Small-angle neutron scattering of aqueous dispersions of lipids and lipid mixtures. A contrast variation study

The present study presents the first small-angle neutron scattering experiment of lipid bilayers in excess water. The method of solvent contrast variation was applied. The variation of scattering intensity with the scattering angle could be analysed in terms of the Kratky-Porod model of scattering by quasi-two-dimensional systems. From this the bilayer thickness of unilamellar vesicles of dimyristoyllecithin was determined to be d = 41 ± 1 Å at 310 K. Several typical mixtures of different lecithins with varying chain lengths and of lecithin with phosphatidic acid were studied. By deuteration of one lipid component a very large contrast between segregated phases could be achieved. It was thus possible to distinguish clearly between homogeneous mixtures and mixtures which exhibit a heterogeneous lipid organization. In the latter case no intensity matching upon solvent contrast variation was possible. It is shown that phase boundaries may be determined very accurately by performing contrast variation experiments with at most two mixtures of different initial composition. The main advantage of the present method is that lateral phase separation may also be clearly studied in fluid states of the bilayer. From the average scattering-length densities one can determine the density of the lipid layers and thus the excess volumes of the mixtures. For lecithin mixtures positive excess volumes are observed.