Structure formation of surfactants in concentrated sulphuric acid: a light scattering study

A common cationic surfactant,n-hexadecylammonium hydrogensulphate, dissolved in concentrated sulphuric acid, has been studied by static and dynamic light scattering. Micelle formation has been observed even in this unusual solvent. An apparent molar mass of 45 500±4.5% was found for the aggregates. A translational diffusion coefficientD₀=5.5×10^–9 cm²/s was measured which gave a hydrodynamically effective radius ofR_h=17.7 nm. The geometric radius of gyration wasR_g=76.2 nm. The ratioR_g/R_h=4.33 is indicative for rodlike structures. Assuming a polydispersity ofL_w/L_n=2 this corresponds to a cylinder ofL_w=152 nm. An axial ratiop_w=(L_w/d)=60.4 nm was estimated which leads to a cylinder diameter of 2.53 nm. At surfactant concentrations higher than 5% (w/vol) the rod-like micelles aggregate to form more globular structures. The time correlation function, recorded by dynamic light scattering, exhibited a two-step decay which indicates a bimodal distribution of particle sizes. The fast motion coincides with that of the micelles at low concentrations while the other is slower than the fast one by three orders of magnitude and corresponds to the translational motion of large clusters.