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

• Published in: Journal of molecular liquids 2009-05, Vol.145 (3), p.188-196
• Main Authors: Vodolazkaya, Natalya A., Gurina, Yuliya A., Salamanova, Natalya V., Mchedlov-Petrossyan, Nikolay O.
• Format: Article
• Language: English
• Subjects: Apparent ionization constants; Direct microemulsions; Fluorescein derivatives; Vis-absorption
• ISSN: 0167-7322; 1873-3166
• DOI: 10.1016/j.molliq.2008.11.007

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 p K a a 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 p K a a values are compared with p K a s 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.