Micro- to Nano- and from Surface to Bulk: Influence of Halogen-Free Ionic Liquid Architecture and Dissociation on Green Oil Lubricity

Akepati Bhaskar Reddy,Georgia A Pilkington,Sergei Glavatskih,Mark W Rutland,Bulat Munavirov,Gabriel Calderon Salmeron

doi:10.1021/acssuschemeng.1c04854

Abstract

Four nonhalogenated ionic liquids (ILs) based on the same phosphonium cation are investigated in terms of the anion suitability for enhancing the lubricity of a biodegradable oil. For all test conditions, typical for industrial machine components, the lubrication is shown to be governed by nonsacrificial films formed by the physisorption of ionic species on the tribo-surfaces. The anionic structure appears to have an important role in the formation of friction modifying films. The orthoborate ILs exhibit the formation of robust ionic boundary films, resulting in reduced friction and better wear protection. On the contrary, the surface adsorption of phosphinate and phosphate ILs appears to antagonistically disrupt the intrinsic lubrication properties of the biodegradable oil, resulting in high friction and wear. Through additional investigations, it is postulated that the higher dissociation of orthoborate ILs in the biodegradable oil allows the formation of hierarchical and electrostatically overscreened layer structures with long-range order, whereas the ILs with phosphate and phosphinate anions exhibit low dissociation in biodegradable oil, possibly due to the ion pairs being surrounded by a hydrocarbon halo, which presumably results in weak adsorption to form a mixed interfacial layer with no long-range order.

Highlights

The environmental footprint of lubricants depends on their ability to reduce losses of energy and material in the lubricated systems
It has been suggested that reactions between tribo-surfaces and certain organic friction modifiers (OFMs) can lead to the formation of friction-reducing sacrificial tribofilms.[2]
We have investigated the ability of four ionic liquids (ILs) to form ionic boundary films when used as additives in a lowviscosity biodegradable fatty acid ester, 2-ethylhexyl laureate (2EHL)

Summary

Introduction

The environmental footprint of lubricants depends on their ability to reduce losses of energy (friction) and material (wear) in the lubricated systems. Another key factor to consider is the origin of materials from which the lubricants are derived. A gradual transition toward low-viscosity lubricants contributes further to machine efficiency through reduction of churning and pumping losses. To maintain contact separation even at lower speeds, lubricant additives must be used. These additives must be capable of facilitating low friction in the boundary lubrication regime. Oil-soluble organo-molybdenum friction modifiers are another class of friction modifiers that have been shown to reduce friction by forming MoS2 nanosheets through stress-induced intramolecular reactions, rather than reactions with the surface.[2]

Methods

Results

Conclusion