Colloids and Surfaces A: Physicochemical and Engineering Aspects. 70 (1993) 207-212 Elsevier Science Publishers B.V., Amsterdam 207 Oxidation of monogly~~rid~, diglyceride, and triglyceride monolayers by aqueous potassium permanganate solution’ C.P. Singh and D.O. Shah* Center for Surface Science & Engineering, Departments of Chemical Engineering & Anesthesiology, University of Florida, Gainesville, FL 32611, USA (Received I7 February 1992; accepted 20 November 1992) Abstract Studies were carried out on the oxidative degradation of monoolein, mono~inolein, diolein, dilinolcin, trio&n and triiinolein monolayers by injecting KMnO, solution under the monolayers. The effect of the initial surface pressure on oxidative degradation was also studied by measuring changes in the surface pressure and surface potential with time. The surface shear viscosities of these six lipids were measured in order to predict their molecular interactions at the air/water interface. The rates of oxidation for these lipids were found to be in the following order: dilinoleinrmonolinolein> monoolein z trilinolein bdiolein > triolein. Interestingly, the surface shear viscosities of these six lipids were found to decrease in the same order. In the present study, an attempt has been made to correlate the effect of the initial surface pressure, the number of double bonds and the number of hydroxyl groups with the oxidation of these lipids by potassium permanganate solution. Keywords: Lipid; monolayer; potassium permanganate; surface potential; surface pressure. Introduction The oxidation of insoluble monolayers by aque- ous potassium ~rmanganate solution has been studied by many investigators [I-S] by measuring simultaneously the time t dependences of the apparent molecular area (A’) and the surface pres- sure. As one spreads a monolayer on the aqueous potassium permanganate solution, the oxidation of the monolayer and evaporation/solubilization of the solvent occur simultaneously; this affects the area/surface pressure at the very beginning of the oxidation process. Therefore, in the present study, the KMnO, solution was injected under the ‘Dedicated to the memory of K. Meguro. *Corresponding author. monolayer after compressing it to the desired surface pressure. It has been reported 173 that the intermediate products formed during the cleavage of the double bonds contain c&epoxy groups. As some of the products after oxidation are insoluble in water, the surface potential of the monolayer is considered to be a more reliable parameter to predict the rate of the oxidation reaction. The oxidation of the unsaturated monoglycer- ides, diglycerides or triglycerides is responsible for rancidity in oil-containing products. The effects of various monoglycerides and diglycerides on the oxidative stability of soybean oil has been investi- gated by measuring the rates of disappearance of headspace oxygen in purified oils containing 0, 0.25, and 0.5% monostearin, distearin, monolino- lein or dilinolein [9]. The mono~ycerides and 0927-7757/93/R%.OO 0 1993 - Elsevier Science Publishers B.V. All rights reserved.