title: Counterion-induced transformations of cationic surfactant micelles studied by using the displacing effect of solvatochromic pyridinium N-phenolate betaine dyes.
abstract: In this paper, we demonstrate that the behavior of a set of eight large-sized negatively solvatochromic pyridinium N-phenolate betaine dyes reflects the principle transformations, occurring in aqueous micellar solutions of three cationic surfactants. As surfactants, cetyltrimethylammonium bromide (CTAB), n-octadecyltrimethylammonium chloride (OTAC), and N-cetylpyridinium bromide (CPB) were used. Normally, for such probes coupled with micelles, a red shift of the vis absorption band is expected as a result of a hydrophobization ("drying") of the micellar interface. However, under addition of electrolytes with anions such as tosylate, salicylate, and some n-alkanesulfonates or n-alkanecarboxylates to the micellar solutions, an unexpected effect was observed. Instead of a red shift, a blue shift of the vis absorption band of some of the dissolved betaine dyes was registered, as compared with the spectrum measured in pure aqueous micellar solutions of CTAB, OTAC, or CPB (Deltalambda(max) up to ca. 80 nm). This blue shift, indicating an increase in the polarity of the dye microenvironment, is explained by displacing the large dye dipoles from the thinned micelles toward the aqueous phase. The effect is well expressed at concentrations of C(betaine dye) approximately 10(-5) M, C(cationic surfactant) approximately 0.001 M, and C(organic anion) approximately 0.01 M. Transmission electron microscopy of dried samples confirms the distinct changes occurring in the studied micellar systems upon the addition of organic anions. The excess of inorganic salts [C(NaBr, KBr, or KCl) = 0.5-4.0 M] restored the position of the vis absorption band or even shifted it toward the red. Moreover, some of the betaine dyes studied (i.e., the more hydrophobic ones) stay in the micellar pseudophase or precipitate under the aforementioned concentration conditions. The peculiarities of the behavior of these betaine dyes are in agreement with their molecular structure.
authors.affiliation: Department of Physical Chemistry, V.N. Karazin Kharkov National University, 61077 Kharkov, Ukraine. mchedlov@univer.kharkov.ua
authors.forename: Nikolay O
authors.initials: NO
authors.lastname: McHedlov-Petrossyan
authors.forename: Natalya A
authors.initials: NA
authors.lastname: Vodolazkaya
authors.forename: Anna A
authors.initials: AA
authors.lastname: Kornienko
authors.forename: Eleonora L
authors.initials: EL
authors.lastname: Karyakina
authors.forename: Christian
authors.initials: C
authors.lastname: Reichardt