Coaggregation of Two Anionic Azo Dyestuffs: A Combined Static Light Scattering and Small-Angle X‑ray Scattering Study

The formation of azo dyestuff aggregates in dilute aqueous solution induced by the addition of Mg²⁺, Ca²⁺, Sr²⁺, or Ba²⁺ ions is followed by time-resolved static light scattering (SLS) and time-resolved small-angle X-ray scattering (SAXS). Time-dependent molar mass data of the growing aggregates is interpreted by means of a kinetic model introduced by Lomakin et al. (Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 1125) for the description of β-amyloid aggregation. This interpretation reveals significant trends within the homologous series of alkaline earth cations. The trends refer to the nucleation and the growth rate of the dyestuff fibers. Time-resolved SAXS experiments indicate that these first two stages are followed by a third one during which a network forms by partial lateral alignment of fibers. At high enough dyestuff concentrations, this network formation even leads to a gel-like phase. Anomalous SAXS (ASAXS) on such a gel phase formed upon the addition of Sr²⁺ revealed the extent of neutralization of the dyestuff molecules within the gel by the specifically interacting alkaline earth cations

Formation Mechanism of Silver Nanoparticles Stabilized in Glassy Matrices

In any given matrix control over the final particle size distribution requires a constitutive understanding of the mechanisms and kinetics of the particle evolution. In this contribution we report on the formation mechanism of silver nanoparticles embedded in a soda-lime silicate glass matrix. For the silver ion-exchanged glass it is shown that at temperatures below 410 °C only molecular clusters (diameter <1 nm) are forming which are most likely silver dimers. These clusters grow to nanoparticles (diameter >1 nm) by annealing above this threshold temperature of 410 °C. It is evidenced that the growth and thus the final silver nanoparticle size are determined by matrix-assisted reduction mechanisms. As a consequence, particle growth proceeds after the initial formation of stable clusters by addition of silver monomers which diffuse from the glass matrix. This is in contrast to the widely accepted concept of particle growth in metal–glass systems, in which it is assumed that the nanoparticle formation is predominantly governed by Ostwald ripening processes

Distribution of Counterions around Lignosulfonate Macromolecules in Different Polar Solvent Mixtures

Lignosulfonate is a colloidal polyelectrolyte that is obtained as a side product in sulfite pulping. In this work we wanted to study the noncovalent association of the colloids in different solvents, as well as to find out how the charged sulfonate groups are organized on the colloid surface. We studied sodium and rubidium lignosulfonate in water–methanol mixtures and in dimethyl formamide. The number average molecular weights of the Na- and Rb-lignosulfonate fractions were 7600 g/mol and 9100 g/mol, respectively, and the polydispersity index for both was 2. Anomalous small-angle X-ray scattering (ASAXS) was used for determining the distribution of counterions around the Rb-lignosulfonate macromolecules. The scattering curves were fitted with a model constructed from ellipsoids of revolution of different sizes. Counterions were taken into account by deriving an approximative formula for the scattering intensity of the Poisson–Boltzmann diffuse double layer model. The interaction term between the spheroidal particles was estimated using the local monodisperse approximation and the improved Hayter–Penfold structure factor given by the rescaled mean spherical approximation. Effective charge of the polyelectrolyte and the local dielectric constant of the solvent close to the globular polyelectrolyte were followed as a function of the methanol content in the solvent and lignosulfonate concentration. The lignosulfonate macromolecules were found to aggregate noncovalently in water–methanol mixtures with increasing methanol or lignosulfonate content in a specific directional manner. The flat macromolecule aggregates had a nearly constant thickness of 1–1.4 nm, while their diameter grew when counterion association onto the polyelectrolyte increased. These results indicate that the charged groups in lignosulfonate are mostly at the flat surfaces of the colloid, allowing the associated lignosulfonate complexes to grow further at the edges of the complex

Block Copolymer Hollow Fiber Membranes with Catalytic Activity and pH-Response

We fabricated block copolymer hollow fiber membranes with self-assembled, shell-side, uniform pore structures. The fibers in these membranes combined pores able to respond to pH and acting as chemical gates that opened above pH 4, and catalytic activity, achieved by the incorporation of gold nanoparticles. We used a dry/wet spinning process to produce the asymmetric hollow fibers and determined the conditions under which the hollow fibers were optimized to create the desired pore morphology and the necessary mechanical stability. To induce ordered micelle assembly in the doped solution, we identified an ideal solvent mixture as confirmed by small-angle X-ray scattering. We then reduced <i>p</i>-nitrophenol with a gold-loaded fiber to confirm the catalytic performance of the membranes

Solution Small-Angle X-ray Scattering as a Screening and Predictive Tool in the Fabrication of Asymmetric Block Copolymer Membranes

Small-angle X-ray scattering (SAXS) analysis of the diblock copolymer poly­(styrene-<i>b</i>-(4-vinyl)­pyridine) in a ternary solvent system of 1,4-dioxane, tetrahydrofuran, and <i>N</i>,<i>N</i>-dimethylformamide, and the triblock terpolymer poly­(isoprene-<i>b</i>-styrene-<i>b</i>-(4-vinyl)­pyridine) in a binary solvent system of 1,4-dioxane and tetrahydrofuran, reveals a concentration-dependent onset of ordered structure formation. Asymmetric membranes fabricated from casting solutions with polymer concentrations at or slightly below this ordering concentration possess selective layers with the desired nanostructure. In addition to rapidly screening possible polymer solution concentrations, solution SAXS analysis also predicts hexagonal and square pore lattices of the final membrane surface structure. These results suggest solution SAXS as a powerful tool for screening casting solution concentrations and predicting surface structure in the fabrication of asymmetric ultrafiltration membranes from self-assembled block copolymers

Turkevich in New Robes: Key Questions Answered for the Most Common Gold Nanoparticle Synthesis

This contribution provides a comprehensive mechanistic picture of the gold nanoparticle synthesis by citrate reduction of HAuCl₄, known as Turkevich method, by addressing five key questions. The synthesis leads to monodisperse final particles as a result of a seed-mediated growth mechanism. In the initial phase of the synthesis, seed particles are formed onto which the residual gold is distributed during the course of reaction. It is shown that this mechanism is a fortunate coincidence created by a favorable interplay of several chemical and physicochemical processes which initiate but also terminate the formation of seed particles and prevent the formation of further particles at later stages of reaction. Since no further particles are formed after seed particle formation, the number of seeds defines the final total particle number and therefore the final size. The gained understanding allows illustrating the influence of reaction conditions on the growth process and thus the final size distribution

Size-Controlled Synthesis of Colloidal Silver Nanoparticles Based on Mechanistic Understanding

Metal nanoparticles have attracted much attention due to their unique properties. Size control provides an effective key to an accurate adjustment of colloidal properties. The common approach to size control is testing different sets of parameters via trial and error. The actual particle growth mechanisms, and in particular the influences of synthesis parameters on the growth process, remain a black box. As a result, precise size control is rarely achieved for most metal nanoparticles. This contribution presents an approach to size control that is based on mechanistic knowledge. It is exemplified for a common silver nanoparticle synthesis, namely, the reduction of AgClO₄ with NaBH₄. Conducting this approach allowed a well-directed modification of this synthesis that enables, for the first time, the size-controlled production of silver nanoparticles 4–8 nm in radius without addition of any stabilization agent

A One-Pot Approach to Mesoporous Metal Oxide Ultrathin Film Electrodes Bearing One Metal Nanoparticle per Pore with Enhanced Electrocatalytic Properties

The controlled incorporation of single metal nanoparticles within the pores of mesostructured conducting metal oxide ultrathin films is demonstrated, taking advantage of the controlled metal precursor loading capacities of PS-<i>b</i>-P4VP inverse micellar templates. The presented one-pot approach denoted as Evaporation-Induced Hydrophobic Nanoreactor Templating (EIHNT) unusually involves the nanostructuration of the metal oxide via the hydrophobic shell of the micellar template, while the concomitant nanostructuration of the metal is achieved via its confinement in the hydrophilic micellar core. This approach is applied to tin-rich ITO and gold, to yield unique mesoporous tin-rich ITO ultrathin film electrodes remarkably loaded with one size-controlled gold nanoparticle per pore. Interestingly, the resulting tin-rich ITO-supported gold nanoparticles exhibit improved catalytic activity and durability in electrocatalytic CO oxidation compared to similarly sized gold nanoparticles supported on conventional ITO coatings