A new insight on the dynamics of sodium dodecyl sulfate aqueous micellar solutions by dielectric spectroscopy

Aqueous sodium dodecyl sulfate micellar solutions were investigated by a recently developed double-differential dielectric spectroscopy technique in the frequency range 100 MHz–3 GHz at 22 °C, in the surfactant concentration range 29.8–524 mM, explored for the first time above 104 mM. The micellar contribution to dielectric spectra was analyzed according to three models containing, respectively, a single Debye relaxation, a Cole–Cole relaxation and a double Debye relaxation. The single Debye model is not accurate enough. Both Cole–Cole and double Debye models fit well the experimental dielectric spectra. With the double Debye model, two characteristic relaxation times were identified: the slower one, in the range 400–900 ps, is due to the motion of counterions bound to the micellar surface (lateral motion); the faster one, in the range 100–130 ps, is due to interfacial bound water. Time constants and amplitudes of both processes are in fair agreement with Grosse's theoretical model, except at the largest concentration values, where interactions between micelles increase. For each sample, the volume fraction of bulk water and the effect of bound water as well as the conductivity in the low frequency limit were computed. The bound water increases as the surfactant concentration increases, in quantitative agreement with the micellar properties. The number of water molecules per surfactant molecule was also computed. The conductivity values are in agreement with Kallay's model over the whole surfactant concentration range.

Volume fraction of bulk water in micellar solutions as a function of surfactant concentration. Triangles and circles (ϕe)(ϕe) are the experimental values according to Cole–Cole and the double Debye models, respectively, as obtained by dielectric spectroscopy. Stars (ϕm)(ϕm) and black squares (ϕb)(ϕb) are the values calculated taking into account respectively the micelles volume from SANS measurements or the amount of water added to surfactant in the sample preparation, assuming that all water behaves as bulk water. ϕmϕm values are very close to the results ϕeϕe. Comparing ϕbϕb with ϕmϕm and ϕeϕe, we argued that some water does not behave as bulk water, rather it is to be considered bound water. Calculations from experimental data ϕeϕe give 8 bound water molecules per surfactant molecule in agreement with SANS results.Figure optionsDownload as PowerPoint slide