An induction current method for determining the critical micelle concentration and the polarity of surfactants

A novel and simple method of determining both the critical micelle concentration (CMC) and polarities of ionic surfactants at the same time is presented in this paper. This method is based on the dependence of electrical potential at the liquid-air interface on the aggregation of the surfactant. When the measuring electrode contacts the liquid-air interface, the difference between the electrical potential of the electrode surface and the electrical potential of the liquid-air interface generates an electrical current through an electrical circuit. The dependences of the magnitudes of the measured signals on the concentrations of sodium dodecyl sulfate and hexadecyltrimethylammonium bromide were measured. The results show that the magnitude of the measured signal increases with the increasing surfactant concentration before it reaches to the CMC. The signal magnitude remains constant after the CMC. The value of CMC can be determined from the intersection point of the two lines before and after CMC. The CMC values calculated with this method are in good agreement with those reported in the reference papers. The experimental results also show that the directions of the signals generated by anionic and cationic surfactants are in opposite direction, which can be used to determine the polarities of the surfactants. The method described in this paper provides a simple and reliable tool to determine the CMC value and the polarity of ionic surfactants.