Role of Electrostatic Interactions in Charge Regulation of Weakly Dissociating Polyacids

Abstract We investigate theoretically charge regulation of weakly dissociating polyacids by potentiometric titration of their aqueous solutions. By treating deprotonation and cation binding to the polyacids as reversible reactions in our model, the ionization constant of acid groups along the polyacid chain is adjusted from its intrinsic value by electrostatic correlations. If electrostatic correlations are modeled with a Debye-Huckel electrostatic free energy that ignores the connectivity of acid groups, the theory follows Henderson-Hasselbalch behavior for monoacids at low polyacid concentrations, and does not capture the shift of the ionization constant from its intrinsic value with increasing pH. Using a random phase approximation (RPA) that captures the chain connectivity, the shift of the ionization constant is predicted, which is found to directly originate from the electrostatic repulsions along the polyacid chain. We make predictions for titration of rodlike and Gaussian coil polyacids, and find that the former is followed by hydrophilic poly(acrylic acid), while the latter is followed more closely by hydrophobic poly(acrylamido-2-methyl-1-propanesulfonic acid). The results suggest that development of more advanced free energy models that allow chain configurations to self adjust with changing pH and chain hydrophobicity could provide accurate a priori modeling of charge regulation of polyacids.