Abstract

The development of environmentally sustainable lubricants is increasingly important as industries seek to reduce friction, wear, energy losses, and environmental impact. Choline amino acid protic ionic liquids (PILs) have emerged as promising candidates due to their bio-based constituents, tunable molecular structures, and favorable lubrication properties. However, the influence of amino acid structure on their lubrication mechanisms and tribological performance remains poorly understood. This dissertation investigates the physicochemical properties, tribological behavior, and tribofilm formation mechanisms of choline amino acid PILs to establish structure–property–performance relationships for sustainable lubricant design. Ten choline amino acid PILs containing glycine, alanine, lysine, leucine, isoleucine, aspartic acid, proline, phenylalanine, histidine, and tyrosine anions were synthesized and characterized. Thermal stability, glass transition behavior, viscosity, ionic conductivity, wettability, miscibility, and corrosion behavior were evaluated. Tribological performance was assessed in steel–steel and aluminum–steel contacts using the PILs as neat lubricants and as 1 wt.% additives in polar and non-polar oils. Surface characterization was performed using scanning electron microscopy, energy-dispersive spectroscopy, Raman spectroscopy, and three-dimensional profilometry. The results demonstrated that amino acid structure strongly influences both physicochemical and tribological properties. Compact amino acid anions produced low-viscosity, high-conductivity PILs with strong surface activity but limited wear protection. In contrast, multifunctional and aromatic amino acid anions generated higher viscosities, stronger intermolecular interactions, and superior anti-wear performance. Among all investigated lubricants, [CHO][ASP], [CHO][HIS], and [CHO][TYR] exhibited the best wear protection. Surface analyses revealed that improved tribological performance was associated with the formation of carbonaceous and oxide-containing tribofilms that reduced direct asperity contact and minimized material removal. Additive performance was found to depend strongly on base-oil polarity and compatibility between the PIL and lubricant matrix. The results further demonstrated that contact material significantly influences lubrication performance, with differences in surface chemistry and tribofilm formation leading to distinct tribological responses in steel–steel and aluminum–steel contacts.This work establishes a comprehensive structure–property–performance framework for choline amino acid PILs and demonstrates that amino acid side-chain functionality governs physicochemical properties, interfacial behavior, tribofilm formation, and tribological performance. These findings provide a foundation for the rational design of next-generation environmentally friendly ionic liquid lubricants.

Publication Date

7-2026

Document Type

Dissertation

Student Type

Graduate

Degree Name

Mechanical and Industrial Engineering (Ph.D)

College

Kate Gleason College of Engineering

Advisor

Patricia Iglesias

Advisor/Committee Member

Rui Liu

Advisor/Committee Member

Michael Schertzer

Campus

RIT – Main Campus

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