Investigation on the aggregation behavior of photo-responsive system composed of 1-hexadecyl-3-methylimidazolium bromide and 2-methoxycinnamic acid

Article abstract of DOI:10.1039/c5ra08164e

A novel fluid system composed of 2-methoxycinnamic acid (trans-OMCA) and 1-hexadecyl-3-methylimidazolium bromide (C₁₆mimBr) in an aqueous solution was investigated. The compounds trans-OMCA and C₁₆mimBr in an aqueous solution can self-assemble and form viscoelastic worm-like micelles. The concentrations of trans-OMCA and C₁₆mimBr have a significant influence on the rheological properties of the system. The samples were characterized by rheological measurements. The structural isomerization of trans-to-cis for trans-OMCA occurred after UV light irradiation. The transformation of the system after UV light irradiation was determined by UV-vis absorption spectroscopy, rheological measurement and cryo-TEM observation. Surface tension measurements were carried out to investigate the role of trans-OMCA and UV light in C₁₆mimBr aqueous solution. Critical aggregation concentration (cac), effectiveness of surface tension reduction (Π_cac), maximum excess surface concentration (Γ_max) and minimum area occupied per surfactant molecule (a_s) were investigated. Critical packing parameter was introduced to express the mechanism of aggregation behavior transition.

Full text of DOI:10.1039/c5ra08164e

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Investigation on Aggregation Behavior of Photo-Responsive
System Composed of 1-Hexadecyl-3-methylimidazolium
Bromide and 2-Methoxycinnamic Acid
Mingyong Du, Caili Dai,* Ang Chen, Xuepeng Wu, Yuyang Li, Yifei
Liu, Weitao Li, Mingwei Zhao
School of Petroleum Engineering, China University of Petroleum (East China),
Qingdao, Shandong, 266580, P. R. China
*
Caili Dai
Email: daicl@upc.edu.cn
Tel: +86-532-86981183 Fax: +86-532-86981161
 Mingwei Zhao Email: zhaomingwei@upc.edu.cn
Tel: +86-532-86981183 Fax: +86-532-86981161

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Abstract
A novel fluid system composed of 2-methoxycinnamic acid (trans-OMCA) and 1-
hexadecyl-3-methylimidazolium bromide (C16mimBr) in aqueous solution was
investigated. The compounds trans-OMCAand C16mimBr in aqueous solution can self-
assemble and form viscoelastic wormlike micelles. Concentration of trans-OMCA and
C16mimBr has significant influence on its rheological properties. The samples were
characterized by rheological measurements. Structural isomerization of trans-to-cis of
trans-OMCA would come into existence after UV light irradiation. Transformation of
the system after UV light irradiation was determined by UV-vis absorption spectra,
rheological measurement and cryo-TEM observation. Surface tension measurements
were carried out to investigate the role of trans-OMCA and UV light of C16mimBr
aqueous solution. Critical aggregation concentration (cac), effectiveness of surface
tension reduction (Πcac), maximum excess surface concentration (Гmax) and minimum
area occupied per surfactant molecule (as) were obtained. Critical packing parameter
was introduced to express the mechanism of aggregation behavior transition.
Keywords: wormlike micelles; rheological property; isomerization; UV light
irradiation, surface tension, critical packing parameter

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1. Introduction
Surfactants could aggregate into highly organized structures, for example, micelles,
fibers, vesicles, and lamellar phases.1-4 As one of the most important structures,
wormlike micelles have received considerable attention because of their particular
micellar structures and unique rheological response.5-9 These micelles with flexible and
elongated characters can easily entangle into a dynamic reversible network structure.
The structures can break and reform, in this way, they are usually called “living” or
“equilibrium” polymers.10-12 Wormlike micelles have extensive application fields and
offer great potential in numerous aspects because of their fascinating properties, for
example, rheology control, drag reduction agent, heat transfer, drug delivery, and
enhanced oil recovery.13-15
Scientists have already focused on craft adaptable materials inspired by nature.16-18
The structure or functionality of these materials can dynamically alter once stimulated
by these factors, for example, light, temperature, electric, and pH.19-27 Among these
systems formed by the environment stimulated materials, smart wormlike micelles are
a particular area of highly promising development.28 The smart wormlike systems
usually rely on small amphiphilic molecules and their self-assembly. The structural
change may lead to macroscopic characteristics variations, such as viscosity and
elasticity. Smart wormlike micelles have attracted considerable interest due to their
promising characteristics, and have many technological application.27,29-31 Compared
with other factors, such as pH, salinity, redox reagent, and temperature, light plays a
vital important role and have a set of significant advantages.32-34 This can be operated
in a clean environment without any other additional chemicals, avoiding any changes
of thermodynamic conditions. Besides, light is a reliable and cheap energy resource,
and can be performed at an accurate distance, which is precious in research field, such
as nanoscience, nanotechnology, and medical application.35,36
Therefore, light has always been used as a stimuli by exploiting trans-to-cis
isomerization in wormlike micelles.34,37-40 The investigation of light-responsive
wormlike micelles was first reported in 1980s.41,42 There are mainly two ways forming

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light-responsive wormlike micelles: adding certain light-sensitive additives into surface
active ionic liquid solutions or introducing functional groups into the molecules.27,43-45
Light-responsive wormlike micelles always take advantage of some light-induced
trans-to-cis isomerization of additives containing certain functional groups or
surfactants.46,47 Obviously, these changes could affect the aggregation behavior of the
molecules inside the aggregates, thus properties of wormlike micelles would alter either.
In this paper, we studied the aggregation behavior of 2-methoxycinnamic acid (trans-
OMCA) and 1-hexadecyl-3-methylimidazolium bromide (C16mimBr) in aqueous
solution, and transition of micelles induced by UV irradiation was observed. Structures
of C16mimBr and trans-OMCA were shown in Scheme 1. We just want to construct a
photo-responsive system with the help of the surfactant and the photo-isomerized
molecule. In the present work, the system is composed of C16mimBr and trans-OMCA,
which contains aromatic rings in their molecules. The complex interactions exist in the
system, such as electrostatic interactions, hydrophobic interactions and π-π stacking
interactions, which may result in the rich phase behaviors. This inspires us to do this
work. We expect that this work will help to better understand the photo-responsive
system and expand their application.
2. Experimental section
2.1 Materials.
The compounds 1-methylimidazole (99%), 1-bromohexadecane (97%),
dichloromethane (99.5%), tetrahydrofuran (THF) (99%), and 2-methoxycinnamic acid
(trans-OMCA) (98%) were purchased from Aladdin Chemical Reagent Co., Ltd.
(Shanghai, China). Ultrapure deionized water was made from Eree water purification
systems. Solutions contain trans-OMCA were prepared with a slight excess of base
(NaOH).
Synthesis of 1-hexadecyl-3-methylimidazolium bromide (C16mimBr)
C16mimBr was synthesized according to a paper published before.48 The solution of 1-
bromohexadecane in dichloromethane was added to 1-methylimidazole dissolved in
dichloromethane. The mixture was stirred under a nitrogen atmosphere for 48 hours at

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75–80 °C. Then the product was cooled to room temperature and dichloromethane was
evaporated. The product was recrystallized from THF at least three times and dried
1
under vacuum for 48 hours at 50 °C. The product was characterized by H-NMR
1
spectroscopy (400 MHz) using D2O as solvent. The H-NMR result is recorded as
following:
C16mimBr δH: 9.03 (s, 1H), 7.37 (t, 2H), 4.00 (t, 2H), 3.72 (s, 3H), 1.68 (s, 2H), 1.13
(m, 26H), 0.61 (t, 3H).
2.2 Characterization
UV irradiation
Samples were irradiated by a CHF-XM500-500W ultrahigh pressure short arc mercury
lamp with a 365nm filter for two hours. The experiment was carried out in a water bath
of 25 °C under stirring.
UV-vis spectroscopy measurement
The UV-vis spectroscopy measurements of the samples before and after UV irradiation
were employed on a UNIC-2802 UV-vis spectrophotometer at room temperature. In
this section, water was used as the solution.
Rheology measurements
An AR2000 rheometer (TA Instruments, New Castle, DE) was employed to investigate
rheology properties of the samples. Samples were run on a cone and plate geometry
(40mm diameter, 2° cone angle). Dynamic frequency sweep measurement was
conducted in the linear viscoelastic region. The region was determined from the
dynamic strain sweep measurement. The shearing rate ranges from 0.001 to 1000s-1.
The zero-shear viscosity (η0) was obtained from viscosity curve to the vertical
coordinates. All experiments were conducted at 25 °C .
Cryo-TEM
Loading 5μL solution onto a TEM carbon grid using a micropipette. The TEM carbon
grid was blotted with two pieces of filter paper in order to form a thin film suspended
on the mesh hole. About 5 seconds later, samples were plunged into a reservoir of liquid

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enhance at -165 °C . The samples were stored in liquid nitrogen till they were transferred
to a cryogenic sample holder (Gatan 626) and examined with a JEOL JEM-1400 TEM
(120KV) at -174 °C . The phase contrast was enhanced by underforce. Images were
recorded on a Gatanmultiscan CCD and processed with a Digital Micrograph. The cryo-
TEM observations were performed in China University of Petroleum (Eastern China).
Surface tension measurements
Surface tension measurements were carried out by a model JYW-200B surface
tensiometer (Chengde Dahua Testing Instrument Co., Ltd. Hebei, China). The
temperature was kept at 25 °C with the help of a thermostatic bath. Surface tension was
measured through a ring method and all tests were repeated at least twice until the
results were repeatable.
3. Results and discussion
3.1 Phase behavior of C16mimBr/trans-OMCA system
In order to investigate the phase behavior of C16mimBr/trans-OMCA system, different
amounts of trans-OMCA were added into a series of C16mimBr solutions with a
constant concentration. Figure 1 shows the phase diagram of C16mimBr/trans-OMCA
system at 25 °C . Phase I and phase II are divided by a solid line, these regions behave
different aggregation behaviors. Just as shown in this figure, samples in phase I are
clear and transparent with no viscoelasticity. Specifically, when C16mimBr
concentration is lower than 80 mM, the liquid is no viscoelasticity liquid even the
concentration of trans-OMCA is up to 100 mM. However, with the addition of trans-
OMCA in certain C16mimBr aqueous solutions, changes can be observed, and the
sample turns into high viscoelastic solution. Just as the figure shown, the fluid in the
bottle cannot flow even the bottle was inverted. It is predicted that wormlike micelles
are formed in this region, which will be proved in the following sections.
3.2 Rheological properties of trans-OMCA and C16mimBr system
3.2.1 Effect of trans-OMCA concentration on rheology properties
In order to investigate the effect of trans-OMCAconcentration on the micelle formation

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of C16mimBr/trans-OMCA system at 25 °C, samples were prepared by adding different
amount of trans-OMCA to a fixed C16mimBr solution (150 mM). Figure 2a shows the
steady-shear rate viscosity curves of these systems. The viscosity of the system is very
49,50
low and displays a Newtonian behavior
under low trans-OMCA concentrations.
However, changes appear when certain amount of trans-OMCA is added. The fluid
exhibits high viscosity at low shear rates and the solution displays a Newtonian property.
Nevertheless, viscosity falls while the shear rate raises, and shear-thinning behavior
presents. High viscosity in the Newtonian plateau can be attributed to the presents of
entangled networks of wormlike micelles and perhaps this is a symbol of long wormlike
micelles, which will be studied in the following section. Many reports have shown that
the entangled network is aligned as the shear rate increases.51 But the fluid cannot reach
an equilibrium condition above a critical shear rate and viscosity falls at this time.
Zero-shear viscosity (η0) can be obtained according to Carreau model50, and it is the
value of viscosity at the plateau in Figure 2a. Figure 2b presents the curve of η0 against
trans-OMCA concentration for this system. In this figure we can tell that η0 increases
with trans-OMCA concentration and reaches a peak, then decreases. This phenomenon
is similar to the behavior of some ionic surfactant and salt aqueous systems.52 The
increase of η0 may be a symbol of micellar length growth. Addition of trans-OMCA
can stimulate the electrostatic repulsions among the headgroups of the surfactant
molecules. Consequently, the micellar length increases and micelles begin to overlap
and entangled network formed. It has been considered that the decrease of η0 is a result
of branched wormlike micelles.53 The branched structures tend to slide along the
micelles and thus the viscosity will reduce.
Viscoelastic properties of the wormlike micellar samples were studied by frequency
sweep measurement. The dynamic rheological behavior for the C16mimBr/trans-
OMCA system was carried out as a function of trans-OMCA concentration at a certain
C16mimBr concentration, which is shown in Figure 3a. The lines show relationship of
elastic modulus (G’) and viscous modulus (G’’) versus oscillation frequency (ω). At
low ω, G’ is smaller than G” and the sample shows viscous behavior. Elastic behavior
presents at high ω, G’ is higher than G’’ and G’ shows a plateau value at that time. The

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samples show viscoelastic property, which indicates the formation of rigid network of
entangled wormlike micelles. A crossover point existed according to the lines, the
crossover frequency is called ωco and the crossover modulus corresponds to Gco. Just as
Figure 5a shows, ωco moves backward and then forward as trans-OMCA concentration
increase from 40 mM to 60 mM. Ahigher ωco value means poorer viscoelastic behavior.
The values of G’ and G” are fitted as equation (1) and equation (2) below.54
2
(
)
휔휏
퐺′ = 퐺0 [
(1)
2
(
) ]
1+ 휔휏
휔휏
퐺" = 퐺0 [
(2)
2
(
) ]
1+ 휔휏
Where G0 stands for the plateau value of shear modulus and τ stands for the stress
relaxation time. G0 and τ can be calculated from equation (3) and equation (4).
퐺0 = 2퐺푐표
(3)
(4)
1
휏 = 휔
푐표
To better show whether the Maxwell model fits the data, Cole-Cole plots are
commonly investigated. The Cole-Cole plot is a curve of G” as a function of G’. It can
be obtained from equation (5).
2
2
퐺
퐺
0
0
퐺“ + (퐺′ − ) = ( 2 )
(5)
2
The Cole-Cole plots are shown in Figure 3b. It can be seen clearly that at low
frequency, all the experimental points follow the Maxwell fluid, which means that the
solution behaves as an ideal Maxwell material within the lower shear frequency range.
It is a proof that wormlike micelles are formed in this situation. However, when trans-
OMCA concentration is lower than 30 mM, the system has no viscoelastic response.
3.2.2 Effect of surfactant concentration on rheology property
It can be seen clearly that concentration of trans-OMCA has great influence on the
viscosity of C16mimBr/trans-OMCA system. The impact of surfactant (C16mimBr)
concentration on this system is studied. The steady state curves for different surfactant
concentration solutions are shown in Figure 4a. It can be seen from the figure that

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samples are Newtonian fluids at low concentrations, the viscosity of which is low and
could not be affected by the shear rate. However, when C16mimBr concentration is
higher than 80 mM, these samples are non-Newtonian fluids and exhibiting a high
constant viscosity at low shear rate and shear thinning behavior at high shear rates,
which is a typical symbol of wormlike micelles.55 Figure 4b shows the curve of η0
obtained from Figure 4a. According to this figure, the value of η0 increases with the
addition of C16mimBr. Low viscosity at low C16mimBr concentrations represents
spherical or short rodlike micelles and the addition of C16mimBr may favor the growth
of micelles.56
3.3 UV-vis light irradiation effect on C16mimBr/trans-OMCA system
3.3.1 UV-vis absorption spectra of C16mimBr/trans-OMCA system induced by UV
irradiation
Molecules of trans-OMCA and cis-OMCA obtain different wavelength absorptions in
ultraviolet region, the structural transition can be detected by UV-vis spectra. In this
way, absorbance spectra of trans-OMCA before and after UV-irradiation were obtained
to prove the trans-to-cis transition in Figure 5. Figure 5a shows that, before irradiation,
trans-OMCA has absorbance peaks at 269 and 312nm. However, UV light irradiation
results to blue shifts to 255 and 292nm, significant reduction of the intensity is also
presented. As shown in published papers, cis-isomer has higher steric hindrance and
lower conjugation degree, indicating that the maximum absorption wavelength and the
value of intensity are lower than that of trans-isomer.27 This blue shift is a proof of
trans-to-cis transition. Besides, the UV irradiated sample was irradiated by visible light
for 24 hours, there is no change in the absorbance spectra, just the same as the black
line in Figure 5a. This phenomenon is in accordance with previous published papers.42
3.3.2 Rheological properties of C16mimBr/trans-OMCA system after UV irradiation
Then, we studied the effect of UV light irradiation on the system. After irradiated for 2
hours, the sample changed to flowing fluid with low viscosity from viscoelastic fluid.
Figure 6a shows the steady shear rheological curve of a 150 mM C16mimBr and 50 mM

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trans-OMCA solution after UV irradiation. It can be found that, the solution is
Newtonian fluid. The value of η0 decreases sharply compared with the value of 150
mM C16mimBr and 50 mM trans-OMCA solution before irradiation just shown in
Figure 2a. This interesting phenomenon indicates the length of wormlike micelles
decreases and they even turn into spherical micelles. Figure 6b shows dynamic rheology
of this sample. It can be clearly seen that G”>G’ in all frequency regions without a
crossover, and this is a symbol of viscous fluid. We can conclude that UV irradiation
could induce the viscoelastic fluid to a thin, viscous one, and the length of wormlike
micelles may decreases, which could be ascribed to the phase transition from wormlike
micelles to rodlike or spherical micelles.
3.3.3 Cryo-TEM investigation on C16mimBr/trans-OMCA system
Cryo-TEM technique was further employed to confirm the effect of UV irradiation for
this system. Cryo-TEM images before and after 120 min of UV irradiation are shown
in Figure 7. As shown in Figure 7a, a typical wormlike micelle network could be
observed, and the network is formed by numerous of wormlike micelles. After UV
irradiation, just as Figure 7b shows, wormlike micellar structures are damaged and
cannot be detected anymore. The change of wormlike micelles structure could be
attributed to the trans-to-cis isomerization induced by UV irradiation. This
phenomenon is in accordance with the rheological results of the trans-OMCA (50 mM)
+ C16mimBr (150 mM) before and after UV irradiation.
3.3.4 Surface tension measurements
Surface tension measurements were carried out to evaluate the aggregation behavior of
the aqueous solutions. Aqueous solution of trans-OMCAwas irradiated by UV light for
120 min before the experiment and it turned into cis form. Figure 8 shows the surface
tension (γ) of C16mimBr aqueous solution, C16mimBr/trans-OMCA aqueous solution
and the C16mimBr/cis-OMCA aqueous solution. All experiments were carried out at
25 °C. We can tell that surface tensions of all the three solutions decrease with the
addition of surfactant before reaching a plateau at certain C16mimBr concentrations.
The critical aggregation concentration (cac) obtained from this figure are listed in Table

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1. The C16mimBr/trans-OMCA solution and C16mimBr/cis-OMCAsolution have lower
cac values. Both trans-OMCAand cis-OMCAmolecules have opposite charges against
surfactant headgroups. They can compress the electric double layer surrounding the
micelles and finally reduce the electrostatic repulsion effect among the headgroups.
Finally cac values decrease. The effectiveness of surface tension reduction (Πcac)5 can
be calculated from the following equation:
훱푐푎푐 = 훾0 − 훾푐푚푐
(6)
Where γ0 is the surface tension of pure solvent and γcac is the surface tension of the
sample at the cac. Πcac shows the equilibrium surface tension reduction caused by the
introduction of these molecules. The values in Table 1 shows Πcac increases in the order
C16mimBr < cis-C16mimBr/trans-OMCA < C16mimBr/trans-OMCA.
The maximum excess surface tension (Гmax) and the minimum area occupied per
surfactant molecule (as) are introduced to describe the aggregation behavior in aqueous
solution clearly.5 They are obtained from the Gibbs adsorption isotherm:
1
푑훾
훤
= −
(
)
(7)
(8)
푚푎푥
2푅푇 푑ln퐶
푇
1
푎푠 = 푁 훤
× 1023
퐴 푚푎푥
where R is the gas constant (8.314 J·mol−1·K −1), T is the absolute temperature, NA is
Avogadro’s number (6.022 × 1023 mol−1). Just as Table 1 indicates, the C16mimBr/trans-
OMCA system obtains the highest Гmax value and the lowest as value. The C16mimBr
headgroups can adsorb the anionic counterion in trans-OMCA, which makes the
surfactant molecules packing more closely. However, cis-OMCA is more hydrophilic
than trans-OMCA and less molecules are absorbed27, this is the reason why the
C16mimBr/cis-OMCA system is different from the C16mimBr/trans-OMCA system.
3.4 Mechanism of micelle transition induced by UV irradiation
The mechanism of micelle transition can be explained by the theory of molecular
packing parameter.57
휐
0
푃 = 푎 푙
(9)
푠 0
Where υ0 is the hydrocarbon core volume of a surfactant, as is the equilibrium area per

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molecule at the surfactant-water interface, l0 is the length of hydrocarbon chain. The
value of P means the critical packing parameter, which can be used to determine the
shape of micelles. The critical condition for the formation of different micelles are
P=1/3, P=1/2, and P=1 respectively. When P<1/3, spherical micelles can be observed;
when 1/3<P<1/2, rodlike or wormlike micelles are obtained; when 1/2<P<1, vesicles
exists; when P≈1 planar bilayers are observed; when P>1, inverted micelles are
obtained.
In our system, the value of υ0 and l0 can be caculated by the experimental fomular
introduced by Tanford.58
휐0 = 27.4 + 26.9푁푐
(10)
(11)
푙0 = 1.5 + 1.265푁푐
Where Nc is the carbon atom number of alkyl chain.
According to fomular (10) and (11), the value of υ0 and l0 for C16mimBr are 457.8 Å3
and 21.7 Å individually. The value of as has been calculated and shown in Table 1. As
a result, the critical packing parameter can be calculated, and the value is 0.261. It is
apparent that spherical micelles can be observed in C16mimBr aqueous solution and no
wormlike micelles would appear theoretically, which is in correspondence with the
experimental results. We can calculate the value of P for the other two systems and the
result are shown in Table 1, they are 0.420 and 0.379 respectively, and in the extent of
1/3 to 1/2, where rodlike micelles and wormlike micelles can be detected. Once
introduced, the compensation ion with opposite charge in trans-OMCA tends to insert
into the spherical micelles, and the surface charges can be shielded because of
electrostatic repulsion. This will lead to the reduction of as, and the value of P increases
at the same time. As a result, wormlike micelles appear with the addition of certain
amount of trans-OMCA. After UV irradiation, trans-OMCA would transform to cis-
OMCA. As, reported before, cis-OMCA is more hydrophobic than trans-OMCA. The
different hydrophobicity is attributed to the net dipole moment. The dipoles tend to add
up in the cis-OMCA, however, they will cancel each other in the trans-OMCA.27 Less
cis-OMCA could combine with C16mimBr because of the hydrophobic interaction and

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steric hindrance. Thus the value of as increases and P decreases, therefore, spherical or
rodlike micelles will appear after UV irradiation. This process can be clearly seen in
Figure 9.
4. Conclusions
In this paper, we have investigated the rheological behavior and UV light influence of
C16mimBr/trans-OMCA system in aqueous solution. Rheological measurements were
used to observe the influence of trans-OMCA and C16mimBr concentration. UV-vis
absorption spectra, rheological measurement and cryo-TEM observation were carried
out to investigate the influence of UV irradiation on the structural change of trans-
OMCA and the micelle transition from wormlike micelles to spherical or rodlike
micelles. Surface tension measurements were employed to investigate the effect of
trans-OMCA and UV light irradiation on the micelle formation process. Critical
packing parameter theory was used to discover the mechanism of micelle transition.
Acknowledgements
The work was supported by the National Science Fund for Distinguished Young
Scholars (51425406), National Natural Science Foundation of China (51174221).
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Table Captions:
Table 1: surface properties of C16mimBr, C16mimBr/trans-OMCA and C16mimBr/cis-
OMCA systems at 25 °C (trans-OMCA and cis-OMCA concentration are 50 mM
individually)

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Scheme Captions
Scheme 1: Chemical structure of C16mimBr (a) and OMCA (b)

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Figure Captions:
Figure 1: Phase diagram of C16mimBr and trans-OMCA system at 25 °C
Figure 2: Viscosity measurement as a function of trans-OMCA concentration (20-100
mM) at a fixed C16mimBr solution (150mM) at 25 °C. (a), Viscosity versus shear rate
curves; (b), Zero-shear viscosity versus trans-OMCA concentration
Figure 3: (a) Dynamic frequency sweep for different trans-OMCA concentration (40
mM, 50 mM, 60 mM) in a 150 mM C16mimBr solution at 25 °C; (b) cole-cole plots of
this system
Figure 4: (a) Viscosity versus shear rate curves measurement as a function of
C16mimBr (40-200 mM) concentration at a fixed trans-OMCA solution (50mM) at
25 °C; (b) Zero-shear viscosity versus trans-OMCA concentration
Figure 5: UV-vis spectra before and after UV irradiation for 2 hours. (a) trans-OMCA
(50 mM); (b) trans-OMCA (50 mM) + C16mimBr (50 mM). All samples were diluted
100 times before investigation.
Figure 6: Typical rheology results of a 150 mM C16mimBr and 50 mM trans-OMCA
solution after UV irradiation for 2 hours at 25 °C: (a) viscosity versus shear rate; (b)
dynamic frequency sweep
Figure 7: Cryo-TEM images of (a) 150 mM C16mimBr + 50 mM trans-OMCA before
UV irradiation and (b) after UV irradiation
Figure 8: Surface tension (γ) of C16mimBr aqueous solution, C16mimBr/trans-OMCA
(50mM) aqueous solution and the C16mimBr/cis-OMCA (50mM) aqueous solution at
25 °C

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Figure 9: Mechanism of the aggregation behavior for the photo-responsive system

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Table 1
cac
Γcac
Πcmc
Γmax
as
P
(μmol/m2)
(Å2)
(mM)
(mN/m)
(mN/m)
C16mimBr
0.566
0.010
0.019
38.98
37.37
38.60
33.05
2.06
3.31
2.99
80.7
50.2
55.6
0.261
0.420
0.379
C16mimBr/trans
34.63
33.39
-OMCA
C16mimBr/cis-
OMCA

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Scheme 1

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Figure 1

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Figure 2

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Figure 3