Ion-ion correlation and charge reversal at titrating solid interfaces

Confronting grand canonical titration Monte Carlo simulations (MC) with recently published titration and charge reversal (CR) experiments on silica surfaces by Dove et al. and van der Heyden it et al, we show that ion-ion correlations quantitatively explain why divalent counterions strongly promote surface charge which, in turn, eventually causes a charge reversal (CR). Titration and CR results from simulations and experiments are in excellent agreement without any fitting parameters. This is the first unambiguous evidence that ion-ion correlations are instrumental in the creation of highly charged surfaces and responsible for their CR. Finally, we show that charge correlations result in "anomalous" charge regulation in strongly coupled conditions in qualitative desagreement with its classical treatment.Comment: 4 pages, 4 figures, submitted to PR

Interactions of colloidal particles with simple electrolytes and polyelectrolytes

Les interactions des particules colloïdales avec des espèces chargées ont été étudiées par des méthodes de diffusion de la lumière et des méthodes électrochimiques. Une étude sur l'agrégation et le comportement de l'accumulation de la charge des particules colloïdales de silice révèle un comportement qui est quantitativement en accord avec la théorie DLVO. Nous avons étendu cette dernière étude aussi en présence de simples électrolytes divalents. La structure de la couche de polyélectrolytes qui sont absorbés a été étudiée par la méthode de la diffusion de lumière dynamique et par celle de la diffusion de neutrons à bas angle d'incidence. Finalement, nous avons étudié les interactions entre les particules colloïdales et d'autres types de particules à charge opposée. Les valeurs de taux d'agrégation ont été mesurées par les méthodes de diffusion de lumières statique et dynamique en présence d'homo-agrégation

Ion-ion correlations and charge reversal at titrating solid interfaces

International audienc

Protonation of silica particles in the presence of a strong cationic polyelectrolyte

Potentiometric titrations and electrophoresis techniques were used to study the charging behavior of aqueous silica particle suspensions in the presence of poly(N,N-diallyldimethylammonium chloride) (DADMAC). The main finding is that the particles undergo charge reversal with increasing pH, which can be directly seen by electrophoresis and as a common crossing point in the titration curves at different salt levels. These features resemble the charging behavior of an amphoteric oxide surface. With increasing polymer dose, the charge reversal point shifts towards higher pH. These trends are well reproduced by a modified Stern model. The main effect of the adsorbed cationic polyelectrolyte is to further deprotonate the surface silanol groups, and thereby to induce additional negative charge on the surface

Effects of heat treatment on the aggregation and charging of Stöber-type silica

Colloidal silica is known to be stable at high salt concentrations and low pH, where silica is basically uncharged. This observation is in qualitative disagreement with the theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO), which predicts rapid aggregation (or coagulation) under these conditions. This study reports a very different behaviour for Stöber-type silica heated at 800 °C, as these particles follow DLVO theory quantitatively. Unheated samples behave approximatively according to DLVO theory, but they show systematic deviations, in particular, featuring higher stability at low pH. The heat treatment also substantially modifies the charging properties, as heated particles show titratable surface charge densities in the range expected for the water–silica interface, while much higher charge densities are observed for the unheated samples. The electrophoretic mobilities, on the other hand, are hardly influenced by the heat treatment. We suspect that the suspension stability of the unheated particles is influenced by the presence of a hairy-layer of polysilicilic acid chains on the surface

Heteroaggregation in Binary Mixtures of Oppositely Charged Colloidal Particles

Heteroaggregation (or heterocoagulation) rate constants have been measured in mixtures of well-characterized colloidal particles of opposite charge with multiangle static and dynamic light scattering. This technique permits routine measurements of absolute heteroaggregation rate constants, also in the presence of homoaggregation. Particularly with multiangle dynamic light scattering, one is able to estimate absolute heteroaggregation rate constants accurately in the fast aggregation regime for the first time. Heteroaggregation rate constants have also been measured over a wide range of parameters, for example, ionic strength and different surface charge densities. Amidine latex particles, sulfate latex particles, and silica particles have been used for these experiments, and they were well characterized with respect to their charging and homoaggregation behavior. It was shown that heteroaggregation rate constants of oppositely charged particles increase slowly with decreasing ionic strength, and provided the surface charge is sufficiently large, the rate constant is largely independent of the surface charge. These trends can be well described with DLVO theory without adjustable parameters

Thermal Cycling of (RE)BCO-Based Superconducting Tapes Joined by Lead-Free Solders

We prepared overlap soldered joints of high-temperature superconducting tapes, using various materials and preparation conditions. In order to select the joints with optimal performance, we correlated their electrical properties (derived from current–voltage curves) with the microstructure of the respective joint cross-section by scanning electron microscopy. With the first group of joints, we focused on the effect of used materials on joint resistivity and critical current, and we found that the dominant role was played by the quality of the internal interfaces of the superconducting tape. Initial joint resistivities ranged in the first group from 41 to 341 nΩ·cm2. The second group of joints underwent a series of thermal cyclings, upon which the initial resistivity range of 35–49 nΩ·cm2 broadened to 25–128 nΩ·cm2. After the total of 135 thermal cycles, three out of four joints showed no signs of significant degradation. Within the limit of 100 thermal cycles, reliable soldered joints can be thus prepared, with normalized resistivity not exceeding 1.4 and with normalized critical current above the value of 0.85

Structure of an Adsorbed Polyelectrolyte Monolayer on Oppositely Charged Colloidal Particles

An adsorbed layer of a cationic polyelectrolyte, poly(diallyldimethyl-ammonium) chloride (PDADMAC) on negatively charged colloidal latex particles was investigated by small-angle neutron scattering (SANS) and dynamic light scattering (DLS). SANS gives a layer thickness of 8 ± 1 Å and a polymer volume fraction of 0.31 ± 0.05 within the film. DLS gives a somewhat larger thickness of 18 ± 2 Å, and the discrepancy is likely due to the inhomogeneous nature of the layer and the existence of polymer tails or loops protruding into solution. These results show that a highly charged polyelectrolyte adsorbs on an oppositely charged colloidal particle in a flat configuration due to the attractive forces acting between the polyelectrolyte and the substrate

Optimization of REBCO Tapes through Division and Striation for Use in Superconducting Cables with Low AC Losses

This study aimed to enhance the performance of Ag-stabilized high-temperature superconducting (HTS) tapes with a focus on reducing magnetization losses. Two approaches were employed: dividing the tapes into narrower widths and introducing striation at the level of the superconducting layer. The process of laser ablation proved to be an effective method for implementing these modifications. The quality of the cut edges and grooves was assessed using scanning electron microscopy. To evaluate the electrical properties, measurements were conducted on the critical current and magnetization loss in samples at different stages: in their initial state, after cutting, and after the striation process. Of the two modifications, the striation process more effectively reduced the AC losses in the HTS tapes, approximately by one order of magnitude. The retention of critical current remained high after cutting, but varied with the number of created filaments after the striation process. Subsequently, a short cable was wound from the cut and striated HTS tape. This cable demonstrated a remarkable sixfold reduction in AC losses compared to the initial HTS tape