Role of Adsorption Fractionation in Determining the CaCO₃ Scale Inhibition Performance of Polydisperse Sodium Polyacrylate

Results are reviewed from a study examining the influences of concentration and molecular weight of poly(acrylic acid) sodium salt on the inhibition of CaCO₃ formation in solutions similar in ion composition and pH to those used in wood pulping operations. Calcium concentrations and temperatures were monitored in heated supersaturated solutions containing sodium polyacrylate samples to gauge their inhibition of CaCO₃ nucleation. The presence of dissolved polymers raises the temperatures necessary to induce detectable nucleation. Nucleation temperatures were found to be strongly dependent on the molecular weight distribution of the sodium polyacrylates. Significantly higher temperatures were obtained from species with molecular weights estimated to be in the region of 5000-6000 g/mol, and contributions from adsorption fractionation and polymer-metal complexation appear present at higher concentrations. Testing of samples produced by mixing sodium polyacrylates of various molecular weight distributions indicates that high molecular weight species govern the observed inhibition under competitive adsorption conditions. This is consistent with the dominant role of adsorption thermodynamics in determining the influence of polymers on nucleation. Also presented is a semi-empirical equation for estimating the low concentration influence of mixed molecular weight distributions on nucleation temperatures using the inhibition performance of the individual components.