Search

The construction of anticorrosion coatings containing antifouling agents is an effective way to ensure the long-term durability of marine steel infrastructures. In this work, an innovative hybrid coating was prepared by introducing biocide CuO nanoparticles in ordinary zinc coating to improve its protective ability for steel in aggressive salt water environments. The CuO nanoparticles were embedded inside the matrix of chitosan/alginate complexes to prevent spontaneous copper leaching during corrosive attacks. Two procedures were applied for the electrodeposition of hybrid/composite zinc-based coatings on low-carbon steel substrates (DC current): first—the co-electrodeposition of encapsulated CuO nanoparticles with zinc on a cathode (steel) electrode from a sulfate electrolyte with a relatively low pH value of about 4.5–5.0 and second—the encapsulated CuO nanoparticles were electrodeposited from aqueous solution as an intermediate layer between two zinc deposits. The particles size and stability of suspensions were evaluated using dynamic light scattering. Both hybrid coatings were compared in terms of surface morphology and hydrophilicity (SEM and AFM analysis, contact angle measurement) and corrosion resistance (potentiodynamic polarization curves, polarization resistance). The protective characteristics of the coatings were compared in a 3.5% NaCl solution and artificial sea water. The hybrid coating showed 2–4 times higher polarization resistance than the bare zinc coating during a 30 day immersion in artificial sea water, indicating that this coating has the necessary characteristics to be used in a marine environment.

The presented investigations demonstrate the corrosion behavior and protective ability of hybrid zinc coatings specially designed for combined protection of low-carbon steel from localized corrosion and biofouling. Polymer-modified copper oxide (CuO) nanoparticles as widely used classic biocide are applied for this purpose, being simultaneously electrodeposited with zinc from electrolytic bath. The corrosion behavior of the hybrid coatings is evaluated in a model corrosive medium of 5% NaCl solution and in artificial sea water (ASW). Scanning electron microscopy (SEM) and atomic force microscopy (AFM) are used to characterize the surface morphology of pure and hybrid zinc coatings. Contact angle measurements are realized with an aim to determine the hydrophobicity of the surface. X-ray photoelectron spectroscopy (XPS) is applied for evaluation of the chemical composition of the surface products appearing as a result of the corrosion treatment. Potentiodynamic polarization (PDP) curves and polarization resistance (Rp) measurements are used to estimate the protective characteristics in both model corrosive media. The results obtained for the hybrid coatings are compared with the corrosion characteristics of ordinary zinc coating with the same thickness. It was found that the hybrid coating improves the anticorrosion behavior of low-carbon steel during the time interval of 35 days and at conditions of external polarization. The tests demonstrate much larger corrosion resistance of the hybrid coating in ASW compared to 5% NaCl solution. The obtained results indicated that the proposed hybrid zinc coating has a potential for antifouling application in marine environment.

Localized corrosion and biofouling cause very serious problems in the marine industries, often related to financial losses and environmental accidents. Aiming to minimize the abovementioned, two types of hybrid Zn-based protective coatings have been composed. They consist of a very thin underlayer of polymer-modified ZnO or CuO nanoparticles and toplayer of galvanic zinc with a thickness of ~14 µm. In order to stabilize the suspensions of CuO or ZnO, respectively, a cationic polyelectrolyte polyethylenimine (PEI) is used. The polymer-modified nanoparticles are electrodeposited on the steel (cathode) surface at very low cathodic current density and following pH values: 1/CuO at pH 9.0, aiming to minimize the effect of aggregation in the suspension and dissolution of the CuO nanoparticles, 2/ZnO at pH 7.5 due to the dissolution of ZnO. Thereafter, ordinary zinc coating is electrodeposited on the CuO or ZnO coated low-carbon steel substrate from a zinc electrolyte at pH 4.5–5.0. The two-step approach described herein can be used for the preparation of hybrid coatings where preservation of particles functionality is required. The distribution of the nanoparticles on the steel surface and morphology of the hybrid coatings are studied by scanning electron microscopy. The thickness of the coatings is evaluated by a straight optical microscope and cross-sections. The protective properties of both systems are investigated in a model corrosive medium of 5% NaCl solution by application of potentiodynamic polarization (PDP) curves, open circuit potential (OCP), cyclic voltammetry (CVA), and polarization resistance (Rp) measurements. The results obtained allow us to conclude that both hybrid coatings with embedded polymer-modified CuO or ZnO nanoparticles ensure enhanced corrosion resistance and protective ability compared to the ordinary zinc.

Core-shell nanocontainers (NCs) with corrosion inhibitor benzotriazole (BTA) are fabricated by electrostatic self-assembly of poly(diallyldimethylammonium chloride) (PDADMAC) and poly(acrylic acid) (PAA) on kaolinite and hematite particles. BTA is entrapped within the polyelectrolyte shell in one assembly step. The growth of the polymer shells and stability of the NCs suspensions against aggregation are followed by electric light scattering and electrophoresis The NCs loaded with BTA are incorporated into the matrix of ordinary zinc coatings by electrodeposition on steel substrates from acidic zinc sulfate solutions. The influence of the NCs on cathodic and anodic processes is analyzed in the starting zinc sulfate solution with cyclic voltammetry. The scanning electron microscopy shows random (near to uniform) distribution of the NCs in both hybrid coatings. The protective characteristics of the hybrid coatings in model medium of 5% NaCl are followed by means of potentiodynamic polarization curves, polarization resistance measurements and electrochemical impedance spectroscopy. XPS method is applied for determination of the nature of additional products appearing on the coatings as a result of the corrosion treatment. Both hybrid zinc coatings reveal improved corrosion protection of steel as compared to the pure zinc coating, the coating with kaolinite-based NCs showing better protective characteristics in comparison with the one incorporating hematite NCs.

The present work describes one possible way to prepare a stable aqueous suspension of carbon sphere particles with a positive charge that is suitable for simultaneous electrodeposition with zinc on steel substrate. In order to stabilize the suspension against aggregation, tri-block amphiphilic copolymer Pluronic F127, which is commercially available, was adsorbed on the surface of carbon sphere particles. This polymer contained poly (ethylene oxide) blocks as hydrophilic segments and poly (propylene oxide) blocks as the hydrophobic part. Scanning electron microscopy and visual observations confirmed the stability of the obtained suspension. The carbon sphere particles were embedded into the zinc coating by the co-electrodeposition process. The surface morphology of the composite coating was investigated using scanning electron microscopy. The influence of the carbon spheres on the cathodic and anodic processes was evaluated with cyclic voltammetry studies. The electrochemical investigations were realized in a model corrosion medium (5% NaCl solution with pH 6.7) by application of selected methods such as polarization resistance, potentiodynamic polarization, and electrochemical impedance spectroscopy, which revealed higher protective ability of the composite coating against corrosion in an aggressive environment.

Kaolinite nanoparticles are encapsulated with polymers for application as nanocontainers (NCs) that can release the entrapped corrosion inhibitor benzotriazole (BTA) at neutral conditions. The NCs are prepared using layer-by-layer assembly of poly(diallyldimethyl ammonium chloride) (PDADMAC) and poly(acrylic acid) (PAA) on kaolinite surface at different pH and salt conditions. The inhibitor is entrapped within polyelectrolyte multilayer in the assembly step. In the absence of salt, stable NCs with BTA are prepared at pH 8/5 for adsorption of PDADMAC and PAA layers, respectively. In the presence of 0.01 M NaCl, stable suspension of NCs cannot be prepared at these pH values. Instead, stabilized suspension of NCs is obtained at pH 8/8 of both polyelectrolyte solutions, which is attributed to the stronger electrostatic interactions between PDADMAC and more highly charged PAA chains. Larger amount of BTA is measured in the NCs prepared in the presence of salt.

Indomethacin (IMC) nanoparticles are encapsulated through layer-by-layer self-assembly of natural polysaccharides chitosan (CHI) and pectin (PEC). The adsorption of both polymers is realized in aqueous solutions containing 10−2 M NaCl and pH 4.5, where the drug particles are practically insoluble. The stoichiometry of charges and the thickness of the polymer coating on the IMC particles, as well as the stability of their suspensions, are identified using electric light scattering. We found linear increase in the coating thickness with the number of deposited layers. Correspondingly, the electric light scattering is found to increase with increasing number of the layers, showing small excess of positive charges when the coating is terminated with the more highly charged CHI. The release rate of IMC from the nanocapsules is monitored with UV-absorbance in buffer at close to physiological pH 7, when the solubility of IMC increases several times. We found that the encapsulation of the IMC nanoparticles prolong the release time of the drug to about 5 h. The released amount of IMC is also found to depend on the concentration of NaCl in the dipping polysaccharide solutions.

Polysaccharide multilayer film consisting of negatively charged pectin (PEC) and positively charged chitosan (CHI) is built by electrostatic self-assembly onto ellipsoidal iron oxide particles. The thickness growth and electrical properties of the PEC/CHI multilayer film are followed using electric light scattering method and electrophoresis. The main focus is on how the PEC and CHI charge density and addition of divalent calcium salt affect the stiffness of the PEC/CHI multilayer film. We found an excess of positive charges within the PEC/CHI film when it is constructed from highly charged CHI and less charged PEC (at pH 4). The excess of positive charge within the film causes a decrease in the amplitude of the electro-optical effect at high electric fields, more expressive for the films ending in CHI. This is related to operation of an elasto-hydrodynamic effect (dominating at strong fields), which favors perpendicular orientation of “soft” particles with respect to the external electric field. Addition of Ca2+ ions to the films ending in PEC causes small increase in the amplitude of the electro-optical effect at high fields, corresponding to an increase in the film stiffness.

Self-assembled lipid nanotubes (LNTs) could mimic the lipid bilayer of biological membranes and are widely used as substrates for proteins in the biotechnology. Since the protein-LNT binding depends strongly on the physicochemical properties of the LNT, we studied one of the important properties of the “naked” (not associated with protein) LNT—their electrical polarizability. The experiments were performed on galactosyl ceramide nanotubes suspension through electric light scattering method. The average values for the length of the LNT particles as well as the magnitude and type of their electrical polarizability are determinated. The analysis of the experimental results is performed on the basis of the comparison with the theory. The data were taken from experimental curves obtained from the variation of the electric light scattering effect with time, strength, and frequency of the applied electric field. The average values for the total electrical polarizability obtained from the electric light scattering effect in three independent ways give comparable results: 1.9, 1.4, and 0.9 × 10−29 F m2. The average value of the length of the galactosyl ceramide nanotubes was estimated to be about 0.95 μm, and the type of polarization was found to be “charge dependent“(electrokinetic) and of Maxwell-Wagner-O’Konski type. The method used here could be applicable to the most of the organic NTs.

Electric light scattering method is used to study electrical properties and structure of polyelectrolyte multilayer films adsorbed on non-spherical particles in the absence of salt. Attention is focused on the advantage of electro-optics to give information simultaneously on the dynamics of counterions and the thickness of the multilayer films adsorbed on the particles in stabilized suspensions. Based on the electro-optical experiment, it is demonstrated that polarization of counterions along the chains of the last-deposited polymer governs the behavior of the whole multilayer film. Evidence is given that the film thickness can increase exponentially with the number of deposited layers even in the absence of salt when a fully charged polyelectrolyte is combined with a weakly charged one. Examples are presented for electro-optical characterization of multilayer films on colloidal particles with potential applications in anticorrosion coatings and medicine.

A new procedure was applied for preparation of indomethacin (IMC) nano-particles (ca. 200 nm), 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 CaCl 2 after deposition of PEC layers was applied to compact the multilayer films on the IMC particles.

Electric light scattering from aqueous suspensions of positively charged ferric oxide particles is investigated in the presence of poly(sodium styrene sulfonate) and poly(allylamine hydrochloride) of different molecular weights, adsorbed sequentially on the particle surface at different ionic strengths. The adsorption is realized over a wide range of NaCl concentration (10 −4 to 10 −1 M) at pH 4.5 and the suspensions of coated particles are then rinsed to ionic strength of 10 −4 M. Our measurements indicate that the amplitude of the electro-optical effect increases with increasing concentration of NaCl from 10 −4 to 10 −2 M due to the increased amount of polymers, adsorbing in more coiled conformation and on more highly charged particle surface. However, the electro-optical effect started to decrease at further increase of NaCl concentration, more expressively at higher field intensities and for bi-layers with first layer from polyelectrolyte of lower molecular weight. This is accompanied with a significant increase in the thickness of the deposited bi-layers after their rinsing to low ionic strength. We attribute both effects to partial desorption of polyelectrolyte complexes from the particle surface, which disturb the homogeneity of the bi-layer structure. The latter causes an increase in the amount of mechanically trapped water on the particle surface and the thickness of adsorbed bi-layers increases as well. As a result, an elasto-hydrodynamic effect, which favors perpendicular orientation with respect to the external electric field, becomes possible.

Conformational transitions of polyacrylic acid with two different molecular weights (PAA100 and PAA250) have been investigated by means of electric birefringence method. The conformation of the polymer chains has been found to depend on pH, showing slight increase in the chain length up to a degree of neutralization 0.3 (ca. pH 6) and, after an almost plateau value, sharply increase to a maximum near the equivalence point. This was attributed to changes in the degree of polyelectrolyte ionization with variation of pH. A model was proposed that interprets the experimental results in the light of the counterion condensation approach.

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.

Electrical properties of sodium carboxymethyl cellulose (NaCMC) in aqueous solution and after addition to a dilute suspension of β-ferric hydrous oxide particles (β-FeOOH) are studied by means of electric birefringence method. We established extended conformation of the NaCMC chains in a solution at concentration 10 −2 g dm−3, which is found high enough to assure overcharging of the particle surface. The frequency behavior of the electric birefringence of NaCMC solution with concentration 10 −2 g dm −3 is found similar to the behavior of the suspension of β-FeOOH particles containing same amount of polyelectrolyte. The observed decrease in the relaxation frequency of the electro-optical effect in both systems is attributed to polarization of condensed counterions near to the polyion surface. These results are in line with our previous estimation, showing that the condensed counterions are not released from NaCMC because of its adsorption onto weakly charged particle surface. They also reveal that, at overcompensation of the particle charge, the electrical properties of the adsorbed polyion dominate the electro-optical behavior of the weakly charged particle.

Two combinations of sodium poly(4-styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) of different chain length and charge density are employed to construct multilayer films. The polyelectrolytes are assembled layer-by-layer on colloidal particles in the absence of salt. We have investigated the formation and electrical characteristics of the films by using electric light scattering technique. The results show that the film thickness is independent of the chain length when fully charged PAH (at pH 4.6) is combined with fully charged PSS. When the films are prepared with less charged PAH (at pH 6.7) and fully charged PSS, lower thickness is found for the film with shorter polymer chains. In all cases, the thickness increment realized on addition of the polymer with lower molar concentration is partially lost on exposure to the solution with higher concentration of the oppositely charged partner. When the film growth is regular (at equal molar concentrations of the fully charged polyelectrolytes), the ratio of PSS to PAH charge, estimated from the electro-optical effect values, exceeds 1. The electro-optical effect is also higher for the films ending with PSS when fully charged PSS is combined with less charged PAH (at pH 6.7). This reveals the key role of the charge in the last-adsorbed layer for the electro-optical behavior of the whole film.

The buildup of poly( l -glutamic acid) (PGA) and poly( l -lysine) (PLL) multilayers on β-FeOOH colloidal particles was investigated by means of electro-optics and electrophoresis. The films were built at different (acidic) pH in the absence of salt. We found that the thickness of the film grows linearly when the fully charged PLL (at pH 5.5) is combined with almost fully charged PGA (at pH 6.5), with a thickness of about 2 nm per single layer. When the fully charged PLL is combined with weakly charged PGA (at pH 4.5), the film thickness increases exponentially with the number of deposited layers. The thickness of the exponentially growing film increases to 300 nm after deposition of 16 layers. The exponential film growth is attributed to the ability of the PLL to diffuse “in” and “out” of the film bulk at each deposition step. The variation in the electrical polarizability of the film-coated particles was also monitored as a function of the number of adsorbed layers. The result reveals that the PLL chains, which can diffuse into the film bulk, have no measurable contribution to the electro-optical effect of the films terminated with PLL. It is only due to the polarization of counterions of the PLL adsorbed on the film surface.

The adsorption of sodium poly(4-styrene sulfonate) on oppositely charged β-FeOOH particles is studied by electrooptics. The focus of this paper is on the release of condensed counterions from adsorbed polyelectrolyte upon surface charge overcompensation. The fraction of condensed Na+ counterions on the adsorbed polyion surface is estimated according to the theory of Sens and Joanny and it is compared with the fraction of condensed counterions on nonadsorbed polyelectrolyte. The relaxation frequency of the electrooptical effect from the polymer-coated particle is found to depend on the polyelectrolyte molecular weight. This is attributed to polarization of the layer from condensed counterions on the polyion surface, being responsible for creation of the effect from particles covered with highly charged polyelectrolyte. The number of the adsorbed chains is calculated also assuming counterion condensation on the adsorbed polyelectrolyte and semiquantative agreement is found with the result obtained from the condensed counterion polarizability of the polymer-coated particle. Our findings are in line with theoretical predictions that the fraction of condensed counterions remains unchanged due to the adsorption of highly charged polyelectrolyte onto weakly charged substrate.

Layer-by-layer deposition of sodium carboxymethylcellulose (NaCMC) and chitosan (CHI) was used to create polyelectrolyte multilayers on ellipsoidal beta-FeOOH particles at low ionic strength. Using electro-optics, we investigated the formation of films in dependence on the polyelectrolyte charge density by controlling pH of the dipping solutions. We found out a linear growth of the CMC/CHI films when they are constructed from highly charged CHI (at pH 4.0) and weakly charged NaCMC (at pH 4.0 and 5.5). The hydrodynamic thickness of the film constructed at pH 4.0/4.0 is unusually large for a linearly growing film (ca. 220 nm after deposition of 8 bilayers), but it strongly decreases (ca. 4 times) with increasing ionization of NaCMC (at pH 5.5). In both cases, the multilayer buildup proceeded through a series of adsorption-desorption steps. This was explained by a partial loss of CHI from the film surface on exposure to the solution of longer NaCMC molecules. The irregular film growth correlated quite well with the variations in the electrical polarizability of the polymer-coated particles. This correlation enabled us to conclude that the adsorption of both polymers occurs only on the film surface, with no diffusion in and out of the film bulk during deposition of each CMC/CHI bilayer.

Complexation between sodium carboxymethylcellulose (NaCMC) and ellipsoidal particles of oppositely charged beta-FeOOH is studied using electric light scattering and electrophoresis. We focus on the complex behavior for overcharging of the particle surface. In this case, the fraction of condensed counterions on NaCMC is found to remain unchanged during polymer adsorption onto a weakly charged particle surface. Using the theory of Sens and Joanny, we evaluate the fraction of condensed counterions and compare it with results for nonadsorbed NaCMC. The polarization of condensed counterions along the chain of the adsorbed macromolecule is proved to create the electro-optical effect in suspensions stabilized by NaCMC adsorption.

The formation of stable multilayer films by using as constituents sodium poly(4-styrene sulfonate) (PSS) and poly(4-vinyl pyridine) (PVP) was studied by electrooptics. A strong increase in basicity of the pyridine rings in the electrical field of the oppositely charged PSS chains was suggested to be the driving force for multilayer film formation. A linear increase in the film thickness was registered after deposition of the first three layers, with no dependence on the polyelectrolyte molecular weight. The electrooptical effect was found to increase with increasing area of each next layer, but depended on the molecular weights of both polymers. Polarization of “condensed” counterions along the chains of the last-adsorbed layer was suggested to explain this dependence. Following the counterion dynamics, we come to the conclusion that the electrical properties of the top layer govern the electrooptical behavior of the PSS/PVP film.

The electro-optical behavior of a multilayer constructed via layer-by-layer deposition of poly(sodium 4-styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) onto ellipsoidal β-FeOOH particles is examined using electric light scattering method. For fully charged polymers (at pH 4.5), the electro-optical effect is found to increase with polyelectrolyte layer number, showing a tendency to saturation in the linear growth regime. The effect is greater and of lower frequency of relaxation for the films ending with PAH in comparison to those with top PSS layer. Evidence is given that polarization of “condensed” counterions along the chains of the last-adsorbed polymer is mainly responsible for the observed electro-optical behavior of the polyelectrolyte multilayer. Although incorporation of “condensed” small ions into the film bulk seems probable for the PSS/PAH multilayer, their participation in the electro-optical effect is found negligible. The structural changes in the PSS/PAH multilayer due to the PAH deprotonation at pH 7.5 and the corresponding changes in the electro-optical effect confirm the key role of the last-adsorbed polymer for the behavior of the entire PSS/PAH film.

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.

Electric light scattering method is used to study complexation between polyelectrolyte molecules and oppositely charged ellipsoidal particles. Attention is focused on the charge inversion that occurs when polyelectrolytes of different charge density, chain length and flexibility adsorb on oxide particles in water medium. In particular, we are interested in the similarity observed between the electrical properties of adsorbed and nonadsorbed polymers in regime of the surface charge overcompensation. The electro-optical results show that, at plateau adsorption, the particles assume the charge characteristic of the polymer. The extension of the adsorbed layer into a vertical direction, in which case the overcharging is caused by dangling loops and tails, is supposed to explain this observation.

Multilayer films are formed by consecutive adsorption of anionic poly(sodium 4-styrenesulfonate) (NaPSS) and cationic poly(ethylenimine) polyelectrolytes on ellipsoidal β-FeOOH particles at low ionic strength. The electrical and hydrodynamic properties of the multilayers are followed using electric light scattering and a lower mobility of condensed ions near the polyion surface is found for the films ending with PSS. Nearly equal (absolute) electrophoretic mobilities are measured at the same time for the multilayers ending with both polymers. The uniform growth of the film after deposition of each pair of polyelectrolytes and the independence of the electrooptical effect of the number of the layer are discussed. Arguments are presented that the influence of the substrate on the film behavior extends only to the first two layers.

The influence of molecular weight on the counterion polarization of sodium polystyrenesulfonate (NaPSS) is investigated by electric light scattering. The polyelectrolyte is adsorbed in excess on beta-ferric hydrous oxide particles (beta-FeOOH), which are shown to assume the charge characteristics of the polymer. Two dispersion regions of the electro-optical effect (below and above 10 kHz) are found to exist due to the polarization of counterions. The high-frequency (HF) effect is attributed to polarization of free counterions along the polymer-coated particle axis and does not depend on the molecular weight of the adsorbed polymer. The amplitude and critical frequency of relaxation of the low-frequency (LF) effect, which is attributed to polarization of bound counterions along the polymer axis, depend on the polyelectrolyte molecular weight. The polarization of bound counterions seems to be responsible for the permanent-like dipole moment behavior of NaPSS at low frequencies of the applied electric field. Copyright 1999 Academic Press.

The variations of electro-optical behavior and pH of ellipsoidal hematite (alpha-Fe2O3) particle suspension are investigated in the presence of polyethylenimine (PEI). Suspension stability and particle electrophoretic mobility are also followed. Considerable shift of pH and the particle isoelectric point are found at extremely low PEI concentrations, indicating that nearly all amino groups of unfolded PEI molecules contact with the oxide surface. The small polymer layer thickness in stabilized suspension (from electro-optical data) shows "flattening" of the PEI molecules even at higher polymer adsorption. Similar electro-optical behavior of suspensions with either a positive or a negative oxide particle, saturated with PEI, indicates that the colloid-polymer complex assumes charge characteristics of the polymer. Reduced counterion mobility, found in both systems, is attributed to counterion condensation on the polyion surface and, possibly, to conformational transition in adsorbed low-charged polymer molecules. Copyright 1997 Academic Press. Copyright 1997Academic Press

Adsorption isotherms and adsorbed layer thicknesses were determined as a function of polymer charge density for three samples of partially hydrolyzed polyacrylamide on β-ferric hydrous oxide particles. The amount adsorbed was found to decrease with increasing polymer charge density. Equal values of the hydrodynamic adsorbed layer thickness were measured by electric light scattering, whereas the average layer thicknesses, calculated according to R. Varoqui theory, were found to decrease with increasing polymer charge density. Information on counterion mobility was obtained by means of frequency-domain electric light scattering. The results observed are discussed in terms of counterion binding to polyelectrolytes of low charge density.

Electric light scattering by aqueous ferric oxide suspension (β-FeOOH) is investigated as a function of the concentration and degree of hydrolysis of polyacrylamide (HPAM). The kilohertz electro-optical effect (αs) connected with ion polarization in the diffuse part of electric double layers of particles decreases with increasing HPAM concentration. It falls to zero near to the isoelectric point of particles and increases again after surface recharging to a value that does not change with further polymer concentration increase. αsis shown to be independent of HPAM charge density in stabilized suspensions. In contrast, the variations of the low frequency effect (at hertz frequencies) after the isoelectric point of particles correlate well with the charge of macromolecules. Similarity between the frequency behavior of the electro-optical effect in stabilized suspensions and the dielectric dispersion of polyelectrolyte solutions is noted. The hydrodynamic thickness of the adsorbed layer, obtained from electric light scattering, is found to be independent of the charge of macromolecules in stabilized suspensions. Its value (Lr∼ 10 nm) is 5–6 times smaller than the thickness of the adsorbed uncharged polyacrylamide, indicating a flatter conformation of the adsorbed polyelectrolyte molecules.

The electric light scattering of aqueous oxide suspension in the presence of sodium carboxymethylcellulose (Na-CMC) was investigated. The electro-optical effect in the kilohertz frequency range was found to decrease because of the polyelectrolyte adsorption, contrary to the strong increase of the effect in the range of particles rotation (at hertz frequencies). The counterion mobility decrease due to condensation in the polyelectrolyte vicinity was supposed to be responsible for the relaxation frequency shift of the effect to the low-frequency side. The addition of salts (NaCl and CaCl2) to the stabilized suspensions was shown to change the ratio of high frequency to low frequency effects--the first one was seen to increase in contrast to the second. In the presence of salts, the polyelectrolyte charge shielding and counterions mobility increase were found to be different for monovalent and divalent ions, which could be attributed to the specific interaction of Ca2+ with Na-CMC.

The variation of the electro-optic decay times with the intensity and frequency of the applied electric field are investigated for a colloid system of rigid rod-like particles. In the kilohertz frequency range the decay process is consistent with field-free Brownian disorientation and the time constants are sensitive to interparticle electrostatic interactions. The abrupt rise in the electro-optic relaxation times accompanied by a decrease in the saturation value of the effects, observed in the process of deionization of the medium, indicates a phase transition from a gas to a liquid-like state of the colloid system. At low frequencies (hertz range and dc pulses) the decay times are reduced by an additional effect, most probably of a hydrodynamic nature.

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.

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.

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.

An electro-optical study of the role that the outermost layer plays in the behavior of a multilayer film of the weak polyelectrolytes poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) was carried out. The film surface was enriched with the chains of the last adsorbed polyelectrolyte when a fully ionized polymer was combined with nearly fully charged chains of the other polyelectrolyte. A distinct influence of the chain length on the critical frequency of relaxation of the electro-optical effect was demonstrated, indicating that polarization of condensed counterions of the last adsorbed polymer governs the behavior of the whole film. The multilayer growth, which was found to proceed via a series of adsorption-desorption steps, correlated well with the changes in the electro-optical effect. No indication was found, however, for participation of small ions from the film bulk in creation of the electro-optical effect.

Counterion polarization of sodium and magnesium polystyrenesulfonate (NaPSS and MgPSS), adsorbed in an excess onto ellipsoidal beta-ferric hydrous oxide particles (beta-FeOOH), is studied in both the absence and the presence of added simple electrolytes, NaCl and MgCl(2). The amplitude of the low-frequency (10(2)-10(4) Hz) and the high-frequency (10(4)-10(6) Hz) electro-optical effects of NaPSS is found to decrease with increasing NaCl concentration, while the critical frequency of relaxation of both effects remains unchanged. Addition of MgCl(2) to the suspension containing MgPSS also reduces the amplitude of the low-frequency electro-optical effect, but the amplitude of the high-frequency effect increases. The increased number of mobile (not bound) divalent ions, which produce larger charge fluctuation, could explain the high-frequency effect increase in the presence of a small excess of MgCl(2). Substitution of Na(+) counterions of PSS by Mg(2+) is found to increase the critical frequency of relaxation and to reduce the amplitude of the low-frequency effect, probably due to the strong repulsion between bound divalent counterions. The polyion length remains nearly unchanged during this substitution, as evidenced by the constant relaxation time of the polyelectrolyte-coated particles. The increase of counterion valence is not found to affect the amplitude of the high-frequency electro-optical effect in the absence of added salt. Copyright 1999 Academic Press.

The electric light scattering method was used to investigate the frequency behavior of ferric oxide particles in carboxymethylcellulose solutions containing mono- and divalent counterion mixtures. The dispersion curves of suspensions, stabilized by polyelectrolyte adsorption, were analyzed in terms of three relaxations: rotational relaxation of the polymer-coated particles (at frequencies below 1 kHz), low-frequency relaxation of the bound counterions polarization (at frequencies from 1 kHz to 10 kHz) and high-frequency relaxation of the free counterions polarization (at frequencies above 60 kHz) along the particle axis. The frequency behavior of the electro-optical effect from suspensions containing polyelectrolyte with counterions mixtures was explained in accordance with the theory that the fraction of free (mono- and divalent) counterions increases with increasing the equivalent fraction of divalent ions in the mixture. Immobility of the strongly bound divalent counterions at low field strengths is supposed to reduce their contribution in the low-frequency effect in comparison to the effect from the less bound monovalent counterions.