Clustering cosolvent-solute

Supercritical fluids (SCFs) such as carbon dioxide have a "hydrocarbon-like solvent strength at typical conditions, so that they are appropriate solvents for lipophilic substances. The solvent strength may be raised significantly by the addition of small amounts of cosolvents such as ethanol to increase solubilities of moderately polar substances selectivelyQ), sometimes by several hundred percent(2,2 4). The solvent and cosolvent form clusters about solutes, in which the cosolvent concentrations are enhanced significantly( ,fi). The present objective is to explore the effects of considerably more powerful solvent additives, that is surfactants. Since very little is known about surfactants in SCFs, spectroscopic probes were used to measure polarities inside the reverse micelles. Polarity is a key indicator of the ability of a reverse micelle to solvate a hydrophile. Using the... 

In supercritical fluids, the possibility of local composition enhancements of cosolvent about a solute suggests that we should see enhancement of anion fluorescence if the water cosolvent clusters effectively about the 2-naphthol solute. Although in liquids the water concentration must be >30% to see anion emission, the higher diffusivity and density fluctuations in SCFs could allow stabilization of the anion at much lower water concentrations provided that the water molecules provide sufficient structure. Therefore the purpose of these experiments was to investigate 2-naphthol fluorescence in supercritical CO 2 with water cosolvent in the highly compressible region of the mixture to probe the local environment about the solute. 

Finally, coupled with the phase equilibrium problem of concentrating the cosolvent, perhaps the composition of cosolvent around the probe is not sufficient to facilitate proton transfer. This implies that if the cosolvent is indeed "clustering" around the solute, there is insufficient structural integrity to solvate the proton. 

The present paper is devoted to the local composition of liquid mixtures calculated in the framework of the Kirkwood—Buff theory of solutions. A new method is suggested to calculate the excess (or deficit) number of various molecules around a selected (central) molecule in binary and multicomponent liquid mixtures in terms of measurable macroscopic thermodynamic quantities, such as the derivatives of the chemical potentials with respect to concentrations, the isothermal compressibility, and the partial molar volumes. This method accounts for an inaccessible volume due to the presence of a central molecule and is applied to binary and ternary mixtures. For the ideal binary mixture it is shown that because of the difference in the volumes of the pure components there is an excess (or deficit) number of different molecules around a central molecule. The excess (or deficit) becomes zero when the components of the ideal binary mixture have the same volume. The new method is also applied to methanol + water and 2-propanol -I- water mixtures. In the case of the 2-propanol + water mixture, the new method, in contrast to the other ones, indicates that clusters dominated by 2-propanol disappear at high alcohol mole fractions, in agreement with experimental observations. Finally, it is shown that the application of the new procedure to the ternary mixture water/protein/cosolvent at infinite dilution of the protein led to almost the same results as the methods involving a reference state. 

For many supercritical fluids, the solubilities of the compounds of interest, even in the high-density region, may be too low for practical apphcation. This limitation can be overcome through the use of a cosolvent. Early on, researchers discovered that the addition of small amounts of a cosolvent could dramatically enhance the solubility of various analytes.In many cases, the enhancement exceeded that predicted based on the bulk concentration of the cosolvent in the supercritical fluid. The results suggested an enhancement of solvent strength under supercritical conditions. The phenomenon, termed the entrainer effect, is related to solute-solute clustering the entrainer effect served as preliminary evidence of unusual behaviors and pointed researchers toward solute-solute clustering. 

Another means to enhance the basicity of n-butyllithium consists in using it in a mixture with potassium ter-butoxide. Here again, coordination of lithium with the oxygen atom of t-BuOK breaks the tetrameric cluster ( -BuLi)4, which leads to the formation of a very active monomer. Alternatively, it is possible to add a cosolvent such as hexamethylphosphoramide (HMPA) to a commercial solution of n-butyl-lithium in hexane, which has the same effect as TMEDA indicated above. Finally, one can also evaporate the hexane solvent and add a more polar solvent such as ether with or without cosolvent. Note that n-butyllithium is not a bulky base, and thus it also often reacts as a nucleophile. In these cases, it is necessary to prepare a bulky base by reaction of n-butyllithium with a bulky amine such as diisopropylamine, bis(trimethylsilyl)amine or tetramethylpiperidine. 

Electron transfer reactions have also been used in the probing of solute-solute interactions in supercritical fluid solutions. For example, Takahashi and Jonah examined the electron transfer between biphenyl anion and pyrene in supercritical ethane (192). Worrall and Wilkinson studied triplet-triplet energy transfer reactions for a series of donor-acceptor pairs, including anthracene-azulene in supercritical C02-acetonitrile and supercritical C02-hexane and ben-zophenone-naphthalene in supercritical C02-acetonitrile (193). The high efficiency of the energy transfer reactions at low cosolvent concentrations was attributed to the effect of solute-solute clustering. 

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