Rheology of lubricant basestocks: A molecular dynamics study of C-30 isomers

We have performed extensive equilibrium and nonequilibrium molecular dynami cs (EMD and NEMD) simulations of three isomers of C30H62 at temperatures of 311 and 372 K employing a united atom model. Using the rotational relaxati on time calculated from the EMD simulation, the Rouse model predicts a zero -shear viscosity for n-triacontane within 16% of the value determined by NE MD. Compared to experiment, NEMD and the united atom model underpredict the kinematic viscosities of n-triacontane and 9-n-octyldocosane but accuratel y predict the values for squalane (within 15%). In addition, the predicted values of the kinematic viscosity index for both 9-n-octyldocosane and squa lane are in quantitative agreement with experiment and represent the first such predictions by molecular simulation. This same general potential model and computational approach can be used to predict this important lubricant property for potential lubricants prior to their synthesis, offering the p ossibility of simulation-guided lubricant design. (C) 2000 American Institu te of Physics. [S0021-9606(00)50643-6].