Molecular Dynamics Simulation on the Aggregation of Lubricant Oxidation Products

Abstract

The aggregation phenomenon of lubricant oxidation products, fatty acids, fatty alcohols, aliphatic aldehydes and aliphatic ketones in base oil, was probed by molecular dynamics (MD) simulation. The results show that there are aggregates formed in fatty acids or fatty alcohols, and that the aggregation does not occur in aliphatic aldehydes and aliphatic ketones in single component models. Fatty acids or fatty alcohols are connected by hydrogen bonds in the form of dimers or multimers. The hydrogen bond can be formed at a higher temperature in fatty acids, but not in fatty alcohols. In the mixture models, there are dimers and/or multimers constructed by different molecules of lubricant oxidation products, and the multimers have a chain or a ring structure. The number of molecules involved in the formation of aggregates decreases with the increase of temperature. The opportunity to construct hydrogen bonds rises with the increasing concentration of oxidation products. Obvious aggregation occurs at 25 and 100 °C in conditions of high concentration, while it happens only at 25 °C with lower concentration. It is easier for fatty acids and fatty alcohols to form hydrogen bonds and the formed hydrogen bonds are more stable, which result in a higher concentration of fatty acids and fatty alcohols in the aggregates. Fatty acids and fatty alcohols are possibly more significant in the formation of aggregates and it may be beneficial to prevent the aggregation by controlling the concentration of them in lubricating oil.