Film thickness and mechanical properties of adsorbed neutral and basic zinc diisobutyl dithiophosphates

A surface force apparatus (SFA) has been used to determine film thickness and film properties of adsorbed neutral and basic diisobutyl zinc dithiophosphates (ZnDTPs) on cobalt surfaces. Sliding experiments were also carried out and film characteristics determined after sliding. Dodecane was used as the solvent. Experiments with dodecane only show dodecane molecules form an immobile monolayer on cobalt surfaces. When subjected to sliding, this monolayer is displaced, and attractive forces between the cobalt surfaces are observed. Neither the films formed initially upon adsorption of the ZnDTPs nor those films after sliding are displaced by the tangential motion. After 80 nm of sliding, the coefficient of friction was 1.2 in dodecane and 0.4 in the ZnDTP solutions. Elastic compressive and shear moduli for the physisorbed ZnDTP films were similar to 0.1 GPa and 0.01-1 GPa, respectively. The distance of first repulsion between the approaching surfaces corresponds roughly to two monolayers of ZnDTP on each surface. Under compression, ZnDTP molecules that am not firmly bonded to the surface are expelled from the contact. At 15-hour adsorption times for both ZnDTPs, the thickness of the confined layer under load corresponds to one monomolecular layer separating the two surfaces. For the neutral ZnDTP, a monomolecular layer thickness is I nm and for basic ZnDTP it is 1.6 nm. A possible explanation for a single molecular-layer separation is that the adsorbed ZnDTP molecules are sparsely distributed on the surfaces, so sparsely that when the surfaces approach the adsorbed molecules interleave forming a layer of molecules of monomolecular thickness, of which some are attached to one surface, and of which the remainder are attached to the other surface. The SFA affords not only mechanical properties of thin films but also some information on their molecular structure.