Non-covalent interactions in lubricant chemistry
Hubbard, Thomas A.
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Lubricant formulations are highly complex mixtures, containing a multitude of additives, each geared towards improving the efficiency of often highly specialised processes. The study of lubrication, or tribology, is a huge area of research, but is often overlooked by chemists in favour of pharmaceutical or agrochemical research. This thesis lays the foundation for the study and further understanding of additive-additive interactions in a lubricant formulation. Chapter one presents a concise introduction to modern lubricant formulations by providing a historical background and examining current understanding, while focusing not on the more widely publicised engineering aspects, but on the chemical properties and mechanisms of lubricant formulations. In order to dissect out additive-additive interactions from additive-solvent interactions, a study of the biggest single component in any lubricant formulation, the base oil, is performed in Chapter two. By using a series of molecular torsion balances which have been shown to be heavily influenced by solvent effects, it is revealed that the complex multitude of available commercial base oils can be substituted with a single common lab solvent. In Chapter three, a computational reparameterisation and experimental examination of previous semi-empirical models for the prediction of hydrogen-bond energies in solution using electrostatic surface potentials was performed. Surprisingly, it was found that a simple DFT/B3LYP/6-31G* method provides a better prediction of H-bond energies in solution than more complex models or functional-group corrected methods. Chapter four focuses on the fundamental understanding of entropy and conformational flexibility of compounds in solution. Using a simple flexible experimental system, which is capable for forming an internal hydrogen bond in solution, the impact of intramolecular hydrogen bonding on the stability of an intermolecular process was studied using both experimental and computational (DFT/molecular dynamics) methods. A distinct cut-off point was observed experimentally where the intramolecular process is completely out-competed by the intermolecular interaction. A computational model which reproduces the experimental trends has yet to be found.