Phenol blue transition energy

Solvatochromic shift data have been obtained for phenol blue in supercritical fluid carbon dioxide both with and without a co-solvent over a wide range in temperature and pressure. At 45°C, SF CO2 must be compressed to a pressure of over 2 kbar in order to obtain a transition energy, E, and likewise a polarizability per unit volume which is comparable to that of liquid n-hexane. The E,j, data can be used to predict that the solvent effect on rate constants of certain reactions is extremely pronounced in the near critical region where the magnitude of the activation volume approaches several liters/mole. 

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). 

The purpose of these scales is to provide a guide for choosing SCF solvents and co-solvents to achieve a desired solvent strength, for example in a separation or reaction process. An example of a solvatochromic scale is presented in Figure 1 for the UV-vis absorption of phenol blue in ethylene as a function of density at two temperatures (S). The scale is defined as the transition energy, Ej = he/ Xmax where Xmax is the wavelength of maximum absorption. 

Figure 1. Transition energy (E ) of phenol blue in ethylene and isothermal...

Figure 7. Transition energy of phenol blue in binary mixtures of acetone and CXDj at 35 C ( 80 bar, A 200 bar [ref. 36]).

Figure 3 Transition energy ( j) and isothermal compressibility vs. density for phenol blue in ethylene (O) 25°C, (A) 10°C, (-----) calculated E t. (From Ref. 26.)...

Figure 5 Transition energy of phenol blue in SC 1,1-difluoroethane as a function of the bulk density. Measured values (symbols) and values expected on the basis of the dielectric constant of the bulk fluid (line) reprinted from Ref. 15...

One of the most popular and successful scales has been developed by Dimroth and Reichardt. It is based on the pyridlnlum-N-phenoxide betaine [3], which exhibits one of the largest solvatochromic effects ever observed. The solvatochromism of this dye is negative since its ground state is considerably more polar than the excited state and is stabilized by polar solvents. Thus, in diphenylether the dye absorbs at 810 ran and appears blue-green, whereas in water the absorption is centred at 453 nm, giving an orange impression. The transition energy, expressed in kcal mol, is the so-called Ej(30) value of the solvent. Ex(30) values have been determined and tabulated for more than 270 pure solvents and many different solvent mixtures. Protonation converts the dye (Scheme 3) into a phenol as a consequence, Et(30) values cannot be measured for acidic solvents, such as carboxylic acids. 

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