Spreading of solid glycerides and phospholipids at the air—water interface

Abstract

The equilibrium spreading pressures ( π e ) of a series of glycerides and phospholipids have been determined at various temperatures. Pure mono, di- and triglycerides and phosphatidylethanolamines can spread spontaneously from the crystal to form monomolecular films at the air—water interface, whereas phosphatidylcholines (lecithins) only spread from the liquid-crystal (i.e., at temperatures above the crystal or gel to liquid crystal transition temperature, T c ). Film pressures of up to 50 m N m −1 can be attained. Glycerides and phosphatidylcholines form liquid expanded monolayers in equilibrium with the bulk lipid while phosphatidylethanolamines can form both condensed and liquid expanded films. Liquid expanded monolayers are stable with respect to the bulk, lipid phase when π< π e ; compression of such monolayers to pressures π> π e leads to collapse and formation of bulk, lipid phase without significant overcompression. Condensed monolayers of phosphatidylcholine are metastable ( π e = 0 when T < T c ) and significant overcompression is observed before collapse to the gel or the monohydrate occurs. The equilibrium spreading pressures increase linearly with temperature until changes in physical state of the bulk lipid occur. On fusion or transition to a liquid crystalline state, the temperature coefficient of the equilibrium spreading pressure changes sign. The enthalpies and entropies of spreading are used to gain information on the structure of the lipid monolayers. The entropy of spreading and thus the order in the resulting monolayer is about midway between that of the crystal and the melt, depending on whether the compound spreads to a condensed or liquid expanded monolayer.