The Influence of Ionic Strength and Lipid Bilayer Charge on the Stability of Liposomes

Abstract

The effect of including a negative charge-inducing agent (phosphatidic acid) on the zeta potential and vesicle aggregation of large unilamellar vesicle liposomes (LUV) in the presence of neutral electrolytes was investigated with the aim of studying their physicochemical stability. Liposome suspensions were prepared by a reverse phase evaporation method varying the ionic strength of electrolytes. Zeta potential values were obtained by microelectrophoresis measurements. The Eversole and Boardman equations which relate the measured zeta potential to the thickness of the electrical double layer was found to be valid in all cases. The surface charge densities in these liposomes were estimated using the surface potential values calculated from this equation. The lipid compositions used were phosphatidylcholine/phosphatidic acid (PC:PA) 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°C. The surface charge density obtained from the zeta potential determinations increased when the electrolyte concentration in the aqueous medium increased, in all the PC:PA lipid compositions tested, the highest values being obtained for the NaBr and the lowest for the Na2SO4. Similar tendencies were obtained from experiments testing the influence of the investigated salts on liposome stability with respect to the aggregation 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 phospholipid components of liposome suspensions as a function of time.