Characterization of proton affinity reactions in polyelectrolytes: Discrete vs continuous distribution of sites approach

The chemical implications of the fitting parameters employed by two empirical models on the characterization of complexation reactions by dissolved humic substances were evaluated on various polyelectrolyte systems, Potentiometric titrations of polyacrylic acid (PAA), and different mixtures of PAA and polyvinylsulfonic acid, were conducted at different ionic strengths, The Scatchard Quasiparticle model (QM) and the Gaussian distribution model (GM) were utilized to describe the experimental data, Both models provided an adequate representation of the results, However, the QM required three different classes of functional groups (five fitting parameters), whereas the GM required only one type of binding site (two fitting parameters) to achieve that goal, Based on the chemical nature of the polymers, it was concluded that the GM offered a more realistic description of the results, The effects of electrostatics were also evaluated, A successful description of the experimental data suggested that such effects could be adequately accounted for with the model employed, However, geometry optimization calculations indicated that the fitting parameters held little relation to the physical reality of the systems, thus invalidating any assumptions based on the results of the fitting exercise. (C) 1998 Academic Press.