A. Foissy

Use of electrophoretic mobility and streaming potential measurements to characterize electrokinetic properties of ultrafiltration and microfiltration membranes

Abstract The electrophoretic mobility of mineral oxide particles and the streaming potential of UF and MF inorganic membranes both depend on the electrical charge distribution at the oxide–solution interface. They are related to the zeta potential. The aim of this work is to evaluate experimentally how measuring the electrophoretic mobility and streaming potential complement each other for understanding the physical-chemical phenomena linked to filtration. We investigated the effect of pH and ionic strength on electrophoretic mobility and streaming potential values. The isoelectric point (i.e.p.) determined by microelectrophoretic measurements on zirconia particles obtained by membrane scraping is different from the i.e.p. obtained from pure zirconia powder. The i.e.p. obtained from scraped particles is the i.e.p. obtained from streaming potential measurements. Zeta potential values calculated from measurements using both techniques are compared to ascertain the accuracy of the conditions of validity of the Helmoltz–Smoluchowski relation for the streaming potential and zeta potential. Well-known inadequacies of this relation stem from double layer overlapping, the conductivity in the pores, etc. A correction factor, between zero and one is calculated. It is related to the zeta potential obtained from electrophoretic mobility measurements.

Role of polyelectrolyte dispersant in the settling behaviour of barium sulphate suspension

Abstract Settling properties of barium sulphate suspensions have been systematically investigated as a function of particle volume fraction and polyelectrolyte dispersant concentration. The settling kinetics and the microstructure of sediments were examined by turbiscan measurement and scanning electron microscopy at low temperature (open-fractured samples). In the absence of dispersant, following the particle volume fraction, several types of behaviour were observed. The overall experimental results are well described by a model, which takes account of the coupling between aggregation and sedimentation. In the presence of a dispersant in excess, the particles are well dispersed, no aggregation occurs, and the settling kinetics follows classical hydrodynamic laws. The nature of suspension stabilisation was investigated by electrophoretic and sediment compactness measurements all along polyacrylic acid (PAA) and polystyrene sulphonate (PSS) isotherm adsorption at pH 9. The settling results show a sharp transition between the behaviour of aggregating and well-dispersed suspension. For the two polyelectrolyte dispersants used (PAA and PSS), at the transition the adsorbed amounts correspond to a same value of the electrokinetic potential, a value that agrees well with DLVO theory predictions.

Molar mass selectivity in the adsorption of polyacrylates on calcite

Abstract Aqueous solutions of polyacrylates, with a wide molar mass distribution, have been equilibrated with aqueous dispersions of calcium carbonate particles. In mixtures with excess polyacrylates, it is found that only a fraction of the original collection of macromolecules adsorbs on the mineral surfaces. The adsorbed macromolecules have been recovered, and their molar mass distribution has been measured using size exclusion chromatography (SEC). It is found that a narrow fraction with intermediate molar mass is preferentially adsorbed. This selection results from a kinetic process where these macromolecules are the first to bind to the surface. Smaller macromolecules do not bind at all and larger ones could bind, but are repelled by the macromolecules that arrived first at the surface.

Study of Ca2+-induced silica coagulation by small angle scattering

Abstract The intefacial behavior of monodisperse (radius ≌ 15 nm) silica sols has been characterized by the evolution of their surface charge as a function of pH and salt concentration. The Ca2+ coagulation critical concentration, ccc, has been determined by turbidity between pH 7.5 and 9. The pH dependence of the calcium ccc shows that one adsorbed Ca2+ releases one H+ from the surface. Small angle neutron scattering gives a power law profile over one decade of Q for the silica aggregates. Their fractal dimension ranges from 1.7 to 2.1 depending on pH and [Ca2+; the faster the coagulation the greater the compacity. Small angle X-ray scattering (1.7 × 10−2

Effect of surface charge on the adsorption mode of sodium poly(acrylate) on alumina-coated TiO2 used as coating pigment

The surface properties of TiO2 and alumina-coated TiO2 used as pigments in papers and paints were investigated. The surface characterization of the coated TiO2 product by osmotic stress experiments and surface counterion titration reveals a large proportion of condensed ions which increases with ∣PZC-pH∣. Nevertheless, the number of uncondensed ions for a single particle (related to the effective charge number) remains almost the same for the ∣PZC-pH∣ investigated. This behavior is in agreement with modelisation on other systems. The concept of formal charge introduced by the MUSIC model is combined with the concept of effective charge on an (hydr)oxide surface. A mechanistic description of the adsorption mode of the sodium salt of poly(acrylic) acid (PaNa) on the alumina coated surface at pH 9 is suggested. The results show that only 5–6% of the functional groups of PaNa are linked to the surface and take the place of OH− and presumably Cl− counterions. The PaNa adsorption on this surface at pH 9 is MW independent as expected from theoretical models. The loop-conformation of the adsorbed polymer onto the coated TiO2 surface is deduced.

Electrodeposition of particles at nickel electrode surface in a laminar flow cell

Abstract Electrodeposition of polystyrene particles on a nickel electrode was observed in the laminar flow cell. The efficiency (rate) of the electrodeposition was determined depending on flow intensity, electrode polarization vs. Ag/AgCl electrode and ionic strength of the solution. The negative polystyrene latex particles were turned positive by adsorption of cetyl trimethyl ammonium bromide (CTAB) and experiments were performed at fixed concentration of the surfactant. It was found that at the CTAB concentration 5×10−5 mol dm−3 mostly single particles were deposited. It was observed that during the experiment, particles were deposited at a constant rate, which well correlated with the electrode polarization and their zeta potential. When the experiments were performed at various flow conditions in the laminar flow cell, the rate of deposition was found only weakly dependent on the flow intensity. That indicated that the hydrodynamic blocking effects were negligible under conditions encountered in the experiment. It was confirmed by the simulation of the dependence of the hydrodynamic blocking on the shear rate in the simple shear flow. At higher concentration of CTAB 5×10−4 mol dm−3 and for the electrode polarization of −1.5 V (vs. Ag/AgCl), transient aggregation at the electrode surface was observed. Qualitative aspects of this aggregation are presented. When the electrode polarization was removed the aggregates moved away except the initial ones, which were firmly attached to the surface. The observed phenomena can be explained in terms of clustering of particles by electrokinetic flows.

Evaluation of the adsorption trends of a low molecular-weight polyelectrolyte with a site-binding model

The site-binding model is very useful for describing the adsorption of ions and small ionized molecules. It has been slightly modified to include multi-site adsorption of larger molecules such as oligomers and low molecular weight polyelectrolytes. We describe alterations of the classical model and the results of calculations for adsorption of polyacrylic acid onto titanium dioxide as an example. The triple layer model is used to relate charge densities to interfacial potential profiles. Comparison between adsorption trends and the surface layer composition as a function of pH and ionic strength demonstrates the prominent influence of ions binding in the adsorption process. The site-binding model makes it easy to simulate the ions displacement associated with polyelectrolyte adsorption. Strongly bound electrolyte anions prevent polyacrylic acid from adsorbing, and, in contrast, electrostatic screening due to cation condensation makes it easier. Calculations of the pH change in the solution, due to adsorption, are also made by comparing ionization ratios of both the surface and polymer units in the adsorbed layer and before adsorption. Trends in electrokinetic potentials as a function of the solutions parameters are evaluated assuming the identity of the shearing surface and the inner boundary of the diffuse layer. All data compare well with experimental values. The very good agreement betwen the experiment and model calculations supports the fact that (small) polyelectrolyte molecules adsorb essentially flat on the surface.

Electrodeposition of latex particles in the presence of surfactant: Investigation of deposit morphology

The deposition of micrometric latex particles on a polarized nickel surface was investigated using a laminar flow cell equipped with a video assembly used to observe and record particle behavior near the electrode. The effects of the nature of the counterions and the concentrations of surfactants on the deposit structure were studied. Negative polystyrene latex particles were turned positive by adsorption of cetyltrimethylammonium in the form of different salts: bromide (CTAB), chloride (CTAC), and hydrogenosulfate (CTAHS). Image analysis was used to gain information on the mechanisms of particle deposition on the electrode. At CTAB concentration 5x10(-5)M, mostly single particles were deposited on the electrode and their adhesion was irreversible. The adsorption mechanism was shown to be dependent on the succession of electrophoretic migration and attractive particle-surface interactions. At a higher CTAB concentration (5x10(-4)M) a transient 3D aggregation was observed which was attributed to electroosmotic and electrohydrodynamic phenomena in the vicinity of the electrode. In the presence of CTAC, aggregates were formed on the electrode for both concentrations. In the case of CTAHS the deposition rate was very low in comparison with CTAB and CTAC. This result was explained by the lower zeta potential of the particles with respect to the other cases. The formation of the aggregates was reversible; furthermore, their morphologies were strongly dependent on the kind of counterion. The aggregates formed in CTAB solution were dense while more open structures were observed with CTAC.

Adsorption d'un polycondensat d'acide naphtalene sulfonique (PNS) et de formaldehyde sur le dioxyde de titane

Abstract Formaldehyde condensate of naphthalene sulfonate is a polymeric compound widely used as a fluidifying agent in cement pastes. Using titanium dioxide, this study is aimed at an analysis of the influence of some practical adsorption parameters, namely, Ph, ionic strength and ion nature (Na + , Ca ++ ). In acidic media (pH −2 moles.1 −1 leads to an adsorption isotherm similar to that obtained in acidic media. The sedimentation volume and the electrophoretic mobility of the particles are determined: the macroscopic properties of the dispersions can be correlated to the surface charge. The evolution of the polymer adsorption as function of pH and ionic strength is connected to variations of superficial charge density of PNS and mineral oxide. However, naphtalene α-sulfonic acid is never adsorbed.