Molecular dynamics simulations of surface-active molecules under bulk and confined conditions
Creators: name: Tsagkaropoulou, Georgia
Publisher: name: The University of Edinburgh
Year: 2019
Subjects: molecular dynamics
additives
lubricants
friction
adsorption
boundary lubrication
solid-liquid interface
simulations
self-assembly

description: Engine oils are of great significance in the automotive industry as they provide lubrication of the moving mechanical components of the engine, thus decreasing friction, and preventing wear and corrosion. There is little understanding of the underlying molecular-scale mechanisms that contribute to the low-friction environment, and how the additives in lubricant oils behave under engine-like conditions. Previous research is based mainly on experimental work and empirical evidence, although in the past decade, computational techniques have been used successfully to simulate the engine environment and verify experimental results. This work is mainly focused on the computational investigation of certain surface active additives under a range of relevant physical conditions, such as high pressure and high shear rate, in order to provide an insight into the behaviour of these compounds at the atomistic level. The simulations are performed using molecular dynamics (MD) and they explore important microscopic processes such as adsorption and self assembly in bulk and under confined conditions. In the first part of the thesis, the focus is on lubricant additives in oil. Surfactant adsorption helps protect the surface from contact with other engine parts, whereas aggregation plays a major role in preventing impurities and byproducts from depositing onto the surfaces. The frictional and structural properties are studied over production runs. The additives studied are polyisobutylsuccinimide-polyamine (PIBSA-PAM), glycerol monooleate (GMO), synovene, and glyceryl monolinoleate (GML), and squalane is used as the lubricating oil. The additives tend to adsorb onto the surfaces, or form micelles, which break under high shear conditions. Interesting trends are identified, linking self assembly with friction and shear rate, and the co-operative interactions between the additives are highlighted. In the second part of the thesis, MD simulations are used to study self-assembly and surface adsorption of ionic surfactants in water. An investigation on the self assembly properties of the surfactant compounds hexadecyltrimethylammonium bromide (CTAB) and didodecyldimethylammonium bromide (DDAB) in water is performed, and the results are linked to experimental findings. The molecules are confined between muscovite mica surfaces, and are found to adsorb and form lamellar structures on mica. The morphology of the mica surface is deemed crucial for the structure of the adsorbed formations, and the simulation results are compared with recent experimental measurements by calculating the scattering-length density profiles and the reflectivity. This study provides a detailed analysis of a range of surface active molecules, either in bulk or confined conditions, and emphasises the importance of the co-operative interactions between two or more additives in bulk and confined solutions.