Polyacrylic acid adsorption and conformation in concentrated alumina suspensions

Particulate suspensions used in many industrial processes are typically highly concentrated and it is often difficult to keep them well dispersed or flocculated. Polymer adsorption is usually employed to achieve desired colloidal and rheological properties of such suspensions. In this work, adsorption of polyacrylic acid (PAA) in concentrated alumina dispersions has been monitored to determine the effect of solids loading on both the adsorption as well as the conformation of the adsorbed species. Fluorescence and electron spin resonance (ESR) spectroscopy were used to determine the conformation of labeled PAA adsorbed on alumina particles. Solids loading was found to have interesting effects on PAA adsorption and conformation at the alumina-water interface. As the solids loading is increased, the adsorption density of PAA was found to decrease under conditions of a fixed high initial polymer concentration. However, in the case of adsorption using reduced initial polymer concentration, this solids loading dependence was removed. Conformational studies (conducted at a constant adsorption density) showed the polymer molecules to become more stretched with increasing solids loading. The relationship between the polymer conformation and the particle dispersion was also investigated. Stretching of the adsorbed polymer was found in this case to produce an increase in the settling rate obtained, suggesting bridging effects. (C) 2001 Elsevier Science B.V. All rights reserved.