Photorheological response of aqueous wormlike micelles with photocleavable surfactant

Article abstract of DOI:10.1021/la4001939

Recently, we have reported a new cinnamic acid-type photocleavable surfactant, C4-C-N-PEG9 that experiences a photocleavage through UV-induced cyclization in aqueous solution, yielding a coumarin derivative (7-butoxy-2H-chromen-2-one) and an aminated polyoxyethylene compound. Here, we have studied the effects of C4-C-N-PEG9 on the photorheological behavior of viscoelastic wormlike micelles formed by aqueous mixture of nonionic surfactants, polyoxyethylene phytosterol ether (PhyEO₂₀) and tetraoxyethylene dodecyl ether (C₁₂EO₄). The 4.9 wt % PhyEO₂₀/H₂O + 2.4 wt % C₁₂EO₄ solution forms wormlike micelles, and its viscosity is ～10 Pa·s. We have found that the addition of C4-C-N-PEG9 into this viscous, non-Newtonian fluid system decreases the viscosity. Viscosity decreased in parallel to the C4-C-N-PEG9 concentration reaching ～0.003 Pa·s at 2.5 wt % of C4-C-N-PEG9. However, viscosity of the C4-C-N-PEG9 incorporated system increased significantly (～200 times at 1.5 wt % of C4-C-N-PEG9 system) upon UV irradiation. Small-Angle X-ray scattering studies have shown that addition of C4-C-N-PEG9 favors wormlike-to-sphere type transition in the micellar structure. However, UV irradiation in the C4-C-N-PEG9 incorporated system causes one-dimensional micellar growth. Since C4-C-N-PEG9 has relatively bigger headgroup size compared to the C₁₂EO₄, addition of C4-C-N-PEG9 into wormlike micelles reduces the critical packing parameter resulting in the formation of spherical aggregates. UV irradiation induced one-dimensional micellar growth is caused due to photocleavage of the C4-C-N-PEG9 into a less surface-active coumarin derivative and an aminated polyoxyethylene compound, as confirmed by UV-vis spectrometry and HPLC measurements. The hydrophobic coumarin derivative formed after cleavage of C4-C-N-PEG9 goes to the micellar core and is responsible for decreasing the viscosity. However, the hydrophilic aminated polyoxyethylene prefers to reside at the vicinity of headgroup of PhyEO₂₀ reducing the interhead repulsion, increasing the critical packing parameter and the viscosity as well.

Full text of DOI:10.1021/la4001939

Article
pubs.acs.org/Langmuir
Photorheological Response of Aqueous Wormlike Micelles with
Photocleavable Surfactant
Shohei Aikawa,† Rekha Goswami Shrestha,†,* Takashi Ohmori,‡ Yuko Fukukita,‡ Yoji Tezuka,§
Takeshi Endo,† Kanjiro Torigoe,† Koji Tsuchiya,∥ Kazutami Sakamoto,⊥ Kenichi Sakai,† Masahiko Abe,†
and Hideki Sakai†,*
†Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki,
Noda, Chiba 278-8510, Japan
‡Shiseido Research Center (Shin-Yokohama), Shiseido Co. Ltd., 2-2-1 Hayabuchi, Tsuzuki, Yokohama, Kanagawa 224-8558, Japan
§NOF Oleo & Specialty Chemicals Research Laboratory, NOF Co., 3-3 Chidori-cho, Kawasaki, Kanagawa 210-0865, Japan
∥Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601,
Japan
⊥Faculty of Pharmacy, Chiba Institute of Science, 15-8 Shiomi, Choshi, Chiba 288-0025, Japan
ABSTRACT: Recently, we have reported a new cinnamic acid-type
photocleavable surfactant, C4−C−N-PEG9 that experiences a
photocleavage through UV-induced cyclization in aqueous solution,
yielding a coumarin derivative (7-butoxy-2H-chromen-2-one) and an
aminated polyoxyethylene compound. Here, we have studied the
effects of C4−C−N-PEG9 on the photorheological behavior of
viscoelastic wormlike micelles formed by aqueous mixture of
nonionic surfactants, polyoxyethylene phytosterol ether (PhyEO20)
and tetraoxyethylene dodecyl ether (C12EO4). The 4.9 wt %
PhyEO20/H2O + 2.4 wt % C12EO4 solution forms wormlike micelles,
and its viscosity is ∼10 Pa·s. We have found that the addition of
C4−C−N-PEG9 into this viscous, non-Newtonian fluid system
decreases the viscosity. Viscosity decreased in parallel to the C4−C−
N-PEG9 concentration reaching ∼0.003 Pa·s at 2.5 wt % of C4−C−
N-PEG9. However, viscosity of the C4−C−N-PEG9 incorporated system increased significantly (∼200 times at 1.5 wt % of C4−
C−N-PEG9 system) upon UV irradiation. Small-Angle X-ray scattering studies have shown that addition of C4−C−N-PEG9
favors wormlike-to-sphere type transition in the micellar structure. However, UV irradiation in the C4−C−N-PEG9 incorporated
system causes one-dimensional micellar growth. Since C4−C−N-PEG9 has relatively bigger headgroup size compared to the
C12EO4, addition of C4−C−N-PEG9 into wormlike micelles reduces the critical packing parameter resulting in the formation of
spherical aggregates. UV irradiation induced one-dimensional micellar growth is caused due to photocleavage of the C4−C−N-
PEG9 into a less surface-active coumarin derivative and an aminated polyoxyethylene compound, as confirmed by UV−vis
spectrometry and HPLC measurements. The hydrophobic coumarin derivative formed after cleavage of C4−C−N-PEG9 goes to
the micellar core and is responsible for decreasing the viscosity. However, the hydrophilic aminated polyoxyethylene prefers to
reside at the vicinity of headgroup of PhyEO20 reducing the interhead repulsion, increasing the critical packing parameter and the
viscosity as well.
respect to charges, and avoid complex interactions. For practical
applications, stimulus-responsive systems have gained much
attention.3−8Their application as carriers for drug delivery,9−11
nanoreactors for preparing nanosized materials,12 and building
blocks for the fabrication of smart surface coatings13 is highly
recommended. In order to implement wormlike micelles into
actual applications, studying the effects of various stimuli on
rheology is a must. There are reports on reversible changes in
1. INTRODUCTION
Surfactants self-assemble into a variety of microstructures, like
spherical or cylindrical micelles, above the critical micelle
concentration (CMC), and at high concentrations a range of
lyotropic mesophases are formed.1 Under certain conditions,
the spherical micelles can transform into long micelles that
overlap with each other, forming a transient network exhibiting
both viscous and elastic properties, called wormlike micelles.
Highly viscoelastic fluids find a wide range of applications as
household and industrial products.2 It is found that wormlike
micelles formed by nonionic surfactants are preferred over ionic
surfactants as they are environmentally friendly, simple with
Received: January 15, 2013
Revised: April 9, 2013
Published: April 10, 2013
© 2013 American Chemical Society
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Scheme 1. Molecular Structures of Polyoxyethylene Phytosteryl Ether (a) PhyEO20, and Photocleavable Surfactant (b) C4−C−
N-PEG9, and (c) Photocleavage Reaction of C4−C−N-PEG9 into Coumarin Derivative (7-Butoxy-2H-chromen-2-one) and
Animated Polyoxyethylene Parts, Respectively
the viscoelastic properties of wormlike micellar solutions as a
result of the trans−cis photoisomerization of azobenzene-
modified cationic surfactants,14 cinnamic acid, and its
derivatives;15−17 the effect of light on the self-assembly of
aqueous mixtures of sodium dodecyl sulfate and a cationic,
bolaform surfactant containing azobenzene;18 and the photo-
responsive smart surface coatings based on the photo-
isomerization of a spiropyran-modified cationic surfactant
adsorbed at a silica/aqueous solution interface.19 Light-sensitive
surfactants provide interesting opportunities to create stimuli-
responsive giant micellar systems because illumination of a
solution can be easily patterned and modulated, and the
benefits of using light as an external stimulus over pH, thermal,
or electrical stimulation include miniaturization and limited
chemical contamination, promoting the development of green,
sustainable systems.20 There are also reports on the effects of
external stimuli (acid−base chemical reactions, thermal
reactions, photoreactions) on cleavable surfactants.21−30
Recently, we have reported a new photocleavable surfactant,
C4−C−N-PEG9 that undergoes photocleavage through UV-
induced cyclization in aqueous solutions, yielding two func-
tional materials including a coumarin derivative and an
aminated polyoxyethylene compound.31 It is expected that
the cleaved compounds may act as a perfume32 and a
moisturizing agent,33 respectively. To the best of our
knowledge, this photocleavable mechanism has not been
applied earlier in the wormlike micelles studies. In this context,
systematic studies focusing on photoinduced structural
modification have been carried out. Here, we study the effect
of photocleavable surfactant, C4−C−N-PEG9, on the rheo-
logical behavior of environmentally friendly viscoelastic worm-
like micelles formed by aqueous mixture of polyoxyethylene
phytosteryl ether (PhyEO20) and tetraoxyethylene dodecyl
ether (C12EO4).34 The results are supported by rheology, UV−
vis spectroscopy and HPLC, and small angle X-ray scattering
measurements. We expect that this new concept of the
photorheological change with photocleavable surfactant can
be of theoretical and practical importance. It is possible to
design a photocleavable surfactant-based wormlike micellar
system.
2. EXPERIMENTAL SECTION
2.1. Materials. Polyoxyethylene phytosteryl ether, (PhyEO20, high
purity, containing a percentage of isomers with branched structure)
was kindly provided by Nikko Chemicals Co., Japan. Tetraoxyethylene
dodecyl ether (C12EO4, purity >98%) was purchased from Nikko
Chemicals Co., Japan. All chemicals were used as received without
further purification. The water used in this study was deionized with a
Barnstead NANO Pure Diamond UV system and filtered with a
Millipore membrane filter (pore size 0.22 lm). The photocleavable
surfactant C4−C−N-PEG9 was synthesized as previously described.31
Scheme 1 shows the molecular structures of (a) Polyoxyethylene
phytosteryl ether (PhyEO20), (b) C4−C−N-PEG9, and (c) photo-
cleavage of C4−C−N-PEG9, respectively.
2.2. Preparation of Wormlike Micelles and Rheology. In this
study, we focus on wormlike micellar solutions formed by an aqueous
mixture of nonionic surfactants, polyoxyethylene phytosteryl ether
(PhyEO20) and tetraoxyethylene dodecylether (C12EO4), at a
composition of 4.9 wt % PhyEO20/H2O + 2.4 wt % of C12EO4, as
reported by Naito et al.34 This mixture is biodegradable and less toxic.
This composition forms viscoelastic wormlike micelles with the lowest
concentration of surfactants in this system. In addition, if total
concentrations of these surfactants are too high, the viscosity is so high
that the handling of the samples is quite difficult.
Different amounts of C4−C−N-PEG9 were added to this micellar
solution and mixed using a vortex mixer until a homogeneous mixture
was achieved. The effect of UV irradiation as a function of time was
studied on the C4−C−N-PEG9 added samples. The ultraviolet light
irradiation was carried out by placing the 2 mL sample in a quartz cell
(10 ×10 mm), with stirring at room temperature. Then the rheological
measurements were performed after keeping the sample at equilibrium
for 2 to 3 days. It was observed that the rheological parameters were
almost the same when immediately performed. Measurements were
performed in a stress-controlled rheometer, AR-G2 (TA Instruments)
using cone−plate geometries (diameter 40 mm with a cone angle of 2°
0′ 4″). Frequency sweep measurements were performed in the linear
viscoelastic regime of the samples, as previously determined by
dynamic strain sweep measurements.
2.3. UV−vis Spectroscopy and HPLC. For the UV−Vis
spectroscopy and HPLC measurements, light irradiation was carried
out using a San-EI Supercure 203S Hg−Xe lamp (200 W) equipped
with a UV filter. The transmission wavelength region is 260−390 nm;
U-340 at an irradiation intensity of 50 mW cm−2. Ultraviolet light
irradiation was carried out by placing the 2 mL sample in a quartz cell
(10 ×10 mm), with stirring at room temperature. The photocleavable
behavior that occurs in bulk solution was monitored using an Agilent
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Figure 1. (a) Photographs of 4.9 wt % PhyEO20/H2O/2.4 wt % C12EO4 and 4.9 wt % PhyEO20/H2O/2.4 wt % C12EO4 + 1.5 wt % C4−C−N-PEG9
solutions before and after 2 h of UV irradiation, (b) UV−vis absorption spectra of 4.9 wt % PhyEO20/H2O/2.4 wt % C12EO4 + 1.5 wt % C4−C−N-
PEG9 solutions before and after 2 h of UV irradiation, and (c) steady-shear rheology of the corresponding system in parts (a) and (b) at 25 °C.
8453 UV−vis spectrophotometer with a quartz cuvette (0.1 mm path
length). The photocleavable behavior was also characterized using an
Agilent 1200 high-performance liquid chromatography system
(ZORBAX Eclipse XDB C18 column, G1314B VWD variable
wavelength detector, methanol mobile phase) equipped with a JEOL
JMS-T100CS ESI mass spectrometer.
factor P(q) so that SAXS data can be evaluated by indirect Fourier
transformation (IFT) without any problem in the evaluation procedure.
However, in semidilute or concentrated systems, the I(q) deviates
from the ideal particle form factor P(q) due to interactions between
the micelles that are represented by the structure factor S(q). Under
this situation, transformation of I(q) into real-space without separating
out the influence of S(q) leads to a Fourier transform that can hardly
be interpreted because of strong oscillations at higher r-values. This
total distribution function describes the distance distributions of the
overall density fluctuations around the mean value, which are related
to the micellar density distributions, described by the correlation
function γ(r), and to the total correlation function h(r) = [g(r) − 1],
describing the deviation of the particle distribution from the
statistically uncorrelated situation, with g(r) being the radial
distribution function and r the distance between the center of two
micelles.
2.4. Small-Angle X-ray Scattering (SAXS). In order to
investigate the microstructure transitions induced by the addition of
C4−C−N-PEG9 and UV irradiation, SAXS measurements were
performed using a SAXSess camera (Anton Paar, Austria) attached
to a PW3830 laboratory X-ray generator with a long fine focus sealed
glass X-ray tube (Cu −Kα wavelength of 0.154 nm) (PANalytical).
The apparatus was operated at 40 kV and 50 mA. The SAXSess
camera is equipped with a focusing multilayer optic and a block
collimator for an intense and monochromatic primary beam with low
background and a translucent beam stop for the measurement of an
attenuated primary beam at zero scattering vector (q = 0). Samples
were enclosed in a vacuum tight thin quartz capillary with an outer
diameter of 1 mm and thickness of 10 μm. Sample temperature was
controlled with a thermostatted sample holder unit (TCS 120, Anton
Paar). The 2D scattering pattern was collected on an image plate (IP)
detection system Cyclone (Perkin-Elmer, U.S.), and was finally
integrated into one-dimensional scattering curves as a function of the
magnitude of the scattering vector q = (4π/λ)sin(θ/2) using
SAXSQuant software (Anton Paar), where θ is the total scattering
angle, and λ is the wavelength of X-ray. All data were normalized to the
same incident primary beam intensity for the transmission calibration
and were corrected for the background scattering from the capillary
and water. All of the measured I(q) data were corrected for the
background scattering from the capillary and the solvents, and the
absolute scale calibration was made using water as a secondary
standard. The SAXS data were analyzed by the generalized indirect
Fourier transformation (GIFT) method35−38 to determine the pair-
distance distribution function, p(r). The p(r) contains information on
shape, size, internal structure, and inhomogeneity of scattering
elements.39−43 Generally, GIFT relies on a basic equation of one-
component monodisperse globular particle systems, I(q) = nP(q)S(q),
where n is average number density of micelles, P(q) is average form
factor, and S(q) is static structure factor. For the polydisperse systems,
the S(q) is to be replaced with the so-called effective structure factor.
It should be noted that in dilute systems where interparticle
interactions are negligible, I(q) is apparently identical to the form
Theoretically, P(q) is the Fourier transformation of p(r) and can be
described as follows,
∞
sin qr
qr
P(q) = 4π
p(r)
dr
∫
0
(1)
So one needs to calculate the IFT of the experimental P(q) to deduce
the p(r) from the SAXS data. However, for concentrated systems, both
the P(q) and S(q) have to be determined simultaneously within one
procedure taking into account of interparticle interactions. Therefore,
to suppress the influence of interparticle interference scattering on the
evaluation of p(r) an interaction potential model for the structure
factor has to be used during the data analysis. The P(q) is absolutely
model free only a rough information on the maximum dimension of
micelle is required while S(q) is calculated according to the Percus−
Yevick44,45 so it is not model independent. In the present study, we
have selected the averaged structure factor model46,47for polydisperse
hard-spheres and Percus−Yevick approximation is used to solve the
Ornstein−Zernike (OZ) equation for the calculation of S(q).
3. RESULTS AND DISCUSSION
3.1. UV−visible Spectra and Rheology. Figure 1 shows
the visual observation of 4.9 wt % PhyEO20/H2O/2.4 wt %
C12EO4 and 1.5 wt % C4−C−N-PEG9 added 4.9 wt %
PhyEO20/H2O/2.4 wt % C12EO4 solutions before and after UV
irradiation. Visually they are quite different. The solution before
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adding C4−C−N-PEG9 has high viscosity and shows shear
birefringence when viewed under a crossed polarizer; however,
when C4−C−N-PEG9 is added to the solution, the viscosity
drops (samples are water-like that do not show any flow
birefringence). The C4−C−N-PEG9 added system regains
viscosity upon UV irradiation.
Figure 1b shows the UV−vis spectra for 4.9 wt % PhyEO20/
H2O/2.4 wt % C12EO4 + 1.5 wt % C4−C−N-PEG9 solutions
before and after 2 h of UV irradiation corresponding to the
irradiation energy 50 mW/cm2. The sample shows an
absorbance peak at 322 nm before UV. With UV light
irradiation, absorbance at 322 nm decreases (A322 assigned for
functions of frequency) at low frequencies or long time scales.
The G′ and G″ show a crossover at the intermediate frequency
(exhibits viscoelasticity). This behavior resembles that of the
Maxwell model observed for entangled wormlike micelles.34
When C4−C−N-PEG9 is added the crossover is not observed
in the whole frequency range (0.1−100 rad·s−1) measured. The
sample exhibits a purely viscous response over the entire range
of frequencies, a response similar to liquid with short micelles.
Alternatively, after UV irradiation, this solution regains
viscous behavior (G′ < G″) at lower frequencies and elastic
behavior (G′ > G″) at higher frequencies, and a crossover
around intermediate frequency is observed. Thus, dynamic
rheology confirms a UV-induced transition from spherical
micelles to wormlike micelles. It is found that (Figure 2), it is
possible to control the rheology with C4−C−N-PEG9 and UV.
Figure 3a shows the effect of the amount of C4−C−N-PEG9
on the viscosity of the 4.9 wt % PhyEO20/H2O/2.4 wt %
C12EO4. It is found that the η0 decreases gradually with the
continuous addition of C4−C−N-PEG9 and the solution
eventually shows Newtonian flow behavior. It is almost
constant beyond 2.5 wt % of the additive. With UV irradiation
(50 mW/cm2, 2 h), the η0 for any amount of C4−C−N-PEG9
on the other hand switches a Newtonian fluid to a viscous, non-
Newtonian fluid. With 1.5 wt % C4−C−N-PEG9, viscosity of
the solution reduces to 0.01 Pa·s. It should be noted that the
most significant change in η0 before and after UV irradiation is
when 1.5 wt % C4−C−N-PEG9 is added to the system, (200-
fold increase in viscosity from 0.01 Pa·s to 2 Pa·s).
Figure 3, parts b and c, shows the steady-shear rheology and
the corresponding UV−visible spectra for 4.9 wt % PhyEO20/
H2O/2.4 wt % C12EO4 + 1.5 wt % C4−C−N-PEG9 solutions
before and after UV irradiation for various periods of time
corresponding to the irradiation energy 50 mW/cm2,
respectively. The viscosity gradually increases with UV
irradiation time, and the time dependent UV- spectrum
shows a gradual decrease in A322 absorbance peak, assigned
for the C4−C−N-PEG9 and an increased A194 absorbance
peak, originated from the coumarin derivative. It is anticipated
that at the fixed concentration of C4−C−N-PEG9 in the
system, steady increase of viscosity occurs due to the gradual
UV induced photocleavage of C4−C−N-PEG9 into coumarin
derivative and aminated polyoxyethylene compound (decrease
in A322 absorbance peak and increase in A194 absorbance peak
increases with UV irradiation time).
the C4−C−N-PEG9) while absorbance at 194 nm (A194
)
increases.31 The A194 absorption peak originates from the
coumarin derivative formed in the process. It is suggested that
the UV induced increased A194 and decreased A322 peaks are
due to the photocleavage of C4−C−N-PEG9 into the
coumarin derivative and aminated polyoxyethylene compound.
Thus, irradiation will cause the C4−C−N-PEG9 molecule to
absorb light, and this, in turn, can trigger a photocleavage of the
C4−C−N-PEG9 to coumarin derivative and aminated polyoxy-
ethylene compound. The photocleavage reaction of C4−C−N-
PEG9 was also confirmed by LC-MS measurements before and
after UV light irradiation.
The steady shear measurements for 4.9 wt % PhyEO20/H2O
+ 2.4 wt % C12EO4 solution (Figure 1c) show the shear
thinning behavior, η decreases with the increase in shear rate, a
trend commonly observed for solution with wormlike micelle
matrix. It consists of a viscous matrix composed of long
wormlike micelles34 (the zero shear viscosity, η0 ≈ 10 Pa·s).
When photodegradable surfactant, C4−C−N-PEG9 is added to
it the η0 decreases. On UV irradiation, gradually the system
regains the viscosity and non-Newtonian behavior with time
(∼2 h after irradiation), see Figure 1 c.
The dynamic or oscillatory rheological response, which is
one of the sensitive probes of the nanostructure in complex
fluids, has been examined. Figure 2 shows the dynamic
3.2. HPLC and ESI-MS Measurements. HPLC measure-
ments have been performed in order to calculate the degree of
photodegradation of C4−C−N-PEG9 as a function of time and
correlate with the viscosity change. Figure 4a shows the
photocleavage ratio of C4−C−N-PEG9 as a function of
irradiation time. The maximum degree of photocleavage has
been evaluated as 85% (confirmed by HPLC measurements
accompanied by the ESI-MS measurements). About 85% of
photocleavage reaction completed within 30 min and reached
photostationary state. Figure 4b shows the η0 as a function of
photocleavage ratio; the η0 increases with the progress of
photocleavage reaction. The η0 increases abruptly at ∼60%
ratio, suggesting the existence of the critical composition which
induces the abrupt growth of the wormlike micellar network.
The photoinduced cleavage of C4−C−N-PEG9 and ultimately
reduction of the interfacial curvature of the aggregate in the
mixed system seems to be responsible for the increase in the
viscosity.
Figure 2. Variation of elastic modulus (G′, closed symbols), and
viscous modulus (G″, open symbols) as a function of oscillation
frequency (ω) for the sample 4.9 wt % PhyEO20/H2O/2.4 wt %
C12EO4, and 4.9 wt % PhyEO20/H2O/2.4 wt % C12EO4 + 0.5 wt %
C4−C−N-PEG9 before and after UV irradiation at 25 °C.
rheology datavariation of elastic modulus (G′) and viscous
modulus (G″) as a function of oscillation frequency (ω) for the
sample 4.9 wt % PhyEO20/H2O/2.4 wt % C12EO4. The sample
shows a typical viscoelastic response; i.e., the behavior is elastic
(G′ > G″, plateau in G′) at high frequencies or short time
scales, while it is viscous (G″ > G′, both moduli are strong
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Figure 3. (a) Zero shear viscosity of 4.9 wt % PhyEO20/H2O + 2.4 wt % C12EO4 solutions as a function of C4−C−N-PEG9 before and after UV
irradiation, (b) steady-shear rheology and (c) UV−vis absorption spectra of the 4.9 wt % PhyEO20 /H2O/2.4 wt % C12EO4+1.5 wt % C4−C−N-
PEG9 system before and after UV irradiation for various periods of time at 25 °C.
Figure 4. (a) Photocleavable ratio for various periods of UV irradiation time and (b) zero shear viscosity as a function of photocleavage ratio of 4.9
wt % PhyEO20/ H2O/2.4 wt % C12EO4 + 1.5 wt % C4−C−N-PEG9 at 25 °C.
3.3. SAXS Measurements. In order to investigate the
microstructure transitions of phytosterol wormlike micelles
induced by the C4−C−N-PEG9, and upon UV irradiation in
the C4−C−N-PEG9 incorporated system, SAXS measure-
ments were performed on the 4.9 wt % PhyEO20/H2O + 2.4 wt
% C12EO4 system at different concentration of C4−C−N-
PEG9 (0, 1.5, and 2.5%), and on the 4.9 wt % PhyEO20/H2O +
2.4 wt % C12EO4 + 1.5% C4−C−N-PEG9 system after UV
irradiation. All of the measurements were performed at 25 °C
and experimental data were evaluated using the GIFT method
as described in the Experimental Section. Figure 5a shows the
SAXS spectra on absolute scale (symbols) of the 4.9 wt %
PhyEO20/H2O + 2.4 wt % C12EO4 system at different
concentrations of C4−C−N-PEG9 with solid lines representing
GIFT fit to the experimental SAXS data and dashed lines, the
calculated form factors as obtained by the GIFT method. The
results of the GIFT procedure, i.e., the pair-distance
distribution function, p(r), are presented in Figure 5b. Figure
5, parts c and d, shows SAXS spectra and corresponding p(r)
curves of the 1.5% C4−C−N-PEG9 system before and after
UV irradiation, respectively. First we discuss the effect of C4−
C−N-PEG9 on the microstructure of the 4.9 wt % PhyEO20/
H2O + 2.4 wt % C12EO4 system. As can be seen in Figure 5a, all
of the SAXS spectra are typical of core−shell particles
displaying local minimum and maximum in the middle-q
regime. This local minimum (or inner central peak) in the
SAXS spectrum is usually affected by the interparticle
interference and the side maximum represents the particle
form factor. As can be seen in Figure 5a, the inner central peak
(indicated by arrows) and side maximum of SAXS pattern
shifted toward higher-q side upon addition of C4−C−N-PEG9.
Besides, the calculated form factor curve decays differently in
the low-q regime. The low-q slope is roughly “−1” representing
the rodlike structure48−50 of the aggregates in the system
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Figure 5. (a) Normalized X-ray scattered intensities, I(q), for the 4.9 wt % PhyEO20/H2O + 2.4 wt % C12EO4 system at different concentrations of
C4−C−N-PEG9 (0, 1.5, and 2.5%) obtained in absolute scales, (b) the corresponding pair-distance distribution function, p(r), (c) the I(q) curves
for the 4.9 wt % PhyEO20/H2O + 2.4 wt % C12EO4 + 1.5 wt % C4−C−N-PEG9 before and after UV irradiation, and (d) the corresponding p(r)
curves. The solid and broken lines in panels (a) and (c) represent the GIFT fit and the calculated form factor for n particles existing in unit volume,
nP(q), respectively. The downward arrows on the high-r side of panels (b) and (d) represent the maximum dimension, Dmax, of spheroid type
micelles and maximum length, Lmax, of elongated rodlike micelles, respectively. Schematic models show the microstructure transitions.
without C4−C−N-PEG9. Addition of C4−C−N-PEG9
decreased the low-q slope, demonstrating the modulation of
the rodlike structure of aggregates into a globular-type object.
Relatively smaller values of the I(q = 0) in the C4−C−N-PEG9
incorporated systems also indicate smaller size aggregates.
As we have mentioned in the Experimental Section, essential
information on the particle shape and size can be obtained from
the pair-distance distribution function p(r). One can estimate
the maximal dimension (Dmax for sphere and Lmax for rods)
within one particle from the abscissa value where the p(r)
function reaches to zero in the higher-r side. Experimental
SAXS spectra of the 4.9 wt % PhyEO20/H2O + 2.4 wt %
C12EO4 system with different surfactant concentrations of C4−
C−N-PEG9 were therefore transformed from the reciprocal
into the real-space by the GIFT method, which separates out
the disturbing interparticle contribution from the overall
scattering intensity. The shape of the p(r) functions (see
Figure 5b) indicates inhomogeneous (core−shell) rodlike and
globular structures with differences in the core and shell
scattering contrast. The local maximum and minimum in the
lower-r side of the p(r) curves, which originate due to electron
density difference in the core and shell, are the characteristic
features of core−shell type micelles. In the absence of C4−C−
N-PEG9, the shape of p(r) function is characteristic of rodlike
micelles: long tail in the higher-r regime. The maximum length
of the rod, Lmax, is ca. 18.5 nm, which is very interestingly! The
shape of the p(r) functions of the C4−C−N-PEG9
incorporated systems have a characteristic of spheroid type
micelles with maximum dimensions, Dmax, ca. 10.4 and 9.7 nm
for the 1.5 and 2.5 wt % C4−C−N-PEG9, respectively. Thus,
judging from the p(r) function, we have observed a clear rod-
to-sphere microstructure transition induced by the C4−C−N-
PEG9, as illustrated by a schematic model in Figure 5b. These
results obtained from the SAXS measurements well explain the
change in rheological behavior of the phytosterol wormlike
micelles upon addition of C4−C−N-PEG9. As shown in Figure
3a, the η0 of phytosterol womlike micelles decreases by
approximately 4 orders of magnitude upon incorporation of 2.5
wt % C4−C−N-PEG9. This massive decrease in viscosity is
caused by the microstructure transitions. Addition of C4−C−
N-PEG9 destroys the network structures of wormlike micelles
and eventually favors the formation of spherical type micelles.
Next, we discuss the effect of UV irradiation on the
microstructure of the 1.5 wt % C4−C−N-PEG9 incorporated
system. Note that this system consists of spheroid type micelles
having maximum dimension ca. 10.2 nm before UV irradiation
(see Figure 5b). One can clearly see the difference in the SAXS
patterns before and after UV irradiation. The scattering
intensity in the low-q regime (q < 0.6 nm−1) decreased and
the inner central peak shifted slightly toward the forward
direction after UV irradiation. It should be noted that the
decrease in the I(q = 0) value is caused due to poor contrast of
system not due to smaller particle size as the calculated form
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factor decays following approximately q−1 behavior in the low-q
regime indicating rodlike shape of the micelle.
to cylinder micellar transition. The long cylinders are entangled
with each other to form a transient network of wormlike
micelles as the C12EO4 concentration is increased. A mixture of
4.9 wt % PhyEO20/H2O + 2.4 wt % C12EO4 tends to form
wormlike micelles (Figure 6).34 When C4−C−N-PEG9 (with a
large hydrophilic group) is added to the 4.9 wt % PhyEO20/
H2O + 2.4 wt % C12EO4 solution, being a short chain
surfactant, it too tries to reside in the palisade region of
PhyEO20 micelle (competing with C12EO4), ultimately
disturbing the wormlike micellar system. The larger hydrophilic
part of C4−C−N-PEG9 decreases the critical packing
parameter of the surfactant leading back to the spherical
micelles with cone geometry. On UV irradiation, photocleavage
of C4−C−N-PEG9 into the hydrophobic coumarin derivative
(7-butoxy-2H-chromen-2-one) and a hydrophilic aminated
polyoxyethylene moiety occurs. The hydrophobic coumarin
derivative goes into the hydrophobic core of 4.9 wt %
PhyEO20/H2O + 2.4 wt % C12EO4 micelle while the
hydrophilic aminated portion locates itself in the vicinity of
hydrophilic environment. The hydrophobic coumarin deriva-
tive that goes to the core of the micelle plays a role in
decreasing the viscosity of the solution. The N-atom of
aminated polyoxyethylene can bond with the H2O molecule
hydrating the PhyEO20 headgroup or with the bulk H2O near
the PhyEO20 headgroup. The molecule still reduces the
interhead repulsion between PhyEO20 molecules. However,
the net reducing-effect is diminished. The hydrophilic aminated
polyoxyethylene is unlikely to form micelles after the cleavage
of a large hydrophobic part. Therefore, we believe that the
cleaved hydrophilic portion still remains near the hydrophilic
part of PhyEO20 rather than going completely to the bulk. This
again alters the net geometry from a cone to a truncated cone
and thereby induces again the spherical micelles to grow into
flexible cylinders.
The UV induced microstructural transition can best be seen
in the real-space p(r) curves shown in Figure 5d. After UV
irradiation, the p(r) curve decays slowly in the higher-r side,
which is characteristic of elongated scattering object. The
maximum length of elongated micelles after UV irradiation is
ca. 14.5 nm. This demonstrates the fact that UV irradiation in
the C4−C−N-PEG9 incorporated system favors sphere-to-rod
type transition resulting in the formation of long rodlike
micelles, which again strongly supports the results obtained
from the rheometry (viscosity of the sample increased from
0.01 to 2.1 Pa·s upon UV irradiation, see Figure 3a). From
visual observation, we found that the sample of the 4.9 wt %
PhyEO20/H2O/2.4 wt % C12EO4 + 1.5% C4−C−N-PEG9
system flows easily before UV irradiation. However, after 2 h
UV irradiation, the sample did not flow even after the glass vial
was turned upside-down.
The results obtained from the GIFT evaluation of the
scattering data very well support the results derived from
rheological measurements. However, we note that the
maximum length of micelles that we have determined from
the real-space p(r) curves, particularly for the rodlike or
wormlike micelles, may not be the actual length of rods.
Generally, the maximum size that one can detect from
scattering measurements depends on the maximum resolution
of the SAXS measurement (qmin, Dmax = π/qmin) and on the
overall contrast of the system. Nevertheless, with the available
contrast of the systems and sensitivity of our SAXS equipment
(qmin ∼ 0.1 nm−1), we could outline the qualitative information
on microstructure transition induced by the C4−C−N-PEG9
and UV irradiation to explain the rheological behavior.
Figure 6 shows the proposed mechanism for the different
rheological behavior induced by the addition of C4−C−N-
4. CONCLUSIONS
Rheology of viscoelastic wormlike system, 4.9 wt % PhyEO20/
H2O + 2.4 wt % C12EO4 could be modified by incorporating
photocleavable surfactant, C4−C−N-PEG9 into the matrix.
The system loses viscoelasticity with the addition of C4−C−N-
PEG9. The addition of C4−C−N-PEG9 with large headgroup
decreases the Critical Packing Parameter of the micelles. This
leads to a microstructure transition from wormlike to spherical
micelles. The viscoelasticity of the system recovered upon UV
irradiation. UV irradiation induces photocleavage of C4−C−N-
PEG9 (as confirmed by UV−visible spectroscopy and HPLC
measurements) giving hydrophobic coumarin derivative (7-
butoxy-2H-chromen-2-one) and the hydrophilic aminated
polyoxyethylene compound. The net effect of C4−C−N-
PEG9 is minimized by this cleavage. It is anticipated that the
coumarin derivatives are solubilized in the core of the
surfactant. This promotes formation of spherical micelles and
plays a role in reduction of the viscosity. However, the
aminated polyoxyethylene remains in the vicinity of hydrophilic
environment of PhyEO20. This promotes a progressive
reduction of the interfacial curvature of the aggregate in the
mixed system and transforming spherical/short cylinder back to
wormlike micelles. Therefore, the formation of wormlike
micelles is promoted due to the UV-induced decomposition
of C4−C−N-PEG9. A transition of Newtonian to non-
Newtonian fluids has been observed with UV irradiation, the
viscosity increased by 200 times. This viscosity control is far
different from that observed with other photoisomerization
Figure 6. Wormlike micellar structure formed by the 4.9 wt %
PhyEO20/H2O + 2.4 wt % C12EO4 system, and the effect of C4−C−
N-PEG9 and UV irradiation. The PhyEO20 molecule is shown as an
orange head and black tail, the C12EO4 molecule is shown with a blue
head and black tail, while the C4−C−N-PEG9 is shown with a green
head and green tail.
PEG9 into 4.9 wt % PhyEO20/H2O/2.4 wt % C12EO4 and UV
irradiation in C4−C−N-PEG9 added solutions. PhyEO20 forms
spherical micelles with cone geometry at dilute concentration
due to interhead repulsion of its larger polyoxyethylene chain
headgroup.51 When C12EO4 is added to the micellar solution of
PhyEO20, it goes to the palisade layer of the latter. The additive
reduces the effective headgroup area and thus leads to a sphere
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material. The results derived from the rheological measure-
ments are well supported by the small-angle X-ray scattering.
This work presents new way of controlling the rheology of
system using a less expensive type of stimulus, light. We
anticipate that such photocleavable surfactant-based wormlike
micelles will find applications in the formulation of high-
performance chemical products in the field of cosmetics,
personal care, and medicine.
AUTHOR INFORMATION
Corresponding Author
■
*Phone: +81-47-124-1501 ext. 3662; fax: +81-47-121-2439; e-
mail: rekhashrestha3@hotmail.com (R.G.S.); hisakai@rs.noda.
tus.ac.jp (H.S.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
R.G.S. is thankful to Dr. Lok Kumar Shrestha, MANA Scientist
at the World Premier International (WPI) Research Center for
Materials Nanoarchitectonics (MANA) of the National
Institute for Materials Science (NIMS), Japan for the fruitful
discussion.
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