Microemulsions dielectric properties

In the following, after a brief account of the data available in the literature as concerns microemulsion conductive and dielectric properties, results will be reported and analyzed that show how structural transitions in the transparent isotropic water-inoil solubilization area can be put into evidence by means of conductometry and dielectrometry. Mention will be made also of the occurrence, in certain w/o microemulsion systems, of percolative conduction phenomena (30) that appear to depend upon the nature of the alcohol used as the cosurfactant, for a given hydrocarbon (31). 

The possibility of realizing via percolated w/o microemulsion conductor/insulating composite materials with very large dielectric constant and exotic optical properties has been pointed out [284],... 

The dielectric relaxation properties in a sodium bis(2-ethylhexyl) sulfosuc-cinate (AOT)-water-decane microemulsion near the percolation temperature threshold have been investigated in a broad temperature region [47,143,147]. The dielectric measurements of ionic microemulsions were carried out using the TDS in a time window with a total time interval of 1 ps. It was found that the system exhibits a complex nonexponential relaxation behavior that is strongly temperature-dependent (Figure 8). 

Thus, the non-Debye dielectric behavior in silica glasses, PS and AOT microemulsions, has similar properties. These systems exhibit an intermediate temperature percolation process that is associated with the transfer of the electric... 

The low temperature properties of a dodecane-hexanol-K.oleate w/o microemulsion from 20°C to -190°C were studied vs. increasing water content (C,mass fraction) in the interval 0.021+-0.1+, by Differential Scanning Calorimetry and dielectric analysis (5 Hz-100 MHz). A differentiation between w/o dispersions is obtained depending on whether they possess a "free water" fraction. Polydispersity is evidenced by means of dielectric loss analysis. Hydration processes occurring, at constant surface tension, on the hydrophilic groups of the amphiphiles, at the expenses of the free water fraction of the droplets, are shown to develop "on ageing" of samples exhibiting a time dependent behavior. 

We have studied a variety of transport properties of several series of 0/W microemulsions containing the nonionic surfactant Tween 60 (ATLAS tradename) and n-pentanol as cosurfactant. Measurements include dielectric relaxation (from 1 MHz to 15.4 GHz), electrical conductivity in the presence of added electrolyte, thermal conductivity, and water self-diffusion coefficient (using pulsed NMR techniques). In addition, similar transport measurements have been performed on concentrated aqueous solutions of poly(ethylene oxide)... 

In the present study, we have examined other transport properties of 0/W microemulsions containing the nonionic surfactant Tween 60 whose dielectric and conductivity properties have been previously characterized. We have chosen properties (water self-diffusion, ionic conductivity at low frequencies, and thermal conductivity) that can be analyzed using the same mixture theory, and which therefore can be compared in a consistent way. Limited transport data are presented from other microemulsions as well. 

Figure 1 shows the dielectric relaxation properties of the Tween microemulsions plotted on the complex permittivity plane (from Foster et al ( 1). The mean relaxation frequency (corresponding to the peak of each semicircle) decreases gradually from 20 GHz for pure water at 25°C to ca. 2 GHz for a concentrated microemulsion containing 20% water. Since the permittivity of the suspended oil/ emulsifier is 6 or less at frequencies above 1 GHz, this relaxation principally arises from the dipolar relaxation of the water in the system. Therefore, the data shown in Figure 1 clearly show that the dielectric relaxation times of the water in the microemulsions are slower on the average than those of the pure liquid. The depressed semicircles indicate a distribution of relaxation times (9), and were analyzed assuming the presence of two water components (free and hydration) in our previous studies. 

Physical Mechanisms. The simplest interpretation of these results is that the transport coefficients, other than the thermal conductivity, of the water are decreased by the hydration interaction. The changes in these transport properties are correlated the microemulsion with compositional phase volume 0.4 (i.e. 60% water) exhibits a mean dielectric relaxation frequency one-half that of the pure liquid water, and ionic conductivity and water selfdiffusion coefficient one half that of the bulk liquid. In bulk solutions, the dielectric relaxation frequency, ionic conductivity, and self-diffusion coefficient are all inversely proportional to the viscosity there is no such relation for the thermal conductivity. The transport properties of the microemulsions thus vary as expected from simple changes in "viscosity" of the aqueous phase. (This is quite different from the bulk viscosity of the microemulsion.)... 

Yu Alexandrov. Dielectric Spectroscopy Investigation of Dynamic and structural properties of microemulsions. Thesis. Jerusalem, 1998. 

DIELECTRIC AND COI )UCTIVE PROPERTIES OF WATER-IN-OIL TYPE MICROEMULSIONS... 

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