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 co mponents of both microemulsions and microdroplets over a wide temperat ure and frequency range. The dielectric parameters obtained by DS dete rmine the geometry of the association structures as well as the overal l picture of the dynamics of the different polar groups, aggregates, a nd association structures representing dipole modes. In this paper the results of a comprehensive study of the different types of microemuls ions (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 t he frequency range 10(5)-10(10) Hz using time domain dielectric spectr oscopy (TDDS) and over a broad temperature interval enabling us to cov er all the main dynamic processes occurring in such systems. The data treatment for the dynamic behavior of the microemulsions was carried o ut 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 none xponential 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 charact erizes the shape of the relaxation function and the cross-correlation effects, and describes the morphology of the system. The molecular mec hanisms responsible for dielectric polarization in microemulsions of d ifferent nature are discussed. Knowledge of the amount of hydrate wate r and co-surfactant in the interface can be obtained for nonionic micr oemulsions. In the case of ionic microemulsions, TDDS is a powerful te chnique for monitoring the organization of clusters and for investigat ion of relaxation processes involving rearrangement and movement of th e droplets forming the clusters. (C) 1997 Elsevier Science B.V.