Ionization Of Weak Polyelectrolytes Research Articles

Thermodynamic non-ideality in charge regulation of weak polyelectrolytes

Polymer ionization differs from that for their monomeric counterparts due to intramolecular correlations. Such effects are conventionally described in terms of the site-binding model that accounts for short-range interactions between neighboring sites. With an apparent equilibrium constant for each ionizable group and the nearest-neighbor energy as adjustable parameters, the site-binding method is useful to correlate experimental titration curves when the site-site interactions are insignificant at long ranges. This work aims to describe the electrostatic behavior of weak polyelectrolytes in aqueous solutions on the basis of the intrinsic equilibrium constants of the individual ionizable groups and solution conditions underlying the thermodynamic non-ideality. A molecular thermodynamic model is proposed for the protonation of weak polyelectrolytes by incorporating classical density functional theory into the site-binding model to account for the effects of the local ionic environment on both inter-chain and intra-chain correlations. By an extensive comparison of theoretical predictions with experimental titration curves, we demonstrate that the thermodynamic model is able to quantify the ionization behavior of weak polyelectrolytes over a broad range of molecular architectures and solution conditions.

Gradient and asymmetric copolymers: the role of the copolymer composition profile in the ionization of weak polyelectrolytes

The ionization of weak polyelectrolytes can be altered by controlling the composition profile of the comonomers along the chain.

Charge correlation effects on ionization of weak polyelectrolytes

Ionization curves of weak polyelectrolytes were obtained as a function of the chargecoupling strength from Monte Carlo simulations. In contrast to many earlier studies, thepresent work treats counterions explicitly, thus allowing the investigation of chargecorrelation effects at strong couplings. For conditions representing typical weakpolyelectrolytes in water near room temperature, ionization is suppressed because ofinteractions between nearby dissociated groups, as also seen in prior work. A novel findinghere is that, for stronger couplings, relevant for non-aqueous environments in the absence ofadded salt, the opposite behavior is observed—ionization is enhanced relative to thebehavior of the isolated groups due to ion–counterion correlation effects. The fraction ofdissociated groups as a function of position along the chain also behaves non-monotonically.Dissociation is highest near the ends of the chains for aqueous polyelectrolytes and highestat the chain middle segments for non-aqueous environments. At intermediate couplingstrengths, dissociable groups appear to behave in a nearly ideal fashion, eventhough chain dimensions still show strong expansion effects due to ionization.These findings provide physical insights on the impact of competition betweenacid/base chemical equilibrium and electrostatic attractions in ionizable systems.