Irreversible adsorption of particles at random-site surfaces

Irreversible adsorption of negatively charged polystyrene latex particles (averaged diameter 0.9 microm) at heterogeneous surfaces was studied experimentally. The substrate bearing a controlled number of adsorption sites was produced by precovering mica sheets by positively charged polystyrene latex (averaged diameter of 0.45 microm). Positive latex (site) deposition was carried out under diffusion-controlled transport conditions and its coverage was determined by direct particle counting using the optical microscopy. Deposition kinetics of larger latex particles (averaged diameter 0.9 microm) at heterogeneous surfaces produced in this way was studied by direct optical microscope observations in the diffusion cell (under no-convection transport conditions). It was demonstrated that the structure of larger particle monolayers, characterized in terms of the pair correlation function, showed much more short-range ordering than it was predicted for homogeneous surface monolayers at the same coverage. This was found in agreement with theoretical predictions derived from the Monte Carlo simulations. On the other hand, particle adsorption kinetics was quantitatively interpreted in terms of numerical solutions of the governing diffusion equation with the nonlinear boundary condition derived from Monte Carlo simulations. From these kinetic measurements maximum (jamming) coverage of particles was determined in an accurate way by extrapolation. It was concluded that both the monolayer structure and jamming coverage were strongly influenced by the site multiplicity (coordination) effect.     

Deposition of colloid particles on protein layers: fibrinogen on mica

Colloid particle deposition was applied to characterize fibrinogen (Fb) monolayers on mica, which were produced by controlled adsorption under diffusion transport. By adjusting the time of adsorption and the bulk Fb concentration, monolayers of desired surface concentration were obtained. The surface concentration of Fb was determined directly by AFM enumeration of single molecules adsorbed over the substrate surface. It was proven that Fb adsorbed irreversibly on mica both at pH 3.5 and at pH 7.4 with the rate governed by bulk transport. The electrokinetic properties of Fb monolayers produced in this way were studied using the streaming potential method. The dependence of the apparent zeta potential of Fb monolayers was determined as a function of the coverage. It was shown that for pH 3.5 the initial negative zeta potential of the mica substrate was converted to positive for Fb coverage exceeding 0.16. On the other hand, for pH 7.4, the zeta potential of a Fb-covered mica remained negative for the entire coverage range. The charge distribution in Fb monolayers was additionally studied using the colloid deposition method, in which negatively and positively charged polystyrene latex particles (ca. 800 nm in diameter) were used. An anomalous deposition of negative latex particles on substrates exhibiting a negative zeta potential was observed. Results of these experiments were quantitatively interpreted in terms of the fluctuation theory assuming that adsorption sites consisted of two and three Fb molecules, for pH 3.5 and 7.4, respectively. These results suggested that for pH 7.4, the distribution of charge on Fb molecules was heterogeneous, characterized by the presence of positive patches, whereas the average zeta potential was negative, equal to -19 mV. The utility of the colloid deposition method for studying Fb monolayers was further demonstrated in deposition experiments involving positive latex particles. It was shown that for a rather broad range of fibrinogen coverage, both the positive and the negative latex particles can adsorb on surfaces covered by Fb, which behaved, therefore, as superadsorbing surfaces. It was also concluded that the colloid deposition method can be used to determine the Fb bulk concentration for the range inaccessible for other methods.     

Irreversible adsorption of particles on heterogeneous surfaces

Methods of theoretical and experimental evaluation of irreversible adsorption of particles, e.g., colloids and globular proteins at heterogeneous surfaces were reviewed. The theoretical models were based on the generalized random sequential adsorption (RSA) approach. Within the scope of these models, localized adsorption of particles occurring as a result of short-ranged attractive interactions with discrete adsorption sites was analyzed. Monte-Carlo type simulations performed according to this model enabled one to determine the initial flux, adsorption kinetics, jamming coverage and the structure of the particle monolayer as a function of the site coverage and the particle/site size ratio, denoted by lambda. It was revealed that the initial flux increased significantly with the site coverage theta(s) and the lambda 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 significantly, at fixed site coverage, when the lambda parameter increased. Practically, for alpha = lambda2theta(s) > 1 the jamming coverage at the heterogeneous surfaces attained the value pertinent to continuous surfaces. The results obtained prove unequivocally that spherically shaped sites were more efficient in binding particles in comparison with disk-shaped sites. It also was predicted that for particle size ratio lambda < 4 the site multiplicity effect plays a dominant role, affecting significantly the structure of particle monolayers and the jamming coverage. Experimental results validating main aspects of these theoretical predictions also have been reviewed. These results were derived by using monodisperse latex particles adsorbing on substrates produced by covering uniform surface by adsorption sites of a desired size, coverage and surface charge. Particle deposition occurred under diffusion-controlled transport conditions and their coverage was evaluated by direct particle counting using the optical and electron microscopy. Adsorption kinetics was quantitatively interpreted in terms of numerical solutions of the governing diffusion equation with the non-linear boundary condition derived from Monte-Carlo simulations. It was proven that for site coverage as low as a few percent the initial flux at heterogeneous surfaces attained the maximum value pertinent to homogeneous surfaces. It also was demonstrated that the structure of larger particle monolayers, characterized in terms of the pair correlation function, showed much more short-range ordering than predicted for homogeneous surface monolayers at the same coverage. The last part of this review was devoted to detection of polyelectrolyte multilayers on various substrates via particle deposition experiments.     

Brownian Dynamics Simulation of Film Formation of Mixed Polymer Latex in the Water Evaporation Stage

Brownian dynamics simulations of the filming process of a mixed polymer latex in the water evaporation stage were performed in order to explore the effect of surface potential on latex particle packing and distribution at a temperature far below the glass transitions of polymers in bulk. Polymer latex particles are modeled as spheres that interact via DLVO potential with various surface charge densities for emulsifier-free emulsion polymerized particles and dispersion polymerized particles. It is found that the distribution of modeled poly(methyl methacrylate) and polystyrene latex particles in the finally formed film exhibits a noticeable dependence of surface potentials of latex particles. When the difference of the surface potentials between binary mixed latex particles is small, the particles distribute randomly. In contrast, when the difference of the surface potentials between binary mixed latex particles is large, heterocoagulation occurs and the polymer latex in which the repulsive electrostatic potential is weak will form clusters in the film. The results are in agreement with laser confocal fluorescence microscopy observations of fluorescent dye labeled poly(methyl methacrylate) and polystyrene mixed latex films. The correlation between latex particles increases with increasing repulsive electrostatic potential, and the spatial order can be obtained at the end of the water evaporation stage. Copyright 2000 Academic Press.     

Breakdown of colloid filtration theory: role of the secondary energy minimum and surface charge heterogeneities

The mechanisms and causes of deviation from the classical colloid filtration theory (CFT) in the presence of repulsive Derjaguin-Landau-Verwey-Overbeek (DLVO) interactions were investigated. The deposition behavior of uniform polystyrene latex colloids in columns packed with spherical soda-lime glass beads was systematically examined over a broad range of physicochemical conditions, whereby both the fluid-phase effluent particle concentration and the profile of retained particles were measured. Experiments conducted with three different-sized particles in a simple (1:1) electrolyte solution reveal the controlling influence of secondary minimum deposition on the deviation from CFT. In a second series of experiments, sodium dodecyl sulfate (SDS) was added to the background electrolyte solution with the intent of masking near-neutrally charged regions of particle and collector surfaces. These results indicate that the addition of a small amount of anionic surfactant is sufficient to reduce the influence of certain surface charge inhomogeneities on the deviation from CFT. To verify the validity of CFT in the absence of surface charge heterogeneities, a third set of experiments was conducted using solutions of high pH to mask the influence of metal oxide impurities on glass bead surfaces. The results demonstrate that both secondary minimum deposition and surface charge heterogeneities contribute significantly to the deviation from CFT generally observed in colloid deposition studies. It is further shown that agreement with CFT is obtained even in the presence of an energy barrier (i.e., repulsive colloidal interactions), suggesting that it is not the general existence of repulsive conditions which causes deviation but rather the combined occurrence of "fast" and "slow" particle deposition.     

Polyelectrolyte adsorption layers studied by streaming potential and particle deposition

Adsorption of a cationic polyelectrolyte, polyallylamine hydrochloride (PAH), having a molecular weight of 70,000 on mica was characterized by the streaming potential method and by deposition of negative polystyrene latex particles. Formation of PAH layers was followed by determining the apparent zeta potential of surface zeta as function of bulk PAH concentration. The zeta potential was calculated from the streaming potential measured in the parallel-plate channel formed by two mica plates precovered by the polyelectrolyte. The experimental data were expressed as the dependence of the reduced zeta potential zeta/zeta0 on the PAH coverage Theta(PAH), calculated using the convective diffusion theory. It was found that for the ionic strength of 10(-2) M, the dependence of zeta/zeta0 on Theta(PAH) can be reflected by the theoretical model formulated previously for surfaces covered by colloid particles. The electrokinetic measurements were complemented by particle deposition experiments on PAH-covered mica surfaces. A direct correlation between the polymer coverage and the initial deposition rate of particles, as well as the jamming coverage, was found. For ThetaPAH > 0.3 the initial deposition rate attained the value predicted from the convective diffusion theory for homogeneous surfaces. The initial deposition rates for surfaces modified by PAH were compared with previous experimental and theoretical results obtained for heterogeneous surfaces formed by preadsorption of colloid particles. It was revealed that negative latex deposition occurred at surfaces exhibiting negative apparent zeta potential, which explained the anomalous deposition of particles observed in previous works. It was suggested that the combined electrokinetic and particle deposition methods can be used for detecting adsorbed polyelectrolytes at surfaces for coverage range of a percent. This enables one to measure bulk polyelectrolyte concentrations at the level of 0.05 ppm.     

Colloid particle adsorption at random site (heterogeneous) surfaces

Irreversible adsorption of colloid particles and globular proteins at heterogeneous surfaces was studied theoretically. The substrate surface was created by covering a uniform surface by coupling sites (active centers) of a desired coverage. In contrast to previous studies concerned with disks, in our simulations the centers were modeled by spheres having a size smaller than that of the adsorbing particles. Adsorption was assumed to occur due to short-ranged attractive interactions if the colloid particle contacted the center. The Monte-Carlo-type simulations enabled one to determine the initial flux, adsorption kinetics, jamming coverage, and the structure of the particle monolayer as a function of the site coverage and the particle/site size ratio, denoted by lambda. It was revealed that the initial flux increased significantly with the site coverage theta(s) and the lambda 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 significantly, at fixed site coverage, when the lambda parameter increased. Practically, for alpha=lambda(2)theta(s)>1 the jamming coverage at the heterogeneous surfaces attained the value pertinent to continuous surfaces. The results obtained prove unequivocally that the spherically shaped sites are much more effective in binding particles than the disk-shaped sites considered previously.     

Is latex surface charge an important parameter for foam stabilization?

We describe the facile production of highly stable foams stabilized solely by cationic polystyrene latex particles. Three model polystyrene latexes were synthesized using either a cationic 2,2'-azobis(2-diisobutyramidine) dihydrochloride (AIBA) or an anionic ammonium persulfate (APS) radical initiator: a 724 +/- 81 nm charge-stabilized cationic polystyrene latex [AIBA-PS], an 800 +/- 138 nm sterically stabilized cationic latex prepared using a poly(ethylene glycol) monomethacrylate macromonomer [PEGMA-AIBA-PS], and a 904 +/- 131 nm charge-stabilized anionic polystyrene latex [APS-PS], respectively. The effect of particle surface charge, latex concentration, and solution pH on foam stability was studied in detail. The PEGMA-AIBA-PS latex proved to be the best foam stabilizer even at relatively low latex concentrations (3.0 wt %), with long-term foam stabilities being obtained after drying. The AIBA-PS latex also produced stable foams, albeit only at higher latex concentrations. However, the APS-PS latex proved to be an ineffective foam stabilizer. This is believed to be primarily due to the anionic surface character of this latter latex, which prevents its adsorption at the anionic air-water interface. This hypothesis is supported by the observation that the AIBA-PS latex no longer acts as an effective foam stabilizer above its isoelectric point (pH 7.04). Scanning electron microscopy studies revealed the formation of well-defined latex bilayers within dried foams, which indicates that the wet air bubbles are stabilized by latex monolayers prior to drying. However, little or no long-range ordering of the latex particles was observed on the surface of the bubbles, which is presumably related to the latex polydispersity.     

Charge localization in multiply charged clusters and their electrical properties: some insights into electrospray droplets

The surface composition of charged Lennard-Jones clusters A(N) (n+), composed of N particles (55 ≤ N ≤ 1169) among which n are positively charged with charge q, thus having a net total charge Q = nq, is investigated by Monte Carlo with Parallel Tempering simulations. At finite temperature, the surface sites of these charged clusters are found to be preferentially occupied by charged particles carrying large charges, due to Coulombic repulsions, but the full occupancy of surface sites is rarely achieved for clusters below the stability limit defined in this work. Large clusters (N = 1169) follow the same trends, with a smaller propensity for positive particles to occupy the cluster surface at non-zero temperature. We show that these charged clusters rather behave as electrical spherical conductors for the smaller sizes (N ≤ 147) but as spheres uniformly charged in their volume for the larger sizes (N = 1169).     

Controlling deposition and release of polyol-stabilized latex on boronic acid-derivatized cellulose

Poly(glycerol monomethacrylate)-stabilized polystyrene (PGMA-PS) latex particles undergo specific, pH-dependent adsorption onto regenerated cellulose film bearing surface phenylboronic acid groups (cellulose-PBA). Deposition occurs at pH 10 and is driven by the boronate ester formation with the polyol latex surface coating. In contrast, no deposition occurs at pH 4, and previously deposited particles can be readily desorbed at this lower pH. In control experiments, conventional anionic sulfate-stabilized polystyrene latex did not deposit onto the hydrophilic cellulose surface. The distribution of boronate groups in the cellulose was determined by exposure to Alizarin Red S dye, which forms a fluorescent complex with phenylboronic acid; confocal microscopy was used to determine a surface density of 3 nm(2) per boronic acid group on the cellulose surface. Although the boronic acid binding constant with PGMA is relatively low (5.4 L/mol), the cooperative interactions between multiple PBA surface sites and the many PGMA chains per latex particle are sufficient to induce specific latex adsorption, providing a convenient new tool for controlling nanoparticle deposition on surfaces.     

Irreversible adsorption of latex particles on fibrinogen covered mica

Physicochemical properties of bovine plasma fibrinogen (Fb) in electrolyte solutions were characterized. These comprised the diffusion coefficient (hydrodynamic radius), determined by the DLS method, electrophoretic mobility and the isoelectric point. The hydrodynamic radius of Fb was 12 nm for pH<5. The number of uncompensated (electrokinetic) charges on the protein N _c was calculated from the electrophoretic mobility data. It was found that for pH<5.8 the electrokinetic charge was positive, independently of the ionic strength and negative for pH>5.8. For pH=3.5 the value of N _c , was 26 for 10^?3 M. Similar electrokinetic measurements were performed for the mica substrate using the streaming potential cell. It was shown that for pH=3.5 and 10^?3 M, the zeta potential of mica remained negative (?50 mV). This promoted an irreversible, electrostatically driven adsorption of Fb, which was confirmed in experiments carried out under diffusion-controlled transport. The surface concentration of Fb on mica was determined directly by AFM counting. By adjusting the time of adsorption, Fb monolayers of desired coverage were produced. Independently, the presence of Fb on mica was determined quantitatively by the colloid enhancement method, in which negatively charged latex particles were used, having the diameter of 800 nm. It was found that for Fb coverage below 0.05 the method was more sensitive than other indirect methods. The experimental data obtained in latex deposition experiments were adequately interpreted in terms of the random site model used previously for polyelectrolytes. It was shown that adsorption sites consisted of a cluster of two Fb molecules. It was concluded that the colloid enhancement method can be successfully used for detecting the presence of proteins at solid substrates and to determine the uniformity of monolayers in the nanoscale.     

Blocking and ripening of colloids in porous media and their implications for bacterial transport

A model accounting for the dynamics of colloid deposition in porous media was developed and applied to systems containing similarly charged particles and collectors. Colloid breakthrough and intracolumn retention data confirmed that blocking reduced overall colloidal adhesion to soil. The surface coverage at which blocking occurred varied for the type of colloid, as shown by changes in the clean-bed collision efficiency, _0, and the excluded area parameter, β. Excluded area parameters were relatively high due to unfavorable interactions between particles and collectors, and ranged from 11.5 for one bacterium (Pseudomonas putida KT2442) to 13.7 and 24.1 for carboxylated latex microspheres with differing degrees of charged groups on their surfaces. Differences in β values for the three colloids were correlated with electrophoretic mobility, with the most negatively charged colloid (carboxylated latex; CL microspheres) having the highest β. No correlation between hydrophobicity and ₀ or β was found. Besides using colloidal particles capable of blocking, the addition of chemical additives to the soil has been suggested as a means for reducing attachment of colloids to porous media. Dextran addition caused an order-of-magnitude reduction in the overall (for carboxylated-modified latex; CMLs). This reduction was not attributed to blocking, but to the sorption of dextran to the soil which lowered ₀. The filtration-based numerical model used to fit the ₀ and β parameters was used to demonstrate that blocking could result in significantly enhanced bacterial transport in field situations.     

Polystyrene latex adsorption at the gold/electrolyte interface

Irreversible deposition of polystyrene latex particles (average diameter, 1.5 microm) on various solid/electrolyte interfaces was studied experimentally by using the direct microscope observation method. The substrate surfaces included bare mica (reference interface), gold covered mica (layer thickness of 50 nm), and solid gold plate. The morphology and thickness of the gold layer on mica was determined by atomic force microscopy. Well-defined transport conditions of particles were created by using the new impinging-jet cell. A characteristic feature of the cell was that the suspension stream was directed obliquely to the interface. This unique characteristic was advantageous allowing one for direct, in situ, observation of particle deposition at metals and other nontransparent interfaces. Experiments performed for various flow intensities indicated that the initial deposition kinetics at all interfaces was identical within the error bounds, in accordance with the model based on the convective-diffusion theory. It was concluded that the limiting flux was governed by the bulk transport rather than by the specific surface interactions.     

Experimental phase diagram of symmetric binary colloidal mixtures with opposite charges

The phase behavior of equimolar mixtures of oppositely charged colloidal systems with similar absolute charges is studied experimentally as a function of the salt concentration in the system and the colloid volume fraction. As the salt concentration increases, fluids of irreversible clusters, gels, liquid-gas coexistence, and finally, homogeneous fluids, are observed. Previous simulations of similar mixtures of Derjaguin-Landau-Verwey-Overbeek (DLVO) particles indeed showed the transition from homogeneous fluids to liquid-gas separation, but also predicted a reentrant fluid phase at low salt concentrations, which is not found in the experiments. Possibly, the fluid of clusters could be caused by a nonergodicity transition responsible for the gel phase in the reentrant fluid phase. Liquid-gas separation takes a delay time after the sample is prepared, whereas gels collapse from the beginning. The density of the liquid in coexistence with a vapor phase depends linearly on the overall colloid density of the system. The vapor, on the other hand, is comprised of equilibrium clusters, as expected from the simulations.     

Synthesis and properties of latex particles with polyaniline shells

Core–shell polyaniline–latex particles have been obtained via oxidative polymerization of aniline hydrochloride in the presence of charged polystyrene latex. The polymerization conditions have been determined under which polyaniline is predominantly formed on the surface of latex particles to yield closed shells. The dependence of the electrical conductivity of the resulting particles on the initial concentration of aniline hydrochloride in the polymerization medium has been studied. Stable aqueous dispersions of polyaniline–latex particles have been prepared by modifying the particles with sodium 3-mercaptopropane sulfonate.     

SANS and SAXS analysis of charged nanoparticle adsorption at oil-water interfaces

A systematic study of the adsorption of charged nanoparticles at dispersed oil-in-water emulsion interfaces is presented. The interaction potentials for negatively charged hexadecane droplets with anionic polystyrene latex particles or cationic gold particles are calculated using DLVO theory. Calculations demonstrate that increased ionic strength decreases the decay length of the electrostatic repulsion leading to enhanced particle adsorption. For the case of anionic PS latex particles, the energy barrier for particle adsorption is also reduced when the surface charge is neutralized through changes in pH. Complementary small-angle scattering experiments show that the highest particle adsorption for PS latex occurs at moderate ionic strength and low pH. For cationic gold particles, simple DLVO calculations also explain scattering results showing that the highest particle adsorption occurs at neutral pH due to the electrostatic attraction between oppositely charged surfaces. This work demonstrates that surface charges of particles and oil droplets are critical parameters to consider when engineering particle-stabilized emulsions.     

Polystyrene particles with proton-ionisable groups: pH-dependent stability of latices with weakly acidic groups

 Results of colloid chemical characterisation and stability measurements on electrostatically stabilised latex dispersions made from emulsions of styrene and 4,4′-azobis-(4-cyanovaleric acid) are reported. The deviant stability of the hydrophobic polystyrene particles at low pH and low ionic strength is related to a proton “tunable” hydration layer surrounding weakly charged particles. The idea implies the formation of a polymer-supported surface phase that does not have any clear boundary, either towards the polymer moiety or in the direction of the bulk solution. The formation of the surface phase is controlled by Coulombic, hydrophobic and van der Waals interactions and by the contribution from the water structure at the hydrophobic and hydrophilic domain of the polymer particles. Negative charges on the hydrophobic surface badly interfere with the water structure at the hydrophobic moiety of the particle, whereas positive or uncharged surface groups do not damage the balance of free and clustered water molecules at the interface. Because the hydrophobic nature of the surface changes with the degree of dissociation of the surface charges, the degree of hydrophobicity of the carboxylic latices can be adjusted by changing the pH; therefore, it may be concluded that the hydrated and discharged carboxylic particle is apparently more hydrophobic relative to the ionised one. Thus, our concept can also explain differences in the hydrophobicity of colloidal polymer particles. Received: 12 June 1999/Accepted in revised form: 24 September 1999     

Latex and hollow particles of reactive polypyrrole: preparation, properties, and decoration by gold nanospheres

Polypyrrole-coated polystyrene latex particles bearing reactive N-amino functional groups (PS-PPyNH2) were prepared by the in-situ copolymerization of pyrrole (Py) and the active amino-functionalized pyrrole (PyNH2) in the presence of 1.33 microm-diameter polystyrene (PS) latex particles. These particles were prepared by dispersion polymerization of styrene using poly(N-vinylpyrrolidone), PNVP, as a steric stabilizer. The functionalized polypyrrole-coated PS particles (PS-PPyNH2) were characterized in terms of their particle size and surface morphology using transmission electron microscopy (TEM). Infrared and X-ray photoelectron spectroscopy (XPS) detected pyrrole-NH2 repeat units at the surface of the latex particles, indicating that this monomer had indeed copolymerized with pyrrole. The core-shell structure of the PS-PPyNH2 particles was confirmed by etching the polystyrene core in THF, leading to the formation of hollow conducting polymer capsules. The PS-PPyNH2 particles were then decorated with citrate-stabilized gold nanoparticles via electrostatic interactions. Furthermore, etching of the polystyrene core resulted in the formation of gold-decorated PPyNH2 hollow capsules.     

Adsorption of Bovine Serum Albumin onto Polystyrene Latex Particles Bearing Saccharidic Moieties

The adsorption of bovine serum albumin (BSA) onto polystyrene latexes bearing various amounts of sugar moieties has been investigated as a function of pH and ionic strength and the results were compared to those for bare polystyrene latexes having negative surface charges. The functionalized latexes were produced by seeded copolymerization of (0.3 μm) liposaccharidic monomer onto polystyrene particles obtained by soap-free emulsion polymerization of styrene using potassium persulfate as initiator. At first, the electrophoretic mobility behavior of the various latexes was examined as a function of pH: a significant decrease was observed in the case of saccharide-containing latex particles compared to the bare particles. The adsorption of BSA onto these latexes exhibited a reduced amount of adsorbed BSA for those latex particles bearing saccharide groups. This adsorbed amount depends on the yield of saccharidic monomer incorporated onto the surfaces of the latex particles.     

Coupled Influence of Colloidal and Hydrodynamic Interactions on the RSA Dynamic Blocking Function for Particle Deposition onto Packed Spherical Collectors

The influence of electrostatic double-layer and hydrodynamic interactions on random sequential adsorption (RSA) of colloidal particles onto packed spherical collectors was investigated using inverse analysis of colloid breakthrough data obtained from well-controlled particle deposition experiments. Deposition experiments were carried out using monodisperse aqueous suspensions of positively charged latex colloids and packed columns of negatively charged uniform glass beads for different combinations of ionic strength, particle size, and approach velocity. From the experimental particle breakthrough data, the initial particle deposition rates and the virial coefficients of the dynamic blocking function based on RSA mechanics were determined. The magnitudes of the virial coefficients were observed to increase from the hard sphere values with increasing flow rates and decreasing ionic strengths of the background electrolyte. Particle size also plays a significant role in governing the deposition dynamics. The deviation from the hard sphere RSA behavior becomes more prominent for larger particles. Copyright 2000 Academic Press.