AGGREGATION AND DISPERSION OF COLLOIDAL SUSPENSIONS BY INORGANIC SURFACTANTS - EFFECT OF CHEMICAL SPECIATION AND MOLECULAR-CONFORMATION

Polyphosphates are widely used as inorganic dispersants, but the mecha nism of the dispersing is not well understood. In this study, the coll oidal properties of fine alumina suspensions in water were studied at different pH conditions in the presence of linear chain polymetaphosph ates. The chemical speciation and molecular conformation were probed u sing liquid and solid state P-31 and Al-27 nuclear magnetic resonance spectroscopy. Three regimes have been found: (i) Neutral pH-long-chain metaphosphates are partially anchored to the alumina surface. The neg atively charged metaphosphate molecules extend from the particle surfa ce to the solution to provide an effective electrosteric repulsion for ce that acts to disperse the colloidal particles. (ii) Low pH-the alum ina particle surfaces hydrolyze and the dissolved species react with t he surfactant to form amorphous aluminum phosphate gels. The reaction product causes bridging flocculation of the suspension. (iii) High pH- partial dissolution of alumina produces tetrahedrally bonded hydroxyal uminate anions, which induces the long-chain polymetaphosphates to dis sociate into phosphate anions. Under these conditions, the metaphospha te no longer exhibits dispersion properties. A fundamental understandi ng of the particle-surfactant interactions in terms of adsorption, che mical reaction, and molecular conformation will provide insights for t he rational design of more effective inorganic dispersants.