F. J. de las Nieves

Application of the colloidal stability of TiO2 particles for recovery and reuse in solar photocatalysis

TiO2-catalyst suspensions work efficiently in photocatalysis for wastewater treatment. Nevertheless, once photocatalysis is complete, separation of the catalyst from solution becomes the main problem. Catalyst recovery has been enhanced through charge neutralisation and coagulation with electrolytes at lab and pilot-plant scale (40 L) to evaluate the potential for its separation after photocatalytic degradation of pollutants. Zeta-potential analysis showed that the isoelectric point (IEP) of TiO2 suspensions is near pH 7. Settling rates and hydrodynamic diameter of TiO2 particles are maximum at the IEP. However, suspensions are stable at different pH. TiO2 was reused in solar photocatalysis pilot-plant (40 L) for treatment of tetrachloroethylene (C2Cl4) comparing two procedures: reuse of the entire suspension after destruction of the organics without separation of the catalyst, and reuse of the catalyst after it had settled to the bottom and clear water had been removed. Photocatalytic efficiency worsens with successive runs when catalyst and water are reused without separation, whereas, when TiO2 is separated, the photocatalyst is not deactivated.

Electrokinetic properties, colloidal stability and aggregation kinetics of polymer colloids

Abstract The purpose of this article is to present some important advances in the electrokinetic and colloidal characterization of polymer colloids. Special attention is paid to the new electrokinetic techniques: diffusiophoresis, dielectric dispersion and electro-acoustic. Also the most recent theoretical approaches are reviewed with respect to the electrokinetic properties of polymer colloids. Recently there has been intense discussion concerning electrokinetic processes and the theories used for data interpretation. Several concerns have been raised relating to the inability of the different processes and theories to yield the same electrokinetic potential. The most important explanations (shear plane expansion, preferential ion adsorption, osmotic swelling, crossing of the mobility/ξ-potential and anomalous surface conductance) to the electrokinetic behaviour of polymer colloids are discussed and analyzed. Also the effect of heat treatment on the electrokinetic properties of different types of polymer colloids is extensively considered. With regard to the colloidal stability of polymer colloids, three- and two-dimensional aggregations are presented. First, the stability factor W is introduced using the classical theory DLVO and the values obtained of Hamaker constant compared with the theoretical values estimated from the Lifshitz theory. The differences usually found by several authors are explained as due to the hydrodynamic interaction. Special attention is paid to the extended DLVO theory for studying homocoagulation of polymer colloids in three dimensions and to the new expressions for the van der Waals, electrostatic and structural forces that must be deduced to study the colloidal stability of polymer colloids in two dimensions. Also, the heterocoagulation of polymer colloids with different sign of surface charge density and particle size is reviewed, and a new definition of the heterocoagulation stability factor is given. The aggregation kinetics of polymer colloids in three dimensions is analyzed using the Smoluchowski theory (in terms of the reaction kernels k ij ) in the cases where the Smoluchowskis equation is analytically solvable (constant, sum, product kernel and linear combinations thereof). The dynamic scaling in aggregation phenomena with polymer colloids is studied using the classification scheme for homogeneous kernels due to Van Dongen and Ernst based on the relative probabilities of large clusters sticking to large clusters, and small clusters sticking to large clusters. The techniques (multiparticle and single particle detection) enabling us to provide cluster-size distribution of aggregating polymer colloids are also presented. Finally, the aggregation kinetics of two dimensional aggregation of polymer colloids is studied on the basis of the fractal dimension of the aggregates. The different scaling theories for two-dimensional aggregation are also considered.

Electrokinetic Studies on Positively Charged Polystyrene Latices

Abstract Using azo- N,N ′-dimethyleneisobutyramidinehydrochloride as the initiator, positively charged monodisperse polystyrene latices were prepared. Particle diameters ranged between 609 and 873 nm, and surface charge densities between 0.18 and 0.44 C m −2 . Both in electrophoresis of dilute dispersions and in streaming current measurements on concentrated plugs, the zeta potential as a function of ionic strength passes through a maximum. From an electrokinetic point of view, these positive latices thus closely resemble the negative ones. The maximum in ζ is probably due to the presence of a flexible layer on the particle surface. For both types of latex this layer accounts for an additional amount of surface conductance.

Salt effects over the swelling of ionized mesoscopic gels

In this work, the effects of salt concentration over the swelling of ionic mesoscopic gels will be studied theoretically and verified with experiments. We will restrict ourselves to the weak screening limit where the Debye screening length is larger than the mesh size of the gel. Under this condition, direct electrostatic interactions are negligible and the swelling is driven by the osmotic pressure of the ions. The swelling response of a mesoscopic gel is strongly dependent on the ionization degree of the gel. In particular, a maximum in the size–salt concentration curve appears for a partially ionized gel, when the salt concentration equals the network charge concentration. This maximum is removed for a totally ionized polymer network. Despite these facts, a charge independent asymptotic behavior between size and salt concentration becomes apparent, at sufficiently high values of the latter. The Flory–Huggins mean-field approach together with the Donnan relations describe the observed swelling adequatel...

Osmotic de-swelling of ionic microgel particles

In this work, we study experimentally the effect of an external osmotic pressure πext on the swelling of ionic mesoscopic gels in the weak screening limit, where the Debye screening length is larger than the mesh size of the gel. Variations in the osmotic pressure were induced by adding dextran to the solution. The results show that ionic microgels do not respond to πext below a given value of the normal stress; above this value the system de-swells with increasing osmotic pressure. The start of de-swelling is set by the gel charge density. The Flory thermodynamic theory for ionic gels captures the essential characteristics of the de-swelling behavior; in particular, it predicts with fairly good accuracy the value of πext at which de-swelling begins. Finally, due to the colloidal character of the gels, we observe that the system flocculates by a depletion interaction mechanism at high dextran concentrations.

The role of ζ potential in the colloidal stability of different TiO2/electrolyte solution interfaces

Abstract The present paper deals with the electrokinetic characterization and colloidal stability of different TiO2/water interfaces under the presence of several electrolytes. The electrophoretic mobility of the TiO2 particles was measured versus NaCl, NaNO3, Na2SO4 and CaCl2 concentrations, at pHs 4, 5.5 and 10.5. ζ Potential calculations have been carried out with the aid of O’Brien and Whites’ conversion theory. The medium pH and the valence of the ions have proven to be of great influence in our suspension electrokinetic behaviour. The colloidal stability has been studied by means of the Eilers and Korff index. The different flocculant character of the ionic species has been established, which is of great importance in the solar photocatalytic proccess where our titanium dioxide is employed.

Effect of electrolyte type on the electrokinetic behavior of sulfonated polystyrene model colloids

Sulfonated polystyrene latex particles were prepared by a two-stage “shot-growth” emulsion polymerization process in the absence of emulsifier. Sodium styrene sulfonate (NaSS) was used as an ionic co-monomer to produce a series of latex particles with the same particle size but with different surface charge densities. The electrophoretic mobility of this functionalized model colloid was studied in the presence of various types of inorganic electrolytes. The μe curves of these latexes exhibit a pronounced maximum at high electrolyte concentrations: 5·10−2 M for 1∶1 electrolytes and 10−2 M for 2∶1 and 1∶2 electrolytes. When a 3∶1 electrolyte (LaCl3) was used, the electrophoretic mobility changed to positive values at high concentration due to the specific adsorption of lanthanum species. The experimental results for the electrokinetic characterization of these sulfonated polystyrene model colloids suggest that the surface of the particles is covered by a layer of oligomers or polymer chains which shift the shear plane toward the bulk solution, increasing the anomalous surface conductance of the polystyrene microsphere-electrolyte solution interface.

Colloidal aggregation induced by attractive interactions

Colloidal aggregation induced by attractive interactions is tackled experimentally and by Brownian dynamics simulations using a mixture of positive and negative particles. The structure of the aggregates and the aggregation kinetics are used to characterize the aggregation behavior. The clusters show uniform internal structures, with a fractal dimension lower than that of clusters formed in diffusion, indicating a more branched architecture. The aggregation kinetics also differs from the diffusive one, slowing down as time proceeds. Both results are totally confirmed by simulation. The transition from the attractive driven to the diffusion controlled regimes is studied varying the range of interaction. Continuous transitions are observed both for the aggregation kinetics and cluster structure.

Motion of microgel particles under an external electric field

The influence of the swelling of charged microgel particles on their motion under an external electric field has been studied. The selected experimental observable was the electrophoretic mobility of the particles, which was measured as a function of the pH since its value controls the electrical charge of the particles. The mobility-pH curve presents a maximum and a minimum as a consequence of the competition between charge-density and friction coefficient variations during swelling. Ohshimas theory for polyelectrolyte-coated particles was employed, describing qualitatively the experimental results. Quantitative discrepancies suggest that charge renormalization should be considered.

Adsorption of monomeric bovine serum albumin on sulfonated polystyrene model colloids 1. Adsorption isotherms and effect of the surface charge density

Abstract The adsorption of monomeric bovine serum albumin (m-BSA) onto sulfonated polystyrene model colloids was studied by determining the adsorption isotherms under various pH and ionic strength conditions. The amount of m-BSA adsorbed was higher for the latex with a higher surface charge density for all the pH values pH (3–8) and ionic strengths (2 and 50 mM) studied. The maximum amount adsorbed was obtained at pH 5 (near the isoclectric point (IEP) of the protein), when the ionic strength was 2 mM, and shifted to pH 4 when the ionic strength was 50 mM. This amount (3.4mg m−2) was higher compared with that obtained with a conventional sulfate latex, but lower in comparison with that obtained with oligomeric BSA (o-BSA) (5.5 mg m−2). No significant desorption of m-BSA and o-BSA was detected after the redispersion of the latex-protein complexes under several, including extreme, experimental conditions. These results together with those obtained from the adsorption isotherms (with only one plateau) seem to indicate no multilayer formation or aggregation of BSA molecules. The flexibility of the BSA molecules and the asymmetry of the charge distribution seem to be the reasons for the high amount of protein adsorbed on sulfonated latexes.