Specific solvent strength

The final set of solvatochromic data are shown in Figure 6 for phenol blue in SF C02 doped with various amounts of the co-solvent or entrainer, methanol. Consider a pressure of 100 bar where the Et of phenol blue in C02 is 54 kcal/mol. The red shift is increased more by the addition of 3.5 mole percent methanol at constant pressure than by an increase in the pressure of pure C02 of over 200 bar. The large specific "solvent strength" of methanol causes this behavior. The red shift caused by the co-solvent is in... 

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As well as density the solvent strength and selectivity of the fluid will affect both the solubilizing power of the fluid for specific solutes and retention in SFC. Solvatochromic indicators have been used to rank fluids commonly used in SFC in terms of... 

The selection of a solvent for a new separation problem, even today, is a matter of trial and error. The application of theory (2) with the additional application of the solubility parameters (6J-65) makes it possible to estimate the composition of appropriate solvent mixtures for the separation of relatively simple compounds. In order to calculate the necessary solvent strength, however, a set of experimental data concerning the behavior of the sample components, the adsorbent, and the elution strength of the eluents with the specific adsorbent are necessary. Others (J5) recommend a graphical method as a time-saving alternative to bi th calculation and the trial-and-error approach to obtain a first approximation of the eluent composition appropriate for the separation of a givin sample. 

Vibrational spectroscopy, too, has been used to study supercritical fluid systems. Buback reviewed (59) this area however, much of his discussions are on fluid systems that are well removed from ambient conditions or difficult to handle easily (e.g., H20, HC1). In an early report, Hyatt (21) used IR absorbance spectroscopy to determine the influence of several solvent systems, including COz, on the vibrational frequencies ( ) of solute molecules. Specifically, he studied the vc=o of acetone and cyclohexanone and vs.H of pyrrole. The goal of this work was to determine the suitability of supercritical fluids as reaction solvent. Hyatt concluded that the ketones experienced an environment similar to nonpolar hydrocarbons in COz and that there were no differences between liquid and supercritical CO2. In contrast, the pyrrole studies indicated that the solvent strength of CO2 was between ether and ethyl acetate. This apparent anomalous result was a manifestation of the, albeit weak, degree of pyrrole hydrogen bonding to CO2. 

Solvatochromic data, specifically absorption or transition energies (E s), have been obtained for the dye phenol blue in supercritical fluids as a function of both temperature and pressure. These data will be used to compare the "solvent strength" of these fluids with liquid solvents. He will use the terms "solvent strength" and "Et" synonymously in this paper such that they include the magnitude of the polarizability/volume as well as the dipole moment. The "solvent strength" has been characterized by the spectroscopic solvatochromic parameter, E, for numerous liquid solvents (9 JU, J7,JJ3). 

Local solvent compression. The next application of the solvato-chromic data will be to determine the magnitude of the local compression of a supercritical fluid solvent in the immediate environment of the solute. The of a dye such as phenol blue can be predicted in liquids where no specific interactions are present by treating the solvent as a homogeneous polarizable dielectric (22,29). The intrinsic "solvent strength", E, °, describes dispersion, Induction, and dipole-dipole forces and is given by (22). 

Complexation is a measure of how strongly cryptates are formed. There is no assumption that the solution species are identical to the solid state entities. The association constant (A J is a measure of the strength of a host s ability to complex a guest in a specific solvent at a specific temperature. As explained in Section 14.22.2, under fast exchange A is an estimation of the signal position of the fully complexed species in a fast exchange system. Aq must be estimated to determine (see Table 7). 

Equation (34) is a general expression which accounts for the three known contributions to solvent selectivity in LSC (i) solvent strength, (ii) solvent-solute localization (including solvent-specific localization), and (iii) solvent-solute hydrogen bonding. The second term ii) is generally the most important and most general contribution to solvent selectivity. 

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