Dynamics in Anionic Micelles: Effect of Phenyl Ring

Micellar dynamics in sodium dodecyl benzene sulfonate (SDBS) is studied using quasi-elastic neutron scattering (QENS) technique. Results are compared with sodium dodecyl sulfate (SDS), a very similar surfactant except for the presence of a phenyl ring in SDBS. SDBS is a very important system for various industrial usages and variety of other applications. The aim here is to investigate the effect of molecular architecture of the surfactant molecule on micellar dynamics. Analysis of the QENS data showed that there exists two distinct motions in both of the micellar systems: whole micellar motion or global motion and the internal motion of the monomer within the micelles. The global diffusion associated with the whole micelle is found to be Fickian in nature. The diffusion coefficient corresponding to the global motion is found to be significantly lower for SDBS than SDS micelles. As far as internal motion is concerned, the structure factor indicates that the alkyl chains are more flexible in SDS compared with SDBS. Similar behavior is also reported in a recent molecular-dynamics simulation study. Detailed analysis showed that a localized translational diffusion model in which the hydrogen atoms undergo diffusion within a sphere could describe the internal motions in both the micelles. Diffusion coefficients corresponding to internal motions is found to be lower in the case of SDBS micelles compared with SDS micelles, implying that internal motions in SDBS micelles are relatively hindered vis-a-vis SDS micelles. This could be understood in terms of denser packing in SDBS micelles due to the presence of pi-stacking.