Sandor Barany

Field-induced disturbance of the double layer electro-neutrality and non-linear electrophoresis

The influence of applied electric field on the field-induced variation of the electrolyte concentration (concentration polarization) disturbs the electro-neutrality of the system, represented by a dispersed particle and its electric double layer in electrolyte solution. The manifestation of this electro-neutrality disturbance in the non-linear electrophoresis was considered in the framework of a procedure of successive approximations in powers of the applied field strength. Analytic expressions describing the component of the electrophoretic velocity proportional to the cubic power of the applied field strength (cubic electrophoresis) were obtained for arbitrary values of the surface conductivity (of Dukhin number). The model restrictions are spherical non-conducting particle with homogeneous surface and thin double layer.

Complex electrosurface investigations of dispersed microphases

Abstract The structure of the electrical double layer (EDL) of colloidal systems is discussed. The methods of determination of the most important parameters of the EDL, i.e. the value of the surface-potential ψ 0 , the Stern-potential ψ δ and the electrokinetic-potential ζ, the surface charge density σ 0 , the electrokinetic charge σ ζ , as well as the specific surface conductance κ σ and Rel parameter are described. The application of the theory of non-linear electrosurface phenomena developed by Dukhin, Derjaguin and Shilov for calculation of the EDL parameters is discussed. The above data of the EDL characteristics for polystyrene and melamin-formaldehyde latices, suspensions of AgI, Sb 2 S 3 , Fe 2 O 3 , ZrO 2 , hydromica, palygorskyte, yeast cells as well as kerosene-in-water, ET grade lubricating oil technical emulsions in different electrolyte solutions are discussed. It has been shown that in the most of cases (with the exception of AgI solution) ψ 0 or ψ δ >ζ and σ 0 ≫σ ζ . It indicates the presence of a considerable free charge between the surface and slipping plane due to the formation of hydrodynamically immobile water layers on the surface in which the ions retain high mobility. The necessity of complex (integrated) electrosurface measurements for the description of the electrical double layer structure is stressed. A new kind of non-linear electrokinetic phenomena, namely the superfast electrophoresis is described. The phenomenon was predicted theoretically by Dukhin and Mishchuk and investigated experimentally in detail in the authors laboratory. It has been shown that the electrophoretic mobility of large (hundreds μm) ion-type conducting particles like ion-exchanger grains/fibres or electron-type conducting particles like Al/Mg alloy, graphite and activated carbon in strong electric fields (100–1000 V/cm) exceeds the electrophoretic mobility values typical for non-conducting particles by 1–2 orders of magnitude. The mobility of such particles depends on the conductivity ratio between the particles and medium and strongly increases with the electric field gradient and the particle size. This is in contrast to classical electrophoresis. The superfast electrophoresis is due to the interaction of a strong electric field with the space charge near the surface of unipolar conducting particles. This space charge is induced by the strong external field because of the concentration polarisation.

SURFACE PROPERTIES AND AGGREGATION OF BASIC ALUMINIUM CHLORIDE HYDROLYSIS PRODUCTS

Abstract The influence of major variables (working solution concentration, salt dosage, pH, ionic strength of the solution) on the electrokinetic potential, degree of hydration and aggregation of products of hydrolysis of basic aluminium chlorides (BAC) during water purification has been studied. The degree of hydration characterised by the spin-lattice relaxation time of the protons of adsorbed water increases with increasing basicity ([OH]/[Al] ratio) of coagulants, pH value and with decreasing the dose of salts and the concentration of their working solutions; this is reflected in the change of the ζ -potential of particles. It is shown that an increase of water pH and alkalinity, as well as decrease of BAC basicity lead to the formation of primary particles of a larger diameter (3.1–8.7 nm) whose surface area sharply reduces in the first minutes of aggregate formation. The latter is explained by different size-to-density properties (measured by laser diagnostic method) when varying the conditions of their formation. Aggregation of BAC hydrolysis product particles occurs in a discrete way; aggregates of higher orders are formed not from primary particles but from the first-order aggregates. An increase in basicity of BAC specimens leads to formation of more polydisperse and larger aggregates. The role of electrostatic and hydration factors in the mechanism of aggregation of ABC hydrolysis products is discussed.

Coagulation rate of silica dispersions investigated by single-particle optical sizing

Abstract Using a single-particle optical sizer, the kinetics of the rapid coagulation of monodisperse silica particles of radius 265 nm in KCl and CaCl2 solutions was studied. The rate constant k11 of the process is 1.7 × 10−12 cm3 s−1, which is three times smaller than the Smoluchowski value for the overall rate constant. This discrepancy is explained by the retardation effect of viscous interactions and the existence of long-range attractive forces between particles. For suspensions with an initial particle concentration between 2.5 × 108 and 2.4 × 109 cm−3, the k11 values are independent of the pH value (in the range 7–10) and of the particle number concentration. The ratio between the coagulation concentrations of monovalent and bivalent counterions is 50, under both perikinetic and orthokinetic conditions, which indicates that this silica suspension is electrostatically stabilized. This is in contrast to literature data for very small SiO2 particles. We propose that the so-called “structural” forces are much less important for large silica particles.

Mechanism of Methylene Blue adsorption on hybrid laponite-multi-walled carbon nanotube particles

The kinetics of adsorption and parameters of equilibrium adsorption of Methylene Blue (MB) on hybrid laponite-multi-walled carbon nanotube (NT) particles in aqueous suspensions were determined. The laponite platelets were used in order to facilitate disaggregation of NTs in aqueous suspensions and enhance the adsorption capacity of hybrid particles for MB. Experiments were performed at room temperature (298 K), and the laponite/NT ratio (Xl) was varied in the range of 0-0.5. For elucidation of the mechanism of MB adsorption on hybrid particles, the electrical conductivity of the system as well as the electrokinetic potential of laponite-NT hybrid particles were measured. Three different stages in the kinetics of adsorption of MB on the surface of NTs or hybrid laponite-NT particles were discovered to be a fast initial stage I (adsorption time t=0-10 min), a slower intermediate stage II (up to t=120 min) and a long-lasting final stage III (up to t=24hr). The presence of these stages was explained accounting for different types of interactions between MB and adsorbent particles, as well as for the changes in the structure of aggregates of NT particles and the long-range processes of restructuring of laponite platelets on the surface of NTs. The analysis of experimental data on specific surface area versus the value of Xl evidenced in favor of the model with linear contacts between rigid laponite platelets and NTs. It was also concluded that electrostatic interactions control the first stage of adsorption at low MB concentrations.

Kinetics and mechanism of flocculation of bentonite and kaolin suspensions with polyelectrolytes and the strength of floccs

The effects of cationic (Zetag 7589, Zetag 8660, and SNF 528) and anionic (Magnafloc 1011) polyelectrolytes, as well as the regimes and intensity of suspension stirring, on the kinetics of formation, disruption, and subsequent regrowth of aggregates of bentonite and kaolin particles, are studied in a flow system. The optimum polymer doses and the stirring conditions providing the formation of largest and strongest floccules are found. Under comparable conditions, the effect of the adsorption of the above polymers on the electrokinetic potential and the degree of aggregation of particles is studied. It is shown that intense flocculation takes place long before the isoelectric point of particles is reached. This allows one to conclude that the formation of polymer bridges plays a significant role in the flocculation of the examined suspensions with polyelectrolytes. Different dependencies of flocculation on the dose of added polyelectrolytes have been found for diluted and concentrated suspensions. This is explained by different extents to which flocculants adsorbed on the particle surface approach the equilibrium state at different concentrations of dispersed phases.

Electrokinetic potential of bentonite and kaolin particles in the presence of polymer mixtures

In order to elucidate the mechanisms of flocculation by polymer mixtures, the effect of adsorption of non-ionic poly(ethylene oxide) — PEO, two samples of strongly (SNF FO 4800) and medium charged (SNF FO 4350) cationic and two samples of medium (SNF AN 935) and weakly charged (SNF AN 905) anionic polyelectrolytes (PE) as well as their binary mixtures on the electrokinetic potential of bentonite and kaolin particles has been studied. It is shown that in the presence of PEO-anionic/cationic polymer mixture, the electrokinetic potential of particles is determined by the adsorption of the polyelectrolyte; neither cationic nor anionic segments can be displaced by the non-ionic polymer. In mixtures of cationic and anionic polyelectrolytes, the ζ-potential of particles is determined by the adsorbed amount of anionic polymer independently of the charge density of PE and way of addition of the mixture components to the suspension, i.e. (1) first adding the cationic polymer, then the anionic one, or (2) first adding the anionic polymer then the cationic one, or (3) adding an increasing amount of pre-prepared 1: 1 mixture. The highest absolute ζ-potential values are observed for pH 7.5 when the surface of bentonite or kaolin particles is “purely” negatively charged and the anionic PE layer is most extended because of few contacts to the surface. With decreasing the pH, the (negative) ζ-potential of particles decreases due to appearance of a small amount of positive charges on the surface that bond an increasing amount of negative segments and results in shrinking of the adsorbed layer of the anionic PE. It is shown also that the electrokinetic potential of particles in anionic and cationic PE mixtures at all studied pH (4, 5, and 7.5) depends on the spatial distribution of negatively charged segments near the surface. The regularities observed are explained by formation of long loops and tails of anionic segments on the surface because of the small number of contacts to the surface; the cationic polyelectrolyte forms on the surface a thin layer with a big number of contacts and which is hidden behind the more extended anionic polymer layer.

Electrokinetic Potential and Flocculation of Bentonite Suspensions in Solutions of Surfactants, Polyelectrolytes and Their Mixtures

The effects of cationic and anionic surfactants and polyelectrolytes, as well as their binary mixtures, on the electrokinetic potential and the degree of aggregation/sizes of particle flocs in bentonite suspensions have been studied. The amounts of the above reagents, compositions and ratios of positive and negative charges in the mixtures, charge densities of the polyelectrolytes, and the order of the introduction of the components into systems have been varied. It has been shown that the addition of cationic surfactants or polyelectrolytes and mixtures thereof causes a substantial reduction of the negative ζ-potentials of the particles and reversal of the sign of their charge. The addition of anionic surfactants or polyelectrolytes leads to a twofold increase in the ζ-potential as a result of adsorption of negatively charged surfactant ions/polymer segments on negatively charged clay particles via non-Coulomb forces. In the presence of mixtures of anionic and cationic polyelectrolytes, irrespective of their composition and the order of the introduction of the components into a suspension, the particles acquire a negative ζ-potential, which is characteristic of particles that adsorb an anionic polyelectrolyte alone. Synergistic flocculating action is observed for mixtures of weakly and moderately charged anionic and cationic polyelectrolytes. This effect takes place even for mixtures with a manifold excess of negatively charged segments relative to positively charged ones. The observed regularities have been explained by the structural peculiarities of mixed adsorption layers of polyelectrolytes, i.e., the coexistence of a thin layer of adsorbed cationic polymer chains characterized by a large number of contacts with the surface and an extended layer of an anionic polyelectrolyte (long loops and tails of macromolecules), in which the cationic polymer layer is “drowned.” The electrokinetic potential and the ability of the particles to aggregation (via the formation of polymer bridges) in mixed solutions of anionic and cationic polyelectrolytes are primarily determined by the adsorption value of the anionic polymer.

Electrokinetic potential of multilayer carbon nanotubes in aqueous solutions of electrolytes and surfactants

The effect of electrolytes with single-, double- and triple-charged counterions, as well as cationic (cetyltrimethylammonium bromide) and anionic (sodium dodecyl sulfate) surfactants, on the electrokinetic potential of multiwall carbon nanotubes prepared via catalytic pyrolysis of propylene in a gas phase according to the CCVD technology has been studied. It has been shown that the influence of different electrolytes on the electrophoretic mobility of the nanotubes does not differ essentially from their effect on the behavior of well-studied inorganic dispersed particles; i.e., the dependence of the absolute values of the ζ-potential on the concentration of a 1: 1 electrolyte passes through a maximum and the introduction of double-charged counterions dramatically reduces the ζ-potential, while the addition of triple-charged cations and a cationic surfactant causes charge reversal of the nanotube surface. The adsorption of sodium dodecyl sulfate in a neutral medium increases the negative ζ values; this effect evens out in the presence of an alkali due to a rise in the electrostatic repulsion between surfactant anions and nanotube surface, which bears a high negative charge resulting from the dissociation of surface functional groups.

Effect of Hydrodynamic Conditions on the Kinetics of Bentonite Suspension Flocculation by Cationic Polyelectrolytes and the Strength of Formed Flocs

The influence of Zetag and SNF FO cationic polyelectrolytes on the aggregation kinetics of bentonite particles in a flow system is studied in detail as a function of the dose of added polymer, charge density of its macromolecules, the regime and intensity of system stirring, and the type of stirrer (magnetic and mechanical stirrers). It is shown that there is an optimal regime, namely, the alternation of slow and rather rapid stirring, that provides the formation of the largest and strongest flocs. The rate of aggregation and the size of formed flocs increase with the amount of added reagent irrespective of the intensity of system stirring, as well as with an increase in the charge density of polyelectrolyte, which is responsible for the affinity of macromolecules for the surface. The effect of polymer charge on the flocculation kinetics is exhibited first of all upon the slow stirring of suspension. The difference in the sizes of aggregates resulted from the flocculation of macromolecules with different charges is leveled with an increase in stirring intensity. An empirical method for comparing the efficiency of the flocculating action of polymers in suspensions prepared using different types of stirrers is proposed.