Local density augmentation supercritical solutions

The photodimerization reaction of anthracene in supercritical CO2 was studied systematically at different CO2 densities. Unlike in normal liqnid solvents, the reaction in supercritical CO2 is significant even at anthracene concentrations as low as a few micromolar. At comparable anthracene concentrations, the photodimerization reaction is one order of magnitude more efficient in CO2 than in normal liqnid solvents. The results also show that the efficient photodimerization reaction of anthracene is hardly affected by the local density augmentation (or solute-solvent clustering) in supercritical CO2 (Bunker et al., 1997). 

An important topic in supercritical fluid research is the effect of solvent local density augmentation on solute-solute interactions in a supercritical fluid solution. The most important question seems to be whether the supercritical solvent environment facilitates solute-solute clustering, which may be loosely defined as local solute concenttations that are greater than the bulk solute concentration. Unlike solute-solvent clustering discussed in the previous section, solute-solute clustering in supercritical fluid solutions is a more complex and somewhat controversial issue. Following is a summary of the available experimental results and a review of the various explanations and mechanistic proposals on the clustering of solute molecules in supercritical fluid solutions. 

The picture of a solute molecule stabilized in solution by a local environment where the solvent s concentration differs considerably from the bulk value is consistent with experiments and simulation. The encouraging agreement between the basic trends found in experiments and simulations should not obscure the fact that Lennard-Jones atoms are a pedestrian representation of the actual molecules studied in the fluorescence experiments. Caution must therefore be exercised when comparing simulations and experiments. At the same time, the very fact that such a crude model is able to capture the essential physics of the phenomenon under investigation lends further support to the notion that local density augmentations are common to all attractive supercritical systems. 

In this chapter, we will review some of the work that we have been doing in recent years in the context of solvation and dynamical properties in polar and non-polar supercritical solutions using molecular dynamics computer simulations. First we will discuss solvation of alkaloids in SC-CO2 and provide detailed molecular views of the main structural features of the local density augmentation around simple alkaloids... 

Knutson, B. L., D. L. Tomasko, C. A. Eckert, P. G. Debenedetti, and A. A. Chiavlo. 1992. Local density augmentation in supercritical solutions A comparison between fluorescence spectroscopy and molecular dynamics results. In ACS Symp. Ser., 488 60-72. 

JL deGrazia, TW Randolph, JA O Brien. Rotational relaxation in supercritical carbon dioxide revisited a study of solute-induced local density augmentation. J Phys Chem A 102 1674, 1998. 

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