Water and counterion structuring around cyclic anionic surfactant micelles in solution

Wide-angle neutron scattering analysed using empirical potential structure refinement (EPSR) was used to investigate how aromatic ring structure influences micelle self-assembly in two anionic surfactants: sodium 4-heptylbenzene sulfonate (SHBS) and sodium 5-heptylfuran-2-sulfonate (SHFS). Both surfactants formed globular micelles in aqueous solutions at 0.2 M but differed significantly in micelle architecture. SHBS micelles were compact and spherical, while SHFS formed more disordered aggregates with a broader spatial distribution of surfactant molecules within the micelle. EPSR-derived radial distribution functions revealed greater water penetration into the SHFS micelle interface and higher water coordination around the sulfonate headgroup. These differences appear to arise from the structural and electronic properties of the headgroup region, rather than direct water hydrogen bonding to the furan ring. SHFS also exhibited reduced sodium counter ion association compared to SHBS. The results demonstrate that small changes to ring structure can substantially affect micelle packing and interfacial hydration, offering insight into how headgroup design can be used to tailor surfactant behaviour.