The autoxidative contraction of monolayers of linoleic acid at a gas‐water interface

Abstract

A monolayer of linoleic acid spread at an airwater interface was found to undergo an immediate oxygen‐dependent contraction when compressed at a constant high film pressure. This is in contrast to the expansion of monolayers of other unsaturated acids at lower film pressures as reported by other workers. The process shows quasi‐first‐order kinetics, and there is little evidence for an induction period, in contrast to bulk‐phase autoxidation. Initial rate of contraction is a function of pH between 6 and 9. This and other data suggest that contraction is due to the titration of the packed adjacent carboxyl groups in the monolayer and to the solution of short‐chain soaps resulting from the oxidative scission of linoleic acid. Although complete anaerobiasis prevented film contraction, neitherdl‐alpha‐tocopherol deposited in the film, nor hydroquinone in the substrate solution were inhibitory alone. Butylated hydroxyanisole (BHA) present in an aerobic gas phase produced inhibition of contraction of linoleic acid films spread on completely deaerated substrates. Inhibition was released upon removal of the BHA vapor, the restored rate in air approaching that before the BHA treatment. A method of specific testing of antioxidants is thus provided, which will differentiate between those presented in the vapor, the lipid, or the solution phases.

Molecular Structure upon Compression and Stability toward Oxidation of Langmuir Films of Unsaturated Fatty Acids: A Vibrational Sum Frequency Spectroscopy Study

“…This effect is also reflected in the formation of different morphologies in the condensed phases . Nonetheless, unsaturated films were early on recognized to be unstable and different mechanisms, including autoxidation, were put forward to explain this behavior. , The autocatalytic oxidation of unsaturated lipids and fatty acids in the bulk has been studied since the 1800s, but it was not until the 1940s that a chain reaction mechanism involving free radicals was first proposed. , The primary oxidation products are hydroperoxides which eventually degrade, breaking the double bonds and forming chain cleaved compounds. Unsaturated fatty acids which are rather stable in the bulk may undergo faster chemical degradation when spread as a single molecular layer on a surface.…”