Viktoria Milkova

Effect of pectin charge density on formation of multilayer films with chitosan.

The effect of pectin charge density on the formation of multilayer films with chitosan (PEC/CHI) is studied by means of electro-optics. Pectins of low (21%) and high (71%) degrees of esterification, which are inversely proportional to the pectin charge density, are used to form films on colloidal beta-FeOOH particles at pH 4.0 when the CHI is fully ionized. We find that, after deposition of the first 3-4 layers, the film thickness increases linearly with the number of adsorbed layers. However, the increase in the film thickness is larger when the film is terminated with CHI. Irregular increase of the film thickness is more marked for the PEC with higher density of charge. Oscillation in the electrical polarizability of the film-coated particles with the number of deposited layers is also registered in the PEC/CHI films. The charge balance of the multilayers, calculated from electrical polarizability of the film-coated particles, is positive, with larger excess of positive charge within the film constructed from CHI and less charged PEC. This is attributed to the ability of CHI to diffuse into the film at each deposition step. Despite the CHI diffusion, the film thickness increases linearly due to the dissolution of unstable PEC/CHI complexes from the film surface.

Dynamics and Heterogeneity of Pb(II) Binding by SiO2 Nanoparticles in an Aqueous Dispersion

Pb(II) binding by SiO(2) nanoparticles in an aqueous dispersion was investigated under conditions where the concentrations of Pb(2+) ions and nanoparticles are of similar magnitude. Conditional stability constants (log K) obtained at different values of pH and ionic strength varied from 4.4 at pH 5.5 and I = 0.1 M to 6.4 at pH 6.5 and I = 0.0015 M. In the range of metal to nanoparticle ratios from 1.6 to 0.3, log K strongly increases, which is shown to be due to heterogeneity in Pb(II) binding. For an ionic strength of 0.1 M the Pb(2+)/SiO(2) nanoparticle system is labile, whereas for lower ionic strengths there is loss of lability with increasing pH and decreasing ionic strength. Theoretical calculations on the basis of Eigen-type complex formation kinetics seem to support the loss of lability. This is related to the nanoparticulate nature of the system, where complexation rate constants become increasingly diffusion controlled. The ion binding heterogeneity and chemodynamics of oxidic nanoparticles clearly need further detailed research.

Nanocolloids of indomethacin prepared using sonication and subsequent encapsulation with polysaccharide films.

A new procedure was applied for preparation of indomethacin (IMC) nano-particles (ca. 200nm), which includes ultrasonication of micron range IMC crystals in water, followed by short centrifugation to remove the larger drug particles. In order to stabilize the suspension against aggregation and to reduce the release time of the drug, water insoluble IMC particles were coated with chitosan/pectin (CHI/PEC) multilayer film at pH 4. Charge balance in the multilayer film and increase in the film thickness with the number of adsorbed layers was studied by means of electro-optics. The release time of IMC molecules from the encapsulated particles was measured at physiological pH 7, when the solubility of IMC in water increases several times. Addition of small amount of CaCl2 after deposition of PEC layers was applied to compact the multilayer films on the IMC particles.

Electro-optics of colloids coated with multilayers from strong polyelectrolytes:surface charge relaxation.

The electrical properties and structure of polyelectrolyte multilayers prepared by layer-by-layer deposition of poly(sodium 4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMAC) on ellipsoidal beta-ferric hydrous oxide particles are examined in situ using the electric light scattering method. In the initial nonlinear growth regime, the electro-optical effect is found to increase almost in the same manner as the thickness of the multilayer film. This reveals an increase in the multilayer surface charge due to the nonlinear increase in polyelectrolyte adsorption. Above five deposition cycles, the electro-optical effect reaches a plateau and the thickness increment per layer becomes constant, indicating regular film growth. Evidence is given that the concentration of small residual ions incorporated into the PSS/PDADMAC film is negligible. Polarization of counterions, bound near to the surface of the last-adsorbed polyion, is assumed to explain the relaxation of the electro-optical effect in the frequency range 10(3)-10(5) Hz.

Structure of polyelectrolyte layers on colloidal particles at different ionic strengths

Abstract The structure of polystyrenesulfonate (PSS) layer adsorbed onto ellipsoidal β-ferric hydrous oxide (β-FeOOH) particles is examined using electro-optics. Changes in the thickness are investigated by varying the ionic strength of the solution for rinsing the adsorbed PSS layer. An increase in the layer thickness is found after rinsing with water, which turns to a decrease when salt (NaCl) is added to the rinsing solution. No changes are registered if formation and rinsing of the layer is carried out at constant ionic strength. Compression of the PSS layer is also found after adsorption of an oppositely charged polyethylenimine (PEI) on the layer rinsed with water or in the presence of 10 −4 M NaCl. Complex-formation between the two oppositely charged polyions on the particle surface is supposed to explain the behavior of the polyelectrolyte bilayer.

Electrical Properties of Polyelectrolyte Layers Adsorbed on Colloidal Particles at Different Ionic Strength

The investigation presents results on the low-frequency electrical polarization of β-FeOOH particles with adsorbed layers from sodium salts of poly(4-styrene sulfonate), poly(acrylic acid), and carboxymethyl cellulose, obtained by electric light scattering. The adsorption is realized in aqueous NaCl solutions of different concentration, and the suspensions of the coated particles are then rinsed to low conductivity. Several electro-optical parameters are sensitive to the conformation of the adsorbed polyelectrolyte layer. The particle electrical polarizability increases drastically with the concentration of NaCl (correlating with significant increase of the adsorbed amount of polyelectrolyte), while their electrophoretic mobility remains practically unchanged. Two time scales are involved in the stepwise relaxation of the electrical polarizability. The faster process is attributed to movement of ions in the polymer layer, restricted by the coiled conformation to smaller distances. The contribution of the Debye atmosphere outside the polymer layer remains almost unchanged, which correlates with the low sensitivity of the electrophoretic mobility to the increasing amount of adsorbed polyelectrolyte. Abrupt increase in a narrow interval of salt concentrations is observed both for the low-frequency component of the particle polarization and for the hydrodynamic layer thickness, indicating changes in the surface electric state, most probably due to swelling of the adsorbed polymer layer.

Complexation of Ferric Oxide Particles with Pectins of Different Charge Density

The effect of polyelectrolyte charge density on the electrical properties and stability of suspensions of oppositely charged oxide particles is followed by means of electro-optics and electrophoresis. Variations in the electro-optical effect and the electrophoretic mobility are examined at conditions where fully ionized pectins of different charge density adsorb onto particles with ionizable surfaces. The charge neutralization point coincides with the maximum of particle aggregation in all suspensions. We find that the concentration of polyelectrolyte, needed to neutralize the particle charge, decreases with increasing charge density of the pectin. The most highly charged pectin presents an exception to this order, which is explained with a reduction of the effective charge density of this pectin due to condensation of counterions. The presence of condensed counterions, remaining bound to the pectin during its adsorption on the particle surface, is proved by investigation of the frequency behavior of the electro-optical effect at charge reversal of the particle surface.

Electrochemical Performance of Low-carbon Steel in Alkaline Model Solutions Containing Hybrid Aggregates

This work reports on the electrochemical performance of low-carbon steel electrodes in model alkaline solutions in the presence of 4.9.10-4 g/l hybrid aggregates i.e. cement extract, containing PDADMAC (poly (diallyl, dimethyl ammonium chloride) / PAA (Poly (acrylic acid)/ PDADMAC over a CaO core. The main objective was to determine if the addition of hybrid aggregates will lead to increased corrosion resistance of the steel surface layers, generally formed in the hereby investigated environmental medium. Further, it was expected that when chlorides are involved, as corrosion accelerating factor, the presence of hybrid aggregates will delay corrosion initiation and therefore lead to increased corrosion resistance. This investigation forms part of a novel approach to control steel corrosion in reinforced concrete, using self-healing mechanisms. The results from this study denote for indeed superior corrosion performance of steel in chloride-free and chloride containing alkaline solution, when hybrid aggregates are involved. The mechanisms are related to increased barrier effects of the formed layer and CaO release from the core of the aggregates.

Complexation of Ferric Oxide Particles with Pectins of Ordered and Random Distribution of Charged Units

Complexation between ferric oxide particles and pectins with degree of methylation 50% but having ordered or random arrangement of free carboxyl groups is investigated by electric light scattering and electrophoresis. The influence of charge distribution in pectin chain on the electrical properties of oppositely charged oxide particles and stability of their suspensions is examined as a function of pectin concentration. Although the difference in charge density of pectin samples is ~5%, we found small but measurable difference in the behavior of both oxide/pectin complexes. This is attributed to condensation of counterions near the chains of pectin with ordered distribution of charges, leading to a decrease in the effective charge density and to a corresponding decrease in the contour length of the adsorbing pectin chains. Two parameters are sensitive to the conformation of the adsorbed chains in suspensions, stabilized by pectin adsorption (at particle charge reversal). The electro-optical effect is higher for the complex with less charged pectin, which is explained with larger amount of chains, adsorbed in more coiled conformation than the chains of pectin with random distribution of free carboxyl groups. The addition of small amounts of CaCl(2) has no significant influence on the thickness of the layer from the less charged pectin, in agreement with a more compact conformation of the chains in this adsorbed layer. In contrast, the thickness of the layer from pectin with random distribution of charged groups decreases with increasing concentration of CaCl(2), indicating a more loose structure of this layer.