Adsorption of Phosphorus-Type Anti-Wear Agents for Refrigerator Nonpolar-Oil Additives

In this study, we characterized the adsorption of several phosphorus-type anti-wear agents on an iron oxide surface in hexane. The phosphorus-type anti-wear agents employed in this study were triphenyl phosphate and its derivatives. The quartz crystal microbalance with dissipation monitoring technique revealed that the adsorption of the phosphorus-type anti-wear agents was strongly dependent on their chemical reactivity; i.e., the anti-wear agents with reactive functional groups result in large adsorption amount on the iron oxide surface. Additionally, (i) the polarity of the anti-wear agents as well as (ii) the presence of a benzene ring in their chemical structure may also affect the adsorption behavior. Adsorption energy of the anti-wear agent was also estimated by an atomistic level simulator “MATLANTISTM” and the simulation results well supported the experimental findings. Four-ball tests further demonstrated that the chemical or physical adsorption film formed on the iron oxide surface resulted in the wear resistance, as expected. This study provides fundamental knowledge on the structure-performance relationship of phosphorus-type anti-wear agents, giving future perspectives for lubricant formulations.