Unusual findings on studying surfactant solutions: displacing solvatochromic pyridinium N-phenolate towards outlying areas of rod-like micelles?

During studies of colloidal surfactants solutions by means of acid/base and solvatochromic indicators an unusual finding was observed. The properties of micelles of cetyl trimethylammonium bromide (CTAB) and N-cetyl pyridinium bromide (CPB), modified by introducing various organic anions (altogether 24 anions), were examined by means of 4-(2,4,6-triphenylpyridinium-1-yl)-2,6-diphenylphenolate, used to establish the empirical ET(30) solvent polarity scale, at working concentrations of ca. 5×10−5 M. Surprisingly, on addition of electrolytes (0.01 M) containing some organic anions (tosylate, alkyl sulfonates, etc.) to aqueous solutions of a cationic surfactant (0.001 M), in the presence of the betaine dye, instead of the expected red shift, a blue shift (up to 75 nm) of its long-wavelength solvatochromic intramolecular charge-transfer (CT) Vis absorption band was observed. This gives evidence for displacing the zwitter-ionic dye from a less polar to a more polar (i.e. more hydrated) microenvironment. The band shift is more or less expressed for various surfactants and for anions of different structure, and in some cases it was not observed at all. This blue shift can be explained by a micellar ‘sphere→rod’ transitions. Such transformations, caused by the addition of hydrophobic counter-ions, are known to be unfavourable for normal solubilizations and manifest themselves in moving of the large-sized dye towards outlying areas of the micellar palisade. An increase in both, surfactant and anion concentration, results normally in a red shift of the CT band, as compared with its spectrum in CTAB, respectively, CPB micelles. In order to clarify further the behaviour of the dye, its acid/base properties were studied, its concentration was changed, ethanol was added, and both more hydrophilic and more hydrophobic betaine dyes were analogously examined.