Adsorption determination of ionic surfactants using an electrode-separated piezoelectric quartz crystal

An ionic surfactant in solution reacts with an ionic surfactant having the opposite charge to form an ion pair which adsorbs on the quartz plate of an electrode-separated piezoelectric quartz crystal (PQC) in a flow system. The frequency change resulting from the adsorption depends upon pH, concentration of buffer and counter ion, and the flow rate of the solution. By controlling these factors, cationic or anionic surfactants at μM concentrations can be determined with no interference from diverse ions found in a river water. In addition, cationic and anionic surfactants cannot coexist in the same solution, because they react with each other to form the ion pair which deposits from solution. Only the form in excess remains. For this reason, it was found that the ionic surfactant could be detected and determined by adding a definite concentration of cationic surfactant monitoring its change of concentration using the above procedure.