The effects of the thermal prephase transition and salts on the coagulation and flocculation of phosphatidylcholine bilayer vesicles.

Absorbance measurements of sonicated dipalmitoyl phosphatidylcholine vesicles reveal two aggregation processes: flocculation and coagulation. Flocculation is only observed for samples in monovalent cationic salt solutions or in salt-free suspensions. This process is abolished in the presence of di- or trivalent cations. It is also found to be strongly temperature dependent, occurring only below the thermal prephase transition of the lipid. Dispersal of the flocculates is rapid but they re-form at a rate dictated by the hysteresis in the prephase transition. In contrast, coagulation is slow. The extent of coagulation does not seem to be strongly dependent on the temperature, the nature of the electrolyte or its concentration. The relation of the coagulated state to vesicle-vesicle fusion is briefly discussed.