The effect of pH on monoglyceride-caseinate mixed monolayers at the air-water interface

The surface pressure (pi) 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 degreesC were determined as a function of the mass fract ion (X) of monoglyceride (monopalmitin or monoolein) in the mixture. The st ructural characteristics, miscibility, and morphology of monoglyceride-case inate mixed films are very dependent on surface pressure and monolayer comp osition. The monolayer structure was more expanded as the pH and the monogl yceride 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 (eithe r monopalmitin or monoolein) and caseinate form a practically immiscible mo nolayer at the air-water interface. The BAM images and the evolution with t he surface pressure of the relative reflectivity of BAM images give complem entary information on the interactions and structural characteristics of mo noglyceride-caseinate mixed monolayers, which at a microscopic level corrob orated in part the conclusions derived from the pi -A isotherm at a macrosc opic level. Over the overall range of existence of the mixed film the monol ayer presents some heterogeneity due to the fact that domains of monoglycer ide (especially of monopalmitin) and spots of collapsed caseinate residues are present during the monolayer compression-expansion cycle, giving relati ve intensity peaks with high relative film thickness. At higher pi, after t he caseinate collapse, characteristic squeezing-out phenomenon was observed . At the monoglyceride monolayer collapse the mixed film is practically dom inated by the presence of monoglyceride. The prevalence of monoglyceride in the interface increases with the amount of monoglyceride in the mixture an at higher pi. However, some degree of interactions exists between monoglyc eride and caseinate in the mixed film and these interactions are more prono unced as the monolayer is compressed at the highest surface pressures. (C) 2001 Academic Press.