Non-ionic block copolymers and proteins at the air-water interface

Sammanfattning: The behavior of block copolymer and protein films at interfaces is of central importance to their function in many application areas. Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers function e.g. as emulsifiers and in coatings in oral, parenteral, and topical pharmaceutical formulations and are also widely used for cleaning purposes. The globular protein ?-lactoglobulin is the most abundant whey protein and it is used within the food industry as a component of many common foods. To understand the interfacial behavior of block copolymers and proteins in such applications one has to study molecular processes like adsorption, the responses to shear and dilatational deformation of the interface (i.e. surface rheology), conformational changes of the molecules, and forces that are generated between interfacial layers. Such investigations form the focus of the work presented in this thesis.Thin film measurements with a series of block copolymers composed of poly(ethylene oxide) and poly(butylene oxide) revealed that the thickness of foam films (air-aqueous solution-air) is determined by electrostatic and steric repulsion. The range of the steric repulsion is directly dependent on the size of the polymer, in particular the length of the poly(ethylene oxide) chains. Block copolymers attached to the air-water interface change their conformation with increasing surface concentration. The study of the dilatational modulus as a function of the surface pressure of spread and adsorbed block copolymer layers showed that the block copolymers pass from a two-dimensional conformation to a three-dimensional extended structure as the available area per molecule decreases. Relaxation processes occur mainly by a redistribution of segments within the surface layer. Mixing of the hydrophobic poly(propylene oxide) segments in a sublayer of poly(ethylene oxide) and water was proposed to occur at high surface concentration of the polymer. Conformational changes (partial denaturation) of globular proteins upon adsorption at the air-water interface leads to the formation of highly viscoelastic networks, which play an important role for the stabilization of foams and emulsions. Mixed layers, formed by competitive adsorption of ?-lactoglobulin and block copolymers, were also investigated. On increasing the concentration of block copolymer, the viscoelasticity of the mixed protein/block copolymer layer gradually decreased. The polymer was able to displace the protein from the interface. The drainage rate of thin liquid films was also enhanced by the presence of block copolymer. Whether or not the incorporation of the polymer in the ?-lactoglobulin layer has a detrimental effect on foam stability depends on the ability of the polymer to generate long-range steric repulsion across the foam films. For pure block copolymer systems, correlations between foam stability and the thickness of interfacial layers, in the steric regime, were found. The results obtained can be utilized for increasing the understanding of the mechanisms for foam stabilization in both model and industrial systems.