Micelle and Solvent Relaxation in Aqueous Sodium Dodecylsulfate Solutions

Abstract

Dielectric spectra have been measured for aqueous sodium dodecylsulfate (SDS) solutions up to 0.1 mol L⁻¹ at 25 °C over the frequency range 0.005≤ν GHz⁻¹≤89. The spectra exhibit two relaxation processes at approximately 0.03 GHz and 0.2 GHz associated with the presence of micelles in addition to the dominant solvent relaxation process at approximately 18 GHz and a small contribution at approximately 1.8 GHz due to H₂O molecules hydrating the micelles. Detailed analysis reveals that the micelles bind 20 water molecules per SDS unit, but not as strongly as trimethylalkylammonium halide surfactants do. The relaxation times and amplitudes of both micelle relaxation processes can be simultaneously analysed with the theory of Grosse, yielding the effective volume of a SDS unit in the micelle and the lateral diffusion coefficient of the bound counterions. The findings of this investigation fully corroborate recent molecular dynamics simulations on structure and dynamics of SDS micelles.