Variation of the Critical Micelle Concentration with Surfactant Structure

In this laboratory experiment students analyze the role of attractive interactions between the hydrocarbon tails and repulsive interactions between the surfactant head groups on the micellar association process by determining the CMC of two surfactant classes. Using electrical conductivity measurements and pyrene fluorescence emission, the CMCs of two homologous series of sulfobetaines, zwitterionic surfactants, and alkyl trimethyl ammonium bromide, cationic surfactants, were determined. In each surfactant family we use surfactants with 12, 14, and 16 carbon atoms. From the correlation of the head group charge to the methylene group contribution to CMC for the two surfactant classes, students can clearly analyze the influence of the electrostatic repulsions on the self-assembly processes.