Intrinsic relationship between viscosity, viscosity index, and molecular structure of isoalkanes

Viscosity and viscosity index are the crucial properties of lubricant base stocks. Molecular dynamics simulation and quantum calculation were used to simulate the five isomers of C₂₆H₅₄ to study the intrinsic relationship between viscosity, viscosity index, and the molecular structure of isoalkanes. The results showed that the intermolecular interaction energy and the volume of rigid-like groups were the intrinsic factors that affected the viscosity and which could describe the viscosity quantitatively. The molecule conformation was studied by calculating the rotational energy barrier of the dihedral angle in the isoalkane molecule, and combined with molecular dynamics, the effect of temperature on the molecular conformation at 313 K and 373 K was further investigated. The α, β, and γ carbon atoms adjacent to the tertiary carbon in the isoalkane molecule were difficult to rotate due to steric hindrance and could be regarded as rigid-like groups at 313 K. The tertiary carbon and the three adjacent carbon atoms formed a regular tetrahedral rigid-like group at 373 K. The changes in the intermolecular interaction energy and the volume of the rigid-like group with temperatures could better describe the viscosity index and reveal the fundamental reasons that affect the viscosity and the viscosity index. The molecular-level understanding of the relationship between the molecular structure and properties of isoalkanes provided theoretical support and scientific guidance for designing isoalkane molecules with specific properties.

Molecular dynamics simulation and quantum calculation were performed using Material Studio 8.0 software. The Amorphous Cell module was used to create an amorphous cell. The Foricite module was used for molecular dynamics simulation; the forcefield was assigned as COMPASS II. Nose-Hoover thermostat and Berendsen barostat were applied to maintain the temperature and pressure, respectively. To describe the non-bond interactions, the Ewald method was applied to calculate the van der Waals and electrostatic interactions. The Conformers module was used to study the conformation and the Dmol3 module was used to calculate the conformational energy with fine quality; the functional of GGA-PW91 and the basis set of DNP were used to calculate the energy.