Dielectric spectra of ionic water-in-oil microemulsions below percolation: frequency dependence behavior

We have investigated the dielectric properties of water-in-oil microemulsions composed of sodium bis(2-ethyl-hexyl)sulfosuccinate, water, and decane, using radiofrequency impedance spectroscopy, below the percolation threshold, where the system behaves as surfactant-coated individual water droplets dispersed in a continuous oil phase. The analysis of the dielectric spectra has evidenced that the whole dielectric response below percolation is due to two different contributions, which give rise to two partially overlapping dielectric relaxations, approximately in the frequency range from 10 to 500 MHz. The first of these mechanisms is originated by the bulk polarization of counterions distributed in the electrical double layer of the droplet interior. The second mechanism is associated with a correlated motion of the anionic head groups SO3- at the surfactant-water interface. The introduction of this latter contribution allows us to justify the experimentally observed increase in the low-frequency permittivity as a function of temperature up to temperatures very close to percolation. The present study shows that deviations from the expected values on the basis of dielectric theories of heterogeneous systems (Maxwell-Wagner effect) observed when percolation is approaching can be accounted for, in a reasonable way, by the introduction of a further polarization mechanism, which involves the anionic surfactant groups. Only very close to percolation, when microemulsions undergo a scaling behavior, deviations of the permittivity (and electrical conductivity as well) are a print of the structural rearrangement of the whole system and models based on colloidal particle suspension theories fail. Even if the whole picture of the dielectric properties of microemulsion systems does not change in deep, nevertheless, the refinement introduced in this paper demonstrates how different polarization mechanisms could be simultaneously present in these rather complex systems and, above all, how the individual particle colloidal properties are maintained up to very close to the percolation threshold.