Morphological and Structural Characteristics of Monoglyceride Monolayers at the Air−Water Interface Observed by Brewster Angle Microscopy

In this paper we report the morphology of monolayer domains for some typical lipids used as food emulsifiers (monopalmitin, monoolein, and monolaurin). In addition, we propound the utility of BAM for quantitative characterization of the relative film thickness as a function of the lipid and surface density. The surface pressure (π)−area isotherms and the Brewster angle microscopy (BAM) images of monopalmitin, monoolein, and monolaurin monolayers spread on buffered water at pH 7 and at 20 °C indicate that the morphology and structural characteristics of these lipids are very dependent on the hydrocarbon chain length and the presence of a double bond in the hydrocarbon chain. With a camera calibration it is possible to determine the relationship between the gray level and the relative reflectivity. The relative reflectivity allows the determination of the relative thickness of the monolayer. The present studies show the utility of a master curve of relative reflectivity versus surface pressure that is characteristic for any lipid. The results of the relative thickness measurements show that the monolayer thickness increases with the surface pressure and is maximum at the collapse point. The monolayer thickness is higher for monopalmitin monolayers and lower for monoolein monolayers. The thickness of the monolaurin film is halfway between those for monopalmitin and monoolein monolayers. The higher monolayer thickness correlates with the higher long-range lipid−lipid interactions and with closer molecular packing at the air−water interface.