Task-specific boronium ionic liquids as ashless lubricant additives†

Abstract

Modern engines are designed for very close contact between shearing planes, which requires high-performance boundary lubrication, delivered by lubricant base oils formulated with an array of additives. Commercial additive packages typically contain metals, sulfur, and phosphorus, which act as poisons to catalytic converters (thereby increasing emissions), increase wear and contribute to corrosion (which lowers the lifespan of engines). Ionic liquids (ILs), which are low-melting organic salts, have been extensively studied as lubricant additives; although some commercially available ionic liquids perform well as friction modifiers, they suffer from low solubility in the oil matrix and may cause corrosion due to residual chloride content. Here, we report nine new, task-specific ionic liquids, designed to act as ashless lubricant additives, comprising boron-containing cations for enhanced wear reduction, carboxylic acid anions to reduce friction, and modified alkyl chains to enhance solubility in the base oil. All ILs were inherently free from metals, sulfur, and phosphorus, and synthesised through a halide-free route. Their speciation was studied through multinuclear NMR and Raman spectroscopies, followed by studies of solubility in Group III+ base oil. Their performance as lubricant additives was assessed in terms of friction reduction and wear scar reduction, benchmarked against glycerol mono-oleate (GMO), a commercially availabe lubricant additive.