Fluorescence and X-ray diffraction studies on binding and complexes of surfactants and dansylated polyelectrolytes with sulfonate groups

Binding of cationic surfactant cetyltrimethylammonium bromide (CTAB) on polyelectrolytes ADDan containing different amounts of sulfonate groups was studied by fluorescence spectrum of dansyl labels in aqueous solution. With increasing concentration of CTAB, there was an abrupt drop in the emission maximum wavelength λem and a sharp increase in the relative intensity RF defined as the ratio of emission in the presence of CTAB to that without CTAB. The critical aggregation concentration (cac) determined from the former was about 3×10−5 mol/l without obvious dependence on polymer charge density. Solid complex films of polyelectrolyte–surfactant of CTAB or dodecyltrimethylammonium chloride (DTAC) were obtained by precipitation and their fluorescence anisotropy r indicated a high mobility of the dansyl chromophore. The r-value for ADDan–CTAB complexes obviously decreased as the charge density of the polyelectrolyte increased while the r for ADDan–DTAC complexes was slight decline, suggesting the contribution of alkyl tails of the surfactant to local mobility. It was the first time to find that the higher the charge density of the polyelectrolyte, the less polar the micellar aggregation of polyelectrolyte–surfactant complex becomes in dilute aqueous solution, which remains in the solid complex. SAXS of the solid complexes illustrated a long-range ordered lamella structure with the long period d of 3.87 and 3.04 nm for CTAB and DTAC complexes, respectively.