Fluid Interfaces Grazing Angles ReflectOmeter

Example 1: Oppositely Charged Polyelectrolyte/Surfactant Mixtures

Interfacial properties of oppositely charged polyelectrolyte/surfactant mixtures have long been interpreted in an equilibrium framework. These systems are very common as a result of their use in everyday life products such as shampoos and conditioners. A sustained research output based on neutron reflectivity data from FIGARO and complementary data from the Partnership for Soft Condensed Matter has established a different interpretation based on non-equilibrium effects.

It has been shown that the samples can take an extremely long time to equilibrate depending on the aggregation and settling kinetics. In the meantime, steady state interfacial properties may be measured that are far from equilibrium. Even once equilibrium is reached, samples may be perturbed from equilibrium as a result of the way they are handled. If settled aggregates contact the air/water interface as a result of convection applied to the system, they have the possibility to dissociate and spread material to create a trapped film.

Work has also been carried out on the mechanisms by which bulk aggregates can interact with interfaces. Different routes include surface trapping, surface affinity and transport under gravity. Directionality effects are interesting to consider in that one usually considers air to be located above a liquid, yet this is not only the case in foams and hanging droplets.

The project culminated in the publication of a general physical description of the behavior of these systems, and was shown to be robust with respect to parameters including the ionic strength as well as the molecular weight and bulk concentration of the polyelectrolyte.

Data from FIGARO were critically important in establishment of this new framework. Contributions included resolution of the individual polyelectrolyte and surfactant surface excesses of systems as a function of the sample preparation protocol, detection of liquid crystalline aggregates at interfaces from the presence of a Bragg peak and off-specular scattering, and fast/accurate determination of the interfacial composition of films that were spread from the interaction of aggregates with the air/water interface.