Structural and morphological characteristics of β-casein monolayers at the air–water interface

Abstract

Brewster angle microscopy (BAM) coupled with surface pressure ( π)–area ( A) isotherms was used to visualize and determine structural characteristics of β-casein monolayers at the air–water interface as a function of subphase pH. The measurements were performed at 20°C. From the results of π– A isotherms, it can be concluded that β-casein monolayers at the air–water interface adopt two different structures. The monolayer structure was more condensed on acidic aqueous solutions. From BAM images, the domains that the residues of β-casein molecules adopt at the air–water interface appeared to be of uniform reflectivity, suggesting homogeneity in thickness and film isotropy. A method was applied to measure the relative reflectivity and relative film thickness. The relative reflectivity versus surface pressure plots reflects the surface equation of state for the spread β-casein monolayer, and is particularly sensitive of the existence of transitions in β-casein monolayers with compression. The relative film thickness increased with the film compression and was a maximum at the collapse point. At the same surface pressure, the relative film thickness is independent of the pH, but the surface density is higher at pH 5 than at pH 7, as a consequence of the more compact packing of β-casein residues on acidic subphases.