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-t ert-butylphenyl)-3,4,9,10-perylene- carboxodiimide (BTBP) in several l iquids and in three supercritical fluids (fluoroform, carbon dioxide, and ethane). In liquids, BTBP follows near perfect Debye-Stokes-Einste in (DSE) behavior under sticky boundary conditions. However, in superc ritical fluids the rotational dynamics of BTBP are distinctly differen t. In close proximity to the critical density (rho(I) approximate to 1 ), the recovered rotational reorientation times are up to 12-fold grea ter than predicted by simple DSE theory with sticky boundary condition s. Upon increasing the fluid density, the recovered rotational reorien tation times steadily decrease until they fall within hydrodynamic pre dictions (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 augmentati on surrounding the solute is quantified in several ways. By using a mo del 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 co mpressibility region we see that the extent of local density augmentat ion decreases to a value approaching the bulk density. In an alternati ve interpretation, we explain the observed rotational reorientation dy namics in terms of the size of the solute-fluid cluster. At low fluid density (near the critical density) the radius of the ''solute-fluid c luster'' is a factor of 2 greater than the solute alone. Again, as pre ssure is increased, there is a decrease in the cluster size and the ra dius of the reorienting species (BTBP + clustered fluid molecules) app roaches the predicted value based on DSE theory using friction/boundar y terms determined for BTBP in normal liquid solvents.