Photoinduced viscosity control of lecithin-based reverse wormlike micellar systems using azobenzene derivatives

Article abstract of DOI:10.1039/c8ra04690e

This report describes the controlled viscosity changes of photoresponsive reverse wormlike micellar systems formed by soybean lecithin (SoyPC), d-ribose, and azobenzene derivatives in decane. UV light irradiation produces a significant (150-fold) decrease in solution viscosity by triggering a structural transformation of the wormlike micelles. Subsequent visible light irradiation leads to recovery of the initial micellar structure and elevated solution viscoelasticity. This dramatic, reversible variation in solution viscosity by light irradiation can be applied to cosmetics, personal care products, and device components.

Full text of DOI:10.1039/c8ra04690e

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Photoinduced viscosity control of lecithin-based
reverse wormlike micellar systems using
azobenzene derivatives†
Cite this: RSC Adv., 2018, 8, 23742
a
a
*
Masaaki Akamatsu,
Kenichi Sakai,
Mayu Shiina,a Rekha Goswami Shrestha,
ab
b
ab
*
Masahiko Abe
and Hideki Sakai
This report describes the controlled viscosity changes of photoresponsive reverse wormlike micellar systems
formed by soybean lecithin (SoyPC), D-ribose, and azobenzene derivatives in decane. UV light irradiation
produces a significant (150-fold) decrease in solution viscosity by triggering a structural transformation of
the wormlike micelles. Subsequent visible light irradiation leads to recovery of the initial micellar structure
and elevated solution viscoelasticity. This dramatic, reversible variation in solution viscosity by light
irradiation can be applied to cosmetics, personal care products, and device components.
Received 1st June 2018
Accepted 24th June 2018
DOI: 10.1039/c8ra04690e
rsc.li/rsc-advances
have applications in the controlled release of perfumes and
drugs. For example, Eastoe and Abbott demonstrated that vesi-
1. Introduction
cles formed by bolaform or gemini surfactants containing
a photoresponsive azobenzene or a stilbene unit transformed
into micelles upon UV irradiation.4,5 We have reported the
photochemical control of micellar structure and release of an oily
substance upon changes in solubilization amounts in a system
based on a cationic azobenzene-type surfactant.6–9
Wormlike micelles are polymer-like molecular assemblies char-
acterized by an elevated solution viscoelasticity due to micellar
entanglement in an aqueous medium.1 The characteristic struc-
ture of wormlike micelles forms when the critical packing
parameter (CPP), which is dened as the ratio of the volume of
the hydrophobic group (v) to the product of the length (l) of this
group and the cross-sectional area (a) of the hydrophilic group
(CPP ¼ v/al), is approximately 1/2. Appropriate packing of
surfactants leads to the formation of wormlike micelles. Aqueous
wormlike micelle solutions have applications in cosmetics,
personal care products, and paints, because of their solubiliza-
tion behaviour and tunable viscoelasticity. On the other hand,
reverse wormlike micelles, which are formed in organic solvents,
extend these applications to conditions that are incompatible
with water (e.g., extreme temperatures and pressures) or can
regulate the drying speed of materials. Most reverse wormlike
micelles are formed by mixtures of biocompatible lecithin and
polar additives, such as water, bile salts, and glycerols.2 The
additives reduce electrostatic repulsions between the lecithin
head groups and induce formation of the wormlike micelles.
Controlled formation of micelles by external stimuli, such as
light or magnetic eld irradiation, a redox reaction, or a temper-
ature or pH change, enables the modulation of solution proper-
ties.3 Light is a promising stimulus, because it is clean and easily
applied with high spatial resolution. Photoresponsive micelles
We also have demonstrated photoinduced viscosity changes of
an wormlike micellar solution in an aqueous medium with
a photoresponsive surfactant10,11 or an additive.12 In related
studies, Zakin, Raghavan and co-workers demonstrated a highly
efficient regulation of heat transfer properties of aqueous solutions
of photoresponsive wormlike micelles.13,14 This discovery opens the
possible application of wormlike micelles as device components. If
photoirradiation can control the formation of reverse wormlike
micelles in non-aqueous media, the approach can be applied in
organic media and thus be used to control heat-transfer capacities
at extreme temperatures and pressures and the drying speed of
paints and related materials. Development of photoresponsive
reverse wormlike micelles is required to achieve this goal. The
photoswitchable reverse wormlike micelles have been developed
with mixtures of biocompatible lecithin and photoresponsive
additives15–17 or synthetic surfactants.18 For instance, Raghavan
et al. accomplished reversible viscosity changes of an organic
solvent containing
a reverse wormlike micellar system of
spiropyran/soybean lecithin (SoyPC)/sodium deoxycholate/cyclo-
hexane.15 Later on, our groups demonstrated larger viscosity
changes in oil using biologically-originated cinnamic acid deriva-
tives as photoswitchable additives,16,17. However, this photoin-
duced viscosity change is unidirectional due to irreversible
photoisomerization of the cinnamic acid derivatives.
aDepartment of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo
University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan
bResearch Institute for Science and Technology, Tokyo University of Science, 2641
Yamazaki, Noda, Chiba 278-8510, Japan
We demonstrate herein dramatic and reversible variations in
solution viscosity of photoresponsive reverse wormlike micelles
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra04690e
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comprising a biocompatible surfactant (SoyPC) and a simple The TLC plates were examined under UV light at 254 and
azobenzene derivative. To achieve this goal, we chose the 365 nm. Flash column chromatography over silica gel (Wakosil
reverse wormlike formed by D-ribose/SoyPC/decane reported by C-200, 64–210 mm) was used for all separations.
Hashizaki et al.19 Addition of D-ribose to a decane solution of
SoyPC forms spherical or ellipsoidal reverse micelles, which
then grow into reverse wormlikes. D-ribose molecules contain
hydroxyl groups that hydrogen bond to the phosphate groups of
SoyPC, which reduces the curvature of the micelles and
promotes the structural transformation. To achieve reversible
photoswitching of the micellar structure, azobenzene deriva-
tives that isomerize between trans and cis forms upon ultraviolet
(UV) and visible light irradiation (Scheme 1a) were added to the
solution of the lecithin-based reverse wormlike micelles. The
effects of photoisomerization and polar substituents on azo-
benzene derivatives on viscosity changes were examined
systematically. The dramatic reversible viscosity changes using
SoyPC and simple azobenzene derivatives in a non-aqueous
medium promises broad application.
2.2. Measurements
1H- and 13C-NMR spectra were measured at 298 K in CDCl3
solutions using a JEOL model JNM-AL300 (300 MHz) spectrom-
eter with Si(CH3)4 as internal standard. Chemical shis (d) and
coupling constants (J) are given in parts per million (ppm) and
Hertz (Hz), respectively. ESI-MS spectra were obtained with
a JASCO model JMS-T100CS spectrometer. UV/vis absorption
spectra were measured using an Agilent 222 UV/vis spectropho-
tometer with a quartz cuvette (1.0 cm path length). Static and
dynamic rheological measurements were carried out with a TA
Instruments AR-G2 unit using cone-plate geometries (40 mm
diameter with a 2ꢀ cone angle). Frequency sweep measurements
were performed in the linear viscoelastic regime of the samples,
as previously determined by dynamic strain sweep measure-
ments. Infrared absorption spectra (at 1 cmꢁ1 resolution) were
recorded with
a JASCO model FT/IR-6100 spectrometer.
2. Experimental
2.1. General procedures
Measurements were performed by the liquid membrane method
using NaCl plates. Optical microscopic images were obtained
with an IMT-2 microscope (Olympus Optical Co., Ltd.). Polarized
optical micrographs were observed through crossed polarizers.
The resulting images were transferred to a computer equipped
with a Moticam 2000 digital camera tted on the eyepiece.
Solvents and reagents were purchased from Tokyo Chemical
Industry Co., Ltd. (Tokyo, Japan) or Wako Pure Chemical Co.
(Osaka, Japan) and were used without further purication. 4-
Butoxyaniline was purchased from Wako Pure Chemical Co.
(Osaka, Japan). L-a-phosphatidylcholine (SoyPC, 95%) was
purchased from Avanti Polar Lipids, Inc. (Alabama, US). This
product has saturated/unsaturated hydrocarbon chains of
several lengths. The chemical structure shown in Scheme 1 is
the major component of SoyPC (95%). D-ribose was purchased
from Wako Pure Chemical Co. (Osaka, Japan). Azobenzene
derivatives (4–7) were purchased from Tokyo Chemical Industry
Co., Ltd. (Tokyo, Japan). All reaction mixtures and fractions
eluted by column chromatography were monitored using thin
layer chromatography (TLC) plates (Merck, Kieselgel 60 F254).
2.3. Preparation of reverse wormlike micelle solutions
SoyPC (105 mM) and D-ribose (55 mM) were dissolved in
methanol. The solvent was removed under reduced pressure,
and the residue was kept in a vacuum desiccator for 3 days to
dry the mixture completely. Decane was added to the resulting
mixture, which was stirred for 1 day to obtain a homogenous
reverse wormlike micelle solution. Azobenzene derivatives (1–7)
were added to the micellar solution, which was heated at 60 ꢀC
to achieve homogeneity and then stirred by a vortex mixer for
1 min (2500 rpm). The micelle solutions containing azobenzene
derivatives were maintained at room temperature for at least 1
day to reach equilibrium.
2.4. UV and visible light irradiation
UV and visible light irradiations were conducted using a 200 W
Hg-Xe lamp (Supercur UVF-203S, San-Ei Electronic). Irradiation
wavelength ranges (UV: 260–390 nm; visible: >420 nm) were
obtained using U-340 and L-42 colour lters supplied by the
Hoya Candeo Optronics Corp. for UV and visible light, respec-
tively. Each solution was irradiated at 100 mW cmꢁ2 in a 1.0 cm
pathlength quartz cuvette until photostationary states were
observed in the UV/vis absorption spectrum.
2.5. Synthesis
Syntheses of azobenzene derivatives (1–3) were carried out with
4-((4-butoxyphenyl)diazenyl)phenol as starting material, which
was prepared according to the literature.20 Derivative 8 was
synthesized by a published procedure.21
Scheme 1 (a) Structural changes of the azobenzene moiety induced
by UV and visible light irradiations. (b) Molecular structures of
synthesized azobenzene derivatives (1–3, 8), azobenzene with and
without polar substituents (4–7), SoyPC, and D-ribose.
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D-ribose (55 mM) in decane forms reverse wormlike micelles.19
The zero-shear viscosity, h0, of the solution was 200 Pa s. Azo-
benzene derivatives (1–3) at a xed concentration of 15 mM
were added to the micelle solution. The viscosity decreased
slightly in comparison with the original solution following
addition of derivatives 2 and 3 (Fig. 1a). On the other hand,
introduction of 1 bearing a shorter alkyl group showed
a signicant decrease in h0, which indicates a transition from
reverse wormlike micelles to shorter ones or rodlike micelles.
To conrm the photoreactivity of the micellar solutions, UV
light was irradiated to the mixtures to induce the trans-to-cis
photoisomerization of 1–3. Aer UV irradiation, the zero-shear
viscosity of all samples decreased. The solution of 2 gave the
greatest decrease in viscosity from 101 to 0.7 Pa s (approxi-
mately a 150-fold change). Steady-state rheology measurements
on the solution of 2 showed a Newtonian region at low shear
rates, where viscosity is constant and independent of the
applied perturbation, and a non-Newtonian region at higher
shear rates, where viscosity is dependent on the applied shear
(Fig. 1b). This so-called shear-thinning behaviour is typical of
wormlike micellar aggregates. Aer UV irradiation, a Newtonian
response was observed over the entire frequency range. Subse-
quent visible light irradiation restored the original viscosity and
shear-thinning, which means that visible light induces refor-
mation of the reverse wormlike micelles by means of the cis-to-
trans photoisomerization. These results demonstrate that a 2/D-
ribose/SoyPC/decane solution of reverse wormlike micelles
undergoes signicant and reversible viscosity changes upon UV
and subsequent visible light irradiation.
2-(4-((4-Butoxyphenyl)diazenyl)phenoxy)ethan-1-ol (1). K2CO3
(1.58 g, 11.5 mmol) was added to a solution mixture of 4-((4-
butoxyphenyl)diazenyl)phenol (1.25 g, 4.58 mmol), 2-bromoe-
thanol (1.15 g, 9.16 mmol), and catalytic amounts of KI in acetone
(10 mL). The resulting mixture was reuxed overnight. The
reaction mixture was allowed to cool to room temperature, and
the solid residue was ltered. The ltrate was dried over anhy-
drous Na2SO4, and the solvent was removed under reduced
pressure. The crude product was recrystallized from ethyl acetate
to obtain pure 1 (0.128 g, 9%) as a brown powder. Synthesis of
compounds 2 and 3 were carried out by same procedure. 1H-NMR
(300 MHz, CDCl3, 25 ꢀC): d ¼ 0.99 (t, 3H, J ¼ 6.0 Hz), 1.45–1.58 (m,
2H), 1.76–1.85 (m, 2H), 3.98–4.06 (m, 4H), 4.16 (t, 2H, J ¼ 6.0 Hz),
6.97–7.03 (m, 4H), 7.85–7.89 (m, 4H) ppm. 13C-NMR (75 MHz,
CDCl3, 25 ꢀC): d ¼ 14.0, 19.4, 31.4, 61.5, 68.2, 69.6, 114.8, 114.9,
124.5, 124.5, 147.0, 147.5, 160.6, 161.4 ppm. ESI-MS: m/z ¼ 337
[M + Na]+, 651 [2M + Na]+.
4-(4-((4-Butoxyphenyl)diazenyl)phenoxy)butan-1-ol (2). 1H-
NMR (300 MHz, CDCl3, 25 ꢀC): d ¼ 0.99 (t, 3H, J ¼ 6.0 Hz),
1.45–1.58 (m, 2H), 1.73–1.85 (m, 4H), 1.88–1.97 (m, 2H), 3.74–
3.76 (s, 2H), 4.01–4.10 (m, 4H), 6.92–7.00 (m, 4H), 7.84–7.88 (m,
4H) ppm. 13C-NMR (75 MHz, CDCl3, 25 ꢀC): d ¼ 14.0, 19.4, 25.9,
29.5, 31.4, 62.6, 68.2, 114.8, 114.8, 124.4, 147.0, 147.2, 161.0,
161.4 ppm. ESI-MS: m/z ¼ 365 [M + Na]+, 707 [2M + Na]+.
6-(4-((4-Butoxyphenyl)diazenyl)phenoxy)hexan-1-ol (3). 1H-
NMR (300 MHz, CDCl3, 25 ꢀC): d ¼ 0.99 (t, 3H, J ¼ 6.0 Hz), 1.38–
1.55 (m, 6H), 1.56–1.66 (m, 2H), 1.75–1.87 (m, 4H), 3.66 (t, 2H, J ¼
6.0 Hz), 4.00–4.05 (m, 4H), 6.96–7.00 (m, 4H), 7.84–7.88 (m,
13
ꢀ
4H) ppm. C-NMR (75 MHz, CDCl3, 25 C): d ¼ 14.0, 19.4, 25.7,
26.0, 29.3, 31.4, 32.8, 63.0, 68.1, 68.3, 114.8, 124.4, 147.0, 147.1,
161.2, 161.3 ppm. ESI-MS: m/z ¼ 393 [M + Na]+, 763 [2M + Na]+.
Dynamic rheology experiments were performed before and
aer irradiation to characterize the structure of the reverse
wormlike micelles. Fig. 1c shows dynamic rheological
responses in elastic modulus (G0) and viscous modulus (G00) for
the micellar solution of 2 as a function of oscillation frequency
(u). The data before irradiation show a typical viscoelastic
response, which is viscous-dominant (G00 > G0) at low frequen-
cies and elastic-dominant (G0 > G00, plateau in G0) at high
frequencies. The intersection of G0 and G00 denes the relaxation
time, sR, of 1.39 s. The constant value of G0 at high frequencies
(87.9 Pa s in Fig. 1c) is called the plateau modulus, G0, which
3. Results and discussion
3.1. Effect of the alkyl chain length of azobenzene
derivatives on photoresponsive solution viscosity
Azobenzene derivatives bearing a polar hydroxylalkyl with
different chain lengths and a butoxy group were synthesized (1–
3). Alkyl chains were introduced to the azobenzene core to
enhance the affinity for lipids. A mixture of SoyPC (105 mM) and
Fig. 1 Zero-shear viscosity, h0, of 15 mM 1–3/55 mM D-ribose/105 mM SoyPC/decane mixtures before and after UV irradiation (a). Steady-state
rheology of a 2/D-ribose/SoyPC/decane mixture before (black) and after UV (blue) and subsequent visible light irradiation (red) (b). Variations in
elastic modulus, G0 (filled circle), and viscous modulus, G00 (open circle), as a function of oscillation frequency, u, of a 2/D-ribose/SoyPC/decane
mixture before (black) and after UV (blue) and subsequent visible light irradiation (red) (c).
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reects the extent of entanglement of wormlike micelles. The
experimental data t with the Maxwell model,22 which conrms
the existence of entangled wormlike micelles. However, devia-
tion of experimental plots of G00 from the theoretical curves is
observed at high frequencies. This behaviour can be explained
by the presence of faster Rouse modes in addition to the slower
reptational relaxations of the Maxwell model.20 UV irradiation
shis the intersection of G0 and G00 to higher frequencies and
produces a shorter relaxation time (sR ¼ 0.268 s), which indi-
cates that relaxation against shear stress occurs within shorter
time scales aer UV irradiation. Moreover, the smaller value of
G0 (15.9 Pa s) corresponds to a decrease in entanglement of the
reverse wormlike micelles. Subsequent irradiation with visible
light leads to recovery of the initial G0 and G00 curves. These
results conrm that photoirradiation produces a reversible
morphological transformation of the reverse wormlike micelles.
3.3. Effect of substituents on azobenzene derivatives to the
photorheological responses
Viscosity data for reverse wormlike micelle solutions with
a series of azobenzene derivatives (4–8) are shown in Fig. 3.
Addition of non- or primary amine-substituted azobenzene
(4 or 5) to a D-ribose/SoyPC/decane mixture produces viscosity
values similar to that of 2. UV irradiation of these samples
induces a decrease in solution viscosity, but with differences
less than that of 2. Addition of azobenzene analogues bearing
a carboxylic or a hydroxyl group (6 or 7) without light irradiation
leads to a large decrease in h0, which suggests that interaction
of strongly polar substituents with the headgroups of SoyPC
disrupts formation of reverse wormlike micelles. Infrared
absorption spectra of decane solutions containing 6 and 7 show
downshis of a band originating from the stretching vibration
of the P]O group in SoyPC (Fig. S2†), which suggests that the
azobenzene derivatives bearing a carboxylic or a hydroxyl group
strongly penetrate into the micelles and changes the micellar
structures.
3.2. Reversibility of changes in solution viscosity with
photoirradiation
Addition of an azobenzene derivative bearing both a hydroxyl
group and a long alkyl group (8) gives a smaller decrease in
solution viscosity than 7, which indicates that the enhanced
hydrophobicity of 8 weakens crucial effect of the phenolic group
on formation of the reverse wormlike micelles. UV irradiation of
the sample causes an approximately 8-fold decrease in h0.
Taking into account the polarity and the position of the
substituents on azobenzene core, the weakly polar alcoholic
group and the less bulky alkyl chain next to the alcoholic group
on 2 maintains formation of reverse wormlike micelles aer the
addition of azobenzene derivative, compared with the phenolic
group on 8.
These results indicate that an azobenzene derivative with
appropriate hydrophobicity is required to maintain viscosity of
the original D-ribose/SoyPC/decane mixture and produce a large
change of solution viscosity upon UV irradiation. The behav-
iours of 1 and 3 with shorter or longer alkyl chain support this
interpretation. It is also found that hydrophobicity of the azo-
benzene analogs governs its solubilization position in the
micellar solutions and impacts of light irradiation on structure
of the micelles.
The continuous viscosity measurements illustrated in Fig. 2a
demonstrate that the dramatic variations in h0 upon UV and
subsequent visible light irradiation are reversible. Photo-
isomerization of azobenzene derivatives was monitored by UV/
vis spectroscopy. As shown in Fig. 2b and S1,† the 350 nm
absorption maximum of trans-azobenzene decreases, and
a broad band at 430 nm originating from cis-azobenzene
appears upon UV irradiation. Deconvolution of the spectrum
shows that approximately 45% of the trans form isomerizes to
the cis form upon UV irradiation. Subsequent visible light
irradiation results in recovery of the peak at 350 nm and returns
all but 9% of the total azobenzene to the original trans cong-
uration. These results demonstrate that the trans–cis photo-
isomerization of azobenzene is consistent with the reversible
changes in solution viscosity.
Fig. 2 Repeatability of zero-shear viscosity changes of a 15 mM 2/
55 mM D-ribose/105 mM SoyPC/decane mixture upon UV and
subsequent visible light irradiation (a). UV/vis absorption spectra of
solutions after (b) UV and (c) subsequent visible light irradiation (the 2/ Fig. 3 Zero-shear viscosity, h0, for 15 mM azobenzene derivatives (2,
D-ribose/SoyPC/decane mixtures were diluted 400-fold with decane 4–8)/55 mM D-ribose/105 mM SoyPC/decane mixtures before and
before the measurements).
after UV irradiation.
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Fig. 4 Photographs, optical micrographs, and polarized optical micrographs (POM) of a 15 mM 2/D-ribose/SoyPC/decane mixture before and
after UV light irradiation (a). Powder X-ray diffraction patterns of trans- and cis-2 (b).
wormlike micelles is retained since 2 is less bulky and has weak
interaction with the head group of SoyPC. The molecular length
3.4. Impact of isomerization of azobenzene derivatives on
reverse wormlike micellar structure
˚
of trans-2 according to the stabilized structure (ca. 22 A)
Fig. 4a shows photographs of a 15 mM 2/55 mM D-ribose/
105 mM SoyPC/decane mixture. Before UV irradiation, the
solution does not ow when the sample cell is tilted, because
entangled wormlike micelles are present. Exposure to UV light
signicantly decreases the solution viscosity causing the sample
to become liquid-like. As shown on the le of Fig. 4, solid
particles are observed in solution before irradiation. The
particles are needle-like and show birefringence by polarized
optical microscopy (Fig. 4a), which suggests that the trans form
of 2 has poor solubility and a part of them exists as a crystalline
solid in decane. The poor solubility of trans-2 in decane is also
observed in Fig. S3† (approximately 70 mM). These particles
disappear from the micrograph aer UV irradiation, indicating
that the cis form of 2 is soluble in decane (Fig. 4a, right). An X-
ray diffraction prole of a solid sample of the trans form of 2
contains peaks, from which the calculated d-spacing corre-
sponds to the characteristic length of p–p stacking23,24 (Fig. 4b).
These peaks disappear aer UV irradiation, indicating that the
cis form of 2 lacks these intermolecular interactions and
exhibits good solubility in the reverse wormlike micellar
solution.
matches one of SoyPC molecule. The low solubility of 2 in the
decane solution also suppresses change in formation of the
reverse wormlike micelles. Irradiation with UV light triggers the
trans-to-cis isomerization and enhances the solubility of the
azobenzene derivative into the micelles. When cis-2 bearing the
bulky azobenzene moiety and mismatched molecular length
˚
(ca. 14 A) with one of SoyPC is incorporated in the reverse
wormlike micelles, the mean volume of the hydrophobic part of
SoyPC is increased and the morphological transformation of the
micelles occurs (Fig. 5, right). Infrared absorption spectra of the
mixture with 2 shows that UV irradiation produces no shi in
the peak position of the P]O stretching vibration indicating
that the hydrogen bond between the SoyPC headgroup and D-
ribose in the hydrophilic region of the micelles remains
(Fig. S4†). These data support incorporation of the cis form of
azobenzene in the hydrophobic rather hydrophilic region of the
micelles. These results demonstrate that variations in solubility
into the micelles formed by 2 produced by light irradiation, and
appropriate hydrophobicity generates signicant, reversible
variations in the solution viscosity of lecithin-based reverse
wormlike micelles.
Based on these results, we propose that the changes in
solution viscosity upon photoirradiation occur by the following
mechanism (Fig. 5). When the azobenzene derivative 2 is added
into a D-ribose/SoyPC/decane mixture, formation of the reverse
4. Conclusions
We have demonstrated that photoirradiation induces signi-
cant and reversible variations in the solution viscosity of reverse
wormlike micelles. Addition of photochromic azobenzene
derivatives bearing butoxy and hydroxybutyl moieties to a D-
ribose/SoyPC/decane mixture forms reverse wormlike micelles.
UV light irradiation induces a 150-fold decrease in solution
viscosity. This change is reversible upon subsequent irradiation
with visible light. By screening azobenzene derivatives with
polar or apolar substituents, the azobenzene analogue having
appropriate hydrophobicity and variations of solubility into the
micelles with photoirradiation produce the largest variations in
solution viscosity. This wormlike micellar solution exhibiting
photo-switchable solution viscosity can be applied to cosmetics,
personal care products, and device components such as a heat-
transfer regulator.
Fig. 5 A plausible mechanism of the morphological variations in
structure of reverse micelles induced by UV and subsequent visible
light irradiation.
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12 H. Sakai, S. Taki, K. Tsuchiya, A. Matsumura, K. Sakai and
M. Abe, Chem. Lett., 2012, 41, 247–248.
Conflicts of interest
13 H. Shi, Y. Wang, B. Fang, X. Y. Talmon, W. Ge,
S. R. Raghavan and J. L. Zakin, Langmuir, 2011, 27, 5806–
5813.
14 H. Shi, W. Ge, H. Oh, S. M. Pattison, J. T. Huggins,
Y. Talmon, D. J. Hart, S. R. Raghavan and J. L. Zakin,
Langmuir, 2013, 29, 102–109.
There are no conicts to declare.
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