Solute distribution and stability in emulsion-based delivery systems: an EPR study

Adsorption and desorption of human serum albumin (HSA) from aqueous solutions on mica were studied using AFM and in situ streaming potential measurements. A quantitative interpretation of these experiments was achieved in terms of the theoretical model postulating a 3D adsorption of HSA molecules as discrete particles. These measurements, performed for various ionic strength, allowed one to determine the coverage of HSA as a function of the zeta potential of mica. This allowed one to determine the amount of irreversibly bound HSA as a function of the ionic strength. It was found that the coverage of irreversibly adsorbed HSA increased from 0.52 mg m(-2) for I=1.3×10(-3) M to 1.6 mg m(-2) for I=0.15M (pH=3.5). The significant role of ionic strength was attributed to the lateral electrostatic repulsion among adsorbed HSA molecules, positively charged at this pH value. This was quantitatively interpreted in terms of the effective hard particle concept previously used for colloid particles. The experimental results confirmed that monolayers of irreversibly bound HSA of a well-controlled coverage can be produced by adjusting the ionic strength of the suspension.     

Hematite nanoparticle monolayers on mica electrokinetic characteristics

Electrokinetic properties of α-Fe(2)O(3) (hematite) nanoparticle monolayers on mica were thoroughly characterized using the streaming potential method. Hematite suspensions were obtained by acidic hydrolysis of ferric chloride. The average size of particles (hydrodynamic diameter), determined by dynamic light scattering (DLS) and AFM, was 22nm (pH=5.5, I=10(-2)M). The hematite monolayers on mica were produced under diffusion-controlled transport from the suspensions of various bulk concentration. The monolayer coverage, quantitatively determined by AFM and SEM, was regulated within broad limits by adjusting the nanoparticle deposition time. This allowed one to uniquely express zeta potential of hematite monolayers, determined by the streaming potential measurements, in terms of the particle coverage. Such dependencies, obtained for various pH, were successfully interpreted in terms of the three-dimensional electrokinetic model. A universal calibrating graph was produced enabling one to determine hematite monolayer coverage from the measured value of the streaming potential. The influence of the ionic strength, varied between 10(-4) and 10(-2)M, on the zeta potential of hematite monolayers was also studied. Additionally, the stability of monolayers (desorption kinetics) was determined under in situ conditions using the streaming potential method. Our experimental data prove that it is feasible to produce uniform and stable hematite particle monolayers of well-controlled coverage. Such monolayers may find practical applications as universal substrates for protein immobilization (biosensors) and in electrocatalytic applications.     

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.     

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.     

Fibrinogen adsorption on mica studied by AFM and in situ streaming potential measurements

Adsorption of fibrinogen from aqueous solutions on mica was studied using AFM and in situ streaming potential measurements. In the first stage, bulk physicochemical properties of fibrinogen and the mica substrate were characterized for various ionic strength and pH. The zeta potential and number of uncompensated (electrokinetic) charges on the protein surfaces were determined from microelectrophoretic measurements. Analogously, using streaming potential measurements, the electrokinetic charge density of mica was determined for pH range 3-10 and the NaCl background electrolyte concentration of 10(-3) and 10(-2) M. Next, the kinetics of fibrinogen adsorption at pH 3.5 and 7.4 in the diffusion cell was studied using a direct AFM determination of the number of molecules per unit area of the mica substrate. Then, streaming potential measurements were performed to determine the apparent zeta potential of fibrinogen-covered mica for different pH and ionic strength in terms of its surface concentration. A quantitative interpretation of these streaming potential measurements was achieved in terms of the theoretical model postulating a side-on adsorption of fibrinogen molecules as discrete particles. On the basis of these results, the maximum coverage of fibrinogen Θ close to 0.29 was predicted, in accordance with previous theoretical predictions. It was also suggested that anomalous adsorption for pH 7.4, where fibrinogen and the mica substrate were both negatively charged, can be explained in terms of a heterogeneous charge distribution on fibrinogen molecules. It was estimated that the positive charge was 12 e (for NaCl concentration of 10(-2) M and pH 7.4) compared with the net charge of fibrinogen at this pH, equal to -21 e. Results obtained in this work proved that the coverage of fibrinogen can be quantitatively determined using the streaming potential method, especially for Θ < 0.2, where other experimental methods become less accurate.     

Protonation of Lysozymes and Its Consequences for the Adsorption onto a Mica Surface

Several physicochemical properties of chicken egg white lysozyme (LSZ) in electrolyte solutions were determined. The hydrodynamic diameter of LSZ at an ionic strength of 0.15 M was found to be 4.0 nm. Using the determined parameters, the number of uncompensated (electrokinetic) charges, N(c), on the molecule surface was calculated from the electrophoretic mobility data. It was found that the N(c) = 2.8 at pH = 3.0 and an ionic strength of I = 0.15 M. At the lower ionic strength, I = 1 × 10(-3) M, this positive charge increased to N(c) = 5.6 at a pH = 3.0 The physicochemical characteristics were supplemented by the dynamic viscosity measurements. The intrinsic viscosity and the hydrodynamic diameter results were compared with theoretical predictions from Brenner's model. Using this approach, it was found that the effective molecule length of LSZ is equal to L(ef) = 5.6 nm. Additional information on the LSZ adsorbed films was obtained by the contact angle measurements. The notably large contact angles were measured on LSZ films formed under the conditions where both the LSZ and the mica were oppositely charged. The higher the positive zeta potential of LSZ, the greater the contact angle measured, which indicates that LSZ affinity for the adsorption on mica increases with its uncompensated charge. The adsorption dependence on the zeta potential of LSZ was explained, assuming a roughly uniform distribution of the net charge on the molecule surface. This assumption is supported by the results of depositing negatively charged, fluorescent latex particles onto the mica surface, which had been modified by LSZ adsorption. The highest latex coverage was formed on mica surfaces that had first been coated with LSZ solutions of lower pH, as a result of the increasing charge of LSZ monolayers in this condition.     

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.     

Zeta potential of particle bilayers on mica: a streaming potential study

The streaming potential of mica covered by bilayers of latex particles was measured using the parallel-plate channel cell. The size of the first latex (A500) bearing amidine charged groups was 503 nm and the second latex (L800) bearing sulfonate groups was 810 nm (at pH 5.5 and an ionic strength of 10(-2)M). The A500 latex exhibited an isoelectric point at pH 10.5, whereas the L800 latex was strongly negative at all pH. Mica sheets were precovered first by the A500 latex particles under diffusion transport conditions. The coverage of this supporting layer was regulated between 0.02 and 0.5 by changing the bulk concentration of latex and the deposition time. Then, the second layer of the L800 latex of regulated coverage up to 0.55 was deposited under the diffusion transport. The coverage of particles and their distributions in both layers were determined by a direct enumeration of particles by optical microscopy under wet conditions and by AFM. It was shown that the structure of the L800 particle layers and the maximum coverage were in accordance with theoretical simulations performed according to the random sequential adsorption (RSA) model. After forming bilayers of desired composition and structure, streaming potential measurements were carried out. The influence of the mica substrate, the supporting layer coverage, and its zeta potential on the apparent zeta potential of bilayers was systematically studied. It was established that for a bilayer coverage exceeding 0.20, the net zeta potential became independent of the substrate and the supporting layer zeta potentials. Then, the asymptotic values of the zeta potential of the bilayer approach 1/√2=0.71 of the bulk zeta potential of the particles forming the external (second) layer. This behavior was interpreted theoretically in terms of the electrokinetic model derived previously for monolayers. It was also concluded that results obtained in this work can be exploited for interpretation of polyelectrolyte film formation in the layer by layer (LbL) processes and protein adsorption pertinent to the antigen/antibody interactions.     

Characterization of polyelectrolyte multilayers by the streaming potential method

Polyelectrolyte multilayer adsorption on mica was studied by the streaming potential method in the parallel-plate channel setup. The technique was calibrated by performing model measurements of streaming potential by using monodisperse latex particles. Two types of polyelectrolytes were used in our studies: poly(allylamine) hydrochloride (PAH), of a cationic type, and poly(sodium 4-styrenesulfonate) (PSS) of an anionic type, both having molecular weight of 70,000. The bulk characteristics of polymers were determined by measuring the specific density, diffusion coefficient for various ionic strengths, and zeta potential. These measurements as well as molecular dynamic simulations of chain shape and configurations suggested that the molecules assume an extended, wormlike shape in the bulk. Accordingly, the diffusion coefficient was interpreted in terms of a simple hydrodynamic model pertinent to flexible rods. These data allowed a proper interpretation of polyelectrolyte multilayer adsorption from NaCl solutions of various concentrations or from 10(-3) M Tris buffer. After completing a bilayer, periodic variations in the apparent zeta potential between positive and negative values were observed for multilayers terminated by PAH and PSS, respectively. These limiting zeta potential values correlated quite well with the zeta potential of the polymers in the bulk. The stability of polyelectrolyte films against prolonged washing (reaching 26 h) also was determined using the streaming potential method. It was demonstrated that the PSS layer was considerably more resistant to washing, compared to the PAH layer. It was concluded that the experimental data were consistent with the model postulating particle-like adsorption of polyelectrolytes with little chain interpenetration. It also was concluded that due to high sensitivity, the electrokinetic method applied can be effectively used for quantitative studies of polyelectrolyte adsorption, desorption, and reconformation.     

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.     

Colloid particle and protein deposition - electrokinetic studies

Recent developments in the electrokinetic determination of particle, polyelectrolyte and protein deposition at solid/electrolyte interfaces, are reviewed. In the first section basic theoretical results are discussed enabling a quantitative interpretation of the streaming current/potential and microelectrophoretic measurements. Experimental results are presented, pertinent to electrokinetic characteristics of simple (homogeneous) surfaces such as mica, silica and various polymeric surfaces used in protein studies. The influence of the ionic strength, background electrolyte composition and pH is discussed, and the effective (electrokientic) charge of these interfaces is evaluated. In the next section, experimental data obtained by streaming potential measurements for colloid particle mono- and bilayers are presented and interpreted successfully in terms of available theoretical approaches. These results, obtained for model systems of monodisperse colloid particles are used as reference data for discussion of more complicated experiments performed for polyelectrolyte and protein covered surfaces. Results are discussed, obtained for cationic polyelectrolytes (PEI, PAH) and fibrinogen adsorbing on mica, interpreted quantitatively in terms of the theoretical approach postulating a heterogeneous 3D charge distribution. The Gouy-Chapman model, based on the continuous charge distribution proved inadequate. Interesting experimental data are also discussed, obtained by electrophoretic methods in the case of protein adsorption on colloid latex particles. In the last section, supplementary results on particle deposition on heterogeneous surfaces produced by controlled protein adsorption are discussed. Quantitative relationships between the amount of adsorbed protein, zeta potential of the interface and the particle coverage are specified. Possibility of evaluating the heterogeneity of protein charge distribution is pointed out. The anomalous deposition of colloid particles on protein molecules bearing the same sign of zeta potential, which contradicts classical DLVO theory, is interpreted in terms of the fluctuation theory. It is concluded that theoretical and experimental results obtained for model colloid systems and flat interfaces can be effectively used for interpretation of protein adsorption phenomena, studied by electrophoresis. In this way the universality of electrokinetic phenomena is underlined.     

Mechanisms of fibrinogen adsorption on latex particles determined by zeta potential and AFM measurements

The adsorption of fibrinogen on polystyrene latex particles was studied using the concentration depletion method combined with the AFM detection of residual protein after adsorption. Measurements were carried out for a pH range of 3.5-11 and an ionic strength range of 10(-3)-0.15 M NaCl. First, the bulk physicochemical properties of fibrinogen and the latex particle suspension were characterized for this range of pH and ionic strength. The zeta potential and the number of uncompensated (electrokinetic) charges on the protein were determined from microelectrophoretic measurements. It was revealed that fibrinogen molecules exhibited amphoteric characteristics, being on average positively charged for pH <5.8 (isolectric point) and negative otherwise. However, the latex particles did not show any isoelectric point, remaining strongly negative for this pH range. Afterward, systematic measurements of the electrophoretic mobility of fibrinogen-covered latex were carried out as a function of the amount of adsorbed protein, expressed as the surface concentration. A monotonic increase in the electrophoretic mobility (zeta potential) of the latex was observed in all cases, indicating a significant adsorption of fibrinogen on latex for pH below 11. It was also proven that fibrinogen adsorption was irreversible, with the maximum surface concentration varying between 2.5 and 5 × 10(3) μm(-2) (weight concentration of a bare molecule was 1.4 to 2.8 mg m(-2)). These measurements revealed two main adsorption mechanisms of fibrinogen: (i) the unoriented (random) mechanism prevailing for lower ionic strength, where adsorbing molecules significantly penetrate the fuzzy polymeric layer on the latex core and (ii) the side-on adsorption mechanism prevailing for pH > 5.8 and a higher ionic strength of 0.15 M. It was also shown that in the latter case, variations in the zeta potential with the protein coverage could be adequately described in terms of the electrokinetic model, previously formulated for planar substrate adsorption. On the basis of these experimental data, an efficient procedure of preparing fibrinogen-covered latex particles of controlled monolayer structure and coverage was envisaged.     

Dispersion stability of a ceramic glaze achieved through ionic surfactant adsorption

The adsorption of cetylpyridinium chloride (CPC) and sodium dodecylbenzenesulfonate (SDBS) onto a ceramic glaze mixture composed of limestone, feldspar, quartz, and kaolin has been investigated. Both adsorption isotherms and the average particle zeta potential have been studied in order to understand the suspension stability as a function of pH, ionic strength, and surfactant concentration. The adsorption of small amounts of cationic CPC onto the primarily negatively charged surfaces of the particles at pH 7 and 9 results in strong attraction and flocculation due to hydrophobic interactions. At higher surfactant concentrations a zeta potential of more than +60 mV results from the bilayered adsorbed surfactant, providing stability at salt concentrations < or = 0.01 M. At 0.1 M salt poor stability results despite substantial zeta potential values. Three mechanisms for SDBS adsorption have been identified. When anionic SDBS monomers either adsorb by electrostatic interactions with the few positive surface sites at high pH or adsorb onto like charged negative surface sites due to dispersion or hydrophobic interactions, the magnitude of the negative zeta potential increases slightly. At pH 9 this increase is enough to promote stability with an average zeta potential of more than -55 mV, whereas at pH 7 the zeta potential is lower at about -45 mV. The stability of suspensions at pH 7 is additionally due to steric repulsion caused by the adsorption of thick layers of neutrally charged Ca(DBS)2 complexes created when the surfactant interacts with dissolved calcium ions from the calcium carbonate component.     

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.     

Hybrid particles of polystyrene and carboxymethyl cellulose as substrates for copper ions

The synthesis of hybrid particles was carried out by emulsion polymerization of styrene in complexes formed by carboxymethyl cellulose (CMC), a polyanion, and a cationic surfactant, cetyltrimethylammonium bromide (CTAB). CMC chains with variable molecular weights and degrees of substitution were tested. The polymerization condition chosen was that corresponding to CMC chains fully saturated with CTAB and to the onset of pure surfactant micelle formation, namely, at the critical aggregation concentration. The hybrid particles were characterized by zeta potential and light scattering measurements. The period of colloidal stability in the ionic strength of 2.0 mol L(-)(1) NaCl was observed visually. Upon increasing the CMC chain length, the particle characteristics remained practically unchanged, but the colloid stability was increased. The increase in the CMC degree of substitution led to particles with more negative zeta potential values. The adsorption of copper ions (Cu(2+)) on the surface of hybrid particles could be described by the Langmuir model, as determined by potentiometric measurements. The increase in the mean zeta potential values and X-ray absorption near-edge spectra evidenced the immobilization of Cu(2+) ions on the hybrid particles.     

Assemblies of poly(styrene/⇌-tert-butoxy-ω-vinyl-benzyl-poryglycidol) Microspheres with different diameters deposited on mica plates

Formation of poly(styrene/α-tert-butoxy-ω-vinyl-benzyl-polyglycidol) microsphere assemblies on mica plates modified with 3-aminopropyltriethoxysilane was investigated. Microsphere assemblies contained two types of particles similar with respect of their chemical structure but with different diameters (D _n = 1000 and 350 nm). Methods of particle deposition included: deposition from water suspension of a mixture of small and large particles on mica plates placed at the bottom of suspension container, deposition of particles from a drop of ethanol suspension (containing large and small microspheres) placed on the mica substrate, deposition of microspheres on modified mica plates crossing the liquid-air interface-sequential deposition of large and small particles, and one-batch deposition from a mixed water suspension of large and small microspheres. Deposition from water suspension containing large and small microspheres on plates placed on the bottom of suspension container yielded assemblies with large particles randomly distributed among the small ones. Fraction of large particles in adsorbed particle assembly was smaller than fraction of large particles in suspension. Particle assemblies prepared by placement of ethanol suspension of large and small microspheres on mica were composed of quite regularly distributed large particles among the small ones. A two step process consisting of withdrawal of mica plate from water suspension of large particles and then on using this plate as substrate in a second step during which the plate was withdrawn from suspension of small particles yielded particle assemblies containing aggregates of large particles randomly distributed among the small ones. Withdrawal of mica plates from water suspension of large and small microspheres resulted in particle assemblies composed of regularly distributed stripes of large and small microspheres. Formation of the described above microsphere assemblies is possible only in case of reversible adsorption of particles.     

Probing of polyelectrolyte monolayers by zeta potential and wettability measurements

Detection of the very first step of polyelectrolyte adsorption onto a solid support is of great importance for understanding mechanisms of solid surface modification. It was shown that streaming potential and contact angle measurements can be successfully used for polyelectrolyte (PE) adsorption characterization in a broad range of surface coverage. Cationic polyallylamine hydrochloride (PAH) was used for the formation of the layer. The electrokinetic characteristics of the substrate covered by the PAH layer were compared with contact angles measured under wet (captive air bubble/substrate in water) and dry (sessile water droplet/dried substrate) conditions. It has been demonstrated that contact angle values determined under both conditions are in good agreement. The observed rapid increase in the contact angle from zero for the bare mica surface to the value close to one characteristic of the PAH monolayer appears in the same PAH coverage range as zeta potential value changes due to adsorption. These results show that wettability can be as sensitive to the presence of small amounts of adsorbed species as electrokinetic measurements.     

Surface charging of layered double hydroxides during dynamic interactions of anions at the interfaces

In this research, we investigated the effect of dynamic anion adsorption/exchange on the surface charging property of Mg(2)AlClLDH and Mg(2)AlCO(3)LDH particles that show the average zeta potential of 41 and 34 mV in the as-prepared suspension, respectively. The addition of NaCl up to 3x10(-3) M in the suspension does not obviously affect the zeta potential of both LDHs, which can be attributed to the less affinity of Cl(-) to LDH. The introduction of Na(2)CO(3) severely reduces the zeta potential at the CO(3)(2-) concentration higher than 1x10(-4) M, and to the negative value in both LDH systems at ca. 2x10(-3) M, which is presumably resulted from the exchange and the re-orientation of CO(3)(2-) in a tilt/vertical style on the surface. All four organic anions (dodecyl sulfate, folate, citrate and polyacrylate) also significantly affect the zeta potential of the LDH particles. At the lower concentrations of organic anionic groups (<1x10(-4) M), the zeta potential was slightly affected, i.e. limited exchange/adsorption. However, the concentration increasing to some point suddenly decreases and reverses the zeta potential of the LDH particles, which is presumably caused by the hydrophobic interactions that bind the hydrophobic hydrocarbon chains (especially in dodecyl sulfate) into the micelle-like bilayer bunches on the LDH surface. In addition, the effect of pH in 5.5-11.0 on the LDH particle surface charging is mainly reflected through the conversion of CO(3)(2-) to HCO(3)(-)/H(2)CO(3) when pH decreases from ca. 11 to 6, with limited contribution from protonation/deprotonation and exchange/adsorption.