The heat of adsorption of long-chain compounds and their effect on boundary lubrication

Abstract
The frictional behaviour between mild steel surfaces lubricated with solutions in white oil of long-chain halides, acids, $\alpha $-substituted acids, esters, cyanide, thiocyanate and a nitro derivative has been investigated under high loads at low speeds. In all cases a transition from smooth sliding to stick-slips occurs at a temperature characteristic of the particular solution employed. For each substance the transition temperature increases with the concentration. Each solution builds up, and is in equilibrium with, an adsorbed and oriented film of the polar compound on the surface. Assuming that the transition occurs when the surface concentration of this film decreases to a certain value which, for any one material is independent of temperature, an equation has been deduced relating the concentration, and transition temperature with the heat of adsorption U. All the experimental results are in good agreement with this equation. The values of U show that these long-chain polar compounds are adsorbed by the interaction of their dipoles with the atoms in the metal surface, and not by any chemical reaction. The results also suggest that the esters are similarly oriented at metal and at aqueous surfaces.