Isothermal Titration Calorimetry of the Polyelectrolyte/Water Interaction and Binding of Ca2+: Effects Determining the Quality of Polymeric Scale Inhibitors

The interaction of poly(sodium acrylate) (NaPAA) and poly(sodium styrenesulfonate) (NaPSS) with water over a broad range of concentrations was followed by isothermal titration calorimetry (ITC) and compared to the corresponding low molecular weight salts, sodium acetate and sodium sulfate. Astonishingly, the backbone of NaPSS, known to be more hydrophobic, remains unseen in the dilution enthalpy, and NaPAA and NaPSS show very similar and close to ideal dilution curves. The NaPSS dilution is even more exothermic than Na2SO4. This is related to the specific structure of water screening the hydrophobic polystyrene backbone. Counterintuitively, binding of Ca2+ ions to PAA, a spontaneous process, shows to be highly endothermic; i.e., the binding is solely driven by entropy. This suggests that not Coulomb interactions, but liberation of water molecules from the hydration shells of the components is the driving energy source for the binding of multivalent ions onto polyelectrolytes. Analysis of binding titrations of Ca2+ to three different industrial scale inhibitors (poly(acrylic acid), poly(aspartic acid), poly(acrylic-co-maleic acid) (Sokolan)) allows to qualify their performance. The experiments underline that the understanding of polyelectrolytes and counterion condensation on the basis of Coulomb descriptions, i.e., ions as point charges and water as a homogeneous dielectric media, is at least misleading, as it disregards the predominant thermodynamic effects, the chemical specificity of the components in response to the structure of water.