Influence of the cetyltrimethylammonium chloride micellar pseudophase on the protolytic equilibria of oxyxanthene dyes at high bulk phase ionic strength

Abstract

The acid–base equilibria of fluorescein, sulfonefluorescein, 2,7-dichlorofluorescein, eosin and ethyl eosin have been studied spectrophotometrically in the micellar pseudophase of cetyltrimethylammonium chloride, the charge of micelles being strongly shielded by high counter-ion concentrations in the bulk phase (4.00 mol dm^–3 KCl). The ‘apparent’ ionization constants (K^a _a) and molar absorptivities of all the species have been determined. The completeness of solubilization is confirmed by several approaches, including examination of emission and excitation spectra. Conclusions concerning tautomerism of dye molecules and ions were deduced from absorption spectra; the fractions (α) of tautomers as well as apparent microscopic ionization constants (k^a) have been evaluated. The medium effects, ΔpK^a _a(=pK^a _a– pK^w _a), vary from –1.5 to +2.0 and are partly controlled by shifts in the tautomeric equilibria. For the Δpk^a values (= pk^a– pk^w) the relationship Δpk_{CO2H > Δpk^a OH is valid. Considering the pK^a a values at 4.00 mol dm^–3 KCl as the first approximation to the ‘intrinsic’ values (pKⁱ a), and comparing them with the pKa in aqueous acetone and other solvents, although the influence of micellar micro-environment is qualitatively similar to that of the organic solvents, there are some doubts about the possibility of modelling the values of the whole set of parameters in the pseudophase by that in a ‘unique’ water–organic compound mixture.}