Structural, topographical, and rheological characteristics of β-casein–dioleoyl phosphatidylcholine (DOPC) mixed monolayers

The surface pressure (π)–area (A) isotherms and Brewster angle microscopy (BAM) images of β-casein–dioleoyl phosphatidylcholine (DOPC) mixed films spread on buffered water at pH 7 and 9 and at 20 °C were determined as a function of the mass fraction of DOPC in the mixture (XDOPC). The structural characteristics, miscibility, and topography (morphology and reflectivity) of DOPC–β-casein mixed films were very dependent on surface pressure and monolayer composition. The structure in DOPC–β-casein mixed monolayers was liquid-expanded-like, as for pure components. The monolayer structure was more expanded as the pH and the DOPC concentration in the mixture were increased. From the concentration and surface pressure dependence on excess area and elasticity (E) it was deduced that DOPC 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 reflectivity of BAM images give complementary information on the interactions and structural characteristics of DOPC–β-casein mixed monolayers, which corroborate the conclusions derived from the π–A isotherm. After the spreading or just after the expansion at π ≈ 0 we have observed the presence of 2D-foams, typical topography of DOPC monolayers at low π. The 2D-foams disappear after the compression of the monolayer and the topography is homogenous and isotropic. From the reflectivity of BAM images it is possible to distinguish between the coexistence of DOPC and β-casein or β-casein displacement by DOPC, depending on the surface pressure. The surface dilatational properties of the mixed films corroborate the coexistence of DOPC and β-casein at π lower than the equilibrium spreading pressure (πe) of β-casein and β-casein displacement by DOPC at π > πe of β-casein. The phenomena observed appear to be generic for protein and polar (monoglycerides) and ionizable (phospholipid) mixed monolayers.