We have addressed surfactant proteins and polyelectrolytes and their equilibrium as well as non-equilibrium structures at air/water interfaces and their charging state and derived for these systems structure-property relations.
For polyelectrolytes where air/water interfaces were modified by oppositely charged poly(sodium 4-styrenesulfonate) (NaPSS) and hexadecyltrimethylammonium bromide (CTAB) polyelectrolyte/surfactant mixtures and which were studied on a molecular level with vibrational sum-frequency generation (SFG), tensiometry, and other surface methods. In order to deduce structure property relations, our results on interfacial PSS-/CnTA+ complexes (with n=12, 14 and 16) were compared to the stability and structure of macroscopic foam as well as to the bulk. PSS-/CTA+ complexes start to replace free CTA+ surfactants at the interface and thus reduce the interfacial electric and once fully screened at the interface, hydrophobic complexes dominate the interface and where they tend to aggregate. As a consequence, adsorbate layers with the highest film thickness, surface pressure and dilatational elasticity are formed. These surface layers provide much higher stabilities and foamabilities of polyhedral macroscopic foams. Mixtures around this concentration show precipitation after a few days, while their surfaces are in a local equilibrium state. Low concentrations result in a significant decrease in surface pressure and a complete loss in foamability. However, SFG and surface dilatational rheology provide strong evidence for the existence of PSS-/CTA+ complexes at the interface. At high concentrations, air-water interfaces are dominated by an excess of free polyelectrolytes.
For proteins we have done work on beta-lactoglobulin (BLG) and bovine serum albumin adsorption layers at air-water interfaces which were also studied with SFG and other surface methods to determine the interfacial charging state as a function of ion concentration (trivalent, divalent and monovalent) and electrolyte pH. The relation between the interfacial molecular structure of adsorbed BLG and the interactions with the supporting electrolyte is additionally addressed on higher length scales along the foam hierarchy – from the ubiquitous air-water interface to thin-foam films to macroscopic foam and clear structure-property relation are shown to exist in these systems. In fact, for the interaction of ions with BLG proteins, foam film measurements showed the formation of common black films modified by protein aggregates which become dominant at suitable conditions (pH, ionic strength) as micrographs of foam films clearly show. The presence of aggregates increases the stability of foam films and that of macroscopic foam due to Pickering effects, where the aggregates can form presumably a gel-like layer in the lamella which is their preferred location within foam as was demonstrated by fluorescence imaging of foams stabilized with labeled BSA proteins.
In case of surfactants, we have initially studied the mechanism of charge regulation and specific ion interaction at the air-water interfaces using SFG and foam film analysis where we used the experimentally determined disjoining pressure to study the surfactants double-layer potential and their dissociation degrees. This was done by using a new thin-film method which deploys IR interferometry to determine the thickness of the water core unambiguously. Furthermore, we can apply this method to more advanced surfactants that are responsive to external stimuli such as light and pH. In fact, active interfaces and foams that can respond to external stimuli such as light or temperature and change their chemistry on demand. For that we have shown that the photo-isomerization of surfactants such as arylazopyrazole (AAP) derivatives results in an unprecedented monolayer-to-bilayer transition upon photo-switching. This fast and reversible structural transformation helped to explain exceptionally large changes in the surface tension and surface excess upon photo-switching, and was rationalized in terms of the changing amphiphilic distribution of the molecules with respect to their conformations. We went on to show that this new mechanism can have strong effects on macroscopic length scales with superior photo-control of aqueous foams.
