This is due to the phosphate group present in the polar head
of phosphatidylcholine, which is directed towards the outside
of the vesicle. This direction is probably due to the low ionic
strength of the liposomal suspension. The fact that liposomes
are negatively charged is important, because the repulsion
of charges increases stability and prevents coalescence of
vesicles. However, this low negative charge is not enough
to keep them stable for long.12,39,40