KINETICS OF LOCALIZED ADSORPTION OF COLLOID PARTICLES

Methods of analyzing localized adsorption of colloid particles at soli d/liquid interfaces were extensively reviewed. First, the initial adso rption fluxes calculated using the Levich-Smoluchowski approximation w ere discussed. The uniformly, and nonuniformly accessible interfaces w ere distinguished and the superiority of the former in experimental st udies was pointed out. A criterion was introduced for estimating the r elative significance of the bulk transfer and surface adsorption steps . It was shown that for the majority of experimental and practical sit uations the surface mass balance equation can be decoupled from the bu lk continuity equation. Thus, in due course attention was focused on t heoretical and experimental methods of determining the surface blockin g parameter B. It was shown that for low and moderate surface concentr ation range the statistical mechanic approach can be effectively used for predicting B. By introducing the equivalent hard sphere radius r it became possible to analyze quantitatively blocking effects of inter acting as well as nonspherical particles. The analytical solutions wer e compared with numerical simulation methods valid for the entire rang e of surface concentrations. The Monte-Carlo algorithm based on the ra ndom sequential adsorption (RSA) concept was compared with the sequent ial Brownian-Dynamics (SBD) method. Theoretical results obtained using these approaches were extensively discussed especially the role of re pulsive electrostatic interaction among adsorbing particles. It was sh own that these interactions diminish profoundly both the particle adso rption rate and the maximum surface concentration of particles forming ''random'' monolayers. When the electrostatic forces were operating ( lower ionic strength) two distinctive adsorption regimes were predicte d (i) fast Langmuir-type adsorption for short times and then (ii) very slow RSA-type approach to the maximum surface concentrations. As disc ussed such long lasting transient adsorption states could erroneously be interpreted as equilibrium adsorption isotherms. Then, the indirect and direct experimental methods aimed at a quantitative determination of particle adsorption kinetics were described. Illustrative experime ntal results performed for model latex suspensions were evoked. A sati sfactory agreement with theoretical predictions was found for a variet y of important physicochemical parameters studied. The RSA approach wa s found useful for describing particle adsorption kinetics for low and moderate surface concentrations in the case when the flow induced eff ects could be neglected. On the other hand, the SBD method was found o f general validity especially in describing the hydrodynamic scatterin g effect observed experimentally for higher shear rates. Finally, the theoretical and experimental results concerning structure formation in adsorption processes were presented. The experimentally measured two- dimensional (2D) pair correlation function g12 of adsorbed particles s uggested a liquid-like short range ordering occurring for larger surfa ce concentrations. The extent of the 2D ordering was influenced by the adsorption mechanisms of particles, especially the presence of extern al field of forces.