Reversible aggregation, precipitation and re-dissolution of rhodamine 6G in aqueous sodium dodecyl sulfate

Abstract

Study of the interactions between a cationic laser-dye (rhodamine 6G or R6G) and an anionic surfactant (sodium dodecyl sulfate or SDS) in aqueous solution has demonstrated that the so-called premicellar effect on absorbance and fluorescence occurring in oppositely charged dye/surfactant systems is associated with a slow ion-pair aggregation/precipitation process. The red-shift and UV/visible band broadening which occurs during the precipitation process has been attributed to the formation of nanoparticles with size-dependent spectral properties. A slight increase of the SDS concentration (less than four-fold) gives rise to a slow re-dissolution of the previously precipitated particles. From the absorbance vs. time kinetic curves recorded during the aggregation/precipitation processes it can be deduced that a reaction-limited aggregation process dominates the early stages of the growth, while the end of the precipitation is diffusion-limited. Re-dissolution is also diffusion-limited. These findings have been interpreted by the formation and the re-dissolution of nanoparticles with possible fractal geometry. The whole process is likely to include several reversible steps. Among them, dimerization of the dye, ion-pairs and larger aggregates formation, precipitation of optically visible crystals, surfactant micellisation and re-dissolution of previously formed aggregates are the most prominent.