Microemulsions, dielectric relaxation

For both types of microemulsions, dielectric relaxations of the Cole-Cole type were in evidence, along with conduction absorptions. The features of the dielectric relaxations were found to depend strongly upon the composition and, even when nonionic surfactants were used, upon the temperature. 

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). 

The third relaxation process is located in the low-frequency region and the temperature interval 50°C to 100°C. The amplitude of this process essentially decreases when the frequency increases, and the maximum of the dielectric permittivity versus temperature has almost no temperature dependence (Fig 15). Finally, the low-frequency ac-conductivity ct demonstrates an S-shape dependency with increasing temperature (Fig. 16), which is typical of percolation [2,143,154]. Note in this regard that at the lowest-frequency limit of the covered frequency band the ac-conductivity can be associated with dc-conductivity cio usually measured at a fixed frequency by traditional conductometry. The dielectric relaxation process here is due to percolation of the apparent dipole moment excitation within the developed fractal structure of the connected pores [153,154,156]. This excitation is associated with the selfdiffusion of the charge carriers in the porous net. Note that as distinct from dynamic percolation in ionic microemulsions, the percolation in porous glasses appears via the transport of the excitation through the geometrical static fractal structure of the porous medium. 

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)... 

Ionic Conductivity. The electrical conductivity measurements were performed using a Hewlett Packard model 4192 impedance analyzer under computer control, using a conductance cell similar to that described by Pauly and Schwan (5). The conductivity measurements were essentially constant between 1-100 kHz, ruling out electrode polarization or other artifacts. In 0/W microemulsions, no appreciable dielectric relaxation effects are expected or observed below 1 GHz (U. 

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.)... 

This is, however, a macroscopic explanation of changes that occur on a molecular level, and is rather superficial. There is clearly a distribution of dielectric relaxation times in the microemulsion. The timescale of the dielectric relaxation measurement (tens of picoseconds) is too short for the phenomenon of fast exchange. It would appear, therefore, that the motional restriction of the water must vary throughout the microemulsion. 

A semiquantitative explanation of the 2-ns component may be as follows The static polarity or the dielectric constant of the water pool of the AOT microemulsions can be obtained from the position of the emission maximum of the probes (C480 and 4-AP) [165,166]. For both probes, the water pool resembles an alcohol-like environment with an effective dielectric constant of 30-40. If one makes a reasonable assumption that the infinite frequency dielectric constant of water in the water pool of the microemulsions is the same as that of ordinary water, i.e., 5, and that the dielectric relaxation time is 10 ns as obtained for the biological systems [18b], then the solvent relaxation time should be about 1.67 ns, which is close to the observed solvation time in AOT microemulsions. 

In order to understand this complex relaxation behavior of the microemulsions, it is necessary to analyze dielectric information obtained from the various sources of the polarization. For a system containing more than two different phases the interfacial polarization mechanism has to be taken into account. Since the microemulsion is ionic, the dielectric relaxation contributions are related to the movement of surfactant counterions relative to the negatively charged droplet interface. A reorientation of AOT molecules, and of free and bound water molecules, should also be mentioned in the list of polarization mechanisms. In order to ascertain which mechanism can provide the experimental increase in dielectric permittivity, let us discuss the different contributions. 

Figure 30 Dielectric relaxation times at 10°C of (a) dode-cane u-tanol/Brij 97/water microemulsion (system 1) and (b) dodecane/pentanol/Cj2(EO)g/water microemulsions (s5rstem 2), for different water contents. (From Ref 141. With permission from Elsevier Science.)...

The short dielectric relaxation process (presented here by 12) is associated with the anisotropic motion of the monomer alcohol species in a chain cluster (149). In microemulsions, the short process is the superposition of several dielectric relaxation processes, which have similar relaxation times such as movement or rotation of the alcohol monomers, hydrate water, and surfactant polar head groups. The short relaxation time is barely affected by the alcohol concentration in the mixture since it is less sensitive to the aggregation process. 

Fig, 3. Cole-Cole plot characteristic for the dielectric relaxation exhibited by microemulsions using non-ionic surfactants. Mass proportion of surfactant in the undecane phase 12%. Mass fraction of water in the system p = 0.093. Temperature T = 36°C. 

Fig. 4. Cole-Cole plots characteristic for the dielectric relaxations exhibited by water-in-hexadecane microemulsions using potassium oleate and 1-hexanol. Potassium oleate to 1-hexanol mass ratio equal to 3/5. Combined surface active agent to hexadecane mass ratio equal to 2/3. p represents the mass fraction of water. Temperature ...

We have seen earlier that the microemulsion formation is a spontaneous process which is controlled by the nature of amphiphile, oil, and temperature. The mechanical agitation, heating, or even the order of component addition may affect microemulsification. The complex structured fluid may contain various aggregation patterns and morphologies known as microstmctures. Methods like NMR, DLS, dielectric relaxation, SANS, TEM, time-resolved fluorescence quenching (TRFQ), viscosity, ultrasound, conductance, etc. have been used to elucidate the microstructure of microemulsions [25,26]. 

Y. Feldman, N. Kozlovich, I. Nir, and N. Garti 1995 Dielectric relaxation in sodium bis 2(ethyl hexyl) sulfosuccinate-water-decane microemulsions near the percolation temperature threshold, Phys. Rev. E 51, 478-491. 

Peyrelasse, J. and Boned, C. 1990 Conductivity, dielectric relaxation, and viscosity of ternary microemulsions The role of the experimental path and the point of view of percolation theory, Phys. Rev. A 41 938-953. 

Bansal V, Chinnaswamy K, Ramachandran C and Shah D. 1979. Structural aspects of microemulsions using dielectric-relaxation and spin label techniques. Journal of Colloid and Interface Science 524-357. 

Figure 1. Plots of the complex permittivity of the 0/W microemulsions prepared with Tween 60 on the complex dielectric plane ("Cole-Cole" plots), showing the depressed semicircles that indicate a distribution of relaxation times. Figure lb is an expanded portion of Figure la. A few frequencies are indicated for reference. Reproduced with permission from Reference 1. Copyright 1982 Academic Press.

MicroBrownian dynamics of microemulsions can be studied by various techniques including dynamic-mechanical, dielectric, ultrasonic and NMR relaxation, ESR, volume, enthalpy and specific heat relaxation, quasielastic light and neutron scadering, fluorescence-depolarization experiments, and many other methods (90, 102-107). The information thus acquired provides an opportunity to clarify... 

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