The Effect of pH on Monoglyceride–Caseinate Mixed Monolayers at the Air–Water Interface

Abstract

The surface pressure (π) area (A) isotherms and Brewster angle microscopy (BAM) of monoglyceride–caseinate mixed films spread on buffered water at pH 5 and 7 and at 20°C were determined as a function of the mass fraction (X) of monoglyceride (monopalmitin or monoolein) in the mixture. The structural characteristics, miscibility, and morphology of monoglyceride–caseinate mixed films are very dependent on surface pressure and monolayer composition. The monolayer structure was more expanded as the pH and the monoglyceride concentration in the mixture were increased. From the concentration and surface pressure dependence on excess area, free energy, and collapse pressure, it was deduced that, at a macroscopic level, monoglyceride (either monopalmitin or monoolein) and caseinate 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 monoglyceride–caseinate mixed monolayers, which at a microscopic level corroborated in part the conclusions derived from the π–A isotherm at a macroscopic level. Over the overall range of existence of the mixed film the monolayer presents some heterogeneity due to the fact that domains of monoglyceride (especially of monopalmitin) and spots of collapsed caseinate residues are present during the monolayer compression–expansion cycle, giving relative intensity peaks with high relative film thickness. At higher π, after the caseinate collapse, characteristic squeezing-out phenomenon was observed. At the monoglyceride monolayer collapse the mixed film is practically dominated by the presence of monoglyceride. The prevalence of monoglyceride in the interface increases with the amount of monoglyceride in the mixture and at higher π. However, some degree of interactions exists between monoglyceride and caseinate in the mixed film and these interactions are more pronounced as the monolayer is compressed at the highest surface pressures.