Effect of the addition of block copolymers on the formation and stability of vesicles (liposomes) prepared using soybean lecithin

Soybean lecithin disperses into water, forming multilamellar liposomes which, on sonication, produce vesicles of the order of 40–50 nm (diameter), as determined by Photon Correlation Spectroscopy (PCS). The effect of concentration of lecithin and sonication time was systematically investigated. Vesicles were then prepared by incorporation of A-B-A block copolymers of polyethylene oxide (PEO) and polypropylene oxide (PPO), i.e. (PEO-PPO-PEO). Experiments using electrophoresis showed a reduction in the ζ-potential of the vesicles on incorporation of the block 0copolymer which can be attributed to the shift of the shear plane.
Differential Scanning Calorimetry experiments showed changes in the transition temperature of the lipids. Such results provide some evidence of incorporation. The effect of the molecular weight of the PEO and PPO chains on the vesicle size was systematically studied by using various molecules to prepare the vesicles. Initial addition of these block copolymers causes an increase in the size of the vesicles. This increase continues until a certain concentration of block copolymer is reached, after which a decrease in size is observed. The initial increase was thought to be due to the incorporation of the block copolymer onto the vesicle bilayer. Various models are presented to describe this incorporation.