Order-disorder phase transformation of triacylglycerols: effect of the structure of the aliphatic chains.

Plant oils have been used as environmentally benign lubricants since they present high viscosity index and flash points and low evaporation loss. Triacylglycerols (TAG) are the major components of naturally occurring oils and fats and are able to produce high strength lubricant films. One of the main concerns that hinders the usage of triacylglycerols as lubricants, however, is the thermal stability of these molecules. In this paper, we report on the effect of chain structure on density, viscosity, and thermal stability of triacylglycerols using molecular dynamics simulations. The selected triacylglycerols are trilauroylglycerol (LLL-TAG), tristearoylglycerol (SSS-TAG), trans-trioleoylglycerol (trans-OOO-TAG), and trans-trilinolenoylglycerol (trans-LeLeLe-TAG). The first two TAGs are saturated molecules with a different number of carbons in the chain, and the second two TAGs are monounsaturated and polyunsaturated molecules, respectively. The computed results demonstrate that the length of the aliphatic chain influences the physical properties of triacylglycerols. TAGs with short chain (LLL-TAG) show higher density than TAGs with longer chains. Viscosity is determined by the degree of recoil of the aliphatic chains and by the number and location of unsaturated bonds. Thermal stability, as represented by the ability of triacylglycerols to stay in a disordered phase during the cooling process, is related to the order-disorder phase transition temperature. Since the phase transition temperature can be correlated to the thermal stability during the cooling process, LeLeLe-TAG shows the highest thermal stability among the systems considered. These results can aid in the design of molecules with specific lubrication properties.