THE INFLUENCE OF IONIC-STRENGTH AND LIPID BILAYER CHARGE ON THE STABILITY OF LIPOSOMES

The effect of including a negative charge-inducing agent (phosphatidic acid) on the zeta potential and vesicle aggregation of large unilamel lar vesicle liposomes (LUV) in the presence of neutral electrolytes wa s investigated with the aim of studying their physicochemical stabilit y. Liposome suspensions were prepared by a reverse phase evaporation m ethod varying the ionic strength of electrolytes. Zeta potential value s were obtained by microelectrophoresis measurements. The Eversole and Boardman equations which relate the measured zeta potential to the th ickness of the electrical double layer was found to be valid in all ca ses. The surface charge densities in these liposomes were estimated us ing the surface potential values calculated from this equation. The li pid compositions used were phosphatidylcholine/phosphatidic acid (PC:P A) 10:0, 9.5:0.5, 9.0:1.0, and 8.0:2.0 molar ratios. The electrolytes used were Na2SO4, NaCl, and NaBr in the range of concentrations from 1 .0 to 150 mM using a buffered solution (pH 7.20) at 25-degrees-C. The surface charge density obtained from the zeta potential determinations increased when the electrolyte concentration in the aqueous medium in creased, in all the PC:PA lipid compositions tested, the highest value s being obtained for the NaBr and the lowest for the Na2SO4. Similar t endencies were obtained from experiments testing the influence of the investigated salts on liposome stability with respect to the aggregati on at different PC:PA lipid compositions. Stability measurements were made determining the alterations in the vesicle size distribution, the lipid oxidation level, and the possible hydrolysis of the phospholipi d components of liposome suspensions as a function of time. (C) 1994 A cademic Press, Inc.