FORMATION OF DIMERS IN LIPID MONOLAYERS

In this work, we analyse theoretically the hypothesis that zwitterioni c lipids form dimers in adsorption monolayers on water/hydrocarbon pha se boundary. A dimer can be modelled as a couple of lipid molecules wh ose headgroup lateral dipole moments have antiparallel orientation. Pr operties including surface pressure, chemical potentials and activity coefficients are deduced from a general expression for the free energy of the monolayer. The theoretical model is in a good agreement with e xperimental data for surface pressure and surface potential of lipid m onolayers. The results favour the hypothesis about formation of dimers in equilibrium with monomers, with the amount of the species dependin g on the area per molecule and temperature. The reaction of dimerisati on turns out to be exothermic with a heat of about 2.5 kT per dimer. T he results may be applied to the molecular models of membrane structur es and mechanisms.