Kiril A. Streletzky

Study of the crystallization of zeolite NaA by quasi-elastic light-scattering spectroscopy and electron microscopy

A study of the crystallization of the molecular sieve zeolite NaA from clear aluminosilicate solutions is reported. It was determined by powder X-ray diffraction that zeolite NaA was the only phase in the solid precipitate. The syntheses were monitored in situ by quasi-elastic light scattering spectroscopy. Characteristics of the products were examined by scanning and transmission electron microscopy. Our data show that nuclei formed from the clear aluminosilicate solution, that crystal growth occurred from the solution, that crystal growth was accelerated at elevated temperatures, and that aging the solution at room temperature before raising the synthesis temperature increased both the number of nuclei that formed and also the crystal growth rate for aging times up to a limit. Adding crushed seed crystal fragments to an unaged synthesis solution produced similar results to aging the solution, including reduced induction times, increased crystal growth rates, and polycrystalline product formation.

Small-angle neutron scattering of soot formed in laminar premixed ethylene flames

We used small-angle neutron scattering (SANS) to measure soot development in one-dimensional, laminarpremixed flames. Scattering spectra were collected for two sooting ethylene/oxygen/argon flames at six positions above the burner surface. A detailed analysis of the scattering signals yielded the basic properties of the soot size distributions as a function of position. The experiments demonstrate that SANS can be used to extract spatially resolved, quantitative information about incipient soot formation and growth with better particle size resolution than light scattering and extinction.

Study of the effects of initial-bred nuclei on zeolite NaA crystallization by quasi-elastic light scattering spectroscopy and electron microscopy

Synthesis studies have been conducted at 25, 60, and 80 °C with a clear solution batch composition that produces zeolite NaA. Aluminosilicate precursor material was added to promote the synthesis and to evaluate the effects of adding initial-bred nuclei that were created by a precursor “seed” reaction. It is shown that the initial-bred nuclei promoted the crystallization rate, that the crystalline mass produced was increased when using the initial-bred nuclei, and that the nucleation of a second population was not inhibited by the addition of initial-bred nuclei as seeds.

Redox-induced polymerisation/depolymerisation of metallo-supramolecular polymers

Redox-responsive metallo-supramolecular polymers (MSPs), consisting of a ditopic ligand end-capped poly(tetrahydrofuran) (pTHF) macromonomer self-assembled with copper ions, have been developed where the degree of polymerization of the MSP, and therefore the viscosity of its solutions, depends on the oxidation state of the copper ion. We use a combination of UV-Vis, viscosity, light scattering, cyclic voltammetry and SAXS to show that the ligands, 2,6-bis(N-methylbenzimidazolyl)pyridine (MeBip), at the ends of the pTHF, bind copper(II) in a 2 : 1 ratio and copper(I) in a 2 : 2 ratio in solution. Thus at fixed 1 : 1 or 1 : 2 stoichiometry of macromonomer to copper ions the viscosity of their solutions shows dramatic changes in response to addition of a chemical oxidant (nitroso tetrafluoroborate) or reductant (aqueous ascorbic acid).

Small angle neutron scattering from D2O–H2O nanodroplets and binary nucleation rates in a supersonic nozzle

Small angle neutron scattering (SANS) experiments were used to characterize binary nanodroplets composed of D2O and H2O. The droplets were formed by expanding dilute mixtures of condensible vapor in a N2 carrier gas through a supersonic nozzle, while maintaining the onset of condensation at a fixed position in the nozzle. It is remarkable, given the small coherent scattering length density of light water, that even the pure H2O aerosol gave a scattering signal above background. The scattering spectra were analyzed assuming a log-normal distribution of droplets. On average, the geometric radius of the nanodroplets rg was rg=13 (±1) nm, the polydispersity ln σr was ln σr=0.19 (±0.07), and the number density N was N=(2±0.2)⋅1011 cm−3. The aerosol volume fractions derived from the SANS measurements are consistent with those derived from the pressure trace experiments, suggesting that the composition of the droplets was close to that of the initial condensible mixture. A quantitative analysis of the scattering spectra as a function of the isotopic composition gave further evidence that the binary droplets exhibit ideal mixing behavior. Because both the stagnation temperature T0 and the location of onset were fixed, the temperature corresponding to the maximum nucleation rate was constant at TJ max=229 (±1) K. Thus, the experiments let us estimate the isothermal peak nucleation rates as a function of the isotopic composition. The nucleation rates were found to be essentially constant with Jmax equal to (3.6±0.5)⋅1016 cm−3 s−1 at a mean supersaturation of 44 (±3).Small angle neutron scattering (SANS) experiments were used to characterize binary nanodroplets composed of D2O and H2O. The droplets were formed by expanding dilute mixtures of condensible vapor in a N2 carrier gas through a supersonic nozzle, while maintaining the onset of condensation at a fixed position in the nozzle. It is remarkable, given the small coherent scattering length density of light water, that even the pure H2O aerosol gave a scattering signal above background. The scattering spectra were analyzed assuming a log-normal distribution of droplets. On average, the geometric radius of the nanodroplets rg was rg=13 (±1) nm, the polydispersity ln σr was ln σr=0.19 (±0.07), and the number density N was N=(2±0.2)⋅1011 cm−3. The aerosol volume fractions derived from the SANS measurements are consistent with those derived from the pressure trace experiments, suggesting that the composition of the droplets was close to that of the initial condensible mixture. A quantitative analysis of the scattering...

Controlling nucleation and growth of nanodroplets in supersonic nozzles

We present the first results for a new supersonic nozzle that decouples nucleation and droplet growth, and closely controls the supersaturation and temperature during nucleation. We characterize the expansions using pressure trace measurements, and the aerosol properties using light scattering and small angle neutron scattering. We show that when nucleation and droplet growth are separated, the aerosol number density decreases, the average particle size increases, and the aerosol can be more monodisperse than that formed in a conventional nozzle. Under these conditions, we can estimate the nucleation rate J as a function of supersaturation S and temperature T directly from the experimental data. For D2O we find that the nucleation rate is 4.3×1015⩽J/cm−3 s−⩽6.0×1015 at 230.1⩽T/K⩽230.4 and 29.2⩽S⩽32.4.

H2O–D2O condensation in a supersonic nozzle

We examined the condensation of H2O, D2O, and four intermediate mixtures (20, 40, 60, and 80 mol % D2O) in a supersonic nozzle. Because the physical and chemical properties of protonated and deuterated water are so similar, this system is ideal for studying the change in condensation behavior as a function of condensible composition. In our experiments dilute mixtures of condensible vapor in N2 are expanded from three different stagnation temperatures resulting in a broad range of onset temperatures (190–238 K) and pressures (27–787 kPa). For a fixed stagnation temperature, the partial pressure required to maintain the onset of condensation at a given location or temperature in the nozzle is consistently higher for H2O than for D2O. In contrast, the supersaturation at fixed onset temperature is usually higher for D2O than for H2O and this difference increases toward lower temperature. The partial pressure at onset for the intermediate mixtures varied linearly between the values observed for the pure compo...

Mode structure of diffusive transport in hydroxypropylcellulose:water

A systematic analysis of the mode structure of diffusive relaxations in 1 MDa hydroxypropylcellulose(HPC):water is presented. New methods and data include (1) use of integral spectral moments to characterize nonexponential decays, (2) spectra of small probes in concentrated HPC solutions, (3) temperature dependence of the mode structure, and (4) comparison of optical probe spectra and spectra of probe-free polymer solutions. We find that (1) probe and polymer relaxations are in general not the same; (2) the apparent viscometric crossover near ct≈6 g/l is echoed by probe behavior; (3) our HPC solutions have a characteristic dynamic length, namely the 50 nm length that matches the polymer’s hydrodynamic radius; (4) characterization of spectral modes with their mean relaxation time affords simplifications relative to other characterizations; and (5) contrary to some expectations, Stokes–Einsteinian behavior (diffusion rate determined by the macroscopic viscosity) is not observed, even for large probes in rel...

Size and Shape Characterization of Thermoreversible Micelles of Three-Armed Star Elastin-Like Polypeptides

Three-armed star elastin-like polypeptides are shown to have the capability of self-assembling into micellar constructs at certain environmental conditions. Here, a study of the size distribution, shape, and molecular weight of these micelles at different salt concentrations and pH values is presented. Multiangle dynamic light scattering was used to study the formation, reversibility, and size of the micelles at different environmental conditions. On the basis of the salt concentration of the solution, two distinct size distribution regimes and a transition region were observed. Static light scattering was performed to study the molecular weight and geometrical anisotropy of the micelles in each regime. The anisotropic behavior and elongation of the particles were independently confirmed by depolarized dynamic light scattering, and a model for micelles at each regime was proposed. The size and molecular weight of the micelles were verified using viscosity measurements. The results of this study suggest that there is big jump in the size and molecular weight of the micelles from the first salt-dependent regime to the other, and the shape of the micelles changes from spheres to cylindrical micelles with a higher than 10:1 axis ratio.

TRANSLATIONAL DIFFUSION OF SMALL AND LARGE MESOSCOPIC PROBES IN HYDROXYPROPYLCELLULOSE-WATER IN THE SOLUTIONLIKE REGIME

Quasi-elastic light scattering spectroscopy was used to study the translational diffusion of monodisperse spheres in aqueous 1 MDa hydroxypropylcellulose (HPC) at 25 °C. Probe diameters d spanned 14–455 nm; HPC concentrations were 0⩽c⩽7g/L. Light scattering spectroscopy consistently found spectra having the form g(1)(t)=(1−Af)exp(−θtβ)+Af exp(−θftβf). Here θf and βf refer to the “fast” mode; θ and β describe the “slow” mode. We examine the dependence of θ, β, θf, βf, and Af on d, c, scattering vector q, and viscosity η. β=1 for large probes; elsewise, β and βf are ∈(0,1). The slow mode, with short-lived memory function, is diffusive; for large probes θ≈(dη)−1. The fast mode, with long-lived memory function, appears coupled to polymer chain internal dynamics. Probe behavior differs between “small” and “large” probes. Small probes have diameters d<Rh, Rh being the chain hydrodynamic radius. Large probes have d⩾Rg, Rg being the polymer radius of gyration.