The effect of autoxidation on the wettability of a linoleic acid monolayer

Abstract

The effect of autoxidation of a linoleic acid monolayer on the contact angle with water depended upon the conditions of oxidation and the type of surface upon which the monolayer was formed. For monolayers on a glass surface with which the carboxyl group did not react, autoxidation in air of 0.02% R.H. increased the contact angle with water from zero to 56°. However, on copper where reaction between the surface and the carboxyl group occurred, autoxidation under the same conditions decreased the contact angle from 86° to 76°. Autoxidation in air of 50% R.H. caused no change in the initial contact angle of zero with water when the monolayer was on glass. On copper, aging under these conditions caused the angle to increase from an initial value of 86° to 92°. Contact angles between methylene iodide and the monolayers were not greatly affected by autoxidation under any of the afore-mentioned conditions. Ultraviolet spectra and reductive polarography indicated that secondary oxidation of the initially formed 9- or 13-hydroperoxide of the conjugated octadecadienoic acid occurred to a much greater extent in the drier air. The hypothesis presented explains the wettability results in terms of the ability of the oxidation products to produce a solid, coherent monolayer structure, taking into account the location in the hydrocarbon chain of oxygen-containing functional groups introduced by autoxidation.