(281a) Effect of Photoisomer Composition in a Photoresponsive Surfactant System

A nonionic photoresponsive surfactant has been developed
which incorporates the light-sensitive azobenzene group into its tail.  Cis-trans
photo-isomerization of this group changes the ability of the surfactant to pack
into aggregates in solution or into adsorbed monolayers at interfaces, allowing
for surface and bulk properties to be modified by varying illumination
conditions.  NMR studies indicate that a solution removed from light for an
extended period of time consists almost entirely of the trans isomer,
while samples exposed to light of fixed wavelength eventually reach a photostationary
state containing significant amounts of both isomers, with UV illumination
producing a mixture dominated by the cis isomer.  Fluorescence
measurements have been used to determine the CMC of the surfactant under
different illumination conditions (dark, visible light, UV) and the results
suggest segregation of the two isomers into different aggregate phases. Micelles
or vesicles rich in the trans isomer form once a critical amount of trans
is reached, while a cis-rich bicontinuous phase is formed at a larger
critical cis concentration. TEM studies performed previously on this
system demonstrate the differences in the aggregate structure under different
illumination conditions at high concentrations. The dynamic surface tension profiles
of this surfactant under the different illumination conditions are profoundly
different from each other despite reaching the same equilibrium tension in
measurements performed at concentrations well above the CMC. This equilibrium
presumably corresponds to a surface saturated with the trans (more
surface active) isomer. After creation of a fresh interface, the dark-adapted
sample shows a single relaxation in surface tension, while mixtures containing cis
surfactant exhibit a more rapid initial decrease in tension, followed by a
plateau of nearly constant tension, and end with a second relaxation to
equilibrium.  This behavior suggests competitive adsorption between the cis
and trans isomers present in the mixtures, in which the cis
isomer reaches the interface more quickly and initially dominates the surface,
causing the intermediate tension plateau, before ultimately being displaced by
the trans isomer.  Among the isomeric mixtures, the rapidity of the
initial decline in surface tension and the duration of the intermediate plateau
are both found to increase with increasing cis content.  Surface
pressure measurements of adsorbed monolayers of the surfactant under the
various illumination conditions were made using a Langmuir film balance, and
the resulting pressure-area isotherms are found to vary between the different
isomeric compositions, with saturation pressures which lend support to the
competitive adsorption hypothesis. Diffusion models were developed to estimate
the time scales expected for surfactant adsorption and surface tension
relaxation in these systems.  These models account for the role of aggregates
in the adsorption process, and consider limiting behavior for three aggregate
properties: mobility, dissolution rate, and ability to incorporate into the interface. 
Good agreement is found between the model predictions and the experimentally
observed relaxation time scales, though the results are inconclusive regarding
the exact role of aggregates.