Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - SUPERFOAM (Structure-Property Relations in Aqueous Foam and Their Control on a Molecular Level)

Teaser

Foams are ubiquitous in our daily lives be it as milk foam on our cappuccino or as heat insulation of the building we live in. The various technological applications range from lightweight materials, waste water treatment to the recycling of rare earth metals via ion...

Summary

Foams are ubiquitous in our daily lives be it as milk foam on our cappuccino or as heat insulation of the building we live in. The various technological applications range from lightweight materials, waste water treatment to the recycling of rare earth metals via ion flotation, just to mention a few. The vast number of possibilities for the use of foam in industrial processes and products originates from a unique tunability of its optical, mechanical as well as chemical properties. This makes foams to an exciting object of current interdisciplinary research.
Foams are hierarchical materials and as such they are greatly affected by the arrangement and distribution of gas bubbles on a macroscopic scale as well as on thickness and composition of lamella on a mesoscopic scale. Although they are hidden in the bulk, liquid-gas interfaces are a building block of foams with overwhelming importance as their properties on a molecular scale can easily dominate hierarchical elements on larger length scales. Thus, composition, conformation and intermolecular interactions of a few molecular layers at liquid-gas interfaces - that are ubiquitous in aqueous foam - can decide whether the produced macroscopic foam has the desired properties or not. As we will show below, this is not only determined by the actual composition of an interface, but depends dramatically on the interactions of interfacial molecules, ions and water molecules. Making foam - particularly from aqueous solutions – is surprisingly easy as one “simply” has to lower the water’s surface tension e.g. by additions of surface active molecules such as proteins or surfactants. However, in most cases the produced foam is inherently unstable. The latter is actually the bottle-neck in foam formulation which is usually performed purely empirically, because starting from a specially engineered interface; one has to control the actual driving forces throughout the entire hierarchical chain. For that reason understanding and controlling foam properties with a bottom-up approach is a major challenge in current research.
In order, to put foam formulation and also our knowledge on foams on a molecular basis we need to characterize the latter in-situ and on a molecular level. For that reason, our goal is to identify molecular building blocks which are comprised of solutes, ions and solvents at interfaces and their interactions that make the most stable foams or drive other foam properties. Once identified and understood, we can use these building blocks to break the ground for new ways in foam formulation and related fields. Considering the tremendous importance of foams as mentioned above, we expect that our approach will have substantial outreach.

Work performed

So far we have addressed proteins and polyelectrolytes and their equillibrium as well as nonequillibrium structures at air/water interfaces and their charging state and derived for these systems structure property relations. That is in particular 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, surface dilatational rheology and ellipsometry. In order to deduce structure property relations, our results on the interfacial molecular structure and lateral interactions of PSS-/CTA+ complexes were compared to the stability and structure of macroscopic foam as well as to bulk properties. For that, the CTAB concentration was fixed to 0.1 mM, while the NaPSS concentration was varied. At NaPSS monomer concentrations <0.1 mM, PSS-/CTA+ complexes start to replace free CTA+ surfactants at the interface and thus reduce the interfacial electric field in the process. This causes the O-H bands from interfacial H2O molecules in our SFG spectra to decrease substantially, which reach a local minimum in intensity close to equimolar concentrations. Once electrostatic repulsion is fully screened at the interface, hydrophobic PSS-/CTA+ complexes dominate and tend to aggregate at the interface and in the bulk solution. 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 to air are in a local equilibrium state. Concentrations >0.1 mM 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 polyelectrolyte concentrations >10 mM, air-water interfaces are dominated by an excess of free PSS- polyelectrolytes and small amounts of PSS-/CTA+ com-plexes which, however, provide higher foam stabilities compared to CTAB free foams. The foam structure undergoes a transition from wet to polyhedral foams during the collapse.
For proteins we have done so far the following work where beta-lactoglobulin (BLG) adsorption layers at air-water interfaces were studied in situ with vibrational sum-frequency generation (SFG), tensiometry, surface dilatational rheology and ellipsometry as a function of bulk Ca2+ concentration. 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 through thin foam films to macroscopic foam. For concentrations <1 mM, a strong decrease in SFG intensity from O-H stretching bands and a slight increase in layer thickness and surface pressure are observed. A further increase in Ca2+ concentrations above 1 mM causes an apparent change in the polarity of aromatic C-H stretching vibrations from interfacial BLG which we associate to a charge reversal at the interface. Foam film measurements show formation of common black films at Ca2+ concentrations above 1 mM due to considerable decrease of the stabilizing electrostatic disjoining pressure. These observations also correlate with a minimum in macroscopic foam stability. For concentrations >30 mM Ca2+, micrographs of foam films show clear signatures of aggregates which tend to increase the stability of foam films. Here, the interfacial layers have a higher surface dilatational elasticity. In fact, macroscopic foams formed from BLG dilutions with high Ca2+ concentrations where aggregates and interfacial layers with higher elasticit

Final results

So far we could show that directed aggregation of proteins and polyelectrolytes and the self-assembly of aggregates can be used to stabilize foam. This is beyond the classical picture of foam stabilization e.g. via the electrostatic disjoing pressure. In addition, we are currently constructing a new device that will allow us to study the molecular composition of thin foam films (even at thicknesses of only a few nanometers). This experiment will help us further to adress this hiearchical element of foam and provide unprecedented information and hitherto not achivable information of foam film.