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PubMed2006

Polymerization of anionic wormlike micelles.

Polymerizable anionic wormlike micelles are obtained upon mixing the hydrotropic salt p-toluidine hydrochloride (PTHC) with the reactive anionic surfactant sodium 4-(8-methacryloyloxyoctyl)oxybenzene sulfonate (MOBS). Polymerization captures the cross-sectional radius of the micelles (approximately 2 nm), induces micellar growth, and leads to the formation of a stable single-phase dispersion of wormlike micellar polymers. The unpolymerized and polymerized micelles were characterized using static and dynamic laser light scattering, small-angle neutron scattering, 1H NMR, and stopped-flow light scattering. Stopped-flow light scattering was also used to measure the average lifetime of the unpolymerized wormlike micelles. A comparison of the average lifetime of unpolymerized wormlike micelles with the surfactant monomer propagation rate was used to elucidate the mechanism of polymerization. There is a significant correlation between the ratio of the average lifetime to the monomer propagation rate and the average aggregation number of the polymerized wormlike micelles.

Journal Article↗
PubMed2006

Molecularly asymmetric triblock copolymers as a single-molecule route to ordered bidisperse polymer brushes.

The conditions signaling the formation of bidisperse brushes in ordered block copolymers are investigated as an A(2) block is progressively grown onto an A(1)B diblock copolymer to form a series of molecularly asymmetric, isomorphic A(1)BA(2) triblock copolymers. Small-angle scattering and self-consistent field theory confirm that the microphase-ordered period decreases when the A(2) block is short relative to the A(1) block, but then increases as A(1)+A(2) bidisperse brushes develop. The mechanical properties systematically follow the spatial distribution of the A(2) block.

Journal Article↗
PubMed2006

Characterization and surface-reactivity of nanocrystalline anatase in aqueous solutions.

The chemical and electrostatic interactions at mineral-water interfaces are of fundamental importance in many geochemical, materials science, and technological processes; however, the effects of particle size at the nanoscale on these interactions are poorly known. Therefore, comprehensive experimental and characterization studies were completed, to begin to assess the effects of particle size on the surface reactivity and charging of metal-oxide nanoparticles in aqueous solutions. Commercially available crystalline anatase (TiO2) particles were characterized using neutron and X-ray small-angle scattering, electron microscopy, and laser diffraction techniques. The 4 nm primary nanoparticles were found to exist almost exclusively in a hierarchy of agglomerated structures. Potentiometric and electrophoretic mobility titrations were completed in NaCl media at ionic strengths from (0.005 to 0.3) mol/kg, and 25 degrees C, with these two experimental techniques matched as closely as the different procedures permitted. The pH of zero net proton charge (pHznpc, from potentiometric titration) and isoelectric point pH value (pHiep, from electrophoretic mobility titrations) were both in near perfect agreement (6.85 +/- 0.02). At high ionic strengths the apparent pHznpc value was offset slightly toward lower pH values, which suggests some specific adsorption of the Na+ electrolyte ions. Proton-induced surface charge curves of nanocrystalline anatase were very similar to those of larger rutile crystallites when expressed relative to their respective pHznpc values, indicating that the development of positive and negative surface charge away from the pHznpc for nanocrystalline anatase is similar to that of larger TiO2 crystallites.

Electrochemistry↗
PubMed2007

A computational study of the reconstruction of amorphous mesoporous materials from gas adsorption isotherms and structure factors via evolutionary optimization.

A general method for the three-dimensional reconstruction of mesoporous materials by evolutionary optimization against target data is developed. The method is applied specifically in reconstruction of amorphous material models using gas adsorption data, structure factor data, or a combination of both. A recently introduced lattice-gas approach is used to model adsorption in these calculations, and a high-pass limited Fourier representation is used to facilitate evolution of large-scale structures during the optimization. Reconstructions are made of several material models which mimic real materials obtained either by phase separation and etching or by sol-gel processing. Analysis of the reconstructions provides considerable insight into the type and quantity of structural information probed by gas adsorption and small-angle scattering experiments. We find that reconstructions based only on structure factors tend to underestimate the mean pore size. We also find that in many cases excellent reconstructions can be obtained using only adsorption-branch data, and that in all cases reconstructions based jointly on both types of data are superior to those based only on one, suggesting that these measures contain "complementary" information. It is also found that in most cases the use of desorption data is not warranted, and that the use of adsorption data taken at many temperatures will not improve reconstructions. The reproducibility of the method is shown to be satisfactory. The method can be computationally expensive if gas adsorption data are used, but it is easily parallelized, and therefore results can still be obtained in reasonable time. Finally, the possible application of this approach to real systems, including templated porous materials, is discussed.

Journal Article↗
PubMed1980

X-ray kinetic studies of microtubule assembly using synchrotron radiation.

The assembly of microtubules has been investigated by time-resolved X-ray diffraction using synchrotron radiation. The small-angle scattering becomes visible within seconds and thus enables study of the structural transitions of the protein aggregates during assembly from their subunits in solution. The X-ray pattern at 4 degrees C arises from a mixture of tubulin rings, dimers and some other species. Raising the temperature to 36 degrees C induces the breakdown of rings, followed by the growth of microtubules. The results suggest that microtubules may be formed from tubulin oligomers smaller than rings.

Animals↗
PubMed1999

A study of charge distribution in Bi2Sr2CaCu2O8+delta superconductors using novel electron-diffraction and imaging techniques.

We report our study of the distribution of valence electrons in Bi2Sr2CaCu2O8+delta high-temperature superconductors using novel electron-diffraction and imaging techniques. The former method was based on quantitative analyses of the diffraction intensity of many reflections as a function of crystal thickness to determine, with an unprecedented accuracy, the Fourier components of the electron distribution in Bi2Sr2CaCu2O8+delta. The latter was based on examining the effect of charge transfer on many-beam imaging by comparing the observed and calculated low- and high-resolution images of long-period displacive and charge modulation of the cuprate. Our study demonstrates that fast electrons have greater sensitivity than X-rays to valence electrons distribution at small scattering angles, and that electron microscopy is a powerful tool in revealing charge distribution in materials.

Journal Article↗
PubMed2006

Determination of pair correlation functions of dense colloidal systems by means of indirect Fourier transformation.

The interparticle pair correlation gr function is determined by the small angle scattering data of interacting, monodisperse, globular colloidal particles. The new approach to this problem requires that the form factor and the particle number density must be known. Instrumental broadening effects and experimental imperfections are included in a way that makes it unnecessary to deconvolute the scattering function from beam profiles. The applicability of the approach is demonstrated by the simulated data of a charged colloidal dispersion and by way of the neutron scattering data of nonionic micellar solutions. In both cases, gr functions are obtained that are in good agreement with the known behavior of the systems.

Journal Article↗
PubMed1981

Configuration of the four iron atoms in dissolved human hemoglobin as studied by anomalous dispersion.

The anomalous dispersion of iron at its K-absorption edge in small-angle scattering of an aqueous solution of hemoglobin has been used to establish the geometrical arrangement of the four iron atoms in this protein. Although the anomalous contributions are 0.001-0.01 of the total scattering, experiments with synchrotron radiation from the storage ring DORIS have shown that these effects can be measured with an average precision of approximately 10% at each of the 50 points of the scattering curve. The anomalous scattering represents the convolution of the whole structure with the configuration of the four iron atoms of hemoglobin. Analysis in terms of multipoles suggests that tetrahedral symmetry of both the subunit arrangement and the iron structure is a dominant feature. The mean distance between the iron atoms of 26 A derived from this experiment compares well with that derived from crystallographic data.

Hemoglobins↗
PubMed1998

Improvement of the prompt-gamma neutron activation facility at Brookhaven National Laboratory.

The prompt-gamma neutron activation facility at Brookhaven National Laboratory was upgraded to improve both the precision and accuracy of its in vivo determinations of total body nitrogen. The upgrade, guided by Monte Carlo simulations, involved elongating and modifying the source collimator and its shielding, repositioning the system's two NaI(Tl) detectors, and improving the neutron and gamma shielding of these detectors. The new source collimator has a graphite reflector around the 238PuBe neutron source to enhance the low-energy region of the neutron spectrum incident on the patient. The gamma detectors have been relocated from positions close to the upward-emerging collimated neutron beam to positions close to and at the sides of the patient. These modifications substantially reduced spurious counts resulting from the capture of small-angle scattered neutrons in the NaI detectors. The pile-up background under the 10.8 MeV 14N(n, gamma)15N spectral peak has been reduced so that the nitrogen peak-to-background ratio has been increased by a factor of 2.8. The resulting reduction in the coefficient of variation of the total body nitrogen measurements from 3% to 2.2% has improved the statistical significance of the results possible for any given number of patient measurements. The new system also has a more uniform composite sensitivity.

Gamma Rays↗
PubMed2000

Computed tomography of x-ray index of refraction using the diffraction enhanced imaging method.

Diffraction enhanced imaging (DEI) is a new, synchrotron-based, x-ray radiography method that uses monochromatic, fan-shaped beams, with an analyser crystal positioned between the subject and the detector. The analyser allows the detection of only those x-rays transmitted by the subject that fall into the acceptance angle (central part of the rocking curve) of the monochromator/analyser system. As shown by Chapman et al, in addition to the x-ray attenuation, the method provides information on the out-of-plane angular deviation of x-rays. New images result in which the image contrast depends on the x-ray index of refraction and on the yield of small-angle scattering, respectively. We implemented DEI in the tomography mode at the National Synchrotron Light Source using 22 keV x-rays, and imaged a cylindrical acrylic phantom that included oil-filled, slanted channels. The resulting 'refraction CT image' shows the pure image of the out-of-plane gradient of the x-ray index of refraction. No image artefacts were present, indicating that the CT projection data were a consistent set. The 'refraction CT image' signal is linear with the gradient of the refractive index, and its value is equal to that expected. The method, at the energy used or higher, has the potential for use in clinical radiography and in industry.

Models, Statistical↗
PubMed2003

The energy-dependent electron loss model: backscattering and application to heterogeneous slab media.

Electron backscattering has been incorporated into the energy-dependent electron loss (EL) model and the resulting algorithm is applied to predict dose deposition in slab heterogeneous media. This algorithm utilizes a reflection coefficient from the interface that is computed on the basis of Goudsmit-Saunderson theory and an average energy for the backscattered electrons based on Everhart's theory. Predictions of dose deposition in slab heterogeneous media are compared to the Monte Carlo based dose planning method (DPM) and a numerical discrete ordinates method (DOM). The slab media studied comprised water/Pb, water/Al, water/bone, water/bone/water, and water/lung/water, and incident electron beam energies of 10 MeV and 18 MeV. The predicted dose enhancement due to backscattering is accurate to within 3% of dose maximum even for lead as the backscattering medium. Dose discrepancies at large depths beyond the interface were as high as 5% of dose maximum and we speculate that this error may be attributed to the EL model assuming a Gaussian energy distribution for the electrons at depth. The computational cost is low compared to Monte Carlo simulations making the EL model attractive as a fast dose engine for dose optimization algorithms. The predictive power of the algorithm demonstrates that the small angle scattering restriction on the EL model can be overcome while retaining dose calculation accuracy and requiring only one free variable, chi, in the algorithm to be determined in advance of calculation.

Algorithms↗
PubMed2003

Multiple-image radiography.

Conventional radiography produces a single image of an object by measuring the attenuation of an x-ray beam passing through it. When imaging weakly absorbing tissues, x-ray attenuation may be a suboptimal signature of disease-related information. In this paper we describe a new phase-sensitive imaging method, called multiple-image radiography (MIR), which is an improvement on a prior technique called diffraction-enhanced imaging (DEI). This paper elaborates on our initial presentation of the idea in Wernick et al (2002 Proc. Int. Symp. Biomed. Imaging pp 129-32). MIR simultaneously produces several images from a set of measurements made with a single x-ray beam. Specifically, MIR yields three images depicting separately the effects of refraction, ultra-small-angle scatter and attenuation by the object. All three images have good contrast, in part because they are virtually immune from degradation due to scatter at higher angles. MIR also yields a very comprehensive object description, consisting of the angular intensity spectrum of a transmitted x-ray beam at every image pixel, within a narrow angular range. Our experiments are based on data acquired using a synchrotron light source; however, in preparation for more practical implementations using conventional x-ray sources, we develop and evaluate algorithms designed for Poisson noise, which is characteristic of photon-limited imaging. The results suggest that MIR is capable of operating at low photon count levels, therefore the method shows promise for use with conventional x-ray sources. The results also show that, in addition to producing new types of object descriptions, MIR produces substantially more accurate images than its predecessor, DEI. MIR results are shown in the form of planar images of a phantom and a biological specimen. A preliminary demonstration of the use of MIR for computed tomography is also presented.

Absorptiometry, Photon↗
PubMed1977

A neutron investigation of yeast valyl-tRNA synthetase interaction with tRNAs.

A new way of studying RNA-protein complexes, using neutron small angle scattering in solution, is described and was applied in the case of the system, yeast valyl-tRNA synthetase, interacting with its cognate and non cognate yeast tRNAs. It was shown that, when limited amounts of tRNA (either cognate or non cognate) are added to valyl-tRNA synthetase, a complex consisting of two enzyme molecules and one tRNA molecule is first formed. It is subsequently dissociated to a one to one complex when more tRNA is present in the solution. The association curve shows a maximum for a molecular ratio, enzyme over tRNA, equal to 2.

Amino Acyl-tRNA Synthetases↗
PubMed1999

Interaction and percolation in the L64 triblock copolymer micellar system.

We present results of analyses of an extensive set of static light scattering (SLS), small angle neutron scattering (SANS), and viscoelastic (frequency dependent complex moduli) measurements of aqueous solutions of a triblock copolymer micellar system. We investigate Pluronic L64 (PEO(13)PPO(30)PEO(13))-water system in a wide range of composition and temperature. We determine phase diagram of the disordered micellar phase, including a cmc-cmt curve, a cloud point curve, the critical concentration, and the critical temperature by means of SLS and SANS. The microstructure and interaction between micelles are determined by analyses of SANS intensities. SANS intensity distributions are well described by combining the cap-and-gown model for the polymer segmental distribution within a micelle and the sticky hard sphere model for the intermicellar structure factor. The existence of percolation loci at well defined poins in the temperature-concentration plane is inferred from an abrupt increase of the stickiness parameter extracted from SANS data and from two order of magnitude jump of the complex moduli at the percolation point. Study of temperature dependence of real (storage) and imaginary (loss) part of the complex modulus at fixed concentration and frequency lends further support to the existence of a percolation line. We observe an increase of some order of magnitude of the real and imaginary part of viscosity at certain temperature and composition, a phenomenon usually ascribed to a gelation process in a polymer solution. The definitive confirmation of the percolation process is obtained by frequency dependent complex viscosity measured in a frequency range 0-160 (rad/sec). From these measurements we clearly observe a well defined frequency scaling behavior of the complex moduli and a loss angle (delta) independent of the frequency. Scaling exponents, determined for frequency-dependent complex moduli satisfy the scaling relations predicted by the scalar elasticity percolation theory.

Journal Article↗
PubMed2003

Effect of a quenched random field on a continuous symmetry breaking transition: nematic to smectic-A transition in octyloxycyanobiphenyl-aerosil dispersions.

High-resolution x-ray diffraction and ac-calorimetric experiments have been carried out on the liquid-crystal octyloxycyanobiphenyl in which aerosil particles are dispersed. The measurements were made over a temperature range around the bulk nematic to smectic-A transition temperature. At this transition the liquid crystal breaks translational symmetry in a single direction. The silica particles, which hydrogen bond together to form a very low density gel, provide the quenched disorder. The random gel leads to observable broadening of the x-ray reflection from the smectic layers. The structure factor is well described by modeling the effect of the aerosils as a quenched random field. Dispersed aerosils are thought to pin both the direction of the translational ordering and the position of the layers. The latter appears to have the greatest effect on the x-ray line shape. We show that the aerosil surface area, as verified by small-angle scattering, equates to the variance of the random field. Calorimetric results reveal substantial change in the specific heat peak associated with the nematic to smectic-A transition. As the concentration of aerosil increases, the specific heat peak remains sharp yet decreases in magnitude and shifts in temperature in a nonmonotonic fashion. In this regime, the critical exponent alpha becomes progressively smaller. For the samples with the largest concentrations of aerosil particles the C(p)(N-A) peak becomes highly smeared and shifts smoothly to lower temperatures.

Journal Article↗
PubMed2003

Field-induced pseudocrystalline ordering in concentrated ferrofluids.

Concentrated surfactant stabilized cobalt ferrofluids up to 6 vol % Co have been studied by small-angle scattering using polarized neutrons and synchrotron x rays. The combination of these techniques allowed the magnetic and nuclear form factors to be reliably separated from the structure factors. Above 1 vol % Co, inter particle interactions are induced by an applied external magnetic field that gives rise to pseudocrystalline ordering of cobalt core-shell particles. Particles are arranged in hexagonal planes, with the magnetic moments aligned parallel to the [110] direction. Two types of equivalent textures were found to be present simultaneously, corresponding to a stacking of the hexagonal planes in horizontal and vertical direction. The in-plane nearest-neighbor distance is almost independent of the concentration and temperatures, whereas the distance between the neighboring planes, c, strongly varies from sample to sample. In addition, segments of chains of particles with parallel moments are aligned along the magnetic field and frozen-in when the carrier liquid is solidified. The field induced pseudocrystalline lamellar hexagonal particle arrangement, observed experimentally in colloidal magnetic liquids, confirms predictions from molecular-dynamics and Monte Carlo simulations.

Journal Article↗
PubMed2005

Numerical study of a binary Yukawa model in regimes characteristic of globular proteins in solutions.

The main goal of this paper is to assess the limits of validity, in the regime of low concentration and strong Coulomb coupling (high molecular charges), of a simple perturbative approximation to the radial distribution functions (RDF's), based upon a low-density expansion of the potential of mean force and proposed to describe protein-protein interactions in a recent small-angle-scattering (SAS) experimental study. A highly simplified Yukawa (screened Coulomb) model of monomers and dimers of a charged globular protein (beta-lactoglobulin) in solution is considered. We test the accuracy of the RDF approximation, as a necessary complementary part of the previous experimental investigation, by comparison with the fluid structure predicted by approximate integral equations and exact Monte Carlo (MC) simulations. In the MC calculations, an Ewald construction for Yukawa potentials has been used to take into account the long-range part of the interactions in the weakly screened cases. Our results confirm that the perturbative first-order approximation is valid for this system even at strong Coulomb coupling, provided that the screening is not too weak (i.e., for Debye length smaller than monomer radius). A comparison of the MC results with integral equation calculations shows that both the hypernetted-chain (HNC) and Percus-Yevick closures have a satisfactory behavior under these regimes, with the HNC being superior throughout. The relevance of our findings for interpreting SAS results is also discussed.

Binding Sites↗