Tribotronic control of an ionic boundary layer in operando extends the limits of lubrication

Abstract

The effect of electric potential on the lubrication of a non-halogenated phosphonium orthoborate ionic liquid used as an additive in a biodegradable oil was studied. An in-house tribotronic system was built around an instrument designed to measure lubricant film thickness between a rolling steel ball and a rotating silica-coated glass disc. The application of an electric field between the steel ball and a set of customized counter-electrodes clearly induced changes in the thickness of the lubricant film: a marked decrease at negative potentials and an increase at positive potentials. Complementary neutron reflectivity studies demonstrated the intrinsic electroresponsivity of the adsorbate: this was performed on a gold-coated silicon block and made possible in the same lubricant system by deuterating the oil. The results indicate that the anions, acting as anchors for the adsorbed film on the steel surface, are instrumental in the formation of thick and robust lubricating ionic boundary films. The application of a high positive potential, outside the electrochemical window, resulted in an enormous boost to film thickness, implicating the formation of ionic multi-layers and demonstrating the plausibility of remote control of failing contacts in inaccessible machinery, such as offshore wind and wave power installations.

Molecular Architecture Effects on Bulk Nanostructure in Bis(Orthoborate) Ionic Liquids

“…[16,17] Such layers can also be controlled in operando by changing the surface potential. [18] However, many common ILs readily corrode ferrous interfaces, as they contain either halides, or other anions based on halides such as [PF 6 ] − and [BF 4 ] − , which can hydrolyze to form corrosive HF. Boron chemistry offers an attractive solution to these problems: [19] from cheap and naturally-abundant precursors, ionic materials can be synthesized which are analogous to those which are already shown to be effective as antiwear lubricant additives.…”

Small-Angle Neutron Scattering Insights into 2-Ethylhexyl Laurate: A Remarkable Bioester

“…This amphiphilic configuration being favored will confer (weak) surface activity (surfactant behavior), and thus, interfacial adsorption and self-assembly phenomena. This orientation has been previously proposed to explain the beneficial lubricative properties of 2-EHL when it adsorbs at metal interfaces, from experimental neutron reflectivity data investigating 2-EHL as a carrier oil for lubricant formulations. − , The potential to orient into surface-active molecular configurations, where self-assembly processes can actively occur, offers an explanation for the superior performance of 2-ethylhexyl esters in applications such as base oils for drilling fluids and skin moisturizing components in cosmetic formulations, when compared to traditional linear alkanes …”

“…This orientation has been previously proposed to explain the beneficial lubricative properties of 2-EHL when it adsorbs at metal interfaces, from experimental neutron reflectivity data investigating 2-EHL as a carrier oil for lubricant formulations. [13][14][15]37 The potential to orient into surface-active molecular configurations, where self-assembly processes can actively occur, offers an explanation for the superior performance of 2-ethylhexyl esters in applications such as base oils for drilling fluids 12 and skin moisturizing components in cosmetic formulations, when compared to traditional linear alkanes. 11 To further confirm the hypothesis that 2-EHL adopts surfactant-like configurations, blends were prepared of H-2-EHL with d 3 -acetonitrile as a model nonaqueous dispersant, up to the approximate room-temperature solubility limit (ca.…”

“…Moreover, the lubricity of the adsorbed layer can be controlled by the application of an electric field. 15 Despite (or perhaps due to) its widespread commercial intrigue, relatively little data beyond basic physical properties and safety information exist in the literature for 2-EHL. 5 In the context of its structure (see Figure 1), this gap in the knowledge base is an important one to address due to two main factors: (1) 2-EHL contains a central ester linkage and therefore lies somewhere between a traditional linear alkane oil and a surfactant in terms of polarity and polarity distribution (i.e., the hydrophilic−lipophilic balance and dielectric constant).…”

“…Halide-free orthoborate ILs have been formulated for this purpose, and they degrade into friction-reducing, nonsacrificial surface tribofilms in concert with the 2-EHL base oil. Moreover, the lubricity of the adsorbed layer can be controlled by the application of an electric field …”