8 research outputs found
Coaggregation of Two Anionic Azo Dyestuffs: A Combined Static Light Scattering and Small-Angle Xāray Scattering Study
The
formation of azo dyestuff aggregates in dilute aqueous solution
induced by the addition of Mg<sup>2+</sup>, Ca<sup>2+</sup>, Sr<sup>2+</sup>, or Ba<sup>2+</sup> 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 Ī²-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 Sr<sup>2+</sup> 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 Ā°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 Ā°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 metalāglass 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 waterāmethanol 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 PoissonāBoltzmann
diffuse double layer model. The interaction term between the spheroidal
particles was estimated using the local monodisperse approximation
and the improved HayterāPenfold 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 waterāmethanol
mixtures with increasing methanol or lignosulfonate content in a specific
directional manner. The flat macromolecule aggregates had a nearly
constant thickness of 1ā1.4 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
polyĀ(styrene-<i>b</i>-(4-vinyl)Āpyridine) in a ternary solvent
system of 1,4-dioxane, tetrahydrofuran, and <i>N</i>,<i>N</i>-dimethylformamide, and the triblock terpolymer polyĀ(isoprene-<i>b</i>-styrene-<i>b</i>-(4-vinyl)Āpyridine) 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 HAuCl<sub>4</sub>, 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 AgClO<sub>4</sub> with NaBH<sub>4</sub>. Conducting this approach allowed a well-directed modification
of this synthesis that enables, for the first time, the size-controlled
production of silver nanoparticles 4ā8 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