Competitive Adsorption of Lubricant Base Oil and Ionic Liquid Additives at Air/Liquid and Solid/Liquid Interfaces

Abstract

Oil-soluble ionic liquids (ILs) have been proved as effective additives in lubricant oils through tribological experiments and post-test analytical analyses. In this study, surface structures of lubricant base oil, oil-soluble ILs, and their mixtures at the air/liquid and solid/liquid interfaces have been studied using sum frequency generation (SFG) vibrational spectroscopy. At the air/base oil and air/IL interfaces, the alkyl chains of the studied compounds were shown to be conformationally disordered and their terminal methyl groups point outward at the liquid surface. The base oil dominates the air/(base oil + IL) interface due to its higher surface excess propensity and larger bulk concentration. At the solid (silica) surface, ILs adopt a structure with their charged headgroups in contact with the silica surface, while their alkyl chains are more conformationally ordered or packed compared to the air/IL interface. At the interface between silica and (base oil + IL) mixtures, ILs also preferentially adsorb to the silica surface with their layer structures somewhat different from those of ILs alone. These results showed that ILs can adsorb onto the solid surface even before tribological contacts are made. The insights obtained from this SFG study provide a better understanding of the role of ionic liquids in lubrication.

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

“…The chemical structure of OFM was not exactly known and the one shown here was suggested by NMR and Infrared spectroscopy [9]. The structure of PAO4 is a generic one [11,21].…”

“…The windows, SFG liquid cell, and glassware were cleaned following the procedure described in Ref. [11]. [9].…”

“…It is, therefore, important to understand chemical interactions between potential 2 of 18 additives to predict or design lubricant formulations for specific applications. The adsorption of a compound (base oil or additive) to a solid surface can be affected by other components in lubricants [11] and compositional or structural information of molecular layers at the interface between a solid and a mixed liquid (lubricant) is needed to understand the tribological performance of a lubricant.…”

“…In our previous study, the competitive adsorption of a base lubricant oil (PAO4) and two oil-soluble ILs, tetraoctylphosphonium bis(2-ethylhexyl)phosphate ([P 8888 ][DEHP]) and trioctylammonium bis (2-ethylhexyl)phosphate ([N 888 H][DEHP]), at a solid surface (silica) was investigated using vibrational sum frequency generation (SFG) spectroscopy [11]. Being a surface spectroscopic technique [13][14][15], SFG spectroscopy has provided valuable insights into the adsorption behavior of those compounds at the solid surface.…”

“…In practice, ILs would be combined with other additives in lubricants and it is necessary to understand the structure of an adsorbed layer at the solid/lubricant interface when several additives are used together. SFG spectroscopy can differentiate characteristic vibrational bands of specific molecules at the solid/liquid interface [11,14,[16][17][18], thus it can determine the chemical identity or structure of interfacial molecules present at the solid/liquid interface.…”