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

Abstract

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.

Revisiting tetranitrophenolsulfonephthalein

“…Evidently, the dissociation constants of organic acids in methanol and in protophilic DMSO are much higher than in protophobic acetonitrile and acetone, the difference reaching even 10 orders of magnitude [18][19][20] . Therefore, in the first two solvents, the R 2À species of NPC predominates in diluted solutions even without adding a base.…”

Interfacial properties of cetyltrimethylammonium-coated SiO2 nanoparticles in aqueous media as studied by using different indicator dyes

“…Presuming that indicator species are completely bound to the modified silica (Chart 2), the pK a a value may be expressed within the framework of the recognized electrostatic model via following equation [29][30][31]33,34]:…”

Premicellar and micelle formation behavior of aqueous anionic surfactants in the presence of triphenylmethane dyes: protonation of dye in ion pair micelles

“…The above observations are in parallel to the observations made for the photometric sensitivities of the dyes which was found to be in the order RN < CV < EV which was attributed to the increasing hydrophobicity of the dyes in the same order. [47] The pC 20 of MG has been found to be higher than that of RN but less than that of CV as in the case of the spectrophotometric sensitivities reported by Lee et al [47] The excess surface concentration at the surface saturation of the surfactants, viz, G s has been calculated using the Gibbs adsorption equation: [14,16,46] dg ¼ À4:606 RT G s d log C 1 (4) where dg is the change in surface tension at the surface saturation and C 1 is the surfactant concentration. The G s of the surfactants in the presence of the dye has been found to be smaller than that in the absence of the dye (Tables 2 and 3).…”

“…[1,2] The nature of the interactions in such dye-surfactant systems with concentrations of the surfactant above the critical micelle concentration (CMC) is somewhat simple and known. [1][2][3][4] The dyes in their monomeric forms are usually solubilized in the micelles and sometimes there exist specific interactions between the ionic dyes and the oppositely charged surfactants in the micelles. [5] However, there are differences in opinion regarding the nature of interactions between oppositely charged dyesurfactant systems when the concentrations of the surfactant are below the CMC and none of the models developed so far can alone explain the experimental observations.…”