Morphology of semisolid aqueous phosphatidylcholine dispersions, a freeze fracture electron microscopy study

Semisolid aqueous dispersions of soy phosphatidylcholine (PC) with high PC mass fractions between 0.35 and 0.6 were prepared using a high-pressure homogeniser. Their morphologies were investigated by freeze fracture transmission electron microscopy (FF-TEM) with regard to the quantity of water present. With all lipid/buffer ratios studied, vesicular structures in dense packing were seen on FF-TEM micrographs. PC ratios of up to 0.45 yielded small, unilamellar vesicles which were uniform in size. Matrices of higher lipid/buffer-ratios (0.45-0.6) also contained these small unilamellar vesicles plus an increasing fraction of more heterogeneous large multivesicular or multilamellar vesicles. Apparently, organisation of PC into small, homogeneous and unilamellar vesicles is not limited to 'classical' liposome dispersions, i.e. dilute lipid dispersions (< 300 mM), but can also be achieved in more concentrated lipid dispersions of PC fractions of up to 0.45 (600 mM) when lipid swelling is exerted under high mechanical stress conditions. (C) 1997 Elsevier Science Ireland Ltd.