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 characteristics of monoolein−β-casein mixed monolayers, which corroborated the conclusions derived from the π−A isotherm. The morphology of monoolein, β-casein, and monoolein−β-casein domains at surface pressures lower than that for β-casein collapse cannot be observed by BAM due to the fact that pure components and mixed monolayers form isotropic domains at the air−water interface. However, the high relative reflectivity of β-casein domains after the collapse point leads to the conclusion that monoolein was unable to displace totally the protein from the mixed monolayer at the air−water interface, even at higher monoolein concentrations in the mixture and at higher surface pressures.

Effects of a central metal on the organization of 5,10,15,20-tetra-(p-chlorophenyl)–rare earth porphyrin hydroxyl compound at the air/water interface and in Langmuir–Blodgett films

“…(The inset shows the structure of TbOH/GdOH.) low the additivity rule for a completely immiscible or ideally mixed monolayer [25][26][27] and we can obtain the total molecular area of the TbOH/SA mixture by using the equation…”

Iso-branched semifluorinated alkanes in Langmuir monolayers

“…1 during the whole range of compression. It is another proof that in the case of these molecules, 3D-aggregates are present at the interface and during compression there is virtually no change in gray level [33,34], and consistently in I/I 0 .…”

“…The values of I/I 0 for these monolayers, registered at their collapse pressures, are compiled in Table 3. According to Rodriguez Patino et al [33,34], Eq. (1) enables one to estimate changes of the monolayer's thickness upon compression.…”

“…Each image corresponds to a 228 µm × 170 µm monolayer's fragment. The film thickness can be estimated with BAM since the light intensity at each point of the BAM image depends on the local thickness and film optical properties [33,34], which can be measured by determining the light intensity at the camera and analyzing the polarization state of the reflected light following the method based on the Fresnel reflection equation [35]. At Brewster angle:…”

“…C is a constant, d is the film thickness, and R p is the p component of the light. The detailed description of the experimental procedure is described in the papers by Radriguez Patino et al [33,34]. In our measurements we determine the so-called relative intensity: I/I 0 , where I 0 is the intensity of the reflected light for an expanded monolayer at the beginning of compression, while I is the intensity of the reflected light at any point of compression.…”