Percolation phenomenon of calcium bis(2-ethylhexyl) sulfosuccinate water-in-oil microemulsions by conductivity and dielectric spectroscopy measurements

The sodium counterion (Na+) of the sodium bis(2-ethylhexyl) sulfosuccinate (AOT) surfactant was exchanged with calcium Ca2+ to investigate the counter ion charge effect on the structure of water in normal decane microemulsions . Ohmic conductivity and dielectric permittivity measurements were performe d on samples at constant water to surfactant mole ratio [water]/[Ca(AOT)(2) ]=26.6. Increasing the volume fraction of the dispersed phase phi, a percol ation phenomenon was observed at the constant temperature of 25 degrees C. The percolation threshold was found at phi similar to 15% by Ohmic conducti vity and static dielectric permittivity studied as a function of phi, and b y the frequency dependence of the complex permittivity. Critical exponents typical of the static percolation mechanism (formation of bicontinuous micr oemulsions) were found below and above threshold. The comparison of these r esults obtained for the two different counterions, Ca2+ and Na+, in AOT sur factant water in normal decane microemulsions allows detection of an import ant difference. The percolation below threshold is dynamic for the sodium-b ased microemulsions, accounting for the formation of clusters of droplets, whereas calcium-based microemulsions show a static percolation. For this sy stem, the coalescence of droplets begins to occur below threshold at phi si milar to 12%. [S1063-651X(99)12807-1].