A R T I C L E S
Ketner et al.
geometry, should be applicable to similar organic additives
where the photoisomerization can indeed be reversed by
irradiation at a different light wavelength. We hope that this
work will stimulate studies into new types of PR fluids as well
as applications for these fluids in microscale devices.
2. Experimental Section
Materials. CTAB was purchased from Sigma-Aldrich and was
greater than 98% in purity. OMCA in its trans form was purchased
from Acros Chemicals, while the cis form was purchased from TCI
America, and each compound was greater than 98% in purity. All
chemicals were used as received. Ultrapure deionized water from a
Millipore water purification system was used in preparing samples for
2
rheological characterization, while D O (99.95% deuterated, from
Cambridge Isotopes) was used for the SANS studies. Solutions
containing OMCA were prepared with a slight excess of base (NaOH),
and CTAB was then added to these solutions to reach the final
composition. Samples were stirred continuously under mild heat until
they became homogeneous. The solutions were then left to equilibrate
overnight at room temperature before any experiments were conducted.
The pH in the samples was between 9 and 11.
Sample Response before and after UV Irradiation. CTAB/OMCA
samples were irradiated with UV light from a Oriel 200 W mercury
arc lamp. A dichroic beam turner with a mirror reflectance range of
Figure 1. Schematic behavior of photoresponsive (PR) fluids consisting
of CTAB and OMCA. When OMCA is in its trans from, its mixture with
CTAB gives rise to long, entangled wormlike micelles. Upon UV irradiation,
trans-OMCA gets photoisomerized to cis-OMCA, and the corresponding
change in molecular geometry causes a drastic reduction in micellar length.
2
80 to 400 nm was used to access the UV range of the emitted light.
viscosity of these solutions can be made to drop by more than
Samples (5 mL) were placed in a Petri dish with a quartz cover, and
irradiation was done for a specific duration under stirring. Due to the
nature of the OMCA spectra, irradiated samples did not undergo any
changes when stored under ambient conditions, which made it easy
to conduct subsequent tests using appropriate techniques such as
UV-vis spectroscopy, HPLC, rheology, and SANS. UV-vis spec-
troscopy before and after irradiation were carried out using a Varian
Cary 50 spectrophotometer.
4
orders of magnitude. The basis for this viscosity change is
the trans to cis photoisomerization of the double bond in OMCA
Figure 1). The resultant change in the geometry of OMCA alters
(
the molecular packing of the CTAB/OMCA complex, leading
to a drastic reduction in the length of the wormlike micelles
(Figure 1). In turn, the sample is transformed from a highly
viscoelastic, gel-like fluid to a thin, runny fluid with a viscosity
close to that of water. Confirmation of the above microstructural
hypothesis is provided by data from small-angle neutron
scattering (SANS).
HPLC Studies. The irradiated solutions were analyzed using HPLC
(Waters Spherisorb 5 µm ODS2, 4.6 mm × 250 mm C18 column). A
flow rate of 0.8 mL/min was used, and the eluting solvent was 15%
ethanol and 85% water. The solution components were detected using
UV absorption (Dynamax absorbance detector model UV-D II) at 225
and 254 nm. These parameters were based on those of a study of similar
molecules by Imae et al.18 HPLC chromatograms of solutions before
and after irradiation are provided as part of the Supporting Information.
Rheological Studies. Steady and dynamic rheological experiments
were performed on an AR2000 stress controlled rheometer (TA
Instruments, Newark, DE). Samples were run at 25 °C on a cone-and-
plate geometry (40-mm diameter, 2° cone angle) or a couette geometry
Micellar systems based on photosensitive additives have been
investigated for a long time, following the pioneering work of
Wolff and co-workers (indeed, to our knowledge, it was Wolff
4,14-16
who coined the term “photorheological fluid”).
Past studies
have mainly focused on anthracene and stilbene derivatives
added to cationic surfactants. However, the viscosity change
induced by light in these samples was rather modest (∼ a factor
4
,16
of 2-10). Thus, the principal result here is that much larger
viscosity changes (factors of 1000 to 10,000) can be achieved
in CTAB/OMCA mixtures. We should also note that the
photosensitivity of OMCA is analogous to that of coumaric acid,
which in turn is a key component of the “photoactive yellow
(rotor of radius 14 mm and height 42 mm, and cup of radius 15 mm).
Dynamic frequency spectra were obtained in the linear viscoelastic
regime of each sample as determined by dynamic stress-sweep
experiments.
Solubility Studies. The solubility of OMCA isomers in water at 25
1
7
protein”. Although, in principle, photoisomerizations are
reversible, the trans-cis isomerization of OMCA is not easily
reversed, and we will discuss the reasons for this later in the
paper. This means that CTAB/OMCA mixtures can only provide
a one-way viscosity switch. Nevertheless, the simplicity and
ease of preparation of these mixtures should make them
attractive for use in some microfluidic or sensor applications.
Also, the underlying principle behind this work, namely, the
ability to fine-tune micellar assembly by exploiting molecular
13
°C was determined as follows, much like in an earlier study. An excess
of the organic derivative was added to deionized water, and the solution
was stirred under mild heat for 1 day, followed by equilibration at room
temperature for two more days. A sample of this solution in a 1 mm
cuvette was placed in the holder of the UV-vis spectrophotometer,
maintained at 25 °C. The sample was left undisturbed for 1 h to allow
any undissolved material to settle to the bottom of the cuvette. The
absorbance was then measured and converted to a concentration value
using the absorbtivity determined from a calibration curve. The same
procedure was repeated for each of the OMCA isomers.
Small Angle Neutron Scattering (SANS). SANS measurements
were made on the NG-7 (30 m) beamline at NIST in Gaithersburg,
MD. Neutrons with a wavelength of 6 Å were selected. Two sample-
(
13) Davies, T. S.; Ketner, A. M.; Raghavan, S. R. J. Am. Chem. Soc. 2006,
1
28, 6669-6675.
(14) Muller, N.; Wolff, T.; von Bunau, G. J. Photochem. 1984, 24, 37-43.
(15) Wolff, T.; Klaussner, B. AdV. Colloid Interface Sci. 1995, 59, 31-94.
(16) Yu, X. L.; Wolff, T. Langmuir 2003, 19, 9672-9679.
(17) Kort, R.; Vonk, H.; Xu, X.; Hoff, W. D.; Crielaard, W.; Hellingwerf, K. J.
FEBS Lett. 1996, 382, 73-78.
(18) Imae, T.; Tsubota, T.; Okamura, H.; Mori, O.; Takagi, K.; Itoh, M.; Sawaki,
Y. J. Phys. Chem. 1995, 99, 6046-6053.
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