Abstract
The present work intends to provide a brief account of the most recent advances in the use of ionic liquid crystals (ILCs) in the field of tribology, that is, the development of new lubricants with the ability to reduce the coefficients of friction and the wear rates of materials under sliding conditions. After a definition of ILCs and their relationship with neutral liquid crystals (LCs) and ionic liquids (ILs), the review will be focused on the influence of molecular structure and composition on the tribological performance, the combination with base oils, surfactants or water, and the different sliding configuration and potential applications. The main mechanisms proposed in order to justify the lubricating ability of ILCs will be analyzed. Special emphasis will be made for recent results obtained for fatty acid derivatives due to their renewable and environmentally friendly nature.
Highlights
The broad field of liquid crystal materials (LCs) has been an active area of interest for scientists and engineers since the discovery of liquid crystalline behavior by Reinitzer at the end of the 19th century [1]
They have shown that both rheological properties and tribological performance are determined by ionic liquid crystals (ILCs) molecular orientation
This molecular orientation can be modified by shear, influencing friction coefficient values, which reach values as low as 0.02. These authors found that friction coefficient values are a function of molecular orientation, and of the chemical composition of the anion. These results show that there still remains a very relevant research effort to be made in the optimization of ILCs chemistry and molecular design
Summary
The broad field of liquid crystal materials (LCs) has been an active area of interest for scientists and engineers since the discovery of liquid crystalline behavior by Reinitzer at the end of the 19th century [1]. A decade ago, the state of the art of both LCs and ILC in tribology was the subject of two review papers [3,4]. Over this period of time, the focus on IL lubricants has somewhat overshadowed the attention on the tribological applications of neutral LCs. Today, the scope of the field has spread to Lubricants 2019, 7, 72; doi:10.3390/lubricants7090072 www.mdpi.com/journal/lubricants. This review reports conventional ILs among the eco-friendly additives Another very recent review on ILs as boundary lubricants [6] does not include. Some review articles on thermotropic [8] and lyotropic LCs [9], as well as on ILCs [10], have been previously published, but to the best of our knowledge, this is the first review dedicated to ILCs in tribology and lubrication as surface modifiers, as well as friction-reducing and wear-preventing agents
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