Conductivity of mixed surfactant water-in-oil microemulsions

The electrical conductivity of D2O-in-n-heptane (or in-p-xylene) microemulsions stabilized by a mixture of the cationic surfactant di-n-didodecyldimethylammonium bromide, DDAB, and either the CIEJ non-ionic surfactants, or polymeric nonionic surfactants of the type PEO-PPO-PEO, Pluronic®, was investigated as a function of D2O content and surfactant mixture composition. It was found that the change in conductivity as a result of partially replacing the cationic surfactant by the non-ionic is due to the change in droplet size, and not a decrease in ionic strength. This was concluded from the following observations: (1) when the droplet size was held constant, the conductivity was constant even for samples with different water droplet ionic strengths, (2) good agreement was found between the measured conductivities and those predicted by charge fluctuation model (CFM) even though the ionic strength (not considered in CFM) is varied, and finally (3) graphical analyses of conductivities for microemulsions stabilized by different cationic-nonionic surfactant mixtures result in curves similar to those expected for microemulsions stabilized by ionic surfactant only. These results show clearly that mixing cationic and non-ionic surfactants is useful in testing and supporting microemulsion conductivity models.