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.
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.
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.
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