Effect of Pendant Sulfide and Sulfonyl Groups on the Thermal, Flow, and Antioxidative Properties of Lipid-Based Aliphatic Monoesters

Abstract

Thermal, flow, and oxidative stability behavior of oleyl oleate monoesters derivatized with sulfur-containing functional groups (sulfide and sulfonyl) suggests that these compounds may be suitable candidates as sustainable alternative base oils to replace mineral oils in lubricant formulations. A robust thiol-ene addition and oxidation synthesis route was utilized to prepare the materials in high yields and high purity with no byproducts. Changes in the functional properties of oleyl oleate are attributable to the enhanced intermolecular forces and irregular conformations introduced by the bulky polar sulfur-containing groups, which introduce steric hindrances and alter phase behavior. The derivatization of oleyl oleate with sulfide and sulfonyl pendant groups resulted in a lower enthalpy of crystallization (10 kJ/mol compared to 76 kJ/mol for oleyl oleate) and an increased viscosity from 16.3 mPa·s to as high as 175.5 mPa·s at 40 °C. The sulfonyl pendant groups imparted higher viscosity to oleyl oleate due to sulfonyl–sulfonyl interactions. The presence of sulfonyl and sulfide pendant groups increased oleyl oleate oxidation onset (Tonox) by 47 and 149 °C, respectively. The sulfide pendant groups converted hydroperoxides as they are formed into non-radical derivatives and thus delayed oxidation. Overall, the sulfide- and sulfonyl-branched oleyl oleate monoesters of this work presented useful thermal transition and viscosity profiles which are competitive with those of mineral oils, making them suitable alternatives for use in lubricant formulations.