The peculiarities of the structure of the fluorescent dye N,N'-di-n-octadecylrhodamine advantage its using as an interfacial acid-base probe in aqueous micellar solution of colloidal surfactants. Two long hydrocarbon tails of the dye provide similar orientation of both cation and zwitterion on the micelle/water interface, with the ionizing group COOH exposed to the Stern region in all the systems studied. Further, the charge type of the acid-base couple, A þ B AE , ensures similar values of the 'intrinsic' contribution, pK i a , to the 'apparent' pK a a value in micelles of different surfactants. This makes the indicator suitable for determination of electrical surface potentials, C. The pK a a s have been obtained in cationic, anionic, zwitterionic, and nonionic surfactant systems, at various salt background. In total 17 systems were studied. At bulk counterion concentration of ca. 0.05 M, the pK a a values vary from 2.14 AE 0.07 in nÀC 18 H 37 NðCH 3 Þ þ 3 Cl À micelles to 5.48 AE 0.06 in nÀC 16 H 33 OSO À 3 Na þ micelles. The C values, corresponding to the Stern region of micelles, have been evaluated as C¼ 59:16 ðpK i a À pK a a Þ for T ¼ 298.15 K. The pK i a parameter was equated to the average value of 4.23 in nonionic surfactants (4.12-4.32, depending on the surfactant type). For cetyltrimethylammonium bromide and sodium n-dodecylsulfate micelles, the C values (AE(7-11) mV) appeared to be þ118 mVand at bulk Br À concentration 0.019 M and À76 mVat bulk Na þ concentration 0.020 M, respectively. This satisfactorily agrees with the theoretical values þ111 and À84 mV, estimated using the Oshima, Healy, and White equation for these well-defined colloidal systems. Finally, not only absorption, but also fluorescence spectra display the same response to changes in bulk pH.

Among different types of calixarenes, the water–soluble ones are of especial interestbecause of their possible applications in biochemical research. In order to elucidate the natureof aqueous solutions of a cationic amphiphilic calixarene, substituted tetrapropoxycalix[4]arene bearing hydrophilic choline groups at the upper rim, we studied vis–spectroscopically the influence of the above system on the acid–base behavior of threeindicator dyes, namely, 2,4-dinitrophenol, bromophenol blue, and N,N/-dioctadecylrhodamine,at constant ionic strength of 0.05 M, maintained with NaCl addition. Simultaneously,‘apparent’ ionization constants, Kaa , of the same dyes were determined in the presence ofcommon cationic surfactant micelles. Within the concentration range from 1.0×10–5 to 0.01 M,the aforementioned water–soluble calixarene displays effects similar to those of micelles ofcetyltrimethylammonium bromide (or chloride). The shifts of the absorption and emissionbands in the visible region, as well as the alterations of the Kaa values against the ‘aqueous’ones appeared to be very similar in aqueous solutions of both calix[4]arene and cationicsurfactant. A conclusion can be made about aggregation (or association), i.e., micelleformation of the cationic calix[4]arene under study.

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