Spectroscopic study of acid-base ionization and tautomerism of fluorescein dyes in direct microemulsions at high bulk ionic strength

Lyophilic nanosized aqueous dispersions belong to organized solutions widely used as media for various chemical reactions. Stepwise ionization and tautomerism of Fluorescein and its ten derivatives and one thio-analogue were studied by using Vis-spectroscopy in direct microemulsions benzene-1-pentanol−cetyltrimethylammonium bromide−water, at ionic strength of the bulk phase 1.0 M KCl. The apparent pKaa values of the substances were determined spectrophotometrically. Dyes containing halogen atoms in phthalic residue, xanthene moiety, or in the both fragments, as well as nitro-derivatives were examined. The pKaa values are compared with pKas of the dyes determined in 50 mass % aqueous ethanol. Absorption spectra of individual molecular and ionic species were singled out. Strong differentiating action of microemulsions was demonstrated. The results were interpreted using the scheme of detailed protolytic equilibrium of the hydroxyxanthene substances.