Maria Zembala

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.

Plasma surface modification of polyethylene

Oxidative (oxygen and air) RF-plasma treatment of HDPE was found as an effective tool for improving wettability, as well as for increasing its surface micro-hardness. The latter plasmas generate wide range of reactive species in the system, which undergo consecutive chemical reactions, creating thus several oxygen based functionalities at the interface (carbonyl, carboxyl, ether, peroxide etc.) as were detected by ATR FTIR and XPS analysis. An increased negative surface charge of plasma treated polyethylene (PE) confirms the presence of functional ionogenic groups containing oxygen. Simultaneously, the vigorous increase of the surface roughness was found as a result of the successful plasma etching.

Zeta potential of mica covered by colloid particles: a streaming potential study.

The streaming potential of mica covered by monodisperse latex particles was measured using the parallel-plate channel, four-electrode cell. The zeta potential of latex bearing amidine charged groups was regulated by the addition of NaCl (10(-4)-10(-2) M) and MgCl(2) (10(-4)-10(-2) M) at a constant pH 5.5 and by the change in pH (4-12) at 10(-2) M NaCl. The size of the latex particles, determined by dynamic light scattering, varied between 502 and 540 nm for the above electrolyte concentration range. Mica sheets have been covered with latex particles under diffusion transport conditions. The latex coverage was regulated by the bulk suspension concentration in the channel and the deposition time. The coverage was determined, with a relative precision of 2%, by the direct enumeration of particles by optical microscopy and AFM. The streaming potential of mica was then determined for a broad range of particle coverage 0 < theta < 0.5, the particle-to-substrate zeta potential ratio zeta(p)/zeta(i), and 8.8 < kappa a < 143 (thin double-layer limit). These experimental data confirmed that the streaming potential of covered surfaces is well reflected by the theoretical approach formulated in ref 32. It was also shown experimentally that variations in the substrate streaming potential with particle coverage for theta < 0.3 and zeta(p)/zeta(i) < 0 are characterized by a large slope, which enables the precise detection of particles attached to interfaces. However, measurements at high coverage and various pH values revealed that the apparent zeta potential of covered surfaces is 1/2(1/2) smaller than the bulk zeta potential of particles (in absolute terms). This is valid for arbitrary zeta potentials of substrates and particles, including the case of negative particles on negatively charged substrates that mimics rough surfaces. Therefore, it was concluded that the streaming potential method can serve as an efficient tool for determining bulk zeta potentials of colloids and bioparticles.

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.

Influence of adsorbed particles on streaming potential of mica

Abstract Zeta potential of natural mica covered by polystyrene and melamine latex particles, was determined by the streaming potential method. Measurements were carried out using the parallel-plate channel formed by two mica sheets separated by a teflon spacer. The dependence of streaming potential on surface coverage (reaching 0.45) was determined at pH 5.3, when the particles were positively charged and at pH 7.8, when the melamine particles became neutral. The coverage was determined directly by optical microscope counting procedure. It was found that the negative streaming potential for bare mica E s was significantly increased by the presence of adsorbed particles. The experimental data were interpreted in terms of a theoretical model postulating that the streaming potential change was due to flow damping over the interface and by charge transport from the double-layer surrounding adsorbed particles. In contrast to earlier approaches, no assumption of the slip (shear) plane shift upon particle adsorption was made. The experimental data allowed one to determine empirically the universal correction function, which can be exploited for quantitative measurements of bioparticle adsorption kinetics via the streaming potential method.

Effect of selenium on characteristics of rape chloroplasts modified by cadmium.

Selenium appears to be an important protective agent that decreases cadmium-induced toxic effects in animals and plants. The aim of these studies was to investigate the changes of properties of chloroplast membranes obtained from Cd-treated rape seedlings caused by Se additions. Chloroplasts were isolated from leaves of 3-week-old rape plants cultured on Murashige-Skoog media supplied with 2 microM Na(2)SeO(4) and/or 400 microM CdCl(2) under in vitro conditions. The following physicochemical characteristics of chloroplasts were chosen as indicators of Se-effects: average size, zeta potential, ultrastructure, lipid and fatty acid composition and fluidity of envelope membrane. The results suggest that Se can partly counterbalance the destructive effects of Cd. This protective action led to an increase of chloroplast size reduced by Cd treatment and rebuilt, to some extent, the chloroplast ultrastructure. Lipid and fatty acid composition of chloroplast envelopes modified by Cd showed a decrease in digalactosyl-diacylglycerol content and an increase of content of monogalactosyl-diacylglycerol and phospholipid fractions, as well as an increase of fatty acid saturation of all lipids studied. The change in fatty acid saturation correlated well with a decrease of membrane fluidity and with a diminishing of absolute values of zeta potential. The presence of selenium in cultured media caused a partial reversal of the detected changes, which was especially visible in properties related to the hydrophobic part of an envelope, i.e. fatty acid saturation and fluidity.

Statistical properties of surfaces covered by large spheres

Statistical properties of surfaces covered by particles deposited under the influence of gravity are investigated by means of optical microscopy and image analysis. The radial distribution function of the particle configurations is determined over a wide coverage range. Special attention is paid to the fluctuations of the number of particles on small surfaces which contain information upon the deposition process. A first analysis of these fluctuations is presented within a mean field approximation. All results are compared to the ballistic process which proves to be a good model. On the contrary, the observations are not compatible with a simple random sequential adsorption model.

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.