αs-Casein—PE6400 mixtures: Surface properties, miscibility and self-assembly

Surface properties, miscibility and self-assembly of mixtures of a food-grade αs-casein and the triblock copolymer PE6400 (PEO13–PPO30–PEO13) were examined. The properties at the surface were determined by surface pressure measurements for a 1:1 molar mixture. Comparison of the measured with the calculated isotherms show attractive interactions at surface pressures above 9 mN/m. The miscibility gaps of solutions containing 0.004–0.2 mmol/l αs-casein and 0.02–0.1 mol/l polymer were examined. It was found that a one-phase region exists at distinct mixing ratios and temperatures. Comparison of the cloud points of mixtures of αs-casein and PE6400 with pure αs-casein showed that the presence of the triblock copolymer enhanced the solubility of the protein. The ζ-potential of the αs-casein solution decreased by addition of PE6400 to zero. Our results thus suggest that αs-casein and PE6400 are miscible. The results of the cloud point and ζ-potential measurements were explained by formation of a mixed aggregate where the PPO chains are anchored inside the hydrophobic part of the αs-casein while the PEO chains cover the charged hydrophilic part of the αs-casein thereby leading to an increase of the cloud point and a decrease in ζ-potential. This is in agreement with the attractive interactions between αs-casein and PE6400 as observed via surface pressure measurements at the surface.