Polyelectrolyte Titration: Theory and Experiment

Abstract

Titration of methacrylic acid / ethyl-acrylate copolymers is studied experimentally and theoretically. At low salt concentrations, this polyacid exhibits a plateau in the titration curve below the neutralization point. The plateau has been often attributed to a first-order phase transition associated with polymer conformational changes. We argue that the specific shape of titration curves of hydrophobic polyelectrolytes is due to electrostatics and does not necessarily require a conformation change of the polyelectrolyte chains. We calculate the free energy at the mean-field level and its first-order (one loop) correction using a loop expansion. The latter is dominated by Debye-Huckel--like charge-charge correlations as well as by correlations between dissociation sites along the polymer chain. We show that the one-loop corrections to the free energy lead to titration curves that agree with experiments. In particular, the model explains the decrease of the pH at the plateau when the polymer concentration is increased or when salt is added to the solution.