An Undergraduate Physical Chemistry Experiment on Surfactants: Electrochemical Study of a Commercial Soap

A general and physical chemistry laboratory on the electrochemistry of soap solutions is proposed. It involves safe, nontoxic, nonpolluting materials, common laboratory apparatuses, and a combination of theoretical and practical concepts. The critical micelle concentration is measured in weight percent and the mean hydrocarbon chain length and soap mixture molar weight are determined. Conductivity data are also used to determine the micelle ionization degree, the micelle and monomer molar conductivity, and the micellar electrophoretic mobility. The existence of a critical micelle concentration range is also visualized. The hydrolysis degree, the fatty acid ionization constant and water solubility, the formation of acid soap, and the concentration at which the first submicellar aggregates form are determined by pH measurements. These results lead to a discussion of several characteristics of surfactant solutions.