Effects of Aggregate Charge and Subphase Ionic Strength on the Properties of Spread Polyelectrolyte/Surfactant Films at the Air/Water Interface under Static and Dynamic Conditions.

We demonstrate the ability to tune the formation of extended structures in films of poly(sodium styrenesulfonate)/dodecyltrimethylammonium bromide at the air/water interface through control over the charge/structure of aggregates as well as the ionic strength of the subphase. Our methodology to prepare loaded polyelectrolyte/surfactant films from self-assembled liquid crystalline aggregates exploits their fast dissociation and Marangoni spreading of material upon contact with an aqueous subphase. This process is proposed as a potential new route to prepare cheap biocompatible films for transfer applications. We show that films spread on water from swollen aggregates of low/negative charge have 1:1 charge binding and can be compressed only to a monolayer, beyond which material is lost to the bulk. For films spread on water from compact aggregates of positive charge, however, extended structures of the two components are created upon spreading or upon compression of the film beyond a monolayer. The application of ellipsometry, Brewster angle microscopy, and neutron reflectometry as well as measurements of surface pressure isotherms allow us to reason that formation of extended structures is activated by aggregates embedded in the film. The situation upon spreading on 0.1 M NaCl is different as there is a high concentration of small ions that stabilize loops of the polyelectrolyte upon film compression, yet extended structures of both components are only transient. Analogy of the controlled formation of extended structures in fluid monolayers is made to reservoir dynamics in lung surfactant. The work opens up the possibility to control such film dynamics in related systems through the rational design of particles in the future.