Electrical resistance microemulsions

Structural Aspects of Microemulsions. Several investigators have studied the structure of microemulsions using various techniques such as ultracentrifugation, high resolution NMR, spin-spin relaxation time, ultrasonic absorption, p-jump, T-jump, stopped-flow, electrical resistance and viscosity measurements (56-58). The useful compilation of different studies on this subject is found in the books by Robb (68) and Shah and Schechter (69). Several structural models of microemulsions have been proposed and we will discuss only a few important studies here. 

For microemulsion system (hexanol system), since it contains water spheres in a continuous oil medium, the addition of water forms more spherical droplets. The continuous medium is still an oil phase and therefore, the electrical resistance is maintained at a high value in the range of lO-- ohms (70). However, for cosolubilized system (pentanol system), as the amount of water is increased, the average distance between alcohol molecules as well as between water molecules would change and this consequently would influence the hydrogen bonding ability of water and alcohol molecules, which in turn would influence the chemical shift of the resonance peak. Moreover, as one adds more and more water in cosolubilized system, it becomes more electrically conducting and, hence exhibits a continuous decrease in the electrical resistance (70). 

To illustrate this point. Figure 15.10 shows the change in electrical resistance (reciprocal of conductivity) with the ratio of water to oil (V /VJ for a microemulsion system prepared using the inversion method [14]. The data in Figure 15.10 indicate the change in optical clarity and birefringence with the ratio of water to oil. 

A few further photochemically induced changes of properties of solutions containing amphiphilic aggregates are reported in the literature Balasubramanian et al. (163) as well as Kano et al. (164) found changes in the electrical resistance and in the light transmission of microemulsions and membrane systems on photochemical cis-trans isomerization of azobenzene derivatives ... 

Using various physicochemical techniques such as high resolution NMR, viscosity, and electrical resistivity measurements, Chan and Shah [26] proposed that the middle-phase microemulsion in three-phase systems at or near optimal salinity is a water-external microemulsion of spherical droplets of oil. Extended studies to characterize the middle-phase microemulsions by several techniques including freeze-fracture electron microscopy revealed the structure to be a water-external microemulsion [26]. The droplet size in the middle-phase microemulsion decreases with increasing salinity. A freeze-fracture electron micrograph of a middle-phase microemulsion is shown in Fig. 8. It clearly indicates that the discrete spherical structure of the oil droplets in a continuous aqueous phase is consistent with the mechanism proposed in Fig. 7. This system was extensively studied by Reed and coworkers [20-22]. 

---