High-frequency dielectric polarization mechanism in water-in-oil microemulsions below percolation

Abstract

In this note, we report high-frequency dielectric relaxations of water-in-oil microemulsion systems occurring in the frequency range from 1 MHz to 1.8 GHz. In the composition range and temperature interval investigated (water-to-surfactant molar ratio of 40.8 and fractional volumes from 0.05 to 0.30), the system behaves as a collection of isolated water droplets, well below the percolation threshold, where a more complex structural arrangement prevails. The observed dielectric spectra evidence a polarization mechanism occurring within the single water droplet, characterized by a dielectric strength of several units and a relaxation time of the order of 1-2 ns. The results presented here give support to the identification of the dielectric relaxation as being due to the orientational correlation of the surfactan head groups at the water-surfactant interface resulting in the formation of an apparent relaxing dipole moment. This polarization mechanism constitutes a novel contribution to the overall microemulsion dielectric spectra, whose features should be accurately investigated to gain a complete description of the electrical behaviour of these systems.