Is Brewster angle microscopy a useful technique to distinguish between isotropic domains in beta-casein-monoolein mixed monolayers at the air-water interface

The surface pressure (pi)-area (A) isotherms and Brewster angle microscopy (BAM) images of monoolein-beta-casein mixed films spread on buffered water at pH 5 and 7 and at 20 degrees C were determined as a function of the mass fraction of monoolein in the mixture (X). The structural characteristics, miscibility, and morphology of monoolein-beta-casein mixed films were very dependent on surface pressure and monolayer composition. The structure in m onoolein-beta-casein mixed monolayers was liquid-expanded-like, as for pure components. The monolayer structure was more expanded as the pH and the mo noolein concentration in the mixture were increased. From the concentration and surface pressure dependence on excess area, elasticity, and collapse p ressure it was deduced that monoolein and beta-casein form a practically im miscible monolayer at the air-water interface. The BAM images and the evolu tion with the surface pressure of the relative reflectivity of BAM images g ive complementary information on the interactions and structural characteri stics of monoolein-beta-casein mixed monolayers, which corroborated the con clusions derived from the pi-A isotherm. The morphology of monoolein, beta- casein, and monoolein-beta-casein domains at surface pressures lower than t hat for beta-casein collapse cannot be observed by BAM due to the fact that pure components and mixed monolayers form isotropic domains at the air-wat er interface. However, the high relative reflectivity of beta-casein domain s after the collapse point leads to the conclusion that monoolein was unabl e to displace totally the protein from the mixed monolayer at the air-water interface, even at higher monoolein concentrations in the mixture and at h igher surface pressures.