Heats of Preferential Adsorption of Boundary Additives at Iron Oxide/Liquid Hydrocarbon Interfaces

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Abstract
Studies of preferential adsorption of several polar compounds from dilute solutions in n-heptane and n-hexadecane onto iron and iron oxides have been carried out using the flow microcalorimeter. The polar compounds studied included normal alcohols and carboxylic acids with alkyl chains ranging from 4 to 18 carbon atoms and a surface active material extracted from a mineral oil. The heats of adsorption suggest that on several iron oxides the long-chain compounds form adsorbed films composed of vertically oriented molecules, not always close-packed. The orientation of adsorbed molecules appear to differ markedly with the nature of substrate and concentration. Some substrates such as iron oxides, promote vertical orientation of alkyl chains as evidenced by relatively high heats of preferential adsorption. Iron surfaces formed under liquid hydrocarbon in the absence of oxygen possess a relatively low surface energy on which the long-chain molecules are adsorbed with very much less energy than on iron oxides. Octadecanoic acid and the surface active material extracted from a petroleum base oil are adsorbed very strongly on iron oxides, especially at low surface coverages. The results are discussed in relation to the performance of the surface active compounds as anti-wear agents for steel surfaces sliding in liquid hydrocarbons.

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