A novel probe for determination of electrical surface potential of surfactant micelles: N,N'-di-n-octadecylrhodamine

• Published in: Journal of physical organic chemistry 2007-05, Vol.20 (5), p.332-344
• Main Authors: Mchedlov-Petrossyan, Nikolay O., Vodolazkaya, Natalya A., Yakubovskaya, Anna G., Grigorovich, Aleksey V., Alekseeva, Vera I., Savvina, Lyubov P.
• Format: Article
• Language: English
• Subjects: apparent ionization constant; electrical surface potential; fluorescence; N,N′‐di‐n‐octadecylrhodamine; N′-di-n-octadecylrhodamine; surfactant micelles; vis absorption
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.1150

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+B±, ensures similar values of the ‘intrinsic’ contribution, pK ia, to the ‘apparent’ pK aa value in micelles of different surfactants. This makes the indicator suitable for determination of electrical surface potentials, Ψ. The pK aas 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 aa values vary from 2.14 ± 0.07 in n–C18H37N(CH3) 3+Cl– micelles to 5.48 ± 0.06 in n–C16H33OSO 3–Na+ micelles. The Ψ values, corresponding to the Stern region of micelles, have been evaluated as Ψ = 59.16 pK ia–pK aa for T = 298.15 K. The pK ia 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 Ψ values (±(7–11) mV) appeared to be +118 mV and at bulk Br− concentration 0.019 M and −76 mV at bulk Na+ concentration 0.020 M, respectively. This satisfactorily agrees with the theoretical values +111 and −84 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. Copyright © 2007 John Wiley & Sons, Ltd. 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 ionic 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. The apparent ionization constants of the indicator were obtained in seventeen colloidal surfactant systems. The indicator is suitable for determination of electrical surface potentials. Both absorption and fluorescence spectra display response to changes in bulk(aqueous) pH.