A Comparison of Classical Force-Fields for Molecular Dynamics Simulations of Lubricants

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Title: A Comparison of Classical Force-Fields for Molecular Dynamics Simulations of Lubricants
Authors: Ewen, J
Gattinoni, C
Thakkar, F
Morgan, N
Spikes, H
Dini, D
Item Type: Journal Article
Abstract: 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.
Issue Date: 2-Aug-2016
Date of Acceptance: 28-Jul-2016
ISSN: 1996-1944
Publisher: MDPI
Journal / Book Title: Materials
Volume: 9
Issue: 8
Copyright Statement: This is an open access article distributed under the Creative Commons Attribution License (CC BY 4.0).
Sponsor/Funder: Engineering & Physical Science Research Council (EPSRC)
Shell Research Limited
Engineering & Physical Science Research Council (EPSRC)
Funder's Grant Number: EP/G026114/1
PT36160 - PO 4550096336
Keywords: Chemical Sciences
Publication Status: Published
Article Number: 651
Appears in Collections:Faculty of Engineering
Mechanical Engineering
Faculty of Natural Sciences

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