Nanolubrication in deep eutectic solvents.

James E Hallett,Hannah J Hayler,Susan Perkin

doi:10.1039/d0cp03787g

Abstract

We report surface force balance measurements of the normal surface force and friction between two mica surfaces separated by a nanofilm of the deep eutectic solvent ethaline. Ethaline, a 1 : 2 mixture of choline chloride and ethylene glycol, was studied under dry conditions, under ambient conditions and with added water, revealing surface structural layers and quantised frictional response highly sensitive to water content, including regions of super-lubric behaviour under dry conditions and with added water. We also report exceptionally long-ranged electrostatic repulsion far in excess of that predicted by Debye-Hückel theory for a system with such high electrolyte content, consistent with previously reported observations of "underscreening" in ionic liquid and concentrated aqueous electrolyte systems [Smith et al., J. Phys. Chem. Lett., 2016, 7(12), 2157].

Highlights

Deep eutectic solvents (DESs) are a class of liquid of growing importance, and are often described as a sub-category or new class of ionic liquids (ILs).[1]
We report surface force balance measurements of the normal surface force and friction between two mica surfaces separated by a nanofilm of the deep eutectic solvent ethaline
We proceed to describe the interfacial structure of ethaline under dry, ambient and hydrated conditions

Summary

Introduction

Deep eutectic solvents (DESs) are a class of liquid of growing importance, and are often described as a sub-category or new class of ionic liquids (ILs).[1]. Unlike ILs, which are primarily composed of discrete anions and cations of one particular species, DESs contain a complex ionic component and can contain a variety of ionic species.[2] The term deep eutectic solvent originates from the low melting temperature TM obtained at the eutectic composition.[3] DESs are obtained by mixing a hydrogen bond-capable salt (typically a quaternary ammonium salt) with a metal salt or hydrogen bond donor species (HBD). In many cases these two components are crystalline solids at room temperature, whilst the mixture formed is liquid under ambient conditions and with relatively low TM. Deep eutectic solvents have been proposed as environmentally friendly alternatives for processes including electrodeposition,[6] metal oxide processing,[7] organic synthesis,[8] and the purification and manufacture of biodiesel.[9,10]

Methods

Results

Conclusion