Dielectric Spectroscopy of Co-Ion Effects on Microemulsion Dynamics and Phase Behaviour

The effect of added cationic co-ions (Li+, Na+, Cs+, (CH3)4N+) on the dielectric spectra of didodecyldimethylammonium bromide / water / n-dodecane microemulsions is reported. At high water/surfactant mole ratios (W ≈100–120) small amounts of co-ions drastically change the shape of the dielectric spectra in the 5–500 MHz region whereas for W ≈20 almost no effect is observed. It is shown that the sensitivity of the dielectric spectra towards co-ion addition correlates with the percolation transition of the microemulsion structure from reverse micelles at high W to a bicontinuous phase at low W.