Jmr. Patino et al., Morphological and structural characteristics of monoglyceride monolayers at the air-water interface observed by Brewster angle microscopy, LANGMUIR, 15(7), 1999, pp. 2484-2492
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.