A kinetic study of the autoxidation of methyl linoleate and linoleic acid emulsions in the presence of sodium chloride

Autoxidation of linoleic acid and methyl linoleate emulsions in aqueous buffer solutions was studied by the rate of oxygen uptake. The oxidation rates of methyl linoleate emulsions increased with an inerease in the pH of the buffer solution. With linoleic acid, oxidation rates rose until the increase reached its peak at pH 5.50 and then decreased gradually to a minimum at pH 8.00. Oxidation rates of methyl linolente and linoleic acid emulsious decreased with increased concentration of NaCl in the system. The effect of variation of pH of the emulsion in the range investigated was similar to that in emulsions without NaCl. There was no evidence that NaCl accelerated the oxidation rates in the system. The observed inhibitory effect of NaCl may result from the decreased solubility of oxygen in the emulsion with the increased concentration of NaCl. Consequently the availability of oxygen would be a limiting factor in oxidation rates. The activation energy for the monomolecular and bimolecular reactions of methyl linoleate and linoleic acid autoxidation was found to be independent of the pH value and sodium chloride concentration of the system. The energy of activations for the monomolecular and bimolecular reactions of methyl linoleate and of linoleic acid are 22,000, 18,200, 19,600, and 16,400 cal./mol., respectively. Spectrophotometric studies of the autoxidized emulsions of linoleic acid and its methyl ester indicate that the magnitude of the absorption at 2325 Å is the same at different pH values. On the contrary, the secondary products showing absorption at 2775 Å are to some extent dependent on the pH value of the emulsion.