Article
Kumar et al.
chains, much like their aqueous counterparts.17-20 Our goal of
engineering nonaqueous PR fluids can thus be reduced to the goal
of generating light-responsive reverse worms. Importantly, we
would like to design these fluids using chemicals that can be
purchased from commercial vendors rather than new molecules
that require synthesis in the laboratory.
The challenge in this context is that there are only a few known
routes for forming reverse worms. (In comparison, aqueous
worms can be assembled by a large number of surfactants along
with a variety of salts.) The conventional “recipe” for reverse
worms17,18 is to combine the phospholipid, lecithin with a small
amount of water. Lecithin-water mixtures give rise to reverse
worms in many nonpolar solvents. While lecithin itself assembles
into small spherical reverse micelles, it is the addition of water that
induces growth of long, wormlike chains. Instead of water, a few
other types of additives can have the same effect, these include
strongly polar solvents like formamide,18 biosurfactants like bile
salts,19,20 and certain sucrose esters.21 The common thread with
all these additives is their ability to form hydrogen bonds
(H-bonds) with the headgroups of lecithin.18,19
from Cambridge Isotopes. Samples were prepared by dissolving
weighed amounts of lecithin and the chosen coumaric acid in a
given organic solvent. The solutions were then heated to ∼65 °C
under continuous stirring for ∼1 h until they became homoge-
neous. The samples were then stirred continuously for 24 h
and then left to equilibrate overnight in a desiccator at room
temperature before any experiments were conducted.
Sample Response before and after UV Irradiation. Sam-
ples were irradiated with UV light from an Oriel 200 W mercury
arc lamp. To access the UV wavelengths of the emitted light,
a dichroic beam turner with a mirror reflectance range of
280-400 nm was used along with a < 400 nm filter. To nullify
the effects of atmospheric moisture, samples (5 mL) were placed
in borosilicate glass vials with their caps on and were irradiated
through vial walls for a specific duration under stirring. Irradiated
samples did not undergo any changes when stored in the dark
under ambient conditions (to be additionally careful, we covered
sample vials with aluminum foil). This stable behavior made it
easy to conduct subsequent tests on the samples. UV-vis spec-
troscopy studies before and after irradiation were carried out
using a Varian Cary 50 spectrophotometer.
To engineer photoresponsive reverse worms, a new route needs
to be found whereby worms could be induced upon addition of
photosensitive aromatic molecules such as phenylalkenes,
stilbenes, or azobenzenes. Accordingly, we have explored the
addition of various aromatic derivatives to lecithin organosols.
Among more than 10 such derivatives tested, reverse worms were
found in only one case - with para-coumaric acid (PCA). In this
paper, we will describe the properties of lecithin/PCA mixtures in
different organic solvents with focus on their rheological proper-
ties initially and after UV irradiation. We will demonstrate light-
induced viscosity reduction by factors of 1000 or more in these
mixtures. These rheological effects will be correlated with changes
in the length of lecithin/PCA worms. In turn, these microstruc-
tural changes will be shown to be dictated by light-induced
changes in the geometry of the PCA molecule. PCA is known
to undergo trans-cis photoisomerization about its olefinic dou-
ble bond.22-24 In fact, PCA photoisomerization is integral to
the function of proteins such as the photoactive yellow protein
(PYP).23,24 Our studies indicate the preferential ability of trans-
PCA to cause reverse worm growth compared to cis-PCA,
which is attributed to the higher polarity and thereby superior
H-bonding capability of the former.22,25
Rheological Studies. Steady and dynamic rheological experi-
ments were performed using an AR2000 stress controlled rhe-
ometer (TA Instruments). Samples were studied at 25 °C on a
cone-and plate geometry (40 mm diameter, 2° cone angle). A
solvent trapwas usedtominimizesolvent evaporation. Frequency
spectra were conducted in the linear viscoelastic regime of the
samples, as determined from dynamic strain sweep measure-
ments. For the steady shear experiments, sufficient time was
allowed before data collection at each shear rate to ensure that
the viscosity reached its steady-state value.
Small Angle Neutron Scattering (SANS). SANS measure-
ments were made on the NG-7 (30 m) beamline at NIST in
˚
Gaithersburg, MD. Neutrons with a wavelength of 6 A were
selected, and three different sample-to-detector distances were
-1
˚
used to access a range of wave vectors q from 0.004 to 0.4 A
.
Samples were prepared with deuterated cyclohexane and were
studied in 1 mm quartz cells at 25 °C. Scattering spectra were
corrected and placed on an absolute scale using calibration
standards provided by NIST. Data are presented as plots of
the radially averaged scattered intensity I vs the wave vector q =
(4π/λ)sin(θ/2), where λ is the neutron wavelength and θ the
scattering angle.
2. Experimental Section
SANS Data Analysis. SANS data were analyzed by the
indirect Fourier transform (IFT) method, which requires no
a priori assumptions on the nature of the scatterers.14,26 Here, a
Fourier transformation of the scattering intensity I(q) is per-
formed to obtain the pair distance distribution function p(r) in
real space. p(r) provides structural information about the scat-
terers, such as their shape and maximum dimension. IFT analysis
was implemented using the commercial PCG software package.
Materials. Soybean lecithin (95% purity) was purchased from
Avanti Polar Lipids, Inc. and used as received.19,20 The trans
isomers of para-, meta-, and ortho-coumaric acids (denoted as
PCA, MCA, and OCA, respectively) were purchased from Sigma-
Aldrich and used as received (each was >98% in purity).
Cyclohexane, iso-octane, and isopropyl palmitate were purchased
from EM Sciences, Fisher Scientific and TCI America, respec-
tively. n-hexane, 1-hexene, and n-decane were purchased from
Sigma-Aldrich. Deuterated cyclohexane (99.5% D) was obtained
3. Results and Discussion
We first studied mixtures in cyclohexane of lecithin and the
isomers of coumaric acid, i.e., para-coumaric acid (PCA), meta-
coumaric acid (MCA), and ortho-coumaric acid (OCA)
(structures of all three are shown in Figure 1). All three com-
pounds were insoluble in cyclohexane when added directly;
however, each could be dissolved in the presence of lecithin.
Among the three, only PCA increased the solution viscosity and
the corresponding samples exhibited all the hallmarks of reverse
(17) Schurtenberger, P.; Scartazzini, R.; Luisi, P. L. Rheol. Acta 1989, 28, 372.
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(19) Tung, S. H.; Huang, Y. E.; Raghavan, S. R. J. Am. Chem. Soc. 2006, 128,
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(20) Tung, S. H.; Huang, Y. E.; Raghavan, S. R. Langmuir 2007, 23, 372.
(21) Hashizaki, K.; Taguchi, H.; Saito, Y. Colloid Polym. Sci. 2009, 287, 1099.
(22) Caccamese, S.; Azzolina, R.; Davino, M. Chromatographia 1979, 12, 545.
(23) Kort, R.; Vonk, H.; Xu, X.; Hoff, W. D.; Crielaard, W.; Hellingwerf, K. J.
FEBS Lett. 1996, 382, 73.
(24) Takeshita, K.; Hirota, N.; Terazima, M. J. Photochem. Photobiol., A:
Chem. 2000, 134, 103.
(25) Lee, H. S. HPLC analysis of phenolic compounds. In Food Analysis by
HPLC, 2nd ed.; Nollet, L. M. L., Ed.; Marcel Dekker: New York, 2000; p 775.
(26) Glatter, O. J. Appl. Crystallogr. 1977, 10, 415.
5406 DOI: 10.1021/la903834q
Langmuir 2010, 26(8), 5405–5411