Vibrational spectroscopic studies on the model lubricant 2-ethylhexyl benzoate (EHB)
In an attempt to investigate the relationship between microscopic and macroscopic fluid properties, like viscosity and density, a vibrational spectroscopic study of the model lubricant 2-ethylhexyl benzoate (EHB) was initiated. The variation in frequencies and band shapes of the vibrational modes of the molecule have been studied as a function of concentration, temperature and pressure. Estimates of correlation times for the reorientational motion of the phenyl ring of EHB, as a function of temperature, have been made from measurements of the Raman bands of the ring v(C-C) deformation mode and (^13)C N.M.R. spin lattice relaxation times. It has been shown using this data that the viscosity/temperature behaviour of EHB is dependent on the reorientational as opposed to the translational motion of the molecule. A noncoincidence of the Raman isotropic and anisotropic bands of the v(C=0) stretching mode of EHB has been seen and explained in terms of a resonance energy transfer (RET) process via transition dipole-transition dipole interactions, most probably as a consequence of preferential alignment via dipole-dipole interactions. It appears from dilution experiments that the EHB molecules do not become completely separated until below 2% mole-fraction. The infrared band of the v(C=0) stretching mode of EHB shows an unusual red shift with increased pressure to 6.5kbar, followed by a blue shift as the pressure is further increased. This has been explained as due to the competing effects of increased alignment of carbonyl dipoles (red shift) and the increase in the repulsive interactions (blue shift). All the other bands exhibit blue shifts, of varying degree, with increased pressure, showing that the repulsive forces dominate the shifts. The spectral changes are consistent with chain extension and increased interchain interactions with increased pressure. The band widths of all the vibrational modes increase with pressure consistent with an increase in vibrational relaxation rate. The fluid appears to exhibit a phase change close to 6.5kbar.