title: Specific effects of Ca(2+) ions and molecular structure of β-lactoglobulin interfacial layers that drive macroscopic foam stability.
abstract: β-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 Ca(2+) 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 Ca(2+) 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 Ca(2+) 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 Ca(2+), 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 Ca(2+) concentrations where aggregates and interfacial layers with higher elasticity are found, showed the highest stability with much smaller bubble sizes.
authors.affiliation: Institute of Particle Technology (LFG), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Cauerstraße 4, 91058 Erlangen, Germany. bjoern.braunschweig@fau.de and Cluster of Excellence Engineering of Advanced Materials (EAM), Nägelsbachstr. 49b, 91052 Erlangen, Germany and Erlangen Graduate School in Advanced Optical Technologies (SAOT), Paul-Gordan-Straße 6, 91052 Erlangen, Germany and Interdisciplinary Center of Functional Particle Systems, Haberstraße 9a, 91058 Erlangen, Germany.
authors.forename: Björn
authors.initials: B
authors.lastname: Braunschweig
authors.affiliation: Institute of Particle Technology (LFG), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Cauerstraße 4, 91058 Erlangen, Germany. bjoern.braunschweig@fau.de and Erlangen Graduate School in Advanced Optical Technologies (SAOT), Paul-Gordan-Straße 6, 91052 Erlangen, Germany and Interdisciplinary Center of Functional Particle Systems, Haberstraße 9a, 91058 Erlangen, Germany.
authors.forename: Felix
authors.initials: F
authors.lastname: Schulze-Zachau
authors.affiliation: Institute of Particle Technology (LFG), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Cauerstraße 4, 91058 Erlangen, Germany. bjoern.braunschweig@fau.de.
authors.forename: Eva
authors.initials: E
authors.lastname: Nagel
authors.affiliation: Institute of Particle Technology (LFG), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Cauerstraße 4, 91058 Erlangen, Germany. bjoern.braunschweig@fau.de.
authors.forename: Kathrin
authors.initials: K
authors.lastname: Engelhardt
authors.affiliation: Institute of Physical Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria.
authors.forename: Stefan
authors.initials: S
authors.lastname: Stoyanov
authors.affiliation: Institute of Physical Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria and Max-Planck-Institute of Colloids and Interfaces, 14476 Golm/Potsdam, Germany.
authors.forename: Georgi
authors.initials: G
authors.lastname: Gochev
authors.affiliation: Institute of Physical Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria.
authors.forename: Khr
authors.initials: K
authors.lastname: Khristov
authors.affiliation: Institute of Physical Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria.
authors.forename: Elena
authors.initials: E
authors.lastname: Mileva
authors.affiliation: Institute of Physical Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria.
authors.forename: Dotchi
authors.initials: D
authors.lastname: Exerowa
authors.affiliation: Max-Planck-Institute of Colloids and Interfaces, 14476 Golm/Potsdam, Germany.
authors.forename: Reinhard
authors.initials: R
authors.lastname: Miller
authors.affiliation: Institute of Particle Technology (LFG), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Cauerstraße 4, 91058 Erlangen, Germany. bjoern.braunschweig@fau.de and Cluster of Excellence Engineering of Advanced Materials (EAM), Nägelsbachstr. 49b, 91052 Erlangen, Germany and Erlangen Graduate School in Advanced Optical Technologies (SAOT), Paul-Gordan-Straße 6, 91052 Erlangen, Germany and Interdisciplinary Center of Functional Particle Systems, Haberstraße 9a, 91058 Erlangen, Germany.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Peukert