HIGH-PRESSURE NMR-STUDY OF TRANSPORT AND RELAXATION IN COMPLEX LIQUIDS OF 2-ETHYLHEXYL CYCLOHEXANECARBOXYLATE AND 2-ETHYLHEXYL BENZOATE

The self-diffusion coefficients, densities, and shear viscosities of liquid 2-ethylhexyl cyclohexanecarboxylate (EHC) were measured as a function of pressure from 1 to 4500 bar within the temperature range from -20 to 80°C. The Stokes-Einstein equation is applicable over 5 order of magnitude changes in self-diffusion and viscosity. The experimental data obtained are compared to those for the complex liquid of 2-ethylhexyl benzoate (EHB) in order to characterize the molecular structure effect of the replacement of the benzene ring with a saturated cyclohexyl ring. In particular, the low-temperature data suggest that conjugation of the phenyl ring with the ester group in EHB slows down diffusion and increases viscosity in comparison with EHC. Analysis in terms of the rough hard sphere model indicates a high degree of rotational-translational coupling which increases as density increases. By use of high-resolution, high-pressure NMR techniques the natural-abundance 13C spin-lattice relaxation times, T1, and nuclear Overhauser enhancement were measured for each individual carbon in EHC and EHB over the same range of temperatures and pressures. An approximate analysis of the experimental T1 data indicates anisotropic reorientation with multiple internal rotations. © 1990 American Chemical Society.