Aggregation processes in micellar solutions: a Raman study

Ionic surfactants such as sodium dodecyl sulfate (SDS) belong to the amphiphilic family: they possess a long hydrophobic hydrocarbon chain and a polar hydrophilic headgroup. In a polar solvent and over the critical micellar concentration these molecules join the form of micelles. The micellar solutions, in turn, if doped with various ligands tend to aggregate. Solid SDS, micelles of SDS in water and micelles of SDS doped with two types of macrocyclic ligands, Kryptofix 2.2.2 (K222) and crown ether 18-Crown-6 (18C6), at different concentrations are studied by Raman scattering, that represents a new approach to such systems. The experimental Raman spectrum, obtained on crystalline powders of SDS, is compared with the ab initio computed spectrum in order to assign the vibrational bands. After discriminating sensitive peaks by comparing the crystalline powders of the single components and their water solutions, the aggregation process and the action of ligands are analyzed following the evolution of the intensity and wavenumber of these characteristic Raman peaks. This shows that the Raman spectroscopy is sensitive to aggregation dynamics and to the effects induced by the hydration layer on the molecules in solutions. A saturation effect in the aggregation process with the increase of the ligand concentration is observed.