Competitive Adsorption of Ionic Liquids Versus Friction Modifier and Anti-Wear Additive at Solid/Lubricant Interface—Speciation with Vibrational Sum Frequency Generation Spectroscopy

Dien Ngo,Seong H Kim,Xin He,Jun Qu,Huimin Luo

doi:10.3390/lubricants8110098

Abstract

A modern lubricant contains various additives with different functionalities and the interactions or reactions between these additives could induce synergistic or antagonistic effects in tribological performance. In this study, sum frequency generation (SFG) spectroscopy was used to investigate competitive adsorption of lubricant additives at a solid/base oil interface. A silica substrate was used as a model solid surface. The lubricant additives studied here included two oil-soluble ionic liquids (ILs, [N888H][DEHP] and [P8888][DEHP]), an antiwear additive (secondary ZDDP), an organic friction modifier (OFM), and a dispersant (PIBSI). Our results showed that for mixtures of ZDDP and IL in a base oil (PAO4), the silica surface is dominated by the IL molecules. In the cases of base oils containing OFM and IL, the silica/lubricant interface is dominated by OFM over [N888H][DEHP], while it is preferentially occupied by [P8888][DEHP] over OFM. The presence of PIBSI in the mixture of PAO4 and IL leads to the formation of a mixed surface layer at the silica surface with PIBSI as a major component. The SFG results in this investigation provide fundamental insights that are helpful to design the formulation of new lubricant additives of desired properties.

Highlights

Lubrication plays an important role in reliable performance and durability of many engine systems and machineries [1,2,3]
The chemical structures of PAO4, sec-ZDDP, organic friction modifier (OFM), and polyisobutylene succinimide (PIBSI) are not precisely known in this study, and infrared spectroscopy was used to obtain the spectra of main IR active vibrational modes in these compounds
The results from IR spectroscopy are useful for interpreting sum frequency generation (SFG) spectra of base oil, additives as well as their mixtures at silica/liquid interface

Summary

Introduction

Lubrication plays an important role in reliable performance and durability of many engine systems and machineries [1,2,3]. Was added to a lubricant mixture consisting of polyalphaolefin (PAO4) base oil, molybdenum dithiocarbamate (MoDTC), and polyisobutylene succinimide (PIBSI), for example, the friction coefficient and wear of a cast iron were reported to be higher than those in the case without adding sec-ZDDP [10]. Such reduced performances could be due to the reaction or interaction between sec-ZDDP and PIBSI in the mixed liquid. It is, important to understand chemical interactions between potential

Methods

Results

Conclusion