Impact of molecular size on the protonation/deprotonation equilibria of acrylic and polyacrylic acids in aqueous electrolyte solutions

In aqueous colloids, various entities can carry charges such as nanoparticles dispersed in electrolyte medium or polyelectrolytes (ionic macromolecules) dissolved in water. A local electrostatic field is created, which counteracts further charge separation. pH-dependent charges can form in protonation/deprotonation equilibria, which can be quantified by potentiometric acid-base titration. We have developed a general adsorption model to evaluate such titrations. After calibrating the system for both pH and concentration, the difference between the added and equilibrium concentration of H+/OH− ions corresponds to the protonation/deprotonation reactions that generate the charges on colloids. Both the amount and the sign of charge can be derived directly from this difference using the proton material balance equation. The pH- and ionic strength dependent degree of dissociation of polyelectrolytes can also be calculated directly. The experimental data can be successfully fitted using surface complexation models (SCMs). In this work, we present experimental and fitting results of acid base titrations for polyacrylic acids of different molecular weights (1.8, 5, 90 and 700 kDa) and poly(acrylic acid-co-maleic acid) 3 kDa comparing them with each other and with the acrylic acid monomer. This work revealed that pKa values from fitting constant capacitance and diffuse layer models showed a nearly linear dependence on ionic strength and varied significantly with molecular size. The larger the molecules, the weaker the acidity of the same acidic group (COOH) at low ionic strengths, which decreases with increasing ionic strength, while at ∼0.5 M concentration the difference disappears.