Solute-solvent clustering

Finally, the PFR of 1-naphthyl acetate has been used to demonstrate the occurrence of solvent-solute clustering in supercritical carbon dioxide. This is... 

Influence of Solvent-solute Clustering on Reactions Due to Augmentation in Local Polarity... 

Spectroscopy and computer simulation have given an abundance of information regarding solvent/solute clustering (3-8). In fact, it is reasonably... 

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). 

Andrew, D. Des Islet, B. T. Margaritis, A. Weedon, A. C. Photo-Fries Rearrangement of Naphthyl Acetate in Supercritical Carbon Dioxide Chemical Evidence for Solvent-Solute Clustering. J. Am. Chem. Soc. 1995, 117, 6132-6133. 

Carlier, C. Randolph, T. W. Dense-Gas Solvent-Solute Clusters at Near Infinite Dilution EPR Spectroscopic Evidence. AlChE J. 1993, 39, 876-884. 

Figures 2a and 2b show how the predicted solvent-solute pair correlation function, gAB varies with density. At the highest density (p = 0.6) the structure is liquid-like with hrst, second, third,. .. maxima/minima oscillating about 1.0. The size of the solvent-solute cluster for this state was calculated to be about —1 solvent molecule the presence of one solute molecule at this state excludes about one solvent molecule.

These calculations support the notion of a large solvent-solute cluster near the CP as suggested from spectroscopic results (2.3.7V The estimation fl.4) that the cluster approaches 100 solvent molecules based on the partial molar volume data of Eckert et al. (1) for naphthalene in supercritical carbon dioxide at this temperature also appears entirely consistent with the predictions from the integral equation theory. 

The solvation structures which we have determined for the supercritical solutions of LJ molecules appear to be in agreement with the recent theoretical and experimental suggestions of solvent-solute clustering and solute-solute aggregation near the CP. Quantitative testing of theoretical models for solvation structure of supercritical solutions (such as that presented here) may become possible if recent efforts at simulation of supercritical solutions (elsewhere in this volume) prove successful. Clearly, the LJ model used in our theoretical studies to date does not provide an adequate representation of real molecular interactions. However, the method we have demonstrated is potentially capable of application with much more accurate (and complicated) potential functions, and meaningful quantitative interpretation of experiments may then become possible. 

Figure 4 Size of solvent-solute cluster xa. density at T = 1.37, 1.41, and 1.46.

O Brien, J. A., T. W. Randolph, C. Carlier, and S. Ganapathy 1993, Quasicritical behavior of dense-gas solvent-solute clusters at near-infinite dilution . AIChE J. 39, 876. 

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]. 

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