Design, Synthesis, and Structure–Tribological Property Relationships of Benzoazacrown Ethers with Varying Alkyl Chain Configuration

The increasing global concern over environmental issues has presented significant challenges to traditional lubricant additives that contain sulfur, phosphorus or metal components. Accordingly, the development and application of sulfur- and phosphorus-free lubricant additives have become an important research direction. Due to the absence of the aforementioned harmful elements, crown ether compounds show a significant competitive advantage as lubricant additives. In this study, two benzoazacrown ether derivatives were designed and synthesized, and their performance as organic friction modifiers (FMs) in low-viscosity base oil was systematically investigated. The results reveal that benzoazacrown ether derivatives bearing multiple oxygen and amide binding sites display distinct tribological behaviors, which are governed by the type of side-chain alkyl groups. Compared with the conventional benzocrown ether derivative (LC-BCE18-6), the amide-functionalized benzoazacrown ether derivatives achieve superior tribological performance. Among them, the benzoazacrown ether derivative with a long linear alkyl side chain (LC-Aza-BCE18-6) delivers the most prominent friction reduction, with its friction coefficient decreasing by 17.4% and wear rate decreasing by 50.1% compared with the base oil. Such excellent performance originates from the effect of polar adsorption sites and long linear alkyl groups. When the molecules adsorb onto the friction surface via the nitrogen-containing crown ether ring, the long linear alkyl chains can arrange orderly to form a stable adsorbed film. During friction, these adsorbed films further undergo tribochemical reactions to produce protective tribofilms, which further reinforce the friction-reducing and anti-wear capabilities.