Evaporation and flow properties of several hydrocarbon oils

Two properties that delimit the useful temperature range of synthetic lubri cating oils are the vapor pressure and viscosity. This study investigates t he ability of fluid theory to model these two properties for diesters with different diacid chain length and alcohol branching, triesters consisting o f triglycerides and a trimethylol propane ester, and nonpolar oils includin g aromatic hydrocarbons, a polyalphaolefin, and two branched alkanes. Withi n each type of oil there are structural isomers. Isomerism influences the v apor pressure and viscosity, in addition to the well-known dependence on mo lecular weight. The expressions for the vapor pressure and viscosity are co mbined using the absolute rate theory of Eyring and equilibrium thermodynam ics. A modification of the rate theory allowing for a rotational contributi on to the flow-activation entropy was needed to simultaneously fit the vapo r pressure and viscosity data between -20 and 100 degrees C. The flow-activ ation rotational entropy, calculated from the combined vapor pressure and v iscosity data, is a significant contribution to the viscosity. This analysi s provides the background for molecular dynamics simulation to assist in de signing new low viscosity and vapor pressure synthetic oils.