Base oil polarity: A critical factor in the thermal stability of copper nanoparticles modified by dialkyl dithiophosphates

Abstract

Inorganic nanoparticles require surface modification with organic modifiers to achieve long-term stable dispersion in base oil when utilized as lubricating oil additives. The decomposition temperature of the modifier is typically considered the maximum operating temperature of the nano-additive. However, we observed that the sedimentation temperature of copper nanoparticles modified by dialkyl dithiophosphates (DDP) (referred to as CuDDP) in base oil is significantly lower than the decomposition temperature of the modifier itself, and the sedimentation rate varies with the polarity of the base oil. The thermal failure mechanism of CuDDP was analyzed. The results demonstrate that competitive adsorption of the outermost modifier by polar groups of the base oil and the sub-surface modifiers of nanoparticles is the primary cause of thermal instability, which occurs prior to structural changes in the modifier itself. The polar groups of base oil generate stronger electrostatic interactions with the polar end of the modifier, thereby promoting modifier desorption. Consequently, particle sedimentation is attributed to modifier desorption, which disrupts dispersion stability. This study provides critical guidance for designing sustainable nano-additives with high-temperature resistance and offers broad applicability for the practical applications of organic-inorganic hybrid nanoparticles.