Conductivity and relative permittivity of sodium n-dodecyl sulfate and n-dodecyl trimethylammonium bromide

The electrical conductivity and relative permittivity of the amphiphilics sodium n-dodecyl sulfate (SDS) and n-dodecyl trimethylammonium bromide (DTAB) were measured as a function of frequency (200 kHz to 20 MHz) at various concentrations. Both methods lead to similar values of the critical micelle concentration (cmc) for both surfactants despite the greater sensitivity of the relative permittivity to structural transformations derived from the formation of the micelles. The electrical conductivity is seen to be rather independent of frequency whereas the relative permittivity exhibits abrupt transitions in the neighborhood of the cmc. Tridimensional plots of the electrical conductivity as a function of the frequency and molar concentration show a gradual transition to the micellar state that takes place in the cmc concentration range. Relative permittivity values have been analyzed independently for concentrations above and below the cmc. In the micellar state both substances show a quadratic dependence of the relative permittivity on concentration. Below the cmc the relative permittivity shows a linear dependence on concentration in the monodisperse (SDS) region and a quadratic dependence in the polydisperse (DTAB) region. The relative permittivity is a better indicator of the existence of amphiphilic preaggregation than the electrical conductivity.