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.     

Colloid Particle Adsorption on Partially Covered (Random) Surfaces

The random sequential adsorption (RSA) approach was used to model irreversible adsorption of colloid particles at surfaces precovered with smaller particles having the same sign of surface charge. Numerical simulations were performed to determine the initial flux of larger particles as a function of surface coverage of smaller particles θ(s) at various size ratios lambda=a(l)/a(s). These numerical results were described by an analytical formula derived from scaled particle theory. Simulations of the long-time adsorption kinetics of larger particles have also been performed. This allowed one to determine upon extrapolation the jamming coverage θ(l)(infinity) as a function of the lambda parameter at fixed smaller particle coverage θ(s). It was found that the jamming coverage θ(l)(infinity) was very sensitive to particle size ratios exceeding 4. Besides yielding θ(l)(infinity), the numerical simulations allowed one to determine the structure of large particle monolayers at the jamming state which deviated significantly from that observed for monodisperse systems. The theoretical predictions suggested that surface heterogeneity, e.g., the presence of smaller sized contaminants or smaller particles invisible under microscope, can be quantitatively characterized by studying larger colloid particle adsorption kinetics and structure of the monolayer. Copyright 2001 Academic Press.     

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.     

Characterization of poly(ethylene imine) layers on mica by the streaming potential and particle deposition methods

Deposition kinetics of polystyrene latex (averaged particle size of 0.66 microm) on mica covered by poly(ethylene imine) (PEI), a cationic polyelectrolyte having an average molecular mass of 75,000 g mol(-1), was studied using the impinging-jet method. The hydrodynamic radius of PEI, determined by PCS measurements, was 5.3 nm. The electrophoretic mobility of PEI was measured as a function of pH for ionic strengths of 10(-3) and 10 (-2) M, which made it possible one to determine the amount of electrokinetic charge of the molecule and its zeta potential. Formation of the polyelectrolyte layer on mica was followed by measuring the streaming potential in the parallel-plate channel. From these measurements, the dependence of the apparent zeta potential of mica on the surface coverage of PEI was determined. The amount of adsorbed PEI on mica was calculated from the convective diffusion theory. These results were quantitatively interpreted in terms of the theoretical model postulating a particle-like adsorption mechanism for PEI with not too significant shape deformation upon adsorption. On the other hand, the Gouy-Chapman model postulating the adsorption in the form of flat disks was proved inappropriate. After the surface was fully characterized, particle deposition experiments were carried out with the aim of finding the correlation between the polymer coverage and the initial rate of latex particle deposition. In the range of small polyelectrolyte coverage, a monotonic relation between the polymer coverage and the initial deposition rate of particles, as well as the jamming coverage, was found. For Theta(PEI)>0.25, the initial particle deposition rate attained the value predicted from the convective diffusion theory for homogeneous surfaces. These results were interpreted theoretically by postulating that an effective immobilization of colloid particles occurred on local polyelectrolyte assemblages containing between two and three PEI molecules.     

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.     

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.     

High density silver nanoparticle monolayers produced by colloid self-assembly on polyelectrolyte supporting layers

A stable silver nanoparticle suspension was synthesized via the reduction of silver nitrate using sodium borohydride and sodium citrate. The particle's shape and size distribution were measured by various methods. The electrophoretic mobility measurements revealed that the zeta potential of particles was highly negative, increasing slightly with the ionic strength, from -52 mV for I=10(-5) M to -35 mV for I=3×10(-2) M (for pH=5.5). The zeta potential of mica modified by the adsorption of cationic polyelectrolytes: PEI and PAH was also determined using the streaming potential measurements. The modified mica sheets were used as substrates for particle monolayers formed via colloid self assembly. The kinetics of this process, proceeding under diffusion-controlled transport conditions, was quantitatively evaluated by a direct enumeration of particles using the AFM and SEM techniques. Both the kinetics of particle deposition and the maximum surface concentration were determined. From the slope of the initial deposition rates, the equivalent diameter of particles was determined to be 16 nm, in agreement with previous measurements. Based on this finding, an efficient method of determining particle size in suspension was proposed. It was also demonstrated that for higher ionic strengths, the maximum coverage of particle monolayers on PAH modified mica exceeded 0.39. The kinetic data were quantitatively interpreted in terms of the random sequential adsorption (RSA) model using the effective hard particle concept.     

土壤颗粒表面电场作用下固-液界面Mg2+-K+与Ca2+-K+交换动力学的比较研究

通过恒流法研究了不同表面电场作用下Mg2+、Ca2+吸附动力学. 结果发现: (1)实验初期阶段是强静电力作用下的零级动力学过程和一定反应时间后的弱静电力作用下的一级动力学过程, 且零级速率过程和一级速率过程之间存在明显的转折点; (2)不同电解质构成中Ca2+的吸附速率明显快于Mg2+的, 平衡吸附量也大于Mg2+的, 且Ca2+在土壤颗粒表面的覆盖度比Mg2+在土壤颗粒表面的覆盖度高; (3)离子的相对有效电荷系数与土壤颗粒表面电场作用的不同是各体系中Ca2+、Mg2+吸附动力学有差别的根本原因; (4)根据离子吸附的理论模型可以分别计算出速率系数、平衡吸附量、离子在土壤颗粒表面的覆盖度以及固定液的体积, 这些参数可以定量评估土壤颗粒表面电场对离子吸附动力学的影响.     

Particle assembly on surface features (patterned surfaces)

Irreversible adsorption (deposition) of spherical particles on surface features of various shapes (collectors) was studied using the random sequential adsorption (RSA) model. The collectors in the form of linear line segments, semicircles, and circles were considered. Numerical simulation of the Monte Carlo type enabled one to determine particle configurations, the jamming coverage, and the end to end length of particle monolayers for various collector length (L) to particle size (d) ratio L = L/d. It was revealed that the jamming coverage for linear collectors Theta'(infinity) increases for L > 2 according to a linear dependence with respect to 1/L. For 2 > L > 1, a parabolic dependence of Theta'(infinity) on 1/L was predicted, characterized by the maximum value of Theta'(infinity) = 1.125 for L = 4/3. These dependencies allowed one to formulate an equation determining the length of nanostructures on surfaces if the averaged number of adsorbed particles is known. It was also predicted that the end to end length of the monolayer on a linear collector /L increased linearly with 1/L for L > 2. For 2 > L > 1 the dependence of /L on L was approximated by a polynomial expression, exhibiting a maximum of /L = 1.17 for L = 1.45. In the case of circular collectors, the jamming coverage was found to be substantially smaller for the same value of 1/L. It was demonstrated that the theoretical results are in agreement with our preliminary experimental data obtained for latex particles adsorbing on polyelectrolyte modified mica and on patterned surfaces obtained by a polymer-on-polymer stamping technique of gold covered silicon (Zheng et al. Langmuir 2002, 18, 4505).     

Particle assembly on patterned surfaces bearing circular (dots) and rectangular (stripes) surface features

Irreversible and localized adsorption of spherical particles on surface features of various shapes (collectors) was studied using the random sequential adsorption (RSA) model. Collectors in the form of dots and rectangles were considered, including the two limiting cases of squares and stripes. Numerical simulation of the Monte Carlo type enabled one to determine particle configurations, average coverage of particles, and the distribution for various collector length to particle size ratios L = L/d and collector width to particle size ratios B = b/d. It was predicted that particle coverage under the jamming state was highly nonuniform, exhibiting a maximum at the center and at the periphery of the collectors. The averaged number of particles Np adsorbed at the jamming state was also determined as a function of the L and B parameters, as well as the averaged number of particles per unit length in the case of stripes. It was revealed that Np was the highest for the circular and square collectors (for a fixed value of L). On the other hand, for L > 5, our numerical results could be well approximated by the analytical expressions Np = thetainfinityL2 for circles, Np = 4thetainfinityL2/pi for squares, Np = 4thetainfinityBL/pi for rectangles, and Np = 4thetainfinityB/pi for stripes (per unit length). It was demonstrated that the theoretical results are in agreement with experimental data obtained for latex particles adsorbing on patterned surfaces obtained by a polymer-on-polymer stamping technique of gold covered silicon and on photolitographically patterned silane layers on silica.     

Interactions of adsorbed albumin with underpotentially deposited copper on gold piezoelectrodes

The adsorption of a model protein, bovine serum albumin (BSA), on Au electrodes was investigated using the Cu adatom probe method and Electrochemical Quartz Crystal Nanobalance (EQCN) technique. The adsorption of BSA was confirmed by AFM imaging and has been found to be controlled by kinetics. Using the Cu adatom probe method, we were able to reconstruct the entire BSA adsorption transient Theta(BSA) vs. t. The adsorption rate constant k(1), determined from this transient is k(1)=2.45x10(5) L mol(-1) s(-1). We have found that the bulk Cu(0) deposition process is blocked by BSA adsorption and it decays exponentially with time during BSA adsorption. It ceases completely when a full monolayer of BSA is formed. In contrast to that, the mass associated with Cu-u.p.d. decreases only to ca. 50% of that in the absence of BSA, indicating that Cu adatoms can penetrate (wedge) into the space between the surface Au atoms and the adsorbed BSA molecules. In addition to that, we have found that the degree of penetration of Cu adatoms can be controlled by the applied deposition potential. By selecting a sufficiently cathodic potential, we were able to deposit a full Cu-u.p.d. monolayer, independent of the BSA surface coverage extending from Theta(BSA)=0 to Theta(BSA) approximately 1. The positive shift of Cu(ad) desorption peak potential E(p), observed in the presence of adsorbed BSA, has been interpreted in terms of Frumkin exchange interaction forces between Cu(ad) and BSA(ad), on the basis of our earlier theoretical model, expanded here to include adsorbed species in two monolayers. This expansion is possible owing to the fast rate of Cu adatom penetration in the interfacial region. From the plots of E(p) vs. Theta(BSA), the presence of strong attractive interactions between Cu(ad) and BSA(ad) was deduced. These interactions result in a super-shift of the Cu-u.p.d. desorption peak potential, corresponding to the exchange interaction coefficient g(M,X)<-4, indicating on a possibility of the formation of a stable interface complex.     

First-principles study of the adsorption of cesium on Si(001)(2 x 1) surface

First-principles calculations based on density functional theory-generalized gradient approximation method have been performed on cesium adsorption on Si(001)(2 x 1) surface. The optimized geometries and adsorption energies have been obtained and the preferred binding sites have been determined for the coverage (Theta) of one monolayer and half a monolayer. At Theta = 0.5 ML the most stable adsorption site is shown to be T3 site. At Theta = 1 ML two Cs atoms are adsorbed at HH and T3 sites, respectively. It was found that the saturation coverage of Cs for the Si(001)(2 x 1)-Cs surface is one monolayer instead of half a monolayer. This finding supports the majority of experimental observations but does not support recent coaxial impact collision ion scattering spectroscopy investigations [Surf. Sci. 531, L340 (2003)] and He(+) Rutherford backscattering spectroscopy studies [Phys. Rev. B 62, 4545 (2000)]. Mulliken charge and overlap population analysis showed that the Cs-Si bond is indeed ionic rather than polarized covalent as generally assumed for alkali metal (AM) on Si(001)(2 x 1) surface. Geometrical structure analysis seems to have limitations in determining the nature of AM-substrate bond. We also found that the silicon surface is metallic and semiconducting for the coverages of 0.5 and 1 ML, respectively.     

Application of the extended RSA models in studies of particle deposition at partially covered surfaces

This paper reviews the application of the extended random sequential adsorption (RSA) approaches to the modeling of colloid-particle deposition (irreversible adsorption) on surfaces precovered with smaller particles. Hard (noninteracting) particle systems are discussed first. We report on the numerical simulations we performed to determine the available surface function, jamming coverage, and pair-correlation function of the larger particles. We demonstrate the effect of the particle size ratio and the small particle surface coverage. We found that the numerical results were in reasonable agreement with the formula stemming from the scaled-particle theory in 2D with a modification for the sphere geometry. Next, we discuss three approximate models of adsorption allowing electrostatic interaction of colloid particles at a charged interface, employing a many-body superposition approximation. We describe two approaches of the effective hard-particle approximation next. We demonstrate the application of the effective hard-particle concept to the bimodal systems and present the effect of electrolyte concentration on the effective particle size ratio. We present the numerical results obtained from the theoretical models of soft-particle adsorption at precovered surfaces. We used the effective hard-particle approximation to determine the corresponding simpler systems of particles, namely the system of hard spheres and the system of hard discs at equilibrium. We performed numerical computations to determine the effective minimum particle surface-to-surface distance, available surface function, jamming coverage, and pair-correlation function of the larger particles at various electrolyte ionic strengths and particle size ratios. The numerical results obtained in the low-surface coverage limit were in good agreement with the formula stemming from the scaled-particle theory with a modification for the sphere geometry and electrostatic interaction. We compared the results of numerical computations of the effective minimum particle surface-to-surface distance obtained using the 2D, 3D, and curvilinear trajectory model. The results obtained with the 3D and curvilinear trajectory models indicate that large-particle/substrate attractive interaction significantly reduces the kinetic barrier to large, charged-particle adsorption at a surface precovered with small, like-charged particles. The available surface function and jamming-coverage values predicted using the simplified 3D and the more sophisticated curvilinear trajectory models are similar, while the results obtained with the 2D model differ significantly. The pair-correlation function suggests different structures of monolayers obtained with the three models. Unlike the three models of the electrostatic interaction, both effective hard-particle approximations give almost identical results. Results of this research clearly suggest that the extended RSA approaches can fruitfully be exploited for numerical simulations of colloid-particle adsorption at precovered surfaces, allowing the investigation of both hard and soft-particle systems.     

Particle adsorption and deposition: role of electrostatic interactions

This work demonstrates how electrostatic interactions, described in terms of the classical DLVO theory, influence colloid particle deposition phenomena at solid/liquid interfaces. Electrostatic interactions governing particle adsorption in both non-polar and polar media (screened interactions) are discussed. Exact and approximate methods for calculating the interaction energy of spherical and non-spherical (anisotropic) particles are presented, including the Derjaguin method. Phenomenological transport equations governing particle deposition under the linear regime are discussed with the limiting analytical expressions for calculating initial flux. Non-linear adsorption regimes appearing for higher coverage of adsorbed particles are analysed. Various theoretical approaches are exposed, aimed at calculating blocking effects appearing due to the presence of adsorbed particles. The significant role of coupling between bulk transport and surface blocking is demonstrated. Experimental data obtained under well-defined transport conditions, such as diffusion and forced convection (impinging-jet cells), are reviewed. Various experimental techniques for detecting particles at interfaces are discussed, such as reflectometry, ellipsometry, streaming potential, atomic force microscopy, electron and optical microscopy, etc. The influence of ionic strength and flow rate on the initial particle deposition rate (limiting flux) is presented. The essential role of electrostatic interactions in particle deposition on heterogeneous surfaces is demonstrated. Experimental data pertinent to the high-coverage adsorption regime are also presented, especially the dependence of the maximum coverage of particles and proteins on the ionic strength. The influence of lateral electrostatic interactions on the structure of particle monolayers is elucidated, and the links between colloid and molecular systems are pointed out.     

Simultaneous measurement of the maximum oscillation amplitude and the transient decay time constant of the QCM reveals stiffness changes of the adlayer

Interpretation of adsorption kinetics measured with a quartz crystal microbalance (QCM) can be difficult for adlayers undergoing modification of their mechanical properties. We have studied the behavior of the oscillation amplitude, A₀, and the decay time constant, , of quartz during adsorption of proteins and cells, by use of a home-made QCM. We are able to measure simultaneously the frequency, f, the dissipation factor, D, the maximum amplitude, A₀, and the transient decay time constant, , every 300 ms in liquid, gaseous, or vacuum environments. This analysis enables adsorption and modification of liquid/mass properties to be distinguished. Moreover the surface coverage and the stiffness of the adlayer can be estimated. These improvements promise to increase the appeal of QCM methodology for any applications measuring intimate contact of a dynamic material with a solid surface.     

Surface clusters of colloid particles produced by deposition on sites

The possibility of producing surface clusters of well-defined structure formed by colloid particles was analyzed theoretically and experimentally. Theoretical results were derived by performing Monte Carlo-type simulations according to the generalized random sequential adsorption (RSA) mechanism. In these simulations, the jamming coverage of particles adsorbing irreversibly on spherical sites was determined as a function of the particle-to-site size ratio lambda. It was revealed that, by properly choosing lambda, a targeted site coordination can be achieved; for example, there can be one, two, three, and so forth particles attached to one site. The structure of the heterogeneous clusters produced in this way was described in terms of the pair correlation function. It was predicted that the extent of ordering within surface clusters was diminished as the concentration of sites increased. These theoretical predictions were checked by performing deposition experiments of negatively charged polystyrene latex particles (average diameter 0.9 mum) under the diffusion-controlled transport regime. Mica sheets precovered by positively charged polystyrene latex (average diameters 0.45 and 0.95 microm) were used as the substrate surface in these experiments. Positive latex (site) deposition was also carried out under diffusion-controlled transport conditions. The concentration of the sites and the adsorbed particles was determined by direct particle counting using optical microscopy. It was found, in quantitative agreement with theoretical simulations, that the structure of surface clusters produced in this way exhibits a significant degree of short-range ordering. It also was proven experimentally that clusters containing a targeted number of colloid particles (e.g., 2 and 4) could be produced by the deposition procedure.     

Adsorption dynamics of CO2 on Zn-ZnO(0001): a molecular beam study

Presented are initial S(0) and coverage Theta dependent, S(Theta), adsorption probability measurements, respectively, of CO(2) adsorption on the polar Zn-terminated surface of ZnO, parametric in the impact energy E(i), the surface temperature T(s), the impact angle alpha(i), varied along the [001] azimuth, the CO(2) flux, and the density of defects, chi(Ar(+)), as varied by rare gas ion sputtering. S(0) decreases linearly from 0.72 to 0.25 within E(i)=0.12-1.33 eV and is independent of T(s). Above E(i)=0.56 eV, S(0) decreases by approximately 0.2 with increasing alpha(i). The shape of S(Theta) curves is consistent with precursor-mediated adsorption (Kisliuk shape, i.e., S approximately const) for low E(i); above E(i)=0.56 eV, however, a turnover to adsorbate-assisted adsorption (S increases with Theta) has been observed. The initial slope of S(Theta) curves decreases thereby with increasing alpha(i), chi(Ar(+)), and T(s), i.e., the adsorbate-assisted adsorption is most distinct for normal impact on the pristine surface at low T(s) and is independent of the CO(2) flux. The S(Theta) curves have been parametrized by analytic precursor models and Monte Carlo simulations have been conducted as well. The temperature dependence of the saturation coverage shows two structures which could be assigned to adsorption on pristine and intrinsic defect sites, respectively, in agreement with a prior thermal desorption spectroscopy study. The heat of adsorption E(d) for the pristine sites amounts to 34.0-5.4Theta, whereas for adsorption on the intrinsic defect sites E(d) of approximately 43.6 kJ/mol could be estimated. Thus, a kinetic structure-activity relationship was present.     

Colloid stability of sodium dihexadecyl phosphate/poly(diallyldimethylammonium chloride) decorated latex

The colloid stability of supramolecular assemblies composed of the synthetic anionic lipid sodium dihexadecyl phosphate (DHP) on cationic poly(diallyldimethylammonium chloride) (PDDA) supported on polystyrene sulfate (PSS) microspheres was evaluated via turbidimetry kinetics, dynamic light scattering for particle sizing, zeta-potential analysis, and determination of DHP adsorption on PDDA-covered particles. At 0.05 g/L PDDA and 5 x 10(9) PSS particles/mL, PDDA did not induce significant particle flocculation and a vast majority of PDDA covered single particles were present in the dispersion so that this was the condition chosen for determining DHP concentration (C) effects on particle size and zeta-potentials. At 0.8 mM DHP, charge neutralization, maximal size, and visible precipitation indicated extensive flocculation and minimal colloid stability for the DHP/PDDA/PSS assembly. At 0.05 g L(-1) PDDA, isotherms of high affinity for DHP adsorption on PDDA-covered particles presented a plateau at a limiting adsorption of 135 x 10(19) DHP molecules adsorbed per square meter PSS which was well above bilayer deposition on a smooth particle surface. The polyelectrolyte layer on hydrophobic particles was swelled and fluffy yielding ca. 6 +/- 1.5 nm hydrodynamic thickness. Maximal and massive adsorption of DHP lipid onto this layer produced polydisperse DHP/PDDA/PSS colloidal particles with low colloid stability and which, at best, remained aggregated as doublets over a range of large lipid concentrations so that it was not possible to evaluate the mean total thickness for the deposited film. The assembly anionic lipid/cationic PDDA layer/polymeric particle was relatively stable as particle doublets only well above charge neutralization of the polyelectrolyte by the anionic lipid, at relatively large lipid concentrations (above 1 mM DHP) with charge neutralization leading to extensive particle aggregation.     

Effect of particle size of activated clay on the adsorption of paraquat from aqueous solution

This paper describes the effect of particle size on the process of paraquat adsorption from aqueous solution onto an activated clay surface at 25 degrees C and initial pH 11.0. Measurements of the pore properties of the clay adsorbents with three different particle sizes (0.053-0.074 mm, 0.037-0.053 mm, and <0.037 mm) were carried out. The rates and isotherms of adsorption have been also investigated by batch methods under the controlled conditions. From the experimental results obtained, the adsorption process can be well described with the pseudo-second order model and Freundlich model for adsorption kinetics and adsorption isotherm, respectively. In addition, the effect of the particle size of the clay adsorbent on the adsorption kinetics was found to be of considerable significance; namely, the rate constant (k) of paraquat adsorption by the clay adsorbent decreased with increasing particle size. It was concluded that the pore properties (i.e., surface area and total pore volume) and particle size of the clay adsorbent played a significant role in determining adsorption capacity and adsorption rate, respectively.     

Mechanisms of fibrinogen adsorption at solid substrates

Adsorption of fibrinogen, modeled as a linear chain of touching beads of various sizes, was theoretically studied using the random sequential adsorption (RSA) model. The adsorption process was assumed to consist of two steps: (i) formation of an irreversibly bound fibrinogen monolayer under the side-on orientation, which is independent of the bulk protein concentration and (ii) formation of the reversibly bound, end-on monolayer, whose coverage was dependent on the bulk concentration. Calculation based on the RSA model showed that the maximum surface concentration of the end-on (reversible) monolayer equals N(⊥∞) = 6.13 × 10(3) μm(-2) which is much larger than the previously found value for the side-on (irreversible) monolayer, equal to N(∞) = 2.27 × 10(3) μm(-2). Hence, the maximum surface concentration of fibrinogen in both orientations is determined to be 8.40 × 10(3) μm(-2) corresponding to the protein coverage of 5.70 mg m(-2) assuming 20% hydration. Additionally, the surface blocking function (ASF) was determined for the end-on fibrinogen adsorption, approximated for the entire range of coverage by the interpolating polynomial. For the coverage approaching the jamming limit, the surface blocking function (ASF) was shown to vanish proportionally to (θ(⊥∞) - θ(⊥))(2). These calculation allowed one to theoretically predict adsorption isotherms for the end-on regime of fibrinogen and adsorption kinetics under various transport conditions (diffusion and convection). Using these theoretical results, a quantitative interpretation of experimental data obtained by TIRF and ellipsometry was successfully performed. The equilibrium adsorption constant for the end-on adsorption regime was found to be 8.04 × 10(-3) m. On the basis of this value, the depth of the adsorption energy minimum, equal to -17.4 kT, was predicted, which corresponds to ΔG = -41.8 kJ mol(-1). This is in accordance with adsorption energy derived as the sum of the van der Waals and electrostatic interactions. Besides having significance for predicting fibrinogen adsorption, theoretical results derived in this work also have implications for basic science providing information on mechanisms of anisotropic protein molecule adsorption on heterogeneous surfaces.