Solvatochromic parameter of electronic transition energy (ET) provides information on dipolarity/polarizability and hydrogen bonding donor interactions in the solvation shell. In this work, a predictive framework is proposed for estimating ET values of phenol blue indicator in three mixed-solvent systems as (i) liquid nonpolar-polar mixtures, (ii) CO2-expanded liquid solvent and (iii) supercritical carbon dioxide (scCO2)-polar cosolvent mixtures. The equations of the ET framework for binary mixtures of nonpolar solvent with polar hydrogen bond acceptor (HBA) solvent can be directly adopted from the previous framework for Kamlet-Taft dipolarity/polarizability (KT-π*) [Ind. Eng. Chem. Res. 2020, 59, 12319–12330] because of the linearity of the homomorphism line between ET and KT-π* for pure nonpolar solvents and pure polar HBA solvents. However, due to specific hydrogen bonding interactions of the phenol blue with polar hydrogen bond donor (HBD) solvents, the ET framework for binary mixtures of nonpolar solvent with polar HBD solvent was the modification by the addition of HBD contribution factor. The HBD contribution factor can be simply estimated by a deviation of an actual ET value of pure HBD solvents from the homomorphism line. To validate the framework, the ET values of seventeen liquid nonpolar-polar mixtures, (ii) three CO2-expanded solvent mixtures and (iii) five scCO2-polar cosolvent mixtures collected from the literature were used. The framework was found to give a reliable ET value with an overall deviation of 0.26% and was also applicable to ET values of Reichardt, Nile red and HxQMBu2 indicators with an overall deviation of 1.73%, 0.36% and 0.40%, respectively. Only four properties of pure components are required for predictions as: (i) gas-phase dipole moment of polar component, (ii) CO2 density, (iii) ET and KT-π* values and (iv) homomorphism relationship between ET and KT-π* values.
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