Effect of Alkylated Diphenylamine Antioxidants on the Anti-aging Properties of Ester Lubricants at the Molecular Level: Molecular Simulations and Experiments

A combined study of molecular simulation and experimental methods is presented for trimethylolpropane trioleate (TMPTO) with different alkylated diphenylamine antioxidants: 4,4′-dimethyldiphenylamine (DMDPA), 4,4′-dioctyldiphenylamine (ODA), and 4,4′-dinonyldiphenylamine (T558) to investigate the mobility, activity, and O2 migration of antioxidants. The solubility parameter (δ), binding energy (Ebinding), mean square displacement (MSD), hydrogen bond dissociation energy (BDE), and solubility coefficient of O2 (P) were calculated by molecular simulation. The simulation results showed that the antioxidant T558 exhibits lower mobility and better physical resistance to O2 penetration. Fourier transform infrared spectroscopy (FT-IR), 1,1-diphenylamine-2-trinitrophenylhydrazine (DPPH) radical scavenging, pressure differential scanning calorimetry (PDSC), and surface tension were investigated by experiments. The experimental results showed that antioxidant T558 with larger relative molecular mass and longer amine para-alkyl chain exhibited better antioxidant properties, which was in an agreement with simulation results. The aim of this study is to provide a molecular-level theoretical basis for the anti-ageing protection of lubricants.