Interfacial characteristics of diglyceride monolayers at the air/aqueous phase interface

The effect of temperature and subphase composition (water and aqueous solutions of ethanol and glucose) on structural characteristics of some diglycerides (distearin, dipalmitin, and diolein) at the air/water interface, were studied. Monolayers of 1,2-distearin were found to be condensed, while 1,3-distearin monolayers were more expanded and unstable. Distearin monolayers (mixture of both isomers at 50% wt/wt) basically showed a liquid-condensed structure until the collapse point. The additivity rule was not fulfilled and the collapse pressure changed with 1,2-distearin molar fraction, which is indicative of the existence of interactions between molecules of both lipids, the mixture being non-ideal. Addition of glucose stabilized the liquid-condensed structure, while ethanol reduced the interval of existence of this phase. Dipalmitin showed solid, liquid-condensed or liquid-expanded structures, as a function of temperature and surface pressure, but the solid phase is predominant. For diolein films, only the liquid-expanded structure was observed.