The influence of the structure of isomeric octadecanols on their adsorption from solution on iron and their lubricating properties

The influence of the structure of isomeric octadecanols on their adsorption from heptane solutions on iron powder without removal of any oxide film potentially present on the iron surface was studied. The structure of the alcohols was altered by shifting the polar hydroxyl group within the chain and by branching the chain. For comparison stearic acid was included in the investigation. The adsorption isotherms were determined and could be fitted by a Langmuir equation, with the exception of 1-octadecanol. From the adsorption experiments the areas occupied per molecule and the coefficients of adsorption were obtained. These data are related to the lubricating properties of cetane solutions of the compounds.

Lubrication with 1-octadecanol results in a low coefficient of friction and a high durability of the built-up monolayer. The area occupied per molecule is low. Chain branching and/or shifting the hydroxyl group to the middle of the chain results in much higher areas per molecule, higher coefficients of friction and poor durabilities of the monolayers. The coefficients of adsorption of the octadecanols are of the same order of magnitude. The difference in lubricating properties may be explained by the difference in molecular areas occupied. The durability of a monolayer of stearic acid is higher than that of 1-octadecanol although their molecular areas are equal owing to the much higher coefficient of adsorption of stearic acid owing to stronger bonding to the surface.