NEMD Simulations of Viscosity and Viscosity Index for Lubricant-Size Model Molecules

Abstract

Synthetic lubricant research can be facilitated with nonequilibrium molecular dynamics simulations (NEMD) of viscosity and viscosity index (VI). While previously reported simulations using united atom (UA) models for intermolecular interactions have generally under-predicted the viscosity of lubricant-size alkanes, VI values, trends with respect to structure, and viscosity pressure coefficients obtained from simulations are generally accurate, suggesting that simulations of UA models can be used for screening candidate fluids. This requires some reliance on the expected accuracy of the method as established by application to a wide variety of model fluids. We report here the results of NEMD simulations on UA models for 2,2,4,4,6,8,8-heptamethylnonane (HMN) and 11-n-amylheneicosane (NAH). HMN represents a molecule with significant branching, dominated with tertiary carbons; NAH represents a fluid with a single large aliphatic branch off of a long aliphatic chain, and a molecule for which there is some interest as a lubricant component. Additionally, we test here a new parameterization of UA potentials for –CHX groups. VI results from simulations using the new parameterization are in excellent agreement with experiment. Viscosity results are significantly better with the new parameterization, but the UA model still tends to underestimate the viscosity of larger, lubricant-size molecules.