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 mechanism and dispersion efficiency of three anionic additives [poly(acrylic acid), poly(styrene sulfonate) and HEDP] on zinc oxide.

Adsorption on ZnO of sodium poly(acrylate) (PAA), sodium poly(styrene sulfonate) (PSS) and a monomer surfactant [hydroxyethylidene diphosphonate (HEDP)] was investigated in suspensions initially equilibrated at pH 7. Results demonstrate interplay in the adsorption mechanism between zinc complexation, salt precipitation, and ZnO dissolution. In the case of PAA, the adsorption isotherm exhibits a maximum attributed to the precipitation of zinc polyacrylate. PSS and HEDP formed high-affinity adsorption isotherms, but the plateau adsorption of HEDP was significantly lower than that of PSS. The adsorption isotherm of each additive is divided into two areas. At low additive concentration (high zinc/additive ratio), the total zinc concentration in the solution decreased and the pH increased upon addition. At a higher additive ratio, zinc concentration and pH increased with the organic concentration. The increase in pH is due to the displacement of hydroxyl ions from the surface and the increase in zinc concentration results from the dissolution of ZnO due to the complexation of zinc ions by the organics. The stability of the ZnO dispersions was investigated by measurement of the particle size distribution after addition of various amounts of polymers. The three additives stabilized the ZnO dispersions efficiently once full surface coverage was reached.

Early steps in layer-by-layer construction of polyelectrolyte films: the transition from surface/polymer to polymer/polymer determining interactions.

The layer-by-layer deposition of two polyelectrolytes, quaternized poly(dimethylaminoethyl methacrylate chloride) (MADQUAT) and poly(acrylic acid) (PAA) on a silica substrate was investigated using optical reflectometry, as a function of pH (pH 4, 5.5 and 9), ionic strength (10(-3) to 10(-1) M) and type of salt. Attention was given to the successive deposited weights and to the corresponding deposited charge densities within the ten first deposited layers. Results show a change of growth regime between an early stage where the substrate had a dominating influence in the build-up and a second stage where the polymer uptake was ruled essentially by polymer-polymer interactions. The pH was seen to influence the growth via the charge densities of silica (first stage) and PAA (first and second stages). The increase of NaCl concentration induced an increase of the film weight between 10(-3) and 10(-2) M, but the trend was more sophisticated between 10(-2) and 10(-1) M where the polymer uptake increased in the first stage of the growth and decreased in subsequent layers. The film weight increased in accordance with the rank of ions in the Hofmeister series. AFM images revealed a heterogeneous film morphology with bumps and valleys, which was explained by a growth mechanism made of the successive formation and growth of polymer complexes.

Reflectometric study of the adsorption of poly(vinyl imidazole) on a gold electrode, effects of pH, and applied potential.

The adsorption-desorption behavior of poly(vinyl imidazole), a weak polybase (pH-dependent positive charge), on a gold electrode was investigated using optical fixed-angle reflectometry. Using an instrument comprising an impinging-jet system, the hydrodynamic conditions were well defined, making it possible to study the adsorption rate. Comparison between the actual adsorption rate and that of a purely diffusion-controlled process revealed the occurrence and the change of an electrostatic barrier in the adsorption process. The surface charge of the gold electrode was varied by means of an externally applied potential. The surface charge density was evaluated by separate electrochemical impedance spectroscopy. The uptake and the adsorption rate were very sensitive to pH and electrode polarization. At pH 3, the adsorption of the fully charged polymer increased fairly regularly with cathodic polarization, whereas it remained at about 0.4 mg m(-2) in the anodic zone At pH 8, the adsorption of the uncharged polymer decreased with the negative charge of the electrode due to the more favorable adsorption of potassium ions on the charged electrode. Discrepancies in adsorption-desorption measurements taken while cycling the pH were due to an electrostatic adsorption barrier.

Mass and charge balance in self-assembled multilayer films on gold. Measurements with optical reflectometry and quartz crystal microbalance.

This work aimed to the determination of weight uptakes and charge balance in the course of successive deposition of polyelectrolytes, using the so-called self-assembled multilayer technique. Polyelectrolytes were the quaternized polydimethylaminoethyl methacrylate chloride, (MADQUAT) and poly(acrylic acid) (PAA). Experiments were made at pH 5.5 in NaCl solutions between 10(-3) and 10(-1) M. Deposits (5 bilayers) on a gold substrate were monitored using a quartz crystal microbalance (QCM) and optical fixed-angle reflectometry. Analysis of data lead to the determination of the sensitivity factor of the reflectometric output. QCM allowed the direct measurement of weight uptakes in 10(-3) and 10(-2) M solutions, while the viscoelastic properties of the film did not look appropriate for the measurement in 10(-1) M solutions. The layer-by-layer uptakes and charge balances in 10(-3) and 10(-2) M solutions revealed a large contribution of the counterions in the neutralization of the electrical charge in the film, more so for the highly charged MADQUAT polymer. The difference between two successive polymer charge densities increased significantly with the layer number and the electrolyte concentration. The increase of NaCl concentration induced an increase of MADQUAT but reversely a decrease of PAA deposits. The results were consistent with the determining influence of the salt in polyelectrolyte adsorption, both with regards to the concentration and the type of ions that has been well demonstrated in the literature. This work also draws attention to the role of small ions in the structural and application properties of self-assembled multilayer films.

Interaction between poly(vinylimidazole) and sodium dodecyl sulfate: binding and adsorption properties at the silica/water interface.

The adsorption properties (adsorbed amount, kinetics, and reversibility) of poly(vinylimidazole) (PVI) and sodium dodecyl sulfate from PVI/SDS mixed solutions on negatively charged silica substrates were studied at pH 9 using reflectometry and compared to that measured on colloidal silica by the solution depletion method. In this paper, we will try to gain insight into the effect of PVI/SDS complex composition on the adsorption characteristics of the complex and particularly on the kinetics of the complex adsorption and its consequence on the adsorption reversibility. The properties of the complex in solution were characterized by means of potentiometric titration at a constant pH, binding isotherm, and surface tension measurements. On the basis of the experimental results the prevailing mechanism of the SDS/PVI interaction and the properties of the PVI/SDS complex were evaluated. Both the PVI/SDS complex uptake and the kinetics of the adsorption decreased with the amount of SDS bound to PVI. At low PVI/SDS binding ([SDS](0)CAC) the incoming complex experiences a blocking barrier of an electrostatic nature. This barrier has been confirmed by reversibility measurement, and the respective roles of the complex structure and charge were assessed.

Contribution of convection, diffusion and migration to electrolyte transport through nanofiltration membranes.

Transport mechanisms through nanofiltration membranes are investigated in terms of contribution of convection, diffusion and migration to electrolyte transport. A Donnan steric pore model, based on the application of the extended Nernst-Planck equation and the assumption of a Donnan equilibrium at both membrane-solution interfaces, is used. The study is focused on the transport of symmetrical electrolytes (with symmetric or asymmetric diffusion coefficients). The influence of effective membrane charge density, permeate volume flux, pore radius and effective membrane thickness to porosity ratio on the contribution of the different transport mechanisms is investigated. Convection appears to be the dominant mechanism involved in electrolyte transport at low membrane charge and/or high permeate volume flux and effective membrane thickness to porosity ratio. Transport is mainly governed by diffusion when the membrane is strongly charged, particularly at low permeate volume flux and effective membrane thickness to porosity ratio. Electromigration is likely to be the dominant mechanism involved in electrolyte transport only if the diffusion coefficient of coions is greater than that of counterions.

Evaluation of the "DSPM" model on a titania membrane: measurements of charged and uncharged solute retention, electrokinetic charge, pore size, and water permeability.

The DSPM (Donnan steric partitioning pore model) was evaluated in the case of a titania membrane with "nanofiltration properties" by measuring the electrokinetic charge, pore size, and water permeability of the membrane, along with charged and uncharged solute retention. The zeta potential values (zeta) were determined from measurements of the electrophoretic mobility (EM) of titania powder forming the filtering layer of the membrane. Zeta potential values were converted into membrane volume charge (X) by assuming two limiting cases: a constant surface charge (sigma(s)(cst)) and a constant surface potential (psi(s)(cst)). The mean pore radius and thickness/porosity ratio of the membrane were determined by permporometry and from water permeability measurements, respectively. Retention measurements were carried out as a function of the permeate volume flux for both neutral solutes (polyethylene glycol PEG of different size) and salts (KCl, MgSO4, K2SO4, and MgCl2) at various pH values. Ionic retentions showed minimum values near the IEP of the membrane. Retention data were analyzed using the DSPM. Very good agreement was found between the pore radius calculated by the model and that determined by permporometry. X values calculated from fitting retention data using the DSPM were also in satisfactorily agreement with X values calculated from EM measurements assuming a constant surface potential for a large pH range. Furthermore, the DSPM leads to X values (X(DSPM)) between those calculated from EM (X(EM)) using the two limiting bounds. In other words, X(DSPM) was higher than X(EM) assuming psi(s)(cst) at pH values far from the isoelectric point (IEP) and lower than X(EM) assuming sigma(s)(cst). These results show that the DSPM is in qualitative agreement with the charge regulation theory (increase of the pore surface potential and decrease of the pore surface charge density with decreasing the pore size). On the other hand, the thickness/porosity ratio of the membrane calculated from solute retention data differed significantly from that determined from water permeability measurements. Moreover, a single value of Deltax/Ak could not be determined from PEG and salt retention data. This means that the Deltax/Ak parameter loses its physical meaning and includes physical phenomena which are not taken into account by the DSPM. Nevertheless, the model satisfactorily predicted the limiting retention, as this is not influenced by the Deltax/Ak parameter.

Electrokinetic characterization of porous plugs from streaming potential coupled with electrical resistance measurements.

The zeta potential of mixed nickel-iron oxide particles is evaluated by a new laboratory instrument. This latter allows the measurement of streaming potential together with the electrical resistance of porous plugs. The conductivity of electrolyte inside plug (pore conductivity) is deduced from electrical resistance measurements and is used together with streaming potential to evaluate the zeta potential by accounting for the surface conduction phenomenon. It is shown that neglecting the surface conduction phenomenon leads to a substantial underestimation of the zeta potential. The coupled measurements of streaming potential and plug electrical resistance yield zeta potential values that are in very good agreement with those obtained by electrophoresis. The densification of the porous plug with increasing pressure increments is put in evidence by the decrease in measured streaming potentials. Electrical resistance measurements make it possible to account for the increase in surface conductivity resulting from the more compacted structure of the plug. By doing so, the calculated zeta potential is found to be virtually independent of the pressure difference involved in streaming potential experiments, whereas the negligence of surface conduction phenomenon leads to a decrease in the apparent zeta potential with increasing pressure level.

Thin wetting films from aqueous electrolyte solutions on SiC/Si wafer.

The stability and rupture of thin wetting films from aqueous NaCl or Na2SO4 solutions of different concentrations on silicon carbide were investigated. The flat surface of SiC was obtained by plasma-enhanced chemical vapor deposition (PE-CVD) on top of a silicon wafer. The microinterferometric method was used for measuring the film thickness with time. The light reflectance was calculated as a function of film thickness for the four-layer system: air/aqueous solution/SiC/Si wafer. The microinterferometric experiments showed that films from aqueous NaCl and Na2SO4 solutions with concentrations up to 0.01 M were stable independent of the pre-treatment of the substrate. The pre-treatment of the SiC surface was crucial for the wetting film stability at electrolyte concentrations greater than 0.01 M. The films were unstable and ruptured if SiC was washed with 5% hydrofluoric acid and concentrated sulfuric acid, while they were stable if washing was in sulfuric acid only, without immersing SiC in HF. The average equilibrium film thickness was determined as a function of electrolyte concentration. Measurements of the electrokinetic potential zeta were performed by electrophores of SiC powder in 0.001 M NaCl. It was shown that silicon carbide surface was negatively charged. The theory of heterocoagulation was used for the interpretation of the results. Besides the DLVO forces, the structural disjoining pressure (both positive and negative) has been included in the analysis.

Potentiality of reflectometry for the study of the adsorption on dielectric and metal substrates: application to the adsorption of polyvinylimidazole on silica and gold.

Optical fixed-angle reflectometry was examined as a technique to study the adsorption of organics (polymers, surfactants) on metallic substrates. Calculations were made to evaluate the sensitivity of the method with regard to the refractive index and the thickness of the metal and the incidence angle. Theoretical results show that Ti, Cr, V, Zr, and gold have an appropriate sensitivity for detecting the adsorption accurately. Parallel experiments using gold and silica substrates and polyvinylimidazole (PVI) as an organic material were carried out. Adsorption measurements as a function of polymer concentration, pH, and ionic strength were in accordance with expected trends.

Surface characterization of oil-containing polyterephthalamide microcapsules prepared by interfacial polymerization.

Oil-containing polyterephthalamide microcapsules were synthesized by the interfacial polymerization of an oil-soluble monomer (terephthaloyl dichloride, TDC) and a mixture of two water-soluble monomers (diethylenetriamine, DETA, and 1, 6-hexamethylenediamine, HMDA). The influence of several synthesis parameters (e.g. concentration ratio of the two amine monomers, stirring rate, concentration of the steric stabilizer PVA) on the size distribution, the membrane morphology and the electrokinetic properties of the microcapsules, was thoroughly investigated. Morphological analysis by electron microscopy showed a strong dependence of the membrane external morphology on the functionality of the water-soluble amine monomer. High stabilizer concentrations and agitation rates during emulsification favoured the production of smaller microcapsules with non-porous and rigid membranes. The electro-chemical interfacial properties of the microcapsules were investigated using a combination of potentiometric, conductimetric and electrokinetic measurements. The dependence of the mean surface charge density on pH revealed the presence of essentially two kinds of chemical groups (e.g. amino and carboxylic groups) on the microcapsule external surface. The total concentration of surface chemical groups and the isoelectric pH were measured as a function of the microcapsule synthesis conditions. Using the experimental data and an appropriate interfacial ionization model, the ratio of the surface groups densities, R = (S - COOH)/(S NH3(+) ), was evaluated and rationalized with respect to the microcapsules synthesis parameters.

Determining the Zeta Potential of Porous Membranes Using Electrolyte Conductivity inside Pores.

The zeta potential is an important and reliable indicator of the surface charge of membranes, and knowledge of it is essential for the design and operation of membrane processes. The zeta potential cannot be measured directly, but must be deduced from experiments by means of a model. The possibility of determining the zeta potential of porous membranes from measurements of the electrolyte conductivity inside pores (lambda(pore)) is investigated in the case of a ceramic microfiltration membrane. To this end, experimental measurements of the electrical resistance in pores are performed with the membrane filled with KCl solutions of various pHs and concentrations. lambda(pore) is deduced from these experiments. The farther the pH is from the isoelectric point and/or the lower the salt concentration is, the higher the ratio of the electrolyte conductivity inside pores to the bulk conductivity is, due to a more important contribution of the surface conduction. Zeta potentials are calculated from lambda(pore) values by means of a space charge model and compared to those calculated from streaming potential measurements. It is found that the isoelectric points are very close and that zeta potential values for both methods are in quite good agreement. The differences observed in zeta potentials could be due to the fact that the space charge model does not consider the surface conductivity in the inner part of the double layer. Measurements of the electrolyte conductivity within the membrane pores are proved to be a well-adapted procedure for the determination of the zeta potential in situations where the contribution of the surface conduction is significant, i.e., for small and charged pores. Copyright 2001 Academic Press.

Binding and Ion-Exchange Analysis in the Process of Adsorption of Anionic Polyelectrolytes on Barium Sulfate.

The adsorption of poly(acrylic acid) and poly(styrenesulfonic acid) (PSS) is studied on a barium sulfate powder. Comparison of the polyelectrolytes shows that the difference in binding strength corroborates the difference between the ligand strengths with barium ions. The surface excess dependence on pH correlates with the density of the ionized groups. The electrokinetic potential follows the surface coverage and the ionization ratio of the polymer up to the onset of the adsorption plateau, but continues to increase above that point. This peculiarity is explained by the release of barium ions from the adsorption layer into the solution. The phenomenon is attributed to the complexing power of unadsorbed molecules. Analysis of the displacement of small ions (Na(+), SO(2-)(4)) shows that adsorbed PSS releases sulfate ions from the surface and sodium counterions from the polymer. The displacement ratio for sulfate ions (SO(2-)(4)/PSS monomer units) remains constant over the adsorption isotherm, but that for sodium ions is constant only up to about two-thirds of the maximum coverage. From the data we deduced that about half of the monomer units of the adsorbed PSS molecules bind with surface barium ions. The other half form barium and sodium sulfonates whose ratio varies with the concentration of unadsorbed molecules. Copyright 1999 Academic Press.

Surface Complexation of Calcite by Carboxylates in Water.

Small molecules that have two carboxylic functions can adsorb from water onto calcite. The adsorption site is a -Ca+ site. The mechanism of adsorption is a complexation of the -Ca+ site by the two carboxylates, similar to the solution complexation of Ca++ ions. The complex has a ring structure where the two carboxylates are joined on one side by the -Ca+ ion and on the other by the n CH2 groups of the small molecule. Five-bond rings (n = 0) are the most stable, followed by six-bond rings (n = 1) and seven-bond rings (n = 2). Five-bond rings can also be formed with one carboxylate and one hydroxyl group (this is the case for alpha-hydroxycarboxylates) or with one enolate and one hydroxyl group (catechol). The sequence of binding strengths is enolate > carboxylate > hydroxyl; it matches the sequence of complexation efficiencies of these groups in solution and their characters as electron donors toward the metal cation. Copyright 1999 Academic Press.