Characterization of Worm-like Micelles Containing Solubilized Dye-Molecules by Light Scattering Techniques

Abstract

A micellar system consisting of the cationic surfactant tetradecyltrimethylammonium bromide (TTAB) and a water-insoluble bis-diazo dye has been investigated. Static and dynamic light scattering measurements have been performed at two KBr concentrations, 0.25 M and 0.5 M. At constant salt concentration molecular weights, radii of gyration and hydrodynamic radii increase with TTAB/dye concentration, and at any given TTAB/dye concentration micelles are bigger at the higher salt content. This behavior can be explained by the screening effect the salt exerts on the charges of the micelles. At any KBr concentration, TTAB/dye micelles are much larger than TTAB micelles without dye. It is therefore concluded that the dye has a strong cooperative effect on micellar aggregation. The magnitudes of the molecular weights in relation to the radii of gyration indicated a worm-like structure of the micelles. Further analysis of overall dimensions using the formalism of Benoit and Doty yielded persistence lengths of 391 nm for the system in the presence of 0.25 M KBr and 71 nm in the presence of 0.5 M KBr. In the case of 0.25 M KBr, fit quality was best when assuming a monodisperse distribution ( M w/ M N = 1) and in the case of 0.5 M KBr when assuming a polydisperse distribution ( M w/ M N = 2). Particle form factors have been compared with calculations according to the Koyama theory, leading to results consistent with those obtained from the analysis of the overall dimensions. Obviously, flexibility as well as polydispersity of the micelles is higher when salt concentration is increased. From dynamic light scattering measurements, hydrodynamic radii have been obtained and related to the radii of gyration R g. The ratios ρ = R g / R h were between 1.5 and 2.1, also indicating a worm-like structure of the micelles.