Abstract
The polyethylenimine (PEI)-water-Ludox AM colloidal silica system was used as a model system to investigate the relationship of charge density and molecular weight of the polyelectrolyte on the adsorption and subsequent flocculation of oppositely charged colloidal particles. Narrow molecular weight fractions of PEI ranging from 1760 to 18,400 were prepared and characterized. Differences in polymer molecular weight, salt concentration, and pH were employed to vary the number of cationic charges per PEI molecule. At the concentrations of PEI required to initiate flocculation of Ludox AM, complete adsorption of PEI occurred regardless of molecular weight, pH, or ionic strength. However, at higher concentrations, adsorption increased with molecular weight, pH, and ionic strength. The flocculation results indicated that the relative importance between polymer bridging and electrostatic considerations is pH dependent. At pH 9 and greater, polymer bridging is the dominant mechanism of destabilization due to the low cationic charge of the PEI molecule. Howevei, at pH less than 9, charge interaction between PEI and colloidal silica is the dominant consideration. An inverse linear relationship was found between the logarithm of the molar concentration of PEI required to initiate flocculation and the logarithm of the cationic charge on the PEI molecule.
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