Barbara Siwek

Structure and ordering in localized adsorption of particles

Abstract Localized, sequential adsorption of colloid particles interacting via screened Coulomb potential was analyzed theoretically and experimentally. The two-dimensional (2D) pair correlation functions were simulated by using the Monte Carlo technique for various surface concentrations θ and for various screening length parameters κα, characterizing the “softness” of the particle-particle interaction potential. For κα ⪢ 1 the hard-disk limiting behavior was confirmed (maximum surface concentration θmx = 55%). The θmx for soft disks, which is generally much smaller than the above value, was also determined as a function of the κα parameter. A distinctive tendency toward a short-range ordering, analogous to the 3D results obtaining previously for colloid suspensions, was found for surface concentration close to the θmx value. Theoretical predictions were tested by applying the direct experimental method based on microscope observations of particle adsorption. A monodisperse polystyrene suspension was used (particle size 0.90 μm) and the adsorbing surface was made of mica sheets. Experimental results proved to be in good agreement with Monte Carlo simulations illustrating well the tendency towards structurization (2D quasi-liquid phase formation) for surface concentrations close to the predicted θmx values.

Kinetics of latex particle deposition from flowing suspensions

Abstract The kinetics of deposition of spherical latex particles onto smooth surfaces from suspensions undergoing stagnation point flow has been determined experimentally using a direct observation technique. (T. D o broś and T. G. M. Van de Ven, Colloid Polym. Sci. 261, 694, 1983). The barrierless deposition conditions have been achieved by adding a surfactant (CTAB) which at the concentration of 10−5 M produced a positive charge on latex particles, whereas the collector surface (glass) remained negatively charged. The influence of particle size, varying between 0.5 and 4 μm, suspension concentration, and flow intensity on the rate of particle transfer has been systematically studied. A characteristic feature of the results obtained is a minimum deposition rate for particle size around 1 μm. The experimental results have been interpreted in terms of the convective mass transfer theory (Z. Adamczyk, T. D o broś, J. Czarnecki, and T. G. M. Van de Ven, Adv. Colloid Interface Sci. 19, 183, 1983; T. D o broś, Z. Adamczyk, and J. Czarnecki, J. Colloid Interface Sci. 62, 529, 1977; Z. Adamczyk, and T. G. M. Van de Ven, J. Colloid Interface Sci. 89, 497, 1981) and a quantitative agreement within error bounds has been found for all particle sizes and flow rates investigated. This proves also that the numerical analysis of the flow field near the stagnation point is correct.

Enhanced deposition of particles under attractive double-layer forces

Abstract The kinetics of Brownian particle deposition from flowing suspensions in the vicinity of a stagnation point (rotating disk, spherical, cylindrical, and impinging jet collectors) was studied theoretically and experimentally. On the basis of the numerical solutions of governing transport equations a considerable enhancement of particle deposition was predicted for particle and collectors bearing opposite surface charge (zeta potentials). The initial deposition rate of particles can be increased by many times when decreasing the ionic strength of the suspension to the value 10 −5 –10 −6 M . This effect, which we shall refer to as the reverse salt effect, is completely opposite to what one commonly observed for equally charged particle and collector surfaces (as well as in coagulation kinetic studies). These theoretical predictions were confirmed quantitatively by the direct microscope observation method using mica collector (modified by chemisorption of silano compounds) and a monodisperse suspension of submicrometer latex particles (negatively charged). The experiments also furnished a direct evidence of the coupling between macroscopic hydrodynamic flow and the microscopic colloid forces and proved that the asymmetric double-layer dynamics cannot be properly described by the constant surface charge boundary conditions.

Kinetics of localized adsorption of particles on homogeneous surfaces

Abstract The kinetics of localized, irreversible adsorption of particles interacting via exponentially decaying repulsive potentials on homogeneous interfaces was analyzed. Asymptotic analytical equations were derived for the surface blocking parameter B (0) and adsorption kinetics in the limit of low and high surface concentrations. In the general case, particle adsorption kinetics was simulated numerically for various values of the characteristic range of the interaction potential. Based on these calculations, a new fitting function was derived describing adsorption kinetics for long times in terms of the maximum surface concentration 0 mx . Theoretical predictions were experimentally tested by applying the direct observation method based on the stagnation point flow cell. Suspensions of monodisperse latex particles, both positively and negatively charged, were used with mica sheets as the adsorbing surfaces. The experiments performed for various bulk particle concentrations and ionic strengths were in a good agreement with theoretical predictions. They confirmed also the validity of the proposed fitting functions for describing adsorption kinetics of interacting particles for the entire range of the surface concentrations.

Reversible and irreversible adsorption of particles on homogeneous surfaces

Abstract The kinetics of localized reversible and irreversible adsorption of interacting particles on homogeneous surfaces was analysed. Asymptotic analytical equations were derived for the surface blocking parameter B(0), and for adsorption kinetics and adsorption isotherms in the limit of low and high surface concentrations. It was found that the geometrical blocking effect was much more pronounced than the Langmuir model predicts, especially for high surface concentrations and low ionic strengths of suspensions. The new adsorption isotherm formulated indicates that for a large adsorption constant, Ka, the equilibrium surface concentration becomes proportional to K−1/3a, whereas in the Langmuir model this quantity is approached as K−1a (for Ka ≫I). In the case of irreversible adsorption the theoretical predictions were experimentally tested by applying the direct microscope observation method. Monodisperse suspensions of negatively charged latex particles were used in these experiments with silanized mica sheets as the adsorbing surface. Our theoretical predictions were quantitatively confirmed, indicating that the Langmuir model is not appropriate for describing localized adsorption of particles on homogeneous surfaces.

Irreversible adsorption of colloid particles at heterogeneous surfaces

Irreversible adsorption of polystyrene latex particles of micrometer size range at heterogeneous surfaces was studied experimentally. Model substrate surfaces of controlled site coverage (heterogeneity degree) used in these studies were produced by preadsorption of positively charged latex particles on mica sheets. Deposition kinetics of latex was studied as a function of the site coverage, particle to site size ratio λ and ionic strength of the colloid suspension. Particle distributions over surfaces and coverage were quantitatively evaluated by the direct microscope observation techniques using the diffusion cell. In this way, pair correlation function for various coverage degree and particle size ratio was evaluated. It also was determined the dependence of the jamming coverage of colloid particles on site coverage and ionic strength of the suspension. It was demonstrated that the decrease in the ionic strength of the suspension resulted in a significant decrease in the jamming coverage. This was attributed to the effect of the electrostatic field generated by the interface whose range was increased for low ionic strength. These experimental data revealed, in accordance with theoretical predictions derived from numerical simulations, that the multiple site coordination exerted a pronounced effect on the jamming coverage and the structure of adsorbed layers. It also was shown that this effect can be regulated by changes in the ionic strength of particle suspensions. This could allow one to produce particle clusters at the surface of targeted composition.

Fluctuations in the number of particles adsorbed under the influence of diffusion and flow

Fluctuations in the number of colloid particles adsorbed irreversibly under diffusion and flow were determined. The experimental measurements were carried out in the impinging‐jet cells using as model colloids the monodisperse polystyrene latex particles of micrometer size range adsorbing at mica sheets. The surface concentration of adsorbed particles was determined quantitatively using the direct microscope observation method coupled with an image analyzing system. Two series of experiments were performed (i) for diffusion controlled adsorption when the random sequential adsorption (RSA) mechanism was valid and (ii) for flow controlled adsorption. It was found that in the case of RSA the reduced variance of the distributions decreased markedly for increasing surface concentration θ in accordance with theoretical predictions based on the mean‐field approximation. The experimental results were in a good agreement with the numerical simulations performed according to the RSA algorithm. It was also determine...

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.

Particle deposition at electrostatically heterogeneous surfaces

Abstract Irreversible adsorption of negatively charged polystyrene latex particles (averaged diameter 0.9 μm) at heterogeneous surfaces was studied experimentally. The substrate of controlled heterogeneity was produced by covering freshly cleaved mica sheets by positively charged polystyrene latex (averaged diameter of 0.47 μm) to a desired surface coverage. Positive latex deposition was carried out under convection and diffusion-controlled transport conditions and the coverage (defined as heterogeneity degree) was determined by direct particle counting using the optical and electron microscopy. Deposition kinetics of larger latex particles at heterogeneous surfaces produced in this way was studied by using the direct optical microscope observations in the impinging-jet and diffusion cells. It was demonstrated that the initial adsorption rate under the convection-controlled transport attained the limiting value pertinent to homogeneous surfaces for heterogeneity degree of a few per cent. This effect was even more pronounced for diffusion-controlled transport conditions. This behaviour was quantitatively interpreted in terms of the theoretical model considering the coupling between surface and bulk transport of particles. Similarly, the experimental results obtained for higher surface coverage of latex (long adsorption times) were in a good agreement with the generalised random sequential adsorption (RSA) model.

Silver nanoparticle monolayers on poly(ethylene imine) covered mica produced by colloidal self-assembly

Monodisperse silver particles were synthesized according to the method of Creighton et al. by reduction of AgNO(3) solutions with NaBH(4) in the presence of polyvinyl alcohol as the stabilizing agent. Bulk characteristics of silver nanoparticles in aqueous solutions were carried out by measuring their extinction spectrum, fluorescence, diffusion coefficients using the PCS method and the electrophoretic mobilities. The average hydrodynamic diameter of PVA covered silver particles was 44 nm, being fairly independent of ionic strength and pH in the range of 3-9. It was also shown that the hydrodynamic radius did not change within prolonged storage of suspensions (up to 75 days), indicating that the sols were quite stable. A similar value of 45±8 nm was determined from SEM measurements. The electrophoretic mobility measurements showed that the zeta potential of silver nanoparticles was insensitive to pH and decreased with the ionic strength, attaining -45 mV for I=10(-5) M and -25 mV for I=10(-2) M. Additionally, the kinetics of silver particle deposition on mica modified by adsorption of a saturated layer of poly(ethylene imine) (PEI) was studied. Surface concentration was determined directly by counting the number of particles over various surface areas using the atomic force microscopy working in the semicontact mode. The maximum surface concentration for I=10(-3)M was 102 μm(-2), which corresponds to the coverage degree of 16%. The kinetic run and the maximum coverage value was in a good agreement with theoretical predictions stemming from the random sequential adsorption (RSA) model. This kinetic run allowed one to determine the size of Ag core, which was 20 nm and the thickness of the PVA layer, equal to 12 nm.