(5cn) Dynamic Surface Tension in a Photoresponsive Surfactant System

The dynamic surface tension behavior of a nonionic photoresponsive
surfactant that incorporates the light-sensitive azobenzene group into its tail
is investigated and reported.  Cis-trans photo-isomerization of
this group alters the aggregation state of the surfactant in bulk solution and
also changes the adsorption capacity of the surfactant at an air-water
interface.  NMR studies indicate that a solution removed from light for an
extended period of time is comprised 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.  UV-Vis absorption
experiments have established that conversion between photostationary states is
reversible, and is rapid under high-intensity illumination.  Dynamic
surface tension studies performed on this system under different illumination
conditions (dark, UV light, visible light) show profoundly different approaches
to equilibrium.  At concentrations well above the CMC, the same
equilibrium tension is reached in all three cases, presumably corresponding to
a surface saturated with the trans (more surface active) isomer.
 The dark sample shows a single relaxation in surface tension after
creation of a fresh interface, 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 seems to suggest competitive adsorption occurring between the cis
and trans isomers present in the mixtures such that the cis
isomer reaches the interface more quickly and dominates the surface, causing
the plateau of nearly constant tension, before ultimately being displaced by
the trans isomer.  This hypothesis is supported by surface pressure
measurements of adsorbed monolayers of the surfactant under various
illumination conditions made using a Langmuir film balance.  In other
experiments, a pendant bubble of air was formed inside a cuvette containing
surfactant solution and allowed to reach a final trans-saturated
equilibrium state (as determined from the surface tension) before being
illuminated with a high-intensity UV light source.  The surface tension
then rapidly increased to a higher, cis-dominated value and remained at
this value for as long as the illumination persisted.  Upon discontinuing
the UV illumination, the surface tension eventually returned to the lower, trans-dominated
value, further evidence supporting the competitive adsorption and displacement
theory. Diffusion models were developed to estimate the time scales expected
for the surface tension relaxations seen in the dynamic surface tension
measurements. These models account for the presence of aggregates in solution,
and good agreement is found between the experimentally observed relaxation
times and the model predictions, though the precise role of aggregates in the
adsorption process is unclear.