Fluorescence study on the size and shape of sodium dodecyl sulphate-aluminium salt micelles

The application of time-resolved fluorescence methods to study the aggregation behavior in aqueous solution of anionic surfactant sodium dodecyl sulphate (SDS) in presence of trivalent counterions (Al3+) was investigated. Addition of small amounts of aluminum salt to dilute micellar solutions (<10 wt.% SDS) resulted in isotropic fluid solutions, but for a molar ratio [SDS]/[Al(NO3)3] lower than 6.7:1, precipitation occurred. The precipitate was resolubilized by further addition of aluminum salt (>5 wt.% Al(NO3)3). Using the time-resolved fluorescence quenching method and employing pyrene–decylpyridinium chloride as probe–quencher pair, the aggregation numbers and fluorescence quenching rates for spherical micelles were determined. Fluorescence measurements give evidence for different kind of micelles in the two isotropic areas investigated. At low aluminum salt concentration, the micelles are relatively small, but larger than in the absence of salt. At high enough salt concentrations, the precipitate is resolubilized into long rod-like micelles. The strong viscoelastic properties in the linear regime supported the presence of worm-like micelles in the redissolution area.