Solvent-solute clustering, cage effects

Influence of Solvent-solute Clustering on Cage Effects in Photolytic Reactions... 

There is one more unique feature of supercritical fluid solvents that will be a recurring theme in this chapter. Several studies have demonstrated that near the critical point, the density of the solvent about a solute is enhanced relative to the bulk density (solvent/solute clustering). As such, the mobility of the solute may be impeded to an extent greater than expected on the basis of the bulk viscosity. This phenomenon may also affect reactivity for reactions that are diffusion-controlled or for which cage effects are important, particularly near the critical point (vide infra). 

Finally, the PFR of 1-naphthyl acetate has been performed in supercritical carbon dioxide the obtained resnlts point to an enhanced solvent cage effect on the product ratios at lower supercritical fluid bulk densities, which is consistent with the existence of supercritical solvent-solute clusters with a lifetime similar to or longer than that of the primary singlet radical pair [143]. 

R2-R2 was 25%, 50%, and 25%, respectively, and did not vary with pressure. The fact that a statistical 1 2 1 ratio of bibenzyls is produced, at all pressures examined including near the critical pressure, suggests that these products are produced by diffusive encounters of (Ri.)free and (R2.)free rather than by in-cage coupling (which would yield solely Ri R2). Thus, these authors found no evidence for an enhanced cage-effect that might be attributable to solute solvent clustering. 

The triplet-triplet annihilation reaction of benzophenone was investigated by laser flash photolysis for both SCCO2 and scC2H6- The authors reported no evidence of enhanced cage effects either near the critical point or removed from the critical point due to SCF solvent clustering or solute-solute interactions [13]. 

The magnitude of die cage effect in SC-CO2 is as anticipated based upon extrapolations from conventional liquid solvents. No evidence was found for an enhanced cage effect, which might be attributable to so vent/solute clustering. 

That caging is effective even when the solvation shell is not completed, as seen in Figure 11.11, is observed in other clusters and solutes. Simulations suggest that more than one effect is at play. There is the purely mechanical effect that a solvent molecule is in the way and that, particularly when it is sufficiently heavy,... 

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