Physicochemical Characterization of Choline Amino Acid Protic Ionic Liquids as Sustainable Lubricant Candidates

Choline amino acid protic ionic liquids (PILs) have emerged as promising candidates for sustainable lubrication due to their tunable structures, low toxicity, and strong interfacial activity. In this study, a series of ten choline amino acid PILs, incorporating aliphatic, branched, multifunctional, and aromatic amino acid anions, were synthesized and systematically characterized to establish structure-property relationships relevant to lubrication applications. The physicochemical behavior of the PILs was investigated through viscosity measurements, ionic conductivity analyses, thermal characterization, wettability assessment, miscibility evaluation with polar and nonpolar base oils, and corrosion studies on copper surfaces. Their lubricating performance when used as boundary lubricants was assessed. The results reveal a strong dependence of bulk and interfacial properties on the anion structure. PILs containing compact aliphatic anions exhibited low viscosities and high ionic conductivities, reflecting weak intermolecular interactions and high ion mobility. In contrast, PILs with branched, aromatic, or multifunctional anions displayed higher viscosities and reduced conductivities because of enhanced hydrogen bonding, steric effects, and supramolecular structuring. Notably, the aspartic acid-containing PIL showed significantly elevated viscosity compared with other small-anion systems, highlighting the dominant influence of multiple carboxylic acid functionalities. Thermal analysis indicated generally high stability for moderate-temperature lubrication applications across the series, with aromatic and multifunctional systems exhibiting increased glass transition temperatures and decomposition resistance. Wettability and miscibility studies demonstrated that the interfacial behavior is governed by both side-chain chemistry and substrate or solvent polarity, with clear differences observed between steel and aluminum surfaces and between ester- and PAO-based oils. Corrosivity evaluations showed that PILs with small, highly mobile anions promote greater surface reactivity on copper, whereas bulkier or multifunctional anions provide improved surface stability through reduced ion mobility and steric shielding. Tribological evaluation through reciprocating sliding friction and wear tests revealed trends consistent with the observed structure-property relationships, supporting the relevance of the measured physicochemical properties to lubrication performance. This work establishes a comprehensive framework linking molecular structure to physicochemical and interfacial behavior in choline amino acid PILs. The findings provide insight into the design of environmentally compatible ionic liquids with tailored properties and potential for lubrication and related applications.