Effect of cationic surfactants on titration behavior of isotactic and atactic poly(methacrylic acid)

Abstract

By forming strong complexes with oppositely charged polyelectrolytes, cationic surfactants significantly affect their solution properties. We investigated the effect of bound surfactant on titration behavior of two isomer forms of poly(methacrylic acid), PMA, atactic and isotactic PMA, aPMA and iPMA, respectively, which are known for the cooperative change in chain conformation in water. The bound surfactant micelles increase the initial degree of ionization, a, of carboxyl groups on PMA, shift the conformational transition to higher a and make it narrower, and exclude the complexed chains from the solution. The titration curves were analyzed in the framework of the Henderson-Hasselbalch theory and the Gibbs free energy change of the conformational transition was calculated. The results were discussed in the context of a complex influence of surfactant on the PMA chain conformation in solution. It is proposed that the PMA-surfactant interaction changes from a so-called hydrophobic mode, which is responsible for the solubilization of the unionized and water-insoluble iPMA at low α, to a predominantly electrostatic one at high α, which leads to the precipitation of both aPMA and iPMA from solution. Constant pH simulations using simple coarse-grained models for the PMA polymers and individual surfactants were performed to rationalize the titration behavior of PMAs in the absence and presence of surfactant. By inducing an attractive interaction between PMA monomers, an excellent agreement between the experimental and numerical titration curves was obtained. Such agreement becomes poorer upon the addition of surfactant; however, the main features of the PMA titration curves are preserved, which supports the role of hydrophobic interactions in PMA-surfactant association at low pH values and that of electrostatics at higher pH.