Pulsed field gradient NMR spectroscopy: Applications in determining the pressure viscosity coefficient and low-temperature flow properties of lubricant base oils

The pulsed field gradient (PFG) NMR technique was employed to measure the self-diffusion coefficients (D) of hydrocarbon lubricant base fluids under ambient pressure conditions. This parameter was then related to various rheological properties, including the kinematic viscosity and the pressure viscosity coefficient (PVC). The results demonstrate that D varies inversely with the fluid viscosity and PVC. This relationship was then used to predict PVC from D for a series of compositionally diverse base oils, providing an R² value of 0.86. PFG NMR spectroscopy was also used to study the molecular dynamics of the liquid-solid transitions of the oils at low temperatures ranging from +40 to -40°C. The self-diffusion coefficient decreased linearly with decreasing temperature to 0°C and then "leveled off' at lower temperatures. This observation can be explained, using spatially heterogeneous dynamics and other mechanistic models, by the onset of wax crystallization.