We report on combined experimental and theoretical investigations of the water/micelle interface of cationic, anionic, zwitterionic, and non-ionic surfactants using a new hydrophobic acid-base indicator 2,6dinitro-4-n-dodecylphenol. The indices of the so-called apparent ionization constant, app a pK , of the indicator fixed in the micellar pseudophase are determined by the spectrophotometric method. The data allows estimating the Stern layer's electrostatic potential of the ionic micelles . Molecular Dynamics modeling was used to locate the dye molecule and, in particular, its ionizing group OH  Owithin the micelles of the studied surfactants. The comparison of the  values estimated using 2,6-dinitro-4-ndodecylphenol with both our computer simulation and literature experimental results reveals obstacles in monitoring electrical interfacial potentials. In particular, the  values of the surfactant micelles with alkylammonium groups determined via 2,6-dinitro-4-n-dodecylphenol are overestimated. The reason is specific interactions of the indicator anion with the surfactant head groups. For anionic surfactants, however, this indicator is quite suitable, which is confirmed by the location of HA and Aequilibrium forms in the pseudophase.

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Within the course of this spectroscopic research, we revealed novel features of the protolytic behavior, which extend the knowledge of the chemistry of xanthene dyes and rationalize the utilization of these compounds. In addition to the well-known tautomerism of the molecular form, H 2 R, of fluorescein dyes, new aspects of tautomeric transformation of anions are disclosed. First, for the dyes bearing the substituents in the phthalic acid residue, 4′-and 5′-aminofluoresceins and 4′-fluorescein isothiocyanate, the monoanion HR − exists in non-hydrogen-bond donor solvents not only as a tautomer with the ionized carboxylic and nonionized OH group but also as a "phenolate" ion with a nonionized COOH group. Such state of HR − ions is typical for dyes bearing halogen atoms or NO 2 groups in the xanthene moiety but was not observed until now in the case of substitution in the phthalic residue. Second, the possibility of the existence of the HR − species in DMSO in the form of colorless lactone is deduced for the 5′-aminofluorescein using the visible and infrared spectra. This results in a dramatic difference in medium effects. For instance, whereas for fluorescein in DMSO, the inversion of the stepwise ionization constants takes place and the K a1 /K a2 value equals 0.08, the same ratio for 5′-aminofluorescein is as high as ∼800. In addition, the pK a values of sulfonefluorescein, erythrosin, methyl ether of fluorescein, and phenol red were obtained to verify the acidity scale in DMSO and to support the detailed scheme of protolytic equilibria of fluorescein dyes.

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This paper is devoted to the phenolsulphonephthalein nitro derivative 3,3 0 -dinitrophenolsulphonephthalein, also called nitrophenol violet (NPV). The neutral molecular form, H 2 R, was isolated as a sultonic tautomer, and an X-ray crystal structure analysis was carried out. UV-vis absorption spectra in methanol, DMSO, acetonitrile, and water at different pH values were ascribed to the molecular and anionic (H 2 R, HR À , and R 2À ) species. Whereas the pK a values of this acid-base indicator (HR À ¡ R 2À + H + ) in water and DMSO are close to those of 3,3 0 ,5,5 0 -tetrabromophenolsulphonephthalein (or bromophenol blue), replacing the four Br atoms with two NO 2 groups results in a pronounced tendency to carbinol formation. In weakly acidic aqueous media, the HR À anion slowly converts into the colourless carbinol H 2 ROH À . The latter is transformed to the orange carbocation only in concentrated (70-94 wt%) sulphuric acid. The formation of H 2 ROH À is atypical for the common sulphonephthalein indicators and should be ascribed to the enhanced positive charge density on the nodal carbon atom. The reaction mechanism and kinetic equation explaining this pH-dependent process are proposed, in addition to a kinetic study of the common process R 2À + HO À ? ROH 3À in the alkaline region. The numerical characterisation of the protolytic processes obtained for NPV is also helpful in gaining a better understanding of the properties of previously studied 3,3 0 ,5,5 0 -tetranitrophenolsulphonephthalein, which is much less accessible for a quantitative description.

Among the vast set of fluorescein derivatives, the double charged R anions of aminofluoresceins are known to exhibit only low quantum yields of fluorescence, [Formula: see text]. The [Formula: see text] value becomes as high as that of the fluorescein dianion when the lone electron pair of the amino group is involved in a covalent bond. According to Munkholm et al (1990 J. Am. Chem. Soc. 112 2608-12), a much smaller increase in the emission intensity can be observed in the presence of surfactant micelles. However, all these observations refer to aqueous or alcoholic solvents. In this paper, we show that in the non-hydrogen bond donor (or 'aprotic') solvents DMSO and acetone, the quantum yields, φ, of the 4'- (or 5')-aminofluorescein R species amount to 61-67% and approach that of fluorescein (φ  =  87%), whereas in water φ is only 0.6-0.8%. In glycerol, a solvent with an extremely high viscosity, the φ value is only 6-10%. We report on the enhancement of the fluorescence of the aminofluorescein dianions as an indicative process, which allows us to distinguish between the micelle-like aggregates of cationic dendrimers of low generation, common spherical surfactant micelles, and surfactant bilayers. Some of these colloidal aggregates partly restore the fluorescence of aminofluoresceins in aqueous media. By contrast, other positively charged micellar-like aggregates do not enhance the quantum yield of aminofluorescein R species. Results for several related systems, such as CTAB-coated SiO particles and reverse microemulsions, are briefly described, and the possible reasons for the observed phenomena are discussed.

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In this article, we examined the fluorescent properties of 4′- and 5′-aminofluorescein, unsubstituted fluorescein, and its 4′-nitro derivative in a set of solvent systems. Fluorescence lifetimes, quantum yields, time-resolved fluorescence spectra, and quantum chemical calculations allowed clarifying the reasons of the emitting properties in this dye series. In water, the dianions R2– of aminofluoresceins are practically nonfluorescent; in alcohols, the quantum yields are low. In dimethylsulfoxide (DMSO), acetonitrile, and other non-hydrogen bond donor solvents, the bright fluorescence of R2– ions is quenched either on adding small amounts of water which hydrate the carboxylate group or under conditions of protonation of this group (COO– → COOH). The last observation is possible owing to the peculiarities of the tautomerism of the 5′-aminofluorescein monoanion, HR–, which exists in DMSO as an equilibrium mixture of a colorless lactone and colored “phenolate” tautomer with an ionized xanthene moiety and unionized carboxylic group. In contrast, the R2– anion of 4′-nitrofluorescein demonstrates spectral behavior different from that of the amino derivatives. It practically does not emit in aprotic solvents; however, in alcohols or water media, its quantum yield increases to some extent. Such changing spectral properties are explained in terms of the excited-state interfragmental charge transfer.