Raman scattering spectroscopic analysis on the structural changes in the micelles of a CTAB-NapTS system

Condensed micelle solutions are ubiquitously seen and utilized in foods, medicine, oil recovery, and so on. It is well-known that micelles formed from ionic surfactants drastically change their aggregate morphologies from spherical to short, rigid rod-like, and occasionally to long, winding worm-like structures as the concentration of the co-existing salts is varied. In general, these large-scale morphological changes induced by additive salt have been directly assessed by X-ray or neutron scattering spectroscopies. However, industrial processes are often carried out under open-system and atmospheric conditions, thus they show transient and dynamic changes of their structures and resultant viscoelastic properties according to the environmental changes. Here we examined whether Raman scattering spectroscopy is applicable for detecting such large-scale morphological changes in micelles by monitoring the corresponding changes occurring in the local packing environment of neighboring surfactant alkyl chains. As a representative micelle-salt system, cetyltrimethylammonium bromide (CTAB) and sodium p-toluene sulfonate (NapTS) were studied by varying their concentration ratio. As a result, it was proposed that the CH₂ twisting and scissoring modes were useful indices to discriminate the morphological changes in micelles. These results provide a useful index for monitoring the transient structural changes of micelle solutions and elucidating the mechanism of the resultant changes in viscoelastic properties in situ and on-site monitoring.