Effect of counterion substitution on the viscosity anomaly in AOT microemulsions

AOT-based water-in-oil microemulsions display an anomalous maximum in the viscosity with X, the water to surfactant ratio. Several explanations for this phenomenon have been offered. In this work, we investigate viscosity and droplet interactions in Ca(AOT)₂/water/n-decane and KAOT/water/n-decane microemulsions and compare our results with the commonly studied NaAOT/water/oil system. The Ca(AOT)₂ system demonstrates a maximum in relative viscosity and droplet attraction near X = 15, similar to the NaAOT system, although the maximum occurs at a higher value of X in the Ca(AOT)₂ system. By contrast, the viscosity and interparticle interactions in the KAOT system do not strongly depend on the amount of water in the system. We attribute the differences in behavior between the two systems to different hydration characteristics of the counterion, and we believe that our results are consistent with a previously proposed model that attributes interdroplet attractions to charge fluctuations and surfactant exchange. Our findings support the connection between the viscosity anomaly and interparticle interactions.