  Polyelectrolyte surfactant mixtures and their interactions with fluid
interfaces are an important research field due to their use in technological
applications. Most of the existing knowledge on these systems is based on
models in which the polyelectrolyte concentration is around 50 times lower than
that used in commercial formulations. The present work marks a step to close
the gap on the understanding of their behavior under more practically relevant
conditions. The adsorption of concentrated mixtures of
poly(diallyldimethylammonium) chloride and sodium N lauroyl N methyltaurate at
the water-vapor interface with a crude mixing protocol has been studied by
different surface tension techniques, Brewster angle microscopy, neutron
reflectometry, and several bulk characterization techniques. Kinetically
trapped aggregates formed during mixing influence the interfacial morphology of
mixtures produced in the equilibrium one phase region, yet fluctuations in the
surface tension isotherm result depending on the tensiometric technique
applied. At low bulk surfactant concentrations, the free surfactant
concentration is very low, and the interfacial composition matches the trend of
the bulk complexes, which is a behavior that has not been observed in studies
on more dilute mixtures. Nevertheless, a transition to synergistic
co-adsorption of complexes and free surfactant is observed at the higher bulk
surfactant concentrations studied. This transition appears to be a special
feature of these more concentrated mixtures, which deserves attention in future
studies of systems with additional components.
