The Dependence of Absorption Spectrum of Congo Red on the Properties of Media: Solvatochromism, Switch Solvatochromism, Selective Solvation and Polarity Scales.

In this study, the effect of polarity and hydrogen bonding ability of solvents on the absorption spectrum of Congo red was investigated. UV–Visible absorption spectra were recorded in eight neat solvents and four aqueous binary solutions of methanol, ethanol (as HBD solvents), and dimethylsulfoxide and dioxane (as HBA solvents). Congo red has two absorption maxima in the range (250–700 nm) that have been assigned to the azo and hydrazo systems. Molar transition energy (ET) values of Congo red in different solvents were correlated with the solvent parameters using linear solvation energy relationships. Multiparameter analysis shows the dependence of the electronic spectra of Congo red on the non-specific and specific interactions. Preferential solvation was observed in all binary mixtures when ET plotted as a function of mole fraction of organic solvents. The results showed that the Congo red was solvated by water–organic solvent complex species in all binary mixtures except in case of aqueous-ethanol mixtures, whose preferred solvation by ethanol in all mole fractions with f12/2 and f12/1 values less than unity. The dual-parameters model is a powerful tool for explaining binary mixture data with Kamlet–Taft parameters. The findings indicated that the HBD and HBA were responsible for the solvatochromism of the binary mixtures. [ABSTRACT FROM AUTHOR]