Computer simulations of realistic models of the two solutes diphenylbutadiene (“DPB”) and hydroxymethylstilbene (“HMS”) in supercritical CO2 (310 K, 0.25-2.0Fc) are used to explore the interplay between local density augmentation and friction in supercritical solvents. Good agreement is found between the extent of local density augmentation observed in these simulations and that deduced from experimental measurements of electronic spectral shifts (also reported here). To test the accuracy of the solvent model for treating solute friction, the viscosity and self-diffusion constants of the neat solvent were simulated and compared to experiment and good agreement was also found. The rotation times of these solutes are compared to results of prior experimental measurements made by Anderton and Kauffman (J. Phys. Chem. 1995, 99, 13 759). In the case of DPB, the simulated times are larger than experimental estimates by as much as 30% at higher densities (F ≈ 1.8Fc). The origins of this disagreement are presently unknown. The density dependence of the simulated friction on both rotational and translational motions is curious in that it appears insensitive to the considerable density augmentation present in these systems. This insensitivity is shown to result from a cancellation of static and dynamic aspects of the friction, both of which are individually responsive to the excess density in the neighborhood of the solute.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry