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In this work we have analyzed the structural and topographical characteristics of mixed monolayers formed by an adsorbed whey protein isolate (WPI) and a spread monoglyceride monolayer (monopalmitin or monoolein) on the previously adsorbed protein film. Measurements of the surface pressure (pi)-area (A) isotherm were obtained at 20 degrees C and at pH 7 for protein-adsorbed films from water in a Wilhelmy-type film balance. Since the surface concentration (1/A) is actually unknown for the adsorbed monolayer, the values were derived by assuming that the A values for adsorbed and spread monolayers were equal at the collapse point of the mixed film. The pi-A isotherm deduced for adsorbed WPI monolayer in this work is practically the same as that obtained directly by spreading. For WPI-monoglyceride mixed films, the pi-A isotherms for adsorbed and spread monolayers at pi higher than the equilibrium surface pressure of WPI are practically coincident, a phenomenon which may be attributed to the protein displacement by the monoglyceride from the interface. At lower surface pressures, WPI and monoglyceride coexist at the interface and the adsorbed and spread pi-A isotherms (i.e., the monolayer structure of the mixed films) are different. Monopalmitin has a higher capacity than monoolein for the displacement of protein from the air-water interface. However, some degree of interactions exists between proteins and monoglycerides and these interactions are higher for adsorbed than for spread films. The topography of the monolayer corroborates these conclusions.

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The surface pressure (π)−area (A) isotherms and Brewster angle microscopy (BAM) of monoglyceride and whey protein isolate (WPI) 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. From the X and π-dependence on excess area, free energy, collapse pressure, BAM images, and the evolution with π of the relative reflectivity (I) of BAM images, it was deduced that the structural characteristics, miscibility, and morphology of monoglyceride−WPI mixed films are very dependent on surface pressure and monolayer composition. The monolayer was more expanded as the monoglyceride concentration in the mixture was increased. Over the overall range of existence of the mixed film, the monolayer presents some heterogeneities. At higher π, after the WPI collapse, characteristic squeezing-out phenomena were observed. At the monoglyceride monolayer collapse, the mixed film was practically dominated by the presence of monoglyceride. However, some degree of interactions exists between monoglyceride and WPI in the mixed film, and these interactions are more pronounced as the monolayer is compressed at the highest surface pressures.