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 typica l lipids used as food emulsifiers (monopalmitin, monoolein, and monolaurin) . In addition, we propound the utility of BAM for quantitative characteriza tion of the relative film thickness as a function of the lipid and surface density. The surface pressure (pi)-area isotherms and the Brewster angle mi croscopy (BAM) images of monopalmitin, monoolein, and monolaurin monolayers spread on buffered water at pH 7 and at 20 degrees C indicate that the mor phology and structural characteristics of these lipids are very dependent o n the hydrocarbon chain length and the presence of a double bond in the hyd rocarbon chain. With a camera calibration it is possible to determine the r elationship between the g-ray level and the relative reflectivity. The rela tive reflectivity allows the determination of the relative thickness of the monolayer. The present studies show the utility of a master curve of relat ive reflectivity versus surface pressure that is characteristic for any lip id. The results of the relative thickness measurements show that the monola yer thickness increases with the surface pressure and is maximum at the col lapse 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 highe r monolayer thickness correlates with the higher long-range lipid-lipid int eractions and with closer molecular packing at the air-water interface.