Is Brewster Angle Microscopy a Useful Technique To Distinguish between Isotropic Domains in β-Casein− Monoolein Mixed Monolayers at the Air−Water Interface?

Abstract

The surface pressure (π)−area (A) isotherms and Brewster angle microscopy (BAM) images of monoolein−β-casein mixed films spread on buffered water at pH 5 and 7 and at 20 °C were determined as a function of the mass fraction of monoolein in the mixture (X). The structural characteristics, miscibility, and morphology of monoolein−β-casein mixed films were very dependent on surface pressure and monolayer composition. The structure in monoolein−β-casein mixed monolayers was liquid-expanded-like, as for pure components. The monolayer structure was more expanded as the pH and the monoolein concentration in the mixture were increased. From the concentration and surface pressure dependence on excess area, elasticity, and collapse pressure it was deduced that monoolein and β-casein form a practically immiscible monolayer at the air−water interface. The BAM images and the evolution with the surface pressure of the relative reflectivity of BAM images give complementary information on the interactions and structural ...