J. Phys; Chem B (2000),104(47), 11027-11034

Itamar Borukhov¹ , David Andelman^2, Regis Borrega¹, Michel Cloitre¹, Ludwik Leibler¹ and Henri Orland3

1 Unité Mixte de Recherche CNRS Elf Atochem (UMR 167), 95 rue Danton, B.P. 108,
92303 Levallois-Perret Cedex, France,
2 School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact Sciences,
Tel Aviv University, Tel Aviv 69978, Israel,
and 3 Service de Physique Théorique, CE-Saclay, 91191 Gif-sur-Yvette Cedex, France

Received.- May 23, 2000; In Final Form: July 31, 2000

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 bas often been 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 frec energy at the mean-field level and its first-order (one loop) correction using a loop expansion. The latter is dominated by Debye -Hückel-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.