Electrical conductivity, near-infrared absorption, and thermal lens spectroscopic studies of percolation of microemulsions

Microemulsions of negatively charged (sodium bis(2-ethylhexyl)sulfosuccinate, AOT) and positively charged (dodecylmethylbutylammonium bromide and benzyldimethylhexadecylammonium chloride) surfactants were studied below and above the percolation thresholds by electrical conductivity, near-infrared absorption, and thermal lens spectrometry. It was found that the AOT microemulsions undergo percolation at a relatively high concentration (about 27% of water (v:v)) and show no variation in the thermal lens effect (theta/AP(0)) as a function of water concentration. These results seem to indicate that the AOT microemulsions consist of small reversed micelles, and this structure is the same below and above the percolation threshold. Conversely, for microemulsions prepared with positively charged surfactants, the percolation occurs at relatively low concentration (around 10% of water (v:v)), and also it is in this region that the thermal lens effect (i.e., theta/AP(0)) as a function of water undergoes changes. It seems that the structure of these positive microemulsions changes concomitantly with the percolation. Specifically, these positive microemulsions form larger interconnected aggregates or bicontinuous structures in solution above percolation threshold concentration.