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

by Trice, William H.

Information about the structural characteristics of monolayers and how they are related to the resultant free surface energy is basic to an understanding of the mechanism involved in wetting. Although the wettability of saturated fatty acid monolayers has been studied extensively little work has been done on the wettability of unsaturated fatty acid monolayers. This study was undertaken in order to ascertain how autoxidation of a linoleic acid (cis, cis-9, 12-octadecadienoic acid) monolayer would affect the contact angle with water and methylene iodide. The objectives were to relate changes in the contact angle to the chemical changes taking place during autoxidation, and this in turn to the mobility of the oxidized molecules. The effect of autoxidation on the contact angle with water depended upon the conditions of oxidation and the type of surface upon which the monolayer was formed. When autoxidation was carried out in dry air at room temperature, the products contained peroxide, hydroperoxide and possibly carbonyl functional groups. The U.V. spectra and reductive polarography indicated that the products were typical of solid-polymeric material produced during extensive autoxidation of bulk systems and the mobility of the molecules was thus decreased. On a glass surface with which the carboxyl group cannot react, this decrease in molecular mobility resulted in an increase in the contact angle. However, the increase was only from zero to a maximum of 560 because of incomplete oxidation and because the oxygen-containing functional groups were apparently accessible to the water. On the other hand, autoxidation in the presence of moisture produced material which was typical of the initial, monomeric products produced in bulk systems. Evidence was obtained for the 9 or 13-hydroperoxide of linoleic acid and also the equivalent keto-acid produced by dehydration of the hydroperoxide which are characteristic of the initial oxidation products. Thus, the mobility of the linoleic acid was evidently not greatly reduced under these conditions, and as a result the contact angle did not increase. -2- When the monolayer was formed upon a copper surface, the unoxidized molecules became quite immobile through reaction of the carboxyl group and the copper. As a result, the contact angle with water was about 92.0 after reaction between the carboxyl group and the copper was complete. Extensive autoxidation of the already immobilized molecules then decreased the contact angle evidently because of the introduction of oxygen-containing groups into the monolayer surface. The contact angle between methylene iodide and the monolayer formed on a glass surface was not greatly affected by autoxidation. The angle was less susceptible to the motion of the unoxidized acid because of the size of the methylene iodide molecule and its low attraction for the glass. Thus, a decrease in the mobility of the acid molecules upon extensive autoxidation had less of an effect on the contact angle. -3-