ELECTROKINETIC MEASUREMENTS IN AQUEOUS-SOLUTIONS OF WEAK ELECTROLYTE TYPE SURFACTANTS

In aqueous solutions of weak electrolyte surfactants, such as fatty acids and long chain aliphatic amines, either ionized or molecular forms predominate depending on pH. In the pH ranges over which molecular forms predominate, the solubility of the surfactant as monomers is not high enough to initiate micellization before the colloidal precipitate appears in the system. The colloidal species which appear in acidic solutions of fatty acids when the solubility limit of the free acid is exceeded, and the colloidal species which appear in alkaline solutions of aliphatic amines, are different from micelles and have clear isoelectric points. In the case of fatty acids, the isoelectric points of the precipitate are situated around pH 2-3. Such colloidal species apparently coagulate/precipitate onto solid particles and bubbles and determine the zeta potential-pH relationship for the mineral particles and bubbles generated in such solutions. It has been shown that the interpretation of the electrokinetic measurements carried out in aqueous solutions of weak electrolyte surfactants may lead to erroneous conclusions if the presence of such colloidal species is ignored. © 1993 Academic Press, Inc.