Changes of pH and counterion activity during the binding process of cationic surfactants to carboxylic polyions

Abstract

The binding of a cationic surfactant, dodecylpyridinium ions (DP +), by an alternating copolymer of maleic acid with styrene (MASt) and an alternating copolymer of maleic acid with ethylene (MAE) and the pH change in the solutions of the copolymers during the process of cooperative binding have been investigated in the presence of a fixed concentration of NaCl by using potentiometric techniques based on a surfactant-ion-selective solid state membrane electrode and a pH electrode. In the cases of MASt at lower degrees of neutralization, α, and MAE, where the binding isotherms are single-step curves, the pH values increase at first, accompanied by a sharp rise in the binding isotherms, and then decrease in the leveling-off phase of the binding process. In the case of MASt at higher α values, however, the relationship of pH vs. the logarithm of the free surfactant ion concentration exhibits a characteristic double-peaked curve corresponding to the two-step binding isotherm. The change in sodium ion activity during the binding process of DP + ions has been also examined in salt-free aqueous solutions of poly(acrylic acid) (PAA) with use of an Na + ion-selective solid state membrane electrode. It was found that Na + ions are released in proportion to the concentration of bound surfactant ions and almost all the initially condensed Na + ions are released finally through the whole process of the surfactant ion binding.