Photo-induced phase transition from multilamellar vesicles to wormlike micelles

Article abstract of DOI:10.1039/c1sm05949a

A novel complex fluid system of tetradecyldimethylamine oxide (C ₁₄DMAO) mixed with para-coumaric acid (PCA) in water was investigated. The phase behavior was determined in detail; one observed sequentially the L₁-phase (micelles), two-phase L₁/L _α, L_α-phase (vesicles), and L _α/precipitates with increasing PCA concentration at constant concentration of C₁₄DMAO. The samples in the regions of micelles and vesicles were characterized by rheological measurements and cryo-TEM observations. Fascinatingly, we found a phase transition from vesicles to wormlike micelles performed via UV-light irradiation. The properties and structures after UV-light irradiation were determined by rheological measurements and cryo-TEM observations to demonstrate the phase transition. To the best of our knowledge, this transition is rarely reported, which could provide a better understanding of the phase transition via light irradiation and find potential applications in drug delivery, biochemistry, and the oilfield industry.

Full text of DOI:10.1039/c1sm05949a

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PAPER
Photo-induced phase transition from multilamellar vesicles to wormlike
micelles
b
b
ab
Dong Wang,ac Renhao Dong, Pangfeng Long and Jingcheng Hao
*
Received 22nd May 2011, Accepted 22nd August 2011
DOI: 10.1039/c1sm05949a
A novel complex fluid system of tetradecyldimethylamine oxide (C14DMAO) mixed with para-
coumaric acid (PCA) in water was investigated. The phase behavior was determined in detail; one
observed sequentially the L1-phase (micelles), two-phase L1/La, La-phase (vesicles), and La/precipitates
with increasing PCA concentration at constant concentration of C14DMAO. The samples in the
regions of micelles and vesicles were characterized by rheological measurements and cryo-TEM
observations. Fascinatingly, we found a phase transition from vesicles to wormlike micelles performed
via UV-light irradiation. The properties and structures after UV-light irradiation were determined by
rheological measurements and cryo-TEM observations to demonstrate the phase transition. To the best
of our knowledge, this transition is rarely reported, which could provide a better understanding of the
phase transition via light irradiation and find potential applications in drug delivery, biochemistry, and
the oilfield industry.
Light irradiation has been introduced to surfactant sciences
Introduction
for many years, because it has more advantages than other
external stimuli such as pH change, salinity effect, temperature,
etc. Firstly, no additives are added to samples. Secondly, the
simple operation is helpful to accelerate experimentation.
Thirdly, it is easy to get light to be widely applied in various
fields. In the past decade, many scientists have dedicated them-
selves to investigating phase transitions induced via light. For
example, Kitamura et al. found that the formation of columnar
aggregates and further hierarchically organized fibrous assembly
could be controlled by photoswitching the peripheral substitu-
ents.5 Abe et al. used an azobenzene modified cationic surfactant,
which exhibits reversible trans/cis photoisomerization, as
a ‘‘photoswitchable’’ agent to control viscosity variations.6
Whitten et al. described the supramolecular aggregates of azo-
benzene-contained phospholipids and the related compounds in
bilayer assemblies and other micro-heterogeneous media.7
Raghavan et al. observed a new class of photorheological fluids
that undergo a transition from wormlike micelles to short length
wormlike micelles.8 Earlier than 2009, they discussed another
system which underwent a contrary transition.9 Zhang et al.
made use of a-CD and azobenzene-containing surfactant to
achieve aggregation transition via UV-light and visible-light.10
They also reported a UV-light controlled system consisting
of a photoresponsive malachite green molecule (MGCB) and
a pyrene with a surfactant (PYN).11 Huang et al. constructed
Self assembled aggregates formed by surfactants in solution have
attracted widespread interest, largely because of being simple
model systems for biological membranes and their possible
applications in various fields of colloids, bionics, and soft matter,
especially as templates for the synthesis of materials. Various
aggregates such as micelles, vesicles, bilayers, etc. have been
studied by many scientists in terms of phase behavior, charac-
teristic, and applications. As good candidates for drug delivery
and mimicking biological membranes, vesicles formed by natural
and synthetic surfactants have been researched in detail for more
than half a century.1 In particular, the phase transition of vesicles
is considered as a very important process in pharmaceuticals and
cell engineering.2 Phase transition can be induced by changing
the proportion or concentration of components, changing equi-
librium temperature, varying the hydrophilic–lipophilic balance
(HLB) value the by addition of cosurfactants, or altering the
ionic strength by the addition of salts, etc. Wormlike micelles
have found numerous applications in different areas such as in
cosmetic products and in the oilfield industry,3 which could be
because they form transient networks and exhibit viscoelastic
properties.4
aState Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical
Physics, Chinese Academy of Sciences, Lanzhou, 730000, P. R. China.
E-mail: jhao@sdu.edu.cn; Fax: + 86-531-88366074; Tel: + 86-531-
88366074
a
system of photo-modulated multi-state and multi-scale
bKey Laboratory of Colloid and Interface Chemistry (Shandong
University), Ministry of Education, Jinan, 250100, P. R. China
cGraduate School of the Chinese Academy of Sciences, Beijing, 100039,
P. R. China
molecular self-assemblies with sodium (4-phenylazo-phenoxy)-
acetate (AzoNa) and cetyltrimethylammonium bromide
(CTAB).12
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In this article, we constructed a new complex fluid system of
a nonionic surfactant and a photosensitive aromatic molecule, in
which the phase transition from vesicles to wormlike micelles
induced by UV irradiation was obtained. It is really a simple way
to control phase transition, which would bring about the tran-
sition of aggregates or solution fluidity change and may provide
potential applications in the fields of drug delivery, oilfields and
biochemistry. To the best of our knowledge, there are rare
reports on this transition.13–15 The UV-light-induced trans/cis
photoisomerization molecules, para-coumaric acid (PCA), play
an important role in this system. After UV irradiation, trans-
PCA can be converted its cis formation (Scheme 1).16,17 Such
a conversion could have a dramatic influence on the equilibrium
of hydrophilic and hydrophobic, which will further affect the
structure of aggregates.
stored in liquid nitrogen until they were transferred to a cryogenic
sample holder (Gatan 626) and examined with a JEOL JEM-1400
TEM (120 kV) at about ꢁ174 ꢀC. The phase contrast was
enhanced by under-focus. The images were recorded on a Gatan
multiscan CCD and processed with Digital Micrograph. The
cryo-TEM observations were performed at Dr J. Hao’s group.
Rheological measurements
Rheological measurements were performed on a Haake RS6000
rheomꢀeter with a coaxial cylinder sensor system (Z41 Ti) at 25.0
ꢂ 0.1 C. The viscoelastic properties were determined by oscil-
latory measurements from 0.01 to 100 Hz. In steady shear
experiments, the shear rate was typically increased from 0.01 to
1000 sꢁ1 in a step wise mode within approximately 10–35 min.
To achieve equilibrium as far as possible, the rheometer was set
to ensure the gradient was less than 0.5 (Ds/s)/Dt % at each shear
rate step, and the maximum waiting time was 60 s.
Experimental
Chemicals
C14DMAO, 30 wt% aqueous solution, was from Clariant
Company (Germany) in China. The aqueous solution was freeze-
dried in a vacuum and then was crystallized three times in
acetone. trans-PCA was purchased from J&K CHEMICA
Company (China), purity>98%. The deionized water was
distilled three times.
Results and discussion
At the beginning, we studied the phase diagram of tetradecyldi-
methylamine oxide (C14DMAO) mixed with trans-PCA in
various concentrations (Fig. 1). Different amounts of trans-PCA
were added into a series of C14DMAO stock solutions with
a constant concentration. For typical observations, samples of
150 mmol Lꢁ1 C14DMAO with an increase of trans-PCA were
chosen to observe the phase sequence. After 4 weeks, L1-phase
(micelles), two-phase L1/La, La-phase (vesicles determined by
cryo-TEM), and precipitates were observed sequentially with
increasing amounts of trans-PCA (Fig. 2). Less than 50 mmol Lꢁ1
trans-PCA in 150 mmol Lꢁ1 C14DMAO is L1-phase, which is
a clear and transparent solution with no birefringence. The
samples from 50 to 58 mmol Lꢁ1 trans-PCA in 150 mmol Lꢁ1
C14DMAO are two-phase L1/La, in which the upper phase is
birefringent but the lower phase is not birefringent. A vesicle
phase with obvious birefringence is formed subsequently between
58 and 86 mmol Lꢁ1 trans-PCA in 150 mmol Lꢁ1 C14DMAO.
After the vesicle phase, the samples are precipitates. The black
squares in Fig. 2 indicate the turbidity of samples. In the micelle
region, there is almost 100% transmission of light. At the
UV-irradiation
UV-irradiation experiments used a CHF-XM35-500W ultrahigh
pressure short arc mercury lamp without any filter (250–600 nm).
This experiment was performed in a room-temperature water
bath to avoid overheating.
UV-Vis spectroscopy and turbidity experiments
UV-Vis spectroscopy measurements before and after UV-irra-
diation were carried out using a U-4100 UV-Vis spectropho-
tometer. Turbidity experiments were also performed on this
instrument. We obtained the data at l ¼ 600 nm. Water was used
as the blank in these two experiments.
Cryo-TEM observations
Cryo-TEM samples were prepared in a controlled environment
vitrification system (CEVS) at 25 ꢀC. A micropipette was used to
load 5 mL solution onto a TEM copper grid, which was blotted
with two pieces of filter paper, resulting in the formation of thin
films suspended on the mesh holes. After waiting for about 5 s, the
samples were quickly plunged into a reservoir of liquid ethane
(cooled by nitrogen) at ꢁ165 ꢀC. The vitrified samples were then
Fig. 1 A phase diagram of the PCA/C14DMAO/H2O system at 25.0 ꢂ
Scheme 1 The photoisomerization of a PCA molecule from trans to cis.
10714 | Soft Matter, 2011, 7, 10713–10719
0.1 ꢀC.
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which is reported by Cates et al.19 This model can be used to
describe living polymers, e.g., wormlike micelles. The storage
modulus G0 and the loss modulus G00 obey the following eqns (1)
and (2), which consider a single stress relaxation time sR:
2
G0 ¼ G0u2sR2/(1 + u2sR
)
(1)
(2)
2
G00 ¼ G0usR/(1 + u2sR
)
where u, G0, and sR represent angular frequency (rad sꢁ1), high
frequency plateau modulus, and relaxation time, respectively. G0
and sR can be obtained from the critical angular frequency u*
and modulus G* at which the curve of G0 and G00 intersects
according to eqns (3) and (4):
sR ¼ 1/u*
G0 ¼ 2G*
(3)
(4)
Fig. 2 The phase behavior of 150 mmol Lꢁ1 C14DMAO with increasing
trans-PCA, including light transmission data (:), l ¼ 600 nm.
beginning of the vesicle phase, the solutions are slightly blue.
With the increase of PCA, the blue color becomes deeper.
Cryo-TEM observations were used to determine the micro-
structure of samples in the different phase regions in Fig. 1. One
can clearly see wormlike micelles in the micelle region (Fig. 3a)
with a composition of 40 mmol Lꢁ1 trans-PCA/150 mmol Lꢁ1
C14DMAO solution. Uni- and multilamellar vesicles were found
in the vesicle region (Fig. 3b and c) with compositions of 60
mmol Lꢁ1 trans-PCA/150 mmol Lꢁ1 C14DMAO and 70 mmol Lꢁ1
trans-PCA/150 mmol Lꢁ1 C14DMAO birefringent solutions.
From the cryo-TEM images, one can find that largely multi-
lamellar vesicles are deformed in the vesicle region with lower
concentrations of PCA, while small unilamellar vesicles and large
bilayer cylinders are observed at higher concentrations of PCA.
The formation of the multilamellar and largely deformed vesicles
instead of the unilamellar small ones is because more PCA
molecules should be deposited in the bilayer membranes with the
increase of the PCA concentration, which might lead to rigidity
of the membrane because of the increase of the rigid aromatic
structure.18
How well the data correspond to the Maxwell model, which
can be visualized by Cole–Cole plots, i.e., plots of G0 as a func-
tion of G00. From eqns (1) and (2), we can obtain the following
eqn (5):
2
G00 + (G0 ꢁ G00/2)2 ¼ (G0/2)2
(5)
If there was only a single stress relaxation time, the Cole–Cole
plots would be a perfect semicircle. However, at low frequency
nearly all wormlike micelles can fit Cole–Cole plots well, at a little
higher frequencies it will deviate because of the Rouse relaxation
modes. According to our experimental data, as shown in Fig. 4a
and b, we can decide that wormlike micelles exist in solution, due
to the good correspondence with the Maxwell model. From the
steady shear, as shown in Fig. 4c, with the increasing of the PCA
concentration, the zero-shear viscosity rises. At low concentra-
tions of PCA, the viscosity values are almost independent of the
shear rate, like Newtonian fluids, probably because only spherical
and short rod-like micelles exist in the solution. At higher
concentrations of PCA, the viscosity values are independent of
the shear rate at low shear rates, while at high shear rates the
value declines obviously. Steady shear data show a plateau value,
which is caused by a network of wormlike micelles. Shear thin
phenomenon attributes to the alignment of the wormlike chain in
the shear flow at high shear rate. We can deduce that the
wormlike micelles grow long and form networks, induced by the
increase of the PCA concentration.
The rheological properties of the three samples of 40 mmol Lꢁ1
trans-PCA/150 mmol Lꢁ1 C14DMAO, 60 mmol Lꢁ1 trans-PCA/
150 mmol Lꢁ1 C14DMAO, and 70 mmol Lꢁ1 trans-PCA/
150 mmol Lꢁ1 C14DMAO were measured by oscillatory sweep
mode and steady shear mode. At a low shear frequency the
rheological properties of 40 mmol Lꢁ1 trans-PCA/150 mmol Lꢁ1
C14DMAO are in accordance with Maxwell’s mechanical model,
Fig. 3 Cryo-TEM images of 40 mmol Lꢁ1 trans-PCA/150 mmol Lꢁ1 C14DMAO (wormlike micelles, a), 60 mmol Lꢁ1 trans-PCA/150 mmol Lꢁ1
C14DMAO (largely multilamellar vesicles, b), and 70 mmol Lꢁ1 trans-PCA/150 mmol Lꢁ1 C14DMAO (small vesicles, c).
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Fig. 4 Oscillatory shear as a function of the angular frequency (a) and the corresponding Cole–Cole plot at s ¼ 10 Pa (b) of 40 mmol Lꢁ1 trans-PCA and
150 mmol Lꢁ1 C14DMAO, and plots of viscosity (h) against shear rate (g_ ) at different C14DMAO/PCA proportions (c).
The samples in the region of vesicles, 60 mmol Lꢁ1 trans-PCA/
150 mmol Lꢁ1 C14DMAO and 70 mmol Lꢁ1 trans-PCA/
150 mmol Lꢁ1 C14DMAO, were also studied by rheological
measurements. The complex viscosity (h*) decreases over the
whole frequency with a slope of ꢁ1, the storage modulus (G0) and
loss modulus (G00) are almost independent of frequency, and G0 is
higher than G00 (Fig. 5). It is consistent with the characteristic of
vesicles.20 However, the G0 value in 70 mmol Lꢁ1 trans-PCA/
150 mmol Lꢁ1 C14DMAO is bigger than that in 60 mmol Lꢁ1
trans-PCA/150 mmol Lꢁ1 C14DMAO, which indicates that the
former has better elasticity than the latter. This is probably
because of the rigidity of the membrane enhancement and more
vesicles existing in the sample of 70 mmol Lꢁ1 trans-PCA/
150 mmol Lꢁ1 C14DMAO, which was explained before.
Photo-irradiation experiment
Trans- and cis-PCA molecules have different wavelength
absorptions in ultraviolet region.16,17 We investigated the
absorption variation from trans- to cis-PCA via a UV-Vis spec-
trophotometer. It can be found that the peak intensity dropped
and had a slight blue shift after UV-irradiation, which is
Fig. 5 Rheograms of the oscillatory shear as a function of the angular frequency at s ¼ 0.1 Pa for samples of 60 mmol Lꢁ1 trans-PCA/150 mmol Lꢁ1
C14DMAO (a) and 70 mmol Lꢁ1 trans-PCA/150 mmol Lꢁ1 C14DMAO (b).
10716 | Soft Matter, 2011, 7, 10713–10719
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attributable to photoisomerization from trans- to cis-PCA
(Fig. 6). About 66% cis-PCA existed in the sample solution, when
photo stationary state was reached.16
Some samples from the vesicle phase with birefringence were
chosen to irradiate with UV-light. A complete phase transition
was easily found in a low concentration region of trans-PCA.
However, in a much higher concentration region of trans-PCA
mixed with C14DMAO phase transition via UV-light cannot be
induced. As a typical example, 60 mmol Lꢁ1 trans-PCA/
150 mmol Lꢁ1 C14DMAO aqueous solution of the vesicle phase,
undergoing 60 miꢀn UV-irradiation in a water bath at room
temperature (ꢃ20 C), the slightly turbid and birefringent solu-
tion can turn into a clear and transparent solution without
birefringence (Fig. 7). The viscosity also has an obvious
enhancement, which could be observed simply via the method of
turning the tube upside-down. However, under the same condi-
tions, we can not find this similar phase transition for a 70 mmol
Lꢁ1 trans-PCA and 150 mmol Lꢁ1 C14DMAO sample. To extend
the irradiation time to 120 min, the phase transition was still not
found, indicating that the concentration of trans-PCA in
C14DMAO plays an important role in this transition.
Fig. 7 Polarization observation from birefrigent patterns to a non-
birefringent solution after UV-light irradiation, for a sample of 60 mmol
Lꢁ1 trans-PCA and 150 mmol Lꢁ1 C14DMAO vesicle solution.
P ¼ v/la
(6)
where v, l, and a are the volume of the hydrophobic chain, the
length of the hydrophobic chain, and the area of the hydrophilic
head groups, respectively. The value of P can give assistance to
speculate the kind of aggregates formed in aqueous solutions of
surfactants. If P < 1/3, global micelles are obtained; if 1/3 <
P < 1/2, rodlike or wormlike micelles are obtained; if 1/2 < P < 1,
vesicles are obtained; if P z 1, planar bilayers are obtained; if
P > 1, inverted micelles are obtained.
Cryo-TEM observations were used to demonstrate the phase
transition from multilamellar vesicles to wormlike micelles.
Multilamellar vesicles with ca. 500 nm, for a 60 mmol Lꢁ1 trans-
PCA and 150 mmol Lꢁ1 C14DMAO sample were clearly observed
from a cryo-TEM image above (Fig. 3b). As shown in Fig. 8, one
can see that the wormlike micelles formed from the vesicles of
60 mmol Lꢁ1 trans-PCA and 150 mmol Lꢁ1 C14DMAO after UV-
light irradiation for 60 mins.
In our system, C14DMAO can be easily protonated by a weak
acid, trans-PCA, in water, to combine to a cationic–anionic
(catanionic) surfactant mixture system. This combination could
decrease the a value, which makes the P value increase. Thus, the
mixture system of C14DMAO and trans-PCA contains bilayer
aggregates, i.e., multilamellar vesicles, in aqueous solution. After
UV-light irradiation, the trans-PCA should be turned into its cis-
formation. The a value could be increased largely because the
steric hindrance of cis-PCA is much bigger than trans-PCA.
Therefore, the P value will decrease largely, which turns vesicles
to wormlike micelles. The more cis-PCA which existed, the more
complete the transition was. A theoretical proposal of models
of multilamellar vesicles and wormlike micelles is sketched
in Fig. 10.
The rheological properties also proved the phase transition
from vesicles to wormlike micelles after UV-irradiation. Both the
oscillatory and Cole–Cole curves, as shown in Fig. 9, well corre-
spond to Maxwell’s mechanical model at low shear frequency,
which demonstrates the existence of wormlike micelles.
Mechanism of phase transition
The mechanism of phase transition could be explained by the
theory of molecular packing parameter, which was introduced by
Israelachvili et al. in 1976.21
Fig. 6 UV-vis spectra of 60 mmol Lꢁ1 trans-PCA/150 mmol Lꢁ1
C14DMAO solution with and without UV irradiation. The sample
solution was diluted 2000 times before investigation.
Fig. 8 A cryo-TEM image of 60 mmol Lꢁ1 trans-PCA and 150 mmol Lꢁ1
C14DMAO aqueous solution after UV-irradiation for 60 min.
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Fig. 9 Rheograms of the oscillatory shear as a function of the angular frequency at s ¼ 8 Pa (a) and the corresponding Cole–Cole plot for the sample of
60 mmol Lꢁ1 trans-PCA and 150 mmol Lꢁ1 C14DMAO (b), after UV-light irradiation.
Another problem is why the concentration of trans-PCA in
C14DMAO plays an important role in the phase transition. The
trans-PCA structure has a better ability to reduce the area of the
head group than the cis-PCA structure because of its lower steric
hindrance. According to the theory of molecular packing
parameter, P, trans-PCA molecules are favourable in the
formation of vesicles. After a certain time of irradiation, trans-
and cis-PCA will reach the photo stationary state, i.e., the ratio
of trans- and cis-PCA is invariable. For much higher concen-
trations of trans-PCA, after a certain time of UV-irradiation, the
trans-PCA could be transferred into cis-PCA but the amount of
trans-PCA is still too much higher to induce a complete phase
transition.
can be turned into cis-PCA/trans-PCA/C14DMAO one via
photo-irradiation, inducing the bilayers to turn to wormlike
micelles. This simple strategy to accomplish a complete transi-
tion from vesicles to wormlike micelles could help us to achieve
further applications in different areas.
Acknowledgements
This work was financially supported by the NSFC (Grant No.
21033005), the National Basic Research Program of China (973
Program, 2009CB930103), and NFS of Shandong Province
(Z2008B01).
Note and references
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10718 | Soft Matter, 2011, 7, 10713–10719
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