Is Brewster angle microscopy a useful technique to distinguish between isotropic domains in beta-casein-monoolein mixed monolayers at the air-water interface
Jmr. Patino et al., Is Brewster angle microscopy a useful technique to distinguish between isotropic domains in beta-casein-monoolein mixed monolayers at the air-water interface, LANGMUIR, 15(14), 1999, pp. 4777-4788
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.