Irreversible adsorption of colloid particles at heterogeneous surfaces

Irreversible adsorption of colloid particles at heterogeneous surfaces was studied theoretically and experimentally. The substrate surface of controlled heterogeneity was created by covering a homogeneous surface by adsorption sites (active centers) of a desired concentration. The centers were modeled as spheres smaller in size than the adsorbing particles. Adsorption was assumed to occur due to short-ranged attractive interactions if the colloid particle contacted the center. Extensive Monte-Carlo type simulations were performed. This allowed one to determine the initial flux, adsorption kinetics, jamming coverage and the structure of the particle monolayer as a function of the site coverage Θs and the particle/site size ratio, λ. It was revealed that the initial flux increased significantly with Θs and the λ parameter. This behavior was quantitatively interpreted in terms of the scaled particle theory. It also was demonstrated that particle adsorption kinetics and the jamming coverage increased abruptly, at fixed site coverage, with λ, attaining for λ2Θs>>1 the value pertinent to continuous surfaces. The theoretical predictions were tested experimentally using monodisperse colloid particles (negatively charged polystyrene latex) and the direct microscope observation method. The heterogeneous surface was produced by covering mica by positively charged latex particles of smaller dimension. Particle adsorption kinetics was determined as a function of site (small particle) coverage. It was demonstrated, in accordance with theory, that the initial flux attains the limiting value pertinent to homogeneous surfaces for a site coverage of a few percent. In this case adsorption kinetics was well reflected by the random sequential adsorption (RSA) model.