Protonation equilibrium of the poly(allylammonium) cation in an aqueous solution of binary 1 : 1 electrolytes

Abstract

The (de)protonation equilibrium of the poly(allylammonium) cation (PAH) in an aqueous solution of various binary 1 : 1 electrolytes of different concentrations (0.1 ≤ c(NaX)/mol dm⁻³ ≤ 1.0; X = Cl⁻, Br⁻, I⁻, NO₃⁻) was investigated potentiometrically at 25 °C. The mixed and concentration apparent equilibrium deprotonation constants (K_ap) were calculated from the experimentally collected data and concentration profiles of dissociated and undissociated functional groups were obtained. The standard pK value of monomers was estimated by extrapolating the pK_ap values determined at various concentrations of added electrolyte to the degree of dissociation α = 1. The dependence of pK_ap on the degree of dissociation could be well described by the two parameter model according to Mandel. The variation of with monomer dissociation degree was found to be in satisfactory agreement with the cylinder Stern model, based on the Poisson–Boltzmann (PB) equation, and a constant Stern capacitance. Generally, the derived apparent constants showed a pronounced dependence on the concentration of binary electrolytes and a weak dependence on the type of anion counterbalancing the polyion charge. The influence of the PAH chain length (polymers containing on average 150 and 700 monomers were examined) on the protonation equilibrium of PAH could not be observed.