Molecular spectroscopy studies of solvent properties of dispersed 'water pools': Fluorescein and 2,7-dichlorofluorescein in reversed AOT-based microemulsions

Stepwise ionization and tautomerism of fluorescein (Fl) and its 2,7-dichloroderivative (2,7-DCFl) were studied using vis-spectroscopy in AOT-stabilized reversed microemulsions of water in n-octane (water:surfactant ratio W = 20). The 'apparent' pKaa values of dyes (average confidential interval ± 0.08) in microemulsions, at ionic strength of dispersed aqueous phase I = 0.05 M, are as follows: pKa1a = 6.68, pKa2a = 7.85 for Fl; pKa1a = 6.00, pKa2a = 7.17 for 2,7-DCFl. The state of H2O molecules in dispersed 'water pools' was interpreted using IR spectra of O-H stretching at various W = H2O:AOT ratio, as well as by registering fluorescence and excitation spectra and fluorescence lifetimes of the dyes. 'Unusual' solvent properties of dispersed water within reversed microemulsions, i.e. micropolarity, microviscosity and pH gradient and inhomogeneous electrical potential within water pool, create favorable conditions for protolytic reactions as compared with aqueous solutions. Conclusions concerning tautomerism of the molecular and ionic species were deduced from the absorption spectra. The neutral (molecular) forms H2R of the studied dyes in reversed microemulsions are highly converted into colorless lactones. The monoanion HR− of the unsubstituted Fl exists both in aqueous solutions and in AOT-stabilized 'water pools' as the common carboxylate tautomer. In contrast, the form HR− of 2,7-DCFl in reversed AOT microemulsions exists mainly as a deeply and intensively colored structure, with non-ionized carboxylic and ionized hydroxylic group (phenolate tautomer).