We measured the ultrasonic vibration current of aqueous solution of dodecyltrimethylammonium bromide (DTAB) and dodecyltrimethylammonium chloride (DTAC) as a function of surfactant concentration at 298.15 K under atmospheric pressure. The dependence of vibration current on the surfactant concentration exhibited a break point around the critical micelle concentration (CMC) in the DTAB system, whereas no break point was observed in the DTAC system. Taking account of the theory for vibration current, this suggests that the vibration current in the concentration regime higher than CMC depends on the electrostatic properties of micelle surface. From the difference in the slope of vibration current versus molarity curve above CMC, it was found that the z-potential of DTAC micelle is larger than that of DTAB micelle. This can be explained by the dissociation degree of counterions from the micelle surface. In addition, the theoretical equation of vibration current enabled us to estimate the partial molar volume of surfactant ions from the different behavior of vibration current in the concentration regime lower than CMC.

In order to clarify the effect of constituent ions on an electroacoustic phenomenon in an aqueous solution of ionic surfactant, we measured the total vibration current TVI as a function of the molarity of aqueous solution of several ionic surfactants at 298.15 K under atmospheric pressure. The different counterion caused the remarkable change in TVI, while the difference in alkyl chain length of surfactant ion hardly affected TVI. This suggests that the size and/or aggregation number of micelle is not a crucial factor contributing to TVI. Employing the fl uorocarbon surfactant, on the other hand, there appeared a fairly large change compared to the hydrocarbon surfactant. This implies that the density contrast between medium and micelle is signifi cant for TVI.