Dielectric spectroscopy of microemulsions

The dielectric spectroscopy method (DS) has become a useful technique for the investigation of the structural and dynamic features of the components of both microemulsions and microdroplets over a wide temperature and frequency range. The dielectric parameters obtained by DS determine the geometry of the association structures as well as the overall picture of the dynamics of the different polar groups, aggregates, and association structures representing dipole modes. In this paper the results of a comprehensive study of the different types of microemulsions (ionic and nonionic) by DS are presented. The static and dynamic dielectric properties of microemulsions as a function of temperature, frequency and concentration of water, oil, surfactant and cosurfactant are considered. The dielectric properties have been investigated in the frequency range 10(5)-10(10) Hz using time domain dielectric spectroscopy (TDDS) and over a broad temperature interval enabling us to cover all the main dynamic processes occurring in such systems. The data treatment for the dynamic behavior of the microemulsions was carried out in the time domain in terms of dipole correlation functions and in the frequency domain in terms of complex dielectric permittivity. The correlation functions of the investigated systems exhibit complex nonexponential relaxation behavior, which must be deconvoluted into normal modes and represented as a sum of the simple exponential, exp(-t/tau), and nonexponential terms, exp [-(t/tau)(v)]. The parameter v characterizes the shape of the relaxation function and the cross-correlation effects, and describes the morphology of the system. The molecular mechanisms responsible for dielectric polarization in microemulsions of different nature are discussed. Knowledge of the amount of hydrate water and co-surfactant in the interface can be obtained for nonionic microemulsions. In the case of ionic microemulsions, TDDS is a powerful technique for monitoring the organization of clusters and for investigation of relaxation processes involving rearrangement and movement of the droplets forming the clusters. (C) 1997 Elsevier Science B.V.