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

Stepwise ionization and tautomensm of fluorescein (Fl) and its 2 7-dichloroderivative (2 7 DCFI) were studied using vis-spectroscopy in AOT stabilized reversed microemulsions of water in n-octane (water surfactant ratio W=20) The apparent pK(a)(a) values of dyes (average confidential interval +/- 0 08) in microemulsions at ionic strength of dispersed aqueous phase I=0 05 M are as follows pK(a1)(a)=668 pK(a2)(a)=7 85 for Fl pK(a1)(a)=600 pK(a2)(a)=7 17 for 2 7 DCFI 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 micropolanty microviscosity and pH gradient and inhomogeneous electrical potential within water pool create favorable conditions for protolytic reactions as compared with aqueous solutions Conclusions concerning tautomensm 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 27 DCFI in reversed AOT microemulsions exists mainly as a deeply and intensively colored structure with non-ionized carboxylic and ionized hydroxylic group (phenolate tautomer) (c) 2010 Elsevier BV All rights reserved