Effect of ionic strength and molecular weight on electrical properties and thickness of polyelectrolyte bi-layers

Electric light scattering from aqueous suspensions of positively charged ferric oxide particles is investigated in the presence of poly(sodium styrene sulfonate) and poly(allylamine hydrochloride) of different molecular weights, adsorbed sequentially on the particle surface at different ionic strengths. The adsorption is realized over a wide range of NaCl concentration (10−4 to 10−1 M) at pH 4.5 and the suspensions of coated particles are then rinsed to ionic strength of 10−4 M. Our measurements indicate that the amplitude of the electro-optical effect increases with increasing concentration of NaCl from 10−4 to 10−2 M due to the increased amount of polymers, adsorbing in more coiled conformation and on more highly charged particle surface. However, the electro-optical effect started to decrease at further increase of NaCl concentration, more expressively at higher field intensities and for bi-layers with first layer from polyelectrolyte of lower molecular weight. This is accompanied with a significant increase in the thickness of the deposited bi-layers after their rinsing to low ionic strength. We attribute both effects to partial desorption of polyelectrolyte complexes from the particle surface, which disturb the homogeneity of the bi-layer structure. The latter causes an increase in the amount of mechanically trapped water on the particle surface and the thickness of adsorbed bi-layers increases as well. As a result, an elasto-hydrodynamic effect, which favors perpendicular orientation with respect to the external electric field, becomes possible.