Electrical conductivity and permittivity measurements near the percolation transition in a microemulsion: I. Experiment

We have made conductivity and permittivity measurements from 5–13 MHz on the microemulsion system toluene–brine–sodium-dodecyl-sulfate–butanol in the region where, by varying the brine salinity, the volume fraction of brine in the microemulsion varies from 0.1 to 0.9 as the system passes from one two-phase region to another via a three-phase region. In the two-phase region at high salinities, where the microemulsion consists of brine droplets dispersed in oil, a percolation transition, identified by a steep rise in the electrical conductivity, and a peak in the permittivity, is observed at brine volume fraction $f_p$=0.130±0.001. This transition, although very close to the boundary between the two-phase and three-phase regions, clearly lies inside the two-phase region. The data also suggest that there is a discontinuity in the conductivity at the phase boundary, and we argue that the microemulsion undergoes a change in structure at the phase boundary.