ELECTRICAL-CONDUCTIVITY, NEAR-INFRARED ABSORPTION, AND THERMAL LENS SPECTROSCOPIC STUDIES OF PERCOLATION OF MICROEMULSIONS

Microemulsions of negatively charged (sodium bis(2-ethylhexyl)sulfosuc cinate, AOT) and positively charged (dodecylmethylbutylammonium bromid e and benzyldimethylhexadecylammonium chloride) surfactants were studi ed below and above the percolation thresholds by electrical conductivi ty, near-infrared absorption, and thermal lens spectrometry. It was fo und that the AOT microemulsions undergo percolation at a relatively hi gh concentration (about 27% of water (v:v)) and show no variation in t he thermal lens effect (theta/AP(0)) as a function of water concentrat ion. These results seem to indicate that the AOT microemulsions consis t of small reversed micelles, and this structure is the same below and above the percolation threshold. Conversely, for microemulsions prepa red with positively charged surfactants, the percolation occurs at rel atively 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 f unction of water undergoes changes. It seems that the structure of the se positive microemulsions changes concomitantly with the percolation. Specifically, these positive microemulsions form larger interconnecte d aggregates or bicontinuous structures in solution above percolation threshold concentration.