Review of Molecular Dynamics Simulations of Phosphonium Ionic Liquid Lubricants

Phosphonium ionic liquids (ILs) have various uses, including as environmentally benign lubricants and lubricant additives. The properties and behavior of these ILs depend on their chemical composition, i.e., cation and anion combination, and the operating conditions. One approach to understanding the relationships between composition, conditions, and lubricant-relevant properties is classical molecular dynamics simulation. Although this research area is still emerging, it is growing rapidly, so a review of the topic is timely. Here, we review force field-based molecular dynamics simulations of phosphonium ILs, with emphasis on physical, chemical, and thermal properties relevant to lubricants. Properties reported in previous studies are density, viscosity, self-diffusivity, ionic conductivity, heat capacity, and thermal stability, as well as interactions with other compounds, including \(\mathrm {H_2O}\) and \(\mathrm {CO_2}\), and solid surfaces. The effects of anion and cation, as well as conditions such as temperature, on these properties are identified and analyzed in terms of anion-cation structure, orientation, and interactions. Finally, trends are summarized and opportunities for future research are identified.