Oxidation of cod phospholipids in liposomes: Effects of salts, pH and zeta potential
This work examines how different salts and pH influence both the zeta potential and the lipid oxidation rate of liposomes made from cod phospholipids. The rate of Fe 21induced lipid oxidation was measured by consumption of dissolved oxygen by liposomes in a closed vessel. Cations (Na 1 , K 1 , Ca 1 , Mg 1 ) did not influence the rate of oxidation in the tested range [ionic strength (I) 0-0.14 M). Among the tested anions, sulphates and nitrates did not significantly change the oxygen uptake rate, but chlorides (KCl, NaCl, CaCl 2 ) reduced the oxidation rate down to approximately 45%, and dihydrogen phosphate down to 14%, when I = 0.14 M. The effect of Cl 2 and H 2 PO4 4 2 was additive. Addition of salts increased the zeta potential of the liposomes, divalent cation salts even resulted in a positive zeta potential. When the liposomes contained different concentrations of chlorides, a linear relationship between oxygen uptake rate and zeta potential was observed. When phosphate was added, the oxygen uptake rate was not related to the changes in zeta potential. The decrease in pH was followed by an increase in zeta potential. The oxygen uptake rate did not change significantly at different positive zeta potentials (pH ,3). When the zeta potential was negative, the oxygen uptake rate was influenced by the zeta potential and may also be influenced by iron solubility. Absolute values of the zeta potential alone cannot be used to predict oxidation rates.