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.