Local density augmentation

These local density augmentations can affect reactions in several ways. The higher local solvent density may change local values of density-dependent properties such as the dielectric constant, thereby influencing reaction rates. It is also 

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Local solvation in the form of local density augmentation and local composition enhancements can influence reactivity, but this depends on the relative time-scales of the reaction and solvation processes. 

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). 

The local density augmentation caused by the large isothermal compressibility of the fluid may conceivably influence k i or ka. We assume that the lifetime of the clusters is extremely short and thus there is no effect on kd, based on the molecular dynamics study of Petsche and Debenedetti (29) and experimental measurements of binary diffusion coefficients near the critical point. It seems more likely that a higher local density would affect k i due to an increase in the number of... 

Steady-state and multifrequency phase and modulation fluorescence spectroscopy are used to study the photophysics of a polar, environmentally-sensitive fluorescent probe in near- and supercritical CF3H. The results show strong evidence for local density augmentation and for a distribution of cluster sizes. These results represent the first evidence for lifetime distributions in a "pure solvent system. 

Figure 5 Relationship between local density augmentation (local/bulk density) and bulk density. Same system and conditions as in Figure 3. Molecular dynamics simulation.

Figure 6 Comparison between local density augmentation deduced from fluorescence spectroscopy ( ), and the corresponding molecular dynamics simulations at R = 1.94 (Q). Both curves are for a reduced temperature of 1.02. The arrow denotes the critical density of carbon dioxide.

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... 

The theory reflects the solvent properties through the thermody-namic/hydrodynamic input parameters obtained from the accurate equations of state for the two solvents. However, the theory employs a hard sphere solute-solvent direct correlation function (C12), which is a measure of the spatial distribution of the particles. Therefore, the agreement between theory and experiment does not depend on a solute-solvent spatial distribution determined by attractive solute-solvent interactions. In particular, it is not necessary to invoke local density augmentation (solute-solvent clustering) (31,112,113) in the vicinity of the critical point arising from significant attractive solute-solvent interactions to theoretically replicate the data. 

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