SURFACE VIBRATIONAL SPECTROSCOPY OF LUBRICANTS ADSORBED AT THE IRON-WATER INTERFACE

Sum-frequency spectroscopy has been used to obtain the vibrational spe ctra of two molecules, potassium oleate and sodium octanoate, adsorbed at the iron-water interface. These two molecules reduce the friction of iron-steel contacts: the surface structures inferred from the vibra tional spectra indicate how these molecules lubricate iron under aqueo us solutions. Potassium oleate causes a reduction in the friction of i ron surfaces at 10 mM; the strength and phase of the resonances in the vibrational spectra of oleate indicate that a bilayer was adsorbed at the iron surface at this concentation. Sodium octanoate reduces the f riction of iron surfaces under anodic potentials. The vibrational spec tra of adsorbed octanoate did not contain any resonances at any applie d potential: this observation suggests that a completely disordered fi lm was adsorbed at iron. The comparison between the coefficients of fr iction suggests that the more ordered films formed by oleate are more effective at lubricating than the disordered octanoate films.