Effects of surface concentration,metals and acid synergists on autoxidation of linoleic acid monolayers on silica

To approximate in a model system the autoxidation of monomolecular layers of lipids on the cell surfaces of freeze-dried foods, the autoxidation of presumed monolayers of linoleic acid adsorbed from solution onto silica gel has been studied as a function of time and α-tocopherol and acid synergist content. The method of Honn, Bezman and Daubert was used, modified by the substitution of linoleic acid for soybean oil and the use of gas chromatography to follow oxygen disappearance at 80 C. It was found that adsorption of linoleic acid onto silica gel from petroleum ether solution conforms to a Langmuir isotherm, consistent with the formation of a monolayer. Confirming the finding of Honn et al. with soybean oil, it was found that the most rapid uptake of oxygen occurred at a linoleic acid-silica ratio close to that for the monolayer. Without included antioxidant, oxidation commences at a nearly linear rate without observable induction period. Time for consumption of one-half mole of oxygen per mole of linoleic acid is ca. 60 min on acid-washed silica. If very small amounts of α-tocopherol are included in the layer, virtually no oxygen uptake measurable in this system occurs during an induction period, the length of which is approximately proportional to tocopherol content. The inflection point at the commencement of rapid oxidation is very sharp; the ensuing oxidation rate approximates that of the unprotected acid. The induction period of linoleic acid with the same tocopherol content is as much as 100% longer when exposed in monolayer than in a bulk form. However the rate after commencement of rapid oxidation is 8–10 times greater in the monolayer. Acid washing of the silica reduced its iron content by 75%. Acid washing also reduced by 60% the rate of autoxidation without α-tocopherol and increased the length of the induction period four-fold when α-tocopherol was present. The effect of pretreatment of the silica by adsorption of the acid synergists, ascorbic, phosphoric, citric and ethylenediamine tetraacetic acid was qualitatively similar to the effect of acid washing. The synergists extended the induction period in increasing order as listed, EDTA producing a 100-fold extension. For ascorbic acid the rate reduction and increase of induction period were not found on unwashed silica and were dependent on the extent of washing. These findings are consistent with synergist sequestration of metals in a complex that is ineffective in new chain generation by perioxide decomposition.