Charge correlation effects on ionization of weak polyelectrolytes

Abstract

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.