Solute-fluid interactions

Elucidation of Solute—Fluid Interactions in Supercritical CF H by Steady-State and Time-Resolved Fluorescence Spectroscopy... 

BETTS ET AL. Solute—Fluid Interactions in Supercritical CFJH... 

Betts, T. A., J. Zagrobelny, and F. V. Bright 1992, Investigation of solute-fluid interactions in supercritical CF3H A multifrequency phase and modulation fluoresence study . J. Supercritical Fluids 5, 48. 

The operating conditions can also be changed to alter fluid hydrophobicity. This allows the fluid to solubilize a substance depending on the solute-fluid interaction without changing the fluid itself. 

A variety of equations-of-state have been applied to supercritical fluids, ranging from simple cubic equations like the Peng-Robinson equation-of-state to the Statistical Associating Fluid Theoiy. All are able to model nonpolar systems fairly successfully, but most are increasingly chaUenged as the polarity of the components increases. The key is to calculate the solute-fluid molecular interaction parameter from the pure-component properties. Often the standard approach (i.e. corresponding states based on critical properties) is of limited accuracy due to the vastly different critical temperatures of the solutes (if known) and the solvents other properties of the solute... 

Solid-Fluid Equilibria The solubility of the solid is very sensitive to pressure and temperature in compressible regions, where the solvent s density and solubility parameter are highly variable. In contrast, plots of the log of the solubility versus density at constant temperature often exhibit fairly simple linear behavior (Fig. 20-19). To understand the role of solute-solvent interactions on sofubilities and selectivities, it is instructive to define an enhancement factor E as the actual solubihty divided by the solubility in an ideal gas, so that E = ysP/Pf, where P is the vapor pressure. The solubilities in CO2 are governed primarily by vapor pressures, a property of the solid... 

Based on the discrete solvent approach, a perturbed hard fluid model was developed to study dissociation reactions [227], In this model, the solute-solvent interactions are... 

Investigations of Solute—Cosolvent Interactions in Supercritical Fluid Media... 

Kim and Johnston (27), and Yonker and Smith (22) have used solute solvatochroism to determine the composition of the local solvent environment in binary supercritical fluids. In our laboratory we investigate solute-cosolvent interactions by using a fluorescent solute molecule (a probe) whose emission characteristics are sensitive to its local solvent environment. In this way, it is possible to monitor changes in the local solvent composition using the probe fluorescence. Moreover, by using picosecond time-resolved techniques, one can determine the kinetics of fluid compositional fluctuation in the cybotactic region. 

The feasibility of extracting substituted phenols from an aqueous solution with supercritical CO2 is reported A special extraction vessel was used in order to overcome the mechanical difficulty in retaining the liquid matrix in the extraction vessel. Solid phase trapping was utilized with a diol silica bonded phase. Methanol was used to rinse the trap. Below 300 atm extraction recovery paralleled CO2 pressure at fixed temperature. Phenol was least extractable while, 2,4-dichlorophenol yielded the greatest percent recovery. Above 300 atm extraction yield declined with pressure. It is theorized that at high CO2 density there is less mixing with the aqueous phase because of increased fluid-fluid interaction. 

In addition to these extraction parameters the matrix itself will have a large impact upon the choice of the extraction conditions. The matrix, besides containing coextractable materials will impact the solute-solid interaction kinetics and thermodynamics. The coextractants themselves can also act as modifiers and, as their concentrations change over the course of an extraction, the make-up of the extraction fluid will dynamically change. 

Another parameter that can have a large effect on the extraction process is the addition of modifiers. The effects of modifiers are still not well understood, but they will change the solvent power of the fluid and/or change the solute/solid interaction (generally both). After the fluid characteristics have... 

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