Addition of particles of alternating charge

Abstract

The maximum attainable coverage of a given type of particle on a smooth planar surface is limited by the shape and size of the particles, and by their electrostatic charge. By alternating the adsorption of one type of particle with another of opposite charge, a composite layer is formed in which the jamming limit is exceeded. The phenomenon was investigated experimentally using negatively charged silica particles and positively charged iron(III) hydroxide, dispersed in water. Silica is not adsorbed at all on a bare silica–titania surface but, after depositing iron(III) hydroxide particles to the jamming limit, a jammed layer of silica may be subsequently deposited, upon which more iron(III) hydroxide may be deposited. The sequence cannot be continued adlib, however, because at each step progressively less material is deposited. Optical waveguide lightmode spectroscopy (OWLS) was used to monitor the precise number of particles adsorbed at each step, and the corresponding layer thickness. Analysis of the results revealed that true multilayers are not formed, because of incomplete charge reversal at each step. The adsorbed silica provides a fresh surface on which the iron(III) hydroxide can be adsorbed, but the amount of silica is limited by interparticle repulsion and, as the interfacial region becomes progressively more basic due to the presence of the iron(III) hydroxide, the silica becomes more and more negatively charged, interparticle repulsion increases, and its jamming limit ineluctably decreases.