Oppositely charged polyelectrolyte/surfactant mixtures control the properties of many of the consumer products that we use every day. While work has been carried out to understand the properties of these mixtures under dynamic conditions relevant to processing and applications [1–2], there is a growing awareness that also the static properties of such mixtures are strongly influenced by non-equilibrium effects [3–4]. We have worked over the last years to relate the interfacial properties of these systems to different non-equilibrium processes in the bulk and at interfaces [5–10].
In this seminar we describe how non-equilibrium effects in fact dominate the interfacial properties of these mixtures. Our work focuses on the systems Pdadmac/SDS and NaPSS/DTAB measured using a range of bulk and surface techniques including neutron reflectometry, ellipsometry and Brewster angle microscopy. We show that the extremely slow equilibration of the bulk means that these materials inevitably exist out of equilibrium conditions even if steady state interfacial properties exist in the meantime. The situation is complicated by the irreversible formation of liquid crystalline particles in the bulk. Their formation depletes the bulk solution yet their penetration into the interfacial modifies the macroscopic interfacial properties in different ways, e.g. their retention at the interface affects the rheological properties while their dissociation and spreading of material by Marangoni flow affects the surface tension. The penetration of the particles into the interface is mediated by different parallel processes including surface affinity, kinetic trapping and transport under gravity. The static interfacial properties of planar surfaces, droplets and foams are therefore necessarily all different.
We conclude with work on films spread from neutral aggregates. Dynamic perturbations reveal that their interfacial coverage is set by that of a single monolayer at the lowest surface area created. Potential for optimized preparation of memory-responsive films for deposition or encapsulation in a range of applications in the future is discussed.
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