A Comparison of Classical Force Fields for Molecular Dynamics Simulations of Lubricants
For the successful development and application of lubricants, a full understanding of their complex nanoscale behavior under a wide range of external conditions is required, but this is difficult to obtain experimentally. Nonequilibrium molecular dynamics (NEMD) simulations can be used to yield unique insights into the atomic scale structure and friction of lubricants and additives; however, the accuracy of the results depend on the chosen force field. In this study, we demonstrate that the use of an accurate, all atom force field is critical in order to; (i) accurately predict important properties of long chain, linear molecules; and (ii) reproduce experimental friction behavior of multi component tribological systems. In particular, we focus on n hexadecane, an important model lubricant with a wide range of industrial applications. Moreover, simulating conditions common in tribological systems, i.e. high temperatures and pressures (HTHP) allows the limits of the selected force fields to be tested. In the first section, a large number of united atom and all atom force fields are benchmarked in terms of their density and viscosity prediction accuracy of n hexadecane using equilibrium molecular dynamics (EMD) simulations at ambient and HTHP conditions. Whilst united atom force fields accurately reproduce experimental density, the viscosity is significantly under predicted compared to all atom force fields and experiments. Moreover, some all atom force fields yield elevated melting points, leading to significant overestimation of both the density and viscosity. In the second section, the most accurate united atom and all atom force field are compared in confined NEMD simulations which probe the structure and friction of stearic acid adsorbed on iron oxide and separated by a thin layer of n hexadecane. The united atom force field provides an accurate representation of the structure of the confined stearic acid film; however, friction coefficients are consistently under predicted and the friction coverage and friction velocity behavior deviates from that observed using all atom force fields and experimentally. This has important implications regarding force field selection for NEMD simulations of systems containing long chain, linear molecules; specifically, it is recommended that accurate all atom potentials, such as L OPLS AA, are employed.
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