ENHANCED AGGREGATION OF COLLOIDAL HEMATITE RESULTING FROM THE ADSORPTION OF ALGINATE: INFLUENCE OF DIVALENT CATIONS ON GEL-NETWORK FORMATION

The role of adsorbed alginate on the aggregation kinetics of colloidal hematite are compared in the presence of various divalent electrolytes through dynamic light scattering. The coated colloids undergo a conventional aggregation mechanism controlled by electrostatic interactions in the presence of magnesium and monovalent cations such as sodium. However, the presence of calcium, strontium, and barium cations lead to enhanced aggregate growth through alginate-gel network formation which serves as a backbone for colloidal aggregates. This enhanced aggregation mechanism is further investigated by employing AFM to conduct approach and retraction force measurements between an alginate-coated hematite colloidal probe and alginate-coated hematite crystal substrate in the presence of different electrolytes. The approach and retraction force analysis provides information on the interactions between the aggregating alginate-coated colloids and the structure of the aggregates formed. Preliminary results show that an increase in the concentration of monovalent electrolyte reduces the observed decay length, which indicates compression of the alginate layers. This is consistent with our finding that the hydrodynamic radius of the stable coated nanoparticles decreases with an increase in the concentration of monovalent electrolyte due to the collapse of the adsorbed alginate macromolecules at the mineral/water interface. Longer-ranged interaction forces as well as multiple discontinuities in the retraction force profiles are noted when the colloidal probe is retracted from the substrate surface in the presence of calcium, strontium, and barium cations, indicating the “pulling-off” of several alginate polymer linkages between the probe and substrate.