Experimental and simulation study of the high-pressure behavior of squalane and poly-α-olefins

Abstract

The equation of state, dynamical properties, and molecular-scale structure of squalane and mixtures of poly-α-olefins at room temperature are studied with a combination of state-of-the-art, high-pressure experiments and moleculardynamics simulations. Diamond-anvil cell experiments indicate that both materials are non-hydrostatic media at pressures above ∼ 1 GPa. The equation of state does not exhibit any sign of a first-order phase transition. High-pressure Xray diffraction experiments on squalane show that there are no Bragg peaks, and hence the apparent solidification occurs without crystallization. These observations are complemented by a survey of the equation of state and dynamical properties using simulations. The results show that molecular diffusion is essentially arrested above about 1 GPa, which supports the hypothesis that the samples are kinetically trapped in metastable amorphous-solid states. The shear viscosity becomes extremely large at very high pressures, and the coefficient governing its increase from ambient pressure is in good agreement with the available literature data. Finally, simulated radial distribution functions are used to explore the evolution of the molecular-scale structure with increasing pressure. Subtle changes in the shortrange real-space correlations are related to a collapse of the molecular conformations with increasing pressure, while the evolution of the static structure factor shows excellent correlation with the available X-ray diffraction data. These results are of indirect relevance to oil-based lubricants, as the pressures involved are comparable to those found in engines, and hence the ability of lubricating thin films to act as load-bearing media can be linked to the solidification phenomena studied in this work.

Contributions of Molecular Dynamics Simulations to Elastohydrodynamic Lubrication

“…They used the longchain optimised potential for liquid simulations (L-OPLS) all-atom force field [34]. Prentice et al [35] successfully employed the Eyring model to estimate the Newtonian viscosities from NEMD simulations of mixtures of hydrogenated 1-decene trimer and 1-decene tetramer as a model for PAO.…”

Tribochemistry, Mechanical Alloying, Mechanochemistry: What is in a Name?

“…This intermediate state can endure e.g. under continuous mechanical action and provide modified tribological properties ( Prentice et al, 2020 ; Reddyhoff et al, 2021 ), or relax when the mechanical perturbation is released to yield a reaction product ( Boldyrev, 2006 ). Owing to the significant kinetic barriers in solid state reactions, this reaction product may not be always the most thermodynamically stable one ( i.e.…”

Evaluation of Methods for Viscosity Simulations of Lubricants at Different Temperatures and Pressures: A Case Study on PAO-2

“…Previous studies also include a variety of calculations for viscosity with EMD-GK combined with NEMD in most cases [16][17][18][19]. The systems in the literature include simple linear alkanes, cyclic [20] or long-chain hydrocarbons, and more complex structures such as squalane and 1-decene trimer (PAO-4) [21][22][23]. There are fewer research papers on viscosity index [24,25] and pressure-viscosity coefficient calculations, a fact that may indicate that these properties can be difficult to simulate.…”