Structure-Property Relations in Aqueous Foam and Their Control on a Molecular Level
(SUPERFOAM)

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Hierarchy Foam

Soft Matter Interfaces are present in various different systems which we either encounter in our daily lives (foams and emulsions in food products, personal care) but they do also have enormous technological applications (polymer foams e.g. for heat insulation). Often macroscopic properties of soft matter can be controlled by interfacial properties. For that reason, increasing our level of understanding on the physico-chemical properties of soft matter interfaces is at the very heart of current research. That is because the physical chemistry of interfaces is of critical importance for fundamental issues in molecular self-assembly, biomolecule interfaces (e.g. membrane proteins), colloid science, foam stability, catalysis and current topics in nano-technology. Studies of the interfacial molecular structure under relevant conditions are, however, extremely challenging. Many interfaces in soft matter where water plays a crucial role are also electrified interfaces and consequently solvation at interfaces is of great importance. Although the composition and structure of electrified interfaces can be very different they have a common similarity a strong electric field at the interface that drives ions, solvent molecules and other (solvated) molecules into ordered structures at the interface. Here, unifying concepts e. g. for the electric double-layer may apply to every surface. The aims of our group are to use surface specific nonlinear optical spectroscopy such as sum-frequency generation (SFG) and second-harmonic generation (SHG) with femtosecond laser pulses to study the effects of electrolyte composition and solvation in the context of self-organization at soft matter interfaces.

Interface controlled processes in foam

Schaum Schaum

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. There are also similarities to emulsion where the structure of oil-water interfaces drives macroscopic properties. Thus composition, conformation and intermolecular interactions of a few molecular layers at an aqueous interface - which are ubiquitous in foam - determine properties throughout the entire hierarchical chain. In order to control the properties of foam on larger length scales we need to control the surface properties of the gas-water interface which can be achieved by changing the lateral interactions of molecules within the interfacial plane e.g. from a repulsive to an attractive regime [1-4]. In the latter case biomolecules tend to agglomerate at the interface and form thicker layers which can be highly effective in preventing bubble coalescence and stabilizing the foam on a macroscopic scale. In addition, we could show that the foam rheology can be controlled from the molecular level [3].


Methodology of the SUPERFOAM project

The main focus of the ERC Starting Grant project SUPERFOAM will be on the molecular structure of interfaces and their in situ characterization. These respective experiments are a nucleation point for further studies on larger length scales - lamella, bubbles and macroscopic foam - which will be performed with solutions of identical composition. The approach will enable us to trace foam properties, bubble coa-lescence and lamella properties back to the actual molecular structures defining them. For that reason we will divide the project into small work packages, which we can handle experimentally. At the conclusion of this project, we can unite the individual parts into a single concept on how molecular structures at gas-water interfaces should look like in order to make foam with the desired properties.

Superfoam



Contact

Dr. Björn Braunschweig
Research Group: Interface Spectroscopy with Nonlinear Optics
Friedrich-Alexander University Erlangen-Nürnberg (FAU)
Institute of Particle Technology (LFG)
Cauerstrasse 4; D-91058 Erlangen; Germany
Phone: +49-9131-85 29580
bjoern.braunschweig@lfg.fau.de



Acknowledgements

This project recives funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No 638278)



References

[1] F. R. Beierlein, T. Clark, B. Braunschweig, K. Engelhardt, L. Glas, W. Peukert,
Carboxylate Ion Pairing with Alkali-Metal Ions for beta-Lactoglobulin and Its Role on Aggregation and Interfacial Adsorption
J. Phys. Chem. B 119, 5505 (2015)

[2] K. Engelhardt, U. Weichsel. E. Kraft, D. Segets, W. Peukert, and B. Braunschweig,
Mixed Layers of beta-Lactoglobulin and SDS at Air-Water Interfaces with Tunable Intermolecular Interactions
J. Phys. Chem. B; 118, 4098-4105 (2014)

[3] K. Engelhardt, W. Peukert and B. Braunschweig;
Vibrational sum-frequency generation at protein modi-fied air-water interfaces: Effects of molecular structure and surface charging
Curr. Opinion Coll. Int. Sci.;19, 207-215 (2014)

[4] K. Engelhardt, M. Lexis, G. Gochev, C. Konnerth, R. Miller, N. Willenbacher, W. Peukert, and B. Braunschweig;
pH Effects on the Molecular Structure of beta-Lactoglobulin Modified Air-Water Interfaces and Its Impact on Foam Rheology
Langmuir, 2, 11646-11655 (2013)

[5] K. Engelhardt, A. Rumpel, J. Walter, J. Dombrowski, U. Kulozik, B. Braunschweig and W. Peukert.
Protein adsorption at the electrified air-water interface: Implications on foam stability
Langmuir, 28, 7780-7787 (2012).

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