Unravelling molecular origins of improved tribological properties of amino acid ionic liquid water-based lubricants

In the quest for sustainable lubrication, this study explores the potential of water-based lubricants (WBLs) with amino acid ionic liquid (AAIL) additives. The research combines experimental and reactive molecular dynamics (MD) simulations to assess the tribological and rheological enhancements provided by AAILs. Specifically, tetrabutylphosphonium (P4444) cations paired with Serine (Ser) and Tryptophan (Trp) anions were mixed into water to create AAIL WBLs. The introduction of AAILs significantly improved the lubricants' rheological properties and reduced the coefficient of friction by up to 34 %. This reduction is attributed to the formation of hydration shells around the amino acid anions, which create a chemically adsorbed tribofilm on iron oxide surfaces, and P4444 cations that establish a load-bearing layer in the middle regions of the lubricant. This dual-layer structure, supported by hydration lubrication, effectively maintains lubricant fluidity while bearing substantial loads. Among the AAIL WBLs tested, those with a thicker, more stable, hydrated amino acid tribofilm demonstrated more significant friction reduction. These insights into the molecular mechanisms behind AAIL WBLs' tribological behaviour underscore their promise as eco-friendly lubricants, contributing to advancing a circular economy in industries reliant on lubrication.