The variation of the [ potential of liposomes as a function of the ionic strength obtained under different osmotic conditions is studied. Dimyristoyl phosphatidylcholine (DMPC) multilamellar vesicles have been selected and the electrolyte used was NaCl. Three different methods were followed for the preparation of the vesicles.
(a) In aqueous medium at different ionic strengths. In this case, the liposomes are under iso-osmotic conditions. (b) Lipidic vesicles under hypo-osmotic conditions are obtained by diluting with water a suspension of liposomes prepared in iso-osmotic conditions as in (a).
(c) Lipidic vesicles under hyperosmotic conditions are obtained by the addition of NaCl to liposome samples prepared as in (a).
Under iso-osmotic conditions, the results allow us to obtain the ionic strength value corresponding to the maximum of the [ potential through a parabolic fit. The maximum stability and the minimum of aggregation occur at that ionic strength value; this is a similar behaviour to that expected from DLVO theory. Under non-iso-osmotic conditions the results do not follow the above behaviour. Under hyperosmotic conditions, the results follow an opposite behaviour to the above which could be explained by a reduction of liposome size and content. Under hypo-osmotic conditions, a strong decrease of the [ potential values takes place; therefore the tendency towards coalescence and aggregation increases.