Adsorption of polyelectrolytes on colloidal particles -: Electrostatic interactions and stability behaviour

Adsorption isotherms of neutral poly(ethyleneoxide) (PEO), cationic poly-L-lysine (PLL) and homo- and copolymers of diallyl-dimethyl-ammoniumchloride (DADMAC) and N-methyl-N-vinyl-acetamide (NMVA) on colloidal silica and latex particles were determined from the concentration in the supernatant solution. Layer thickness and flocculation of the particles are measured by photon correlation spectrometry. Adsorbed amount, layer thickness and stability of the suspensions are influenced by ionic strength and pH, molar:mass and charge densities of the polymers and of the surface. Correlations between flocculation and adsorption parameters are accomplished. With neutral PEO the stability is determined by the layer thickness adjusted by molar mass and coverage; thicknesses greater than or equal to 4 nm stabilize up to high electrolyte concentrations. With high positively charged PE layers (pH less than or equal to 7) stable latex suspensions are guaranteed and large electrokinetic influences on the diffusion coefficient depending on the PE/electrolyte ratio are observed. With increasing ionic strength, latex covered with PE layers of low molar mass flocculates, while for that covered with PE layers of high molar mass stabilization occurs. Adsorption of PDADMAC and P(DADMAC-co-NMVA) result in spontaneous flocculation if a certain amount of PE depending on the charge density according to a definite coverage is added. Flocculation at increasing ionic strength can be controlled by the adsorption of PDADMAC depending on the molar mass. At full coverage the suspensions are stabilized if the thickness of the layer is large enough, dependent on ionic strength, charge density and molar mass of the PE.