Lubricant Characterization by Molecular Simulation

Lubrication is a phenomenon of immense practical importance and fundamental scientific interest, and the automobile engines of the future are envisioned by the Partnership for a New Generation Vehicle will require the development of improved lubricants that perform well at higher operating temperatures and higher engine speeds. The rheological properties of liquid alkanes of intermediate molecular sizes (C20H42-C40H82) are among the most important properties in lubricant performance. Though realistic study of these systems by molecular simulation has previously been limited by both high computational costs and the lack of potential models accurate over a wide range of physical conditions, the advent of massively parallel supercomputers has now made such studies possible. As an illustration of the ability of molecular simulation as a useful tool for lubricant development, we present the first molecular-simulation-based calculation of the kinematic viscosity index of an alkane liquid, viz. 2,6,10,15, 19,23-hexamethytetracosane, commonly called squalane. In the mass range of interest, squalane is one of the few commercially available isoparaffins and has been the subject of previous studies by molecular simulation (Mondello and Grest 1995; Mundy et al., 1997). Though numerous properties of a lubricant are important to end-use applications, the viscosity is considered most significant. Mundy et al. (1996) investigated the variation of decane’s viscosity with pressure, calculating its pressureviscosity coefficient by equilibrium molecular dynamics simulations. The kinematic viscosity index (VI) is another widelyused industrial characterization of automotive lubricants. It was proposed by Dean and Davis (1929) as an indication of an oil’s viscosity-temperature characteristics in terms of its Saybolt viscosities at 311 K (100°F) and 372 K (210°F). Two series of reference lubricating-oil fractions ( H and L ) were used for comparison. Series H exhibited little change of viscosity with temperature while the viscosities of series L oils exhibited large variation with temperature. Series H and L represented, respectively, the best and worst oils available in 1929. Series H oils were assigned a VI of 100, series L a