Probing the scale of local density augmentation in supercritical fluids: A picosecond rotational reorientation study

We report on the rotational reorientation kinetics of N,N′-bis-(2,5-tert-butylphenyl)-3,4,9,10-perylenecarboxodiimide (BTBP) in several liquids and in three supercritical fluids (fluoroform, carbon dioxide, and ethane). In liquids, BTBP follows near perfect Debye-Stokes-Einstein (DSE) behavior under sticky boundary conditions. However, in supercritical fluids the rotational dynamics of BTBP are distinctly different. In close proximity to the critical density (ρ_r ≈ 1), the recovered rotational reorientation times are up to 12-fold greater than predicted by simple DSE theory with sticky boundary conditions. Upon increasing the fluid density, the recovered rotational reorientation times steadily decrease until they fall within hydrodynamic predictions (i.e., DSE theory). This extraordinary behavior is explained in terms of local solute-fluid density augmentation which is a feature particular only to supercritical fluids. The local density augmentation surrounding the solute is quantified in several ways. By using a model recently developed by Anderton and Kauffman (J. Phys. Chem. 1995, 99, 13759), the local fluid density is found to exceed the bulk by up to 300%. Upon increasing the pressure and moving away from the high compressibility region we see that the extent of local density augmentation decreases to a value approaching the bulk density. In an alternative interpretation, we explain the observed rotational reorientation dynamics in terms of the size of the solute-fluid cluster. At low fluid density (near the critical density) the radius of the "solute-fluid cluster" is a factor of 2 greater than the solute alone. Again, as pressure is increased, there is a decrease in the cluster size and the radius of the reorienting species (BTBP + clustered fluid molecules) approaches the predicted value based on DSE theory using friction/boundary terms determined for BTBP in normal liquid solvents.