide, therefore, the bromide content in
these ionic liquids was determined by a Br- selective electrode, and the data were
given in Table S1 (ESI†). It can be seen that the bromide content was less than 0.0079
mol kg-1. These results suggest that the quality of the ionic liquids prepared in this
work is reliable to investigate the aggregation behavior of ionic liquids in aqueous
solutions.
Photo-isomerization of [Cnmim][CA] ionic liquids in aqueous solution
As a typical example, UV-vis spectra for [C12mim][CA] and trans- cinnamic acid
(H[CA]) at the concentration of 1 mM were shown in Fig. 1 and S1, respectively,
where a slightly excess of base was added to aqueous solutions to increase solubility

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of trans- cinnamic acid in water. It can be seen that the maximal absorption
wavelength of [Cnmim][CA] was at around 267 nm before irradiation and at 254 nm
after irradiation (at photostationary state) in aqueous solutions, and the absorbance
peak heights were also significantly reduced after irradiation. This blue shift of the
maximal absorption wavelength is an indicative of trans to cis photo-isomerization,[35]
which is exactly similar to what has been observed for trans- cinnamic acid (see
Fig.S1). This indicates that the spectral features of these ionic liquids were surely
originated from their [CA] anion, and almost have nothing to do with their cations and
alkyl chain length. Thus the photoisomerization of [Cnmim][CA] ionic liquids can be
represented by the reaction shown in Fig.2. By HPLC measurements, it was
confirmed that percentage of cis-isomer of [Cnmim][CA] (n = 8, 10, 12)
corresponding to photoisomerism equilibrium was ca. 83% at 298.2 K.
Fig. 1. UV-vis spectra of aqueous [C12mim][CA] solution at the concentration
of 1 mM before and after irradiation.
O
CnH2n+1
CnH2n+1
O
UV light
N
N
N
N
O
O
cis-isomer
trans-isomer
Fig. 2. Photoisomerism of [Cnmim][CA] ionic liquids.
Aggregation parameters for the ionic liquids in water before irradiation
The experimental conductivities for aqueous solutions of [Cnmim][CA] (n = 4, 6, 8,
10, 12) before irradiation were shown in Fig. 3 as a function of ionic liquid

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concentration at 298.15 K. It can be seen that a characteristic shape of the curves was
observed in these [Cnmim][CA] (n = 8, 10, 12) solutions, except for [C4mim][CA]
and [C6mim][CA]. These curves exhibited typical behavior with two linear fragments,
and the concentration at which the two linear fragments intersect was assigned to the
critical aggregate concentration (CAC). The degree of ionization (α) of the aggregates
was taken to be the ratio of the values of dκ/dC above and below the CAC, and can be
calculated from the slope of the two straight lines in these regions. Actually, α value
indicates the ability of anion bound on the aggregates. An increase of α value suggests
a decrease in the ability of the anions bound to the aggregates.[38] The CAC and α
values of [Cnmim][CA] ionic liquids calculated from a linear least-squares analysis
were presented in Table 2.
Fig. 3. Plots of the conductivity of aqueous [Cnmim][CA] solutions before irradiation as a function
of the ILs concentration at 298.15K.
Table 2. CAC, α and ΔGm°values of aqueous [Cnmim][CA] (n = 8, 10, 12) before and after
irradiation (at photostationary equilibrium) at 298.15 K
Before irradiation
Photostationary equilibrium
IL
CAC / mol kg-1
α
ΔGm°/ kJ mol-1 CAC / mol kg-1
α
ΔGm°/ kJ mol-1
[C12mim][CA]
[C10mim][CA]
[C8mim][CA]
0.00320
0.0118
0.0454
0.0462
0.0570
0.0625
-47.3
-40.7
-34.2
0.00545
0.0194
0.0767
0.154
0.175
0.180
-42.2
-36.0
-29.7
Obviously, the CAC values decreased with the increase of carbon atom number in
alkyl chain (Nc) of cations of the ionic liquids, just similar to what have been

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observed for conventional ionic surfactants. Interestingly, a linear correlation exists
between log(CAC) and Nc as shown in Fig. 4. This linear correlation can be described
by equation (1) with a correlation coefficient of 0.9998.
log(CAC) = 0.957 – 0.288Nc
(1)
The slope of the straight line was -0.29, which is close to -0.28 obtained from
[Cnmim]Br ( n = 10, 12, 14, 16 ) ionic liquids,[39] and is in the range reported for
[40]
conventional ionic surfactants (-0.27~ -0.33).
This result is consistent with the
findings that the effect of alkyl chain length of the cation on the CAC does not depend
markedly on the structure of the cations and the type of counterions.[22]
Fig. 4. Logarithmic plot of CAC at 298.15K against carbon atom number in alkyl chain, Nc, of
[Cnmim][CA] (n = 8, 10, 12).
It is worth to note that the CAC values of [Cnmim][CA] are far lower than that of
other imidazolium-based ionic liquids with the same carbon atom number (see Table
3). This means that the surface activity of [Cnmim][CA] is advantageous to other
imidazolium-based ionic liquids with the same carbon atom number, and these ionic
liquids are more easier to form aggregates. Furthermore, it can be seen that the α
values of [Cnmim][CA] was decreased with the increase of alkyl chain length of the
cations. Similar phenomenon was reported for some surfactants in aqueous
solutions.[41] On the other hand, it was found that the α values of [Cnmim][CA] (n=8,
10, 12) were much less than those of other [Cnmim]X (n=8, 10, 12) ionic liquids with
different anions such as [CH3COO], Cl, Br, [NO3] and [CF3COO] (see Table 3 for

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example). This indicates that the interactions between imidazolium head group and
[CA] anion is much stronger than those between imidazolium head group and
[CH3COO]-, Cl-, Br-, [NO3]- or [CF3COO]-. This stronger association was resulted
from the stronger hydrophobicity of [CA] anion.
Table 3. Comparison of CAC, α and ΔGm°values of [C8mim][X] ionic liquids at 298.15 K
IL
CAC /(molkg-1)
α
ΔGm°/ kJ mol-1
[C8mim][CH3COO]a
[C8mim]Cla
0.22
0.21
0.16
0.15
0.12
0.045
0.42
0.37
0.34
0.29
0.19
0.063
-21.5
-22.4
-23.6
-25.1
-27.2
-34.2
[C8mim]Bra
[C8mim][NO3]a
[C8mim][CF3COO]a
[C8mim][CA]
a reported in ref.[22]
On the basis of the pesudophase model of micellization, the standard Gibbs energy
of aggregation can be calculated from the following equation:[42]
ΔGm° = (2 -α)RT ln xCAC
(2)
where xCAC is the critical aggregation concentration expressed in mole fraction scale.
The values of ΔGmo calculated for [Cnmim][CA] before irradiation were also included
in Table 2. It was found from Table 2 that the standard Gibbs energy values for
[Cnmim][CA] aggregation became more negative with increasing alkyl chain length
of the ionic liquids. This is similar to the case of other types of ionic liquids,
indicating that the aggregation was driven mainly by hydrocarbon-hydrocarbon
interactions.[22] In addition, the values of standard Gibbs energy for the aggregation of
[C8mim][CA] was unusually more negative than those of the other [C8mim]X (X =
[CH3COO], Cl, Br, [NO3], [CF3COO]) ionic liquids, which suggests that the
aggregation of [C8mim][CA] was more favorable than the ionic liquids mentioned. In
general, aggregate formation was determined by a balance between the repulsive
headgroup interactions and the attractive forces arising from a need to minimize the
exposure of the hydrophobic core to water. The anion acts by altering such a balance.

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o
As shown in Table 3, the more negative values of ΔGm for [C8mim][CA] was
resulted from the stronger absorption of [CA] anion on the aggregates (i.e, lower α),
which decreased the electrostatic repulsion between the imidazolium headgroups, and
facilitated the formation of aggregates in aqueous solutions.
Photo-induced modulation of aggregation behavior of the ionic liquids
Fig.5. Shows the variation of conductivity for [Cnmim][CA] (n = 8, 10, 12) in
aqueous solutions as a function of the ionic liquids concentration before and after
irradiaion. By using the same procedure described before, we calculated the values of
CAC, α and ΔGm°at 298.15 K for [Cnmim][CA] (n = 8, 10, 12) ionic liquids after
irradiation (at photostationary equilibrium). For the sake of comparison, these values
were also included in Table 2.
a
b
c
d
Fig. 5. Plots of the conductivity of [Cnmim][CA] in aqueous solutions as a function of the ionic
liquids concentration before irradiation and after irradiaion. (a), [C8mim][CA]; (b),
[C10mim][CA]; (c), [C12mim][CA]; and (d), [C12mim][CA] at different photoisomerism
percentage.

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It is clear from Fig.5 and Table 2 that UV irradiation significantly increased the
conductivity, CAC, α and ΔGm°values of these aqueous ionic liquids solutions. The
increase in α value means that the association of cis-CA anion with imidazolium head
groups became weak after UV irradiation, which was not beneficial to the formation
of cationic aggregates and resulted in the increase of CAC and ΔGm°values and the
reduction of aggregation ability of the ionic liquids. Actually, the CAC values of
aqueous [Cnmim][CA] solution at photostationary equilibrium were roughly doubled.
In addition, it is evident from Fig.5d that the extent of photo-induced isomerization of
the anion from trans- to cis-isomers had an important affect on both α and CAC values.
The higher the percentage of cis-isomers of the anion is, the higher the α and CAC
values are. Therefore, UV irradiation can be conveniently used to modulate
aggregation behavior of ionic liquids in aqueous solutions.
From the above discussion, it is clear that the difference in the interaction between
counter ion and headgroup before and after light irradiation is responsible for
light-induced aggregation behavior modulation of the ionic liquids in water. Before
irradiation, trans-CA anions are the major existing form of the counter ion, and their
hydrophobic part (aromatic ring) is relatively "distant" from the hydrophilic part
(carboxylate group). This structure feature makes it convenient for the counter ions to
intercalate its hydrophobic part into the micelle interior and remain its hydrophilic
part outside water. That is to say, trans-CA anions can tightly associate with cationic
aggregates, thus promoting the growth of the aggregates. Under these circumstance,
CAC, α and ΔGmo values of [Cnmim][CA] were particularly small. On the other hand,
when trans-CA anions were transformed into cis-CA anions upon UV light irradiation,
molecular skeleton of the anion was curved, and the distance (D) between the
hydrophobic benzene ring and the hydrophilic carboxylate group was greatly
shortened (see Fig.6). Thus, benzene ring was located close to carboxylate group,
resulting in the difficulty for the benzene ring to orient inward to the [Cnmim]+
aggregates interface and the carboxylate group outward. Furthermore, the difference
in the relative hydrophobicity of trans- and cis-CA also led to the difference in the
interactions of two isomers with [Cnmim]+ aggregates. It has been noted for a variety

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of compounds that the trans isomer is more hydrophobic than its cis isomer, this
difference in hydrophobicity has been ascribed to the lower net dipole moment of the
trans isomer compared to the cis isomer.[35] This implies that trans-CA would readily
bind to [Cnmim]+ aggregates, whereas the more hydrohpilic cis-CA could be more
likely to remain in solution and this is not desirable for the growth of aggregates. As a
o
result, CAC, α and ΔGm values of [Cnmim][CA] significantly increased after light
irradiation.
O
O
UV light
D
D
O
O
trans-CA
cis-CA
Fig.6. Alteration of distance (D) between hydrophobic benzene ring and hydrophilic carboxylate
group after irradiation
Photo-induced modulation of the aggregates size of [Cnmim][CA] ionic liquids
In order to further investigate the influence of UV light irradiation on the aggregates
size, DLS measurement was employed to detect the changes of aggregates size of
[Cnmim][CA] (n = 8, 10, 12) before and after UV irradiation in aqueous solution. Fig.
7 shows the size distribution for the aggregates from DLS measurements at 0.1 mol/kg
of [Cnmim][CA] before and after light irradiation for 3 h at 298.2 K. It can be seen
that at the same concentration, the aggregate size increased with increasing alkyl
chain length of the ILs before irradiation. For example, the aggregate size grew from
5.2 to 6.4 nm when the alkyl chain length increased from C8 to C10. In the meantime,
size distribution for [C12mim][CA] aggregate showed obvious polydispersity with two
peaks at 3.56 and 77.5 nm, indicating the formation of larger sized aggregates. It can
also be seen from Fig. 7 that after irradiation for 3 h, the aggregate size was changed
from 5.2, 6.4, and 3.6 nm to 3.5, 3.4 and 2.5 nm for [C8mim][CA], [C10mim][CA] and
[C12mim][CA], respectively, and the aggregate with a diameter of 77.5 nm
disappeared for [C12mim][CA]. These results indicate that the aggregate size of the
ionic liquids became small upon UV irradiation, proving that UV irradiation is not
conducive to the growth of the aggregates. This is highly consistent with the

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conclusion deduced from conductivity measurements.
Fig. 7. The size distribution for the aggregates from DLS measurements at 0.1 mol/kg of
[Cnmim][CA] ( n = 8, 10, 12 ) before and after UV irradiation for 3 h at 298.2 K.
Conclusions
In summary, a new class of light-responsive ionic liquids [Cnmim][CA]( n = 4, 6, 8,
10, 12) was synthesized and characterized for the first time, and their aggregation
behavior was investigated before and after UV light irradiation. It was found that after
UV irradiation, the critical aggregation concentration, ionization degree of the
aggregates, and standard Gibbs energy of aggregation of the ionic liquids increased,
while the size of aggregates decreased. This could be attributed to the
photoisomerization of anion of [Cnmin][CA] from trans- to cis- isomers upon
exposure to UV light. The cis-[CA] has unfavorable geometry and weaker
hydrophobicity, which makes the association interactions of the counter ions with
[Cnmim]+ aggregates to weaken. These findings are expected to be useful in light
controlled drug delivery, microscale photo-control devices and sensors.
Acknowledgements

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RSC Advances
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DOI: 10.1039/C5RA20772J
This work was supported by the National Natural Science Foundation of China (No.
21133009, 21273062 and 21203057).
Electronic Supplementary Information Available: Synthesis and characterization of
the ionic liquids, bromide content in the ionic liquids and UV-vis spectra of trans-
cinnamic acid before and after UV light irradiation. See DOI: 10.1039/ x0xx00000x
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Table of content entry
The aggregation behavior of a new class light-reponseive ionic liquids was modulated
efficiently by UV light irradiation in aqueous solutions.