Type III Deep Eutectic Solvents (DESS) as base lubricants

Mineral oils are the most commonly used fraction of formulated lubricating oils except for some specialised applications where synthetic oils are employed. An alternative approach has been suggested using ionic liquids (ILs) due to their high viscosity index and high thermal stability. The aim of this study is to examine the use a different type of ionic liquids in the form of Deep Eutectic Solvents (DESs) as base lubricants as these have significantly improved environmental credentials. The first stage of the study involved the investigation of thermo-physical properties such as heat capacity, thermal stability, surface tension, viscosity index, melting point, conductivity and density to determine if the liquids are suitable fluids as lubricants. The data for a variety of imidazolium based ILs and standard mineral base oil were also determined and used for comparison sake. In addition the properties of DES mixtures with water were determined and self-diffusivity were measured using NMR spectroscopy. It was shown for the first time that aqueous DES mixtures are not homogeneous but instead they form bicontinuous micro-emulsions. This study has also been the first to quantify the corrosion rates of metals in DESs and ionic liquids. The corrosion of iron, aluminium and nickel was studied in four DESs and four ILs using both Tafel slope analysis and electrochemical impedance spectroscopy. The corrosion rate was found to change over time for some liquids and so the corrosion product films were characterised using Raman spectroscopy. The interfacial properties of DES and ILs are shown to be totally different from mineral base oil and the wettability in terms of contact angle and interfacial energies for various metals have been studied. The friction coefficient and wear volume were measured for DESs for dissimilar sliding couples. Finally, the change of both thermo-physical and mechanical properties due to the inclusion of two common surfactants sodium dodecylsulfate and cetyltrmethylammonium bromide are characterised in three DESs and shown to decrease the wear volume.