pH Effects on the Molecular Structure of β‑Lactoglobulin Modified Air−Water Interfaces and Its Impact on Foam Rheology Kathrin Engelhardt, † Meike Lexis, ‡ Georgi Gochev, §,∥ Christoph Konnerth, † Reinhard Miller, § Norbert Willenbacher, ‡ Wolfgang Peukert, † and Bjö rn Braunschweig †, * † Institute of Particle Technology (LFG), University of Erlangen-Nuremberg, Cauerstrasse 4, 91058 Erlangen, Germany ‡ Institute of Mechanical Engineering, Karlsruhe Institute of Technology (KIT), Gotthard-Franz-Strasse 3, 76131 Karlsruhe, Germany § Max-Planck-Institute of Colloids and Interfaces, Am Mü hlenberg 1, 14476 Potsdam, Germany ∥ Institute of Physical Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria ABSTRACT: Macroscopic properties of aqueous β-lactoglo- bulin (BLG) foams and the molecular properties of BLG modified air−water interfaces as their major structural element were investigated with a unique combination of foam rheology measurements and interfacial sensitive methods such as sum- frequency generation and interfacial dilatational rheology. The molecular structure and protein−protein interactions at the air−water interface can be changed substantially with the solution pH and result in major changes in interfacial dilational and foam rheology. At a pH near the interfacial isoelectric point BLG molecules carry zero net charge and disordered multilayers with the highest interfacial dilatational elasticity are formed at the air−water interface. Increasing or decreasing the pH with respect to the isoelectric point leads to the formation of a BLG monolayer with repulsive electrostatic interactions among the adsorbed molecules which decrease the interfacial dilational elasticity. The latter molecular information does explain the behavior of BLG foams in our rheological studies, where in fact the highest apparent yield stresses and storage moduli are established with foams from electrolyte solutions with a pH close to the isoelectric point of BLG. At this pH the gas bubbles of the foam are stabilized by BLG multilayers with attractive intermolecular interactions at the ubiquitous air−water interfaces, while BLG layers with repulsive interactions decrease the apparent yield stress and storage moduli as stabilization of gas bubbles with a monolayer of BLG is less effective. 1. INTRODUCTION Foams as dispersions of gases in liquids show unique rheological properties: Under the application of comparatively small stresses they behave like a viscoelastic solid, while at higher stresses they become shear thinning and flow like a liquid. This mechanical behavior of foams in combination with a remarkably high surface area and low density leads to a variety of demanding applications. 1,2 Among the latter, protein foams that are present in dairy products 3−5 are in particular interesting since the physical and chemical properties of the inherent air− water interfaces largely determine the macroscopic properties of the foam. 6 As air−water interfaces are a basic structure element of aqueous foams, they can control foam rheology and other macroscopic properties such as foam stability. 7,8 For that reason it is of great importance to increase our level of understanding of protein adsorption and stabilization mecha- nisms at the interface of a foam lamella. The latter information would help to control and to tune foam properties such as foamability, foam stability, or mechanical properties of the macroscopic foam. In general, in situ molecular level studies of protein adsorption are needed to address changes in the composition and molecular structure of protein adsorption layers at the air−water interface directly. In the past, protein interfaces were studied with techniques such as ellipsometry, 9 neutron reflection, 10,11 X-ray reflectiv- ity, 12 Brewster angle microscopy, 13 and with surface tension measurements. 14,15 However, in recent years vibrational sum- frequency generation (SFG) has become a powerful tool for surface science studies of biointerfaces. 6,16−23 In this article we report the use of a combination of established analytical techniques such as bubble profile analysis tensiometry, surface dilational rheology, ellipsometry, and foam rheology measurements with vibrational SFG spectroscopy. This unique approach allows us to address not only single properties of foams or interfaces but also provides information on several length scales. As we will demonstrate the latter approach has enabled us to reveal composition, structure, and mechanical properties of β-lactoglobulin (BLG) interfacial Received: July 18, 2013 Revised: August 19, 2013 Published: August 20, 2013 Article pubs.acs.org/Langmuir © 2013 American Chemical Society 11646 dx.doi.org/10.1021/la402729g | Langmuir 2013, 29, 11646−11655