Application of matched spin probes to the investigation of the overall motion of model lubricant molecules

Abstract

EPR spectroscopy has been used to measure the fast-limit rotational correlation times (and their activation energies) of two spin probes which closely match the structure of two model lubricants, namely a low-traction fluid (di-n-octyl ether) and a high-traction fluid (2,4-dicyclohexyl-2-methylpentane, DCMP). The data have been compared with those obtained from these fluids from ¹³C NMR relaxation measurements. The latter provide values for the rotational correlation times and their activation energies, but they give no insight into the isotropy of the motion. The EPR work showed that the spin probe for di-n-octyl ether executes isotropic motion while that for DCMP is anisotropic. The values obtained by EPR and NMR for the rotational correlation times and their associated activation energies are in reasonable agreement; however, the EPR results show that the spin probe for DCMP rotates with near axial symmetry and that the NMR rotational correlation time corresponds to the faster of the two motions.