Electro-optics of colloids coated with multilayers from strong polyelectrolytes:surface charge relaxation

The electrical properties and structure of polyelectrolyte multilayers prepared by layer-by-layer deposition of poly(sodium 4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMAC) on ellipsoidal beta-ferric hydrous oxide particles are examined in situ using the electric light scattering method. In the initial nonlinear growth regime, the electro-optical effect is found to increase almost in the same manner as the thickness of the multilayer film. This reveals an increase in the multilayer surface charge due to the nonlinear increase in polyelectrolyte adsorption. Above five deposition cycles, the electro-optical effect reaches a plateau and the thickness increment per layer becomes constant, indicating regular film growth. Evidence is given that the concentration of small residual ions incorporated into the PSS/PDADMAC film is negligible. Polarization of counterions, bound near to the surface of the last-adsorbed polyion, is assumed to explain the relaxation of the electro-optical effect in the frequency range 10(3)-10(5) Hz.