Comparisons and correlations between the properties of lipid molecules in crystals, bilayer dispersions in water, and monolayers on a water surface

Abstract

The main types of packing observed in lipid crystals and bilayers under different physical conditions of temperature and water content involve compromise between the influence of the polar groups which tend to organize the molecules on a rectangular lattice and that of the chains which prefer a hexagonal structure. When compressed as a monolayer on top of a water surface, lipids display different phase transitions. The comparison between bilayer and monolayer data allows the following identifications for the monolayer phases. (a) The gas-liquid expanded (LE) transition corresponds to an electrostatic condensation process, the gas phase is a system of noninteracting molecules in which chains are disordered, whereas the liquid expanded phase is a regular rectangular lattice of polar groups with disordered chains. (b) The liquid expanded (LE)-liquid condensed (LC)-solid (S) transition corresponds to a chain ordering, in the liquid condensed and solid phases the polar group lattice is not changing qualitatively from what it was in the liquid expanded phase but the chains are ordered in a hexagonal packing of cylinders. The monolayer phases LE and S are identified with the bilayer phases 11 and 12 and the LC phase appears because the LE and S phases are not in equilibrium. The different behavior of bilayers and monolayers, resulting from an extra air/hydrocarbon chain interface in the monolayer case, has been compared and, from the wealth of structural information known on the bilayer structure, some insight has been gained on identification of monolayer phases.