Phospholipids disrupt the interfacial network of proteins at the oil/water interface

Emulsions are part of everyday life, used, for example, in cosmetics, drug delivery, and food systems. Mixed interfaces in emulsions are quite common, and are composed of proteins and low molecular weight emulsifiers like phospholipids. However, research questions about the interfacial structure of mixed interfaces, such as their composition and arrangement, as well as their interfacial rheology and dynamics, remain unanswered. In this study, we hypothesize that the charge and nature of phospholipid head groups have a strong impact on the interfacial structure and rheology of protein-stabilized emulsions, but barely influence their interfacial dynamics. A combination of conventional methods - such as drop tensiometry and interfacial rheology - and advanced methods - such as small angle neutron scattering and neutron spin echo spectroscopy - helps us to answer research questions about complex interfacial systems. The head group of phospholipids strongly affects the interfacial structure and rheology of a β-lactoglobulin-stabilized emulsion. The interfacial structure was resolved using small-angle neutron scattering with partial structure factor analysis and coarse-grained modeling. The elastic interfacial protein layer is damaged by the addition of phospholipids. Phosphatidylcholine is loosely bound to the interface alongside β-lactoglobulin molecules, whereas phosphatidylglycerol is partially bound to β-lactoglobulin molecules via hydrogen bonds or hydrophobic interactions. The interfacial dynamics are characterized by 2D diffusion within the interfacial layer of the oil droplet and height fluctuations normal to the interfacial layer. The interfacial dynamics of the protein are inert for changes in interfacial structure, composition, and rheology, although structure and rheology have a strong influence on each other. These results provide guidance for the emulsion characteristics of food, cosmetics, and drug delivery systems.