Pure Aggregates Research Articles

Substitutional n-Type Doping of an Organic Semiconductor Investigated by Electron Paramagnetic Resonance Spectroscopy

Doping a perylene diimide organic semiconductor with a one-electron reduced perylene diimide containing a covalently bound counterion provides a well-characterized system for understanding doping in organic semiconductors. We obtain insight into the doping process by electron paramagnetic resonance (EPR) measurements of the dopant solutions, the dopant plus host solutions from which thin films are spin-coated, and the resulting solid films. After correction for some trace impurities in the solutions, the spin density incorporated into the solid films is linearly proportional to the added dopant density. Nevertheless, the film conductivity increases superlinearly with dopant concentration. Although neither pure dopant nor host aggregate in solution, they aggregate when combined. This is presumably a result of the delocalization of the dopant electron over a number of host molecules. Angle-dependent EPR measurements on thin films suggest that the g-tensor symmetry axis is close to the π−π stacking axis, con...

Physicochemical properties of mixed micellar aggregates containing CCK peptides and Gd complexes designed as tumor specific contrast agents in MRI.

New amphiphilic molecules containing a bioactive peptide or a claw moiety have been prepared in order to obtain mixed micelles as target-specific contrast agents in magnetic resonance imaging. The first molecule, C(18)H(37)CONH(AdOO)(2)-G-CCK8 (C18CCK8), contains a C18 hydrophobic moiety bound to the C-terminal cholecystokinin octapeptide amide (CCK 26-33 or CCK8). The second amphiphilic compound, C(18)H(37)CONHLys(DTPAGlu)CONH(2) (C18DTPAGlu) or its gadolinium complex, (C18DTPAGlu(Gd)), contains the same C18 hydrophobic moiety bound, through a lysine residue, to the DTPAGlu chelating agent. The mixed aggregates as well as the pure C18DTPAGlu aggregate, in the presence and absence of Gd, have been fully characterized by surface tension measurements, FT-PGSE-NMR, fluorescence quenching, and small-angle neutron scattering measurements. The structural characterization of the mixed aggregates C18DTPAGlu(Gd)-C18CCK8 indicates a spherical arrangement of the micelles with an external shell of approximately 21 A and an inner core of approximately 20 A. Both the DTPAGlu(Gd) complexes and the CCK8 peptides point toward the external surface. The measured values for relaxivity in saline medium at 20 MHz proton Larmor frequency and 25 degrees C are 18.7 mM(-)(1) s(-)(1). These values show a large enhancement in comparison with the isolated DTPAGlu(Gd) complex.

Study on Aqueous Mixtures of Fullerene-Based Star Ionomers and Sodium Dodecyl Sulfate Using Small Angle Scattering with Contrast Variation

Using small angle neutron scattering (SANS) and small angle X-ray scattering (SAXS), we have studied aqueous mixtures of fullerene-based ionomers, C60[(CH2)4SO3Na]6 (FC4S), and sodium dodecyl sulfate (SDS). With the contrast variation provided by SAXS and a selected deuteration of SDS for SANS, we have identified the formation of complex aggregates of FC4S and SDS. The aggregation structure of the complex obtained is similar to that for pure FC4S aggregates and is stable to the SDS concentration in the mixtures studied. Combining the contrast data sets of small angle scattering, we have obtained detailed aggregation information, including aggregation number, shape, size, and fractional ionization for the FC4S/SDS complex aggregates. It is found that the globular-like complex aggregate, with a radius of gyration of 20 Å, consists of five FC4S and two SDS, in an average.

Characterization of Mixed Micelles of Sodium Dodecyl Sulfate and Tetraoxyethylene Dodecyl Ether in Aqueous Solution

The mixed micelles of sodium dodecyl sulfate (SDS) and tetraoxyethylene dodecyl ether (C12E4 or Brij 30) were studied by surface tension, ζ potential, and fluorescence spectroscopy measurements. In the range of 0−0.5 mole fraction (x) of added Brij 30 to a 30 mM SDS solution, the mixed micellar aggregation number (Nagg) changed from 65 to 171, and the ζ potential of the micelles varied from −60 to −15 mV. The change in size was acompanhied by a 2-fold increase in the internal micellar viscosity, indicating that addition of the nonionic surfactant with similar alkyl chain enhances the surfactant packing in the mixed aggregate. The reduction of the electrical surface potential causes a proportional decrease of the binding constant of the cationic dye acridine orange. The change in temperature from 15 to 45 °C of a solution with x = 0.5 results in a decrease in the Nagg from 175 to 128, but the SDS to Brij 30 monomers ratio remains constant. This fact is typical of ionic micelles, opposite to the usual behavior of pure nonionic micellar aggregates with an oxyethylene surfactant headgroup.

Application of Molecular Dynamic Simulation of Water Clusters with CO and CO2 Molecules to Binary Nucleation Problems

The thermodynamic properties of pure water clusters and aqueous aggregates with either CO or CO2 molecule were calculated by the molecular dynamics method. The resulting size dependence of the surface tension of the clusters was used to determine the size of the critical seeds. The rate of homogeneous and binary nucleation in atmospheric air was estimated. The role of polar and nonpolar impurity molecules at the initial stage of steam condensation is discussed.

Metal nanocluster formation in silica films prepared by rf-sputtering: an experimental study

Composite silica films containing metal nanoclusters were prepared by the rf- sputtering technique, in which SiO2 was co-deposited with gold+copper, gold+silver, or copper+silver. The formation of either pure or alloy clusters was studied by extended X-ray absorption fine structure spectroscopy and transmission electron microscopy. For all systems, the presence of alloy aggregates was evidenced. Moreover, small amounts of pure metal aggregates as well as dispersed or oxidized dopants were observed. 61.46.+w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals – 61.10.Ht X-ray absorption spectroscopy: EXAFS, NEXAFS, XANES, etc. – 81.05.Pj Glass-based composites, vitroceramics

Scattering Behavior of Restructured Aggregates: A Simulation Study

Simulations of diffusion-limited cluster–cluster aggregation (DLCA) with no restructuring, full restructuring, and partial restructuring have been performed. The scattering patterns produced from these aggregates have been simulated using the Rayleigh–Gans–Debye approximation. Pure DLCA aggregates produced a scattering pattern with the slope of the fractal region being about −1.8. In contrast, the slope of the fractal region of the scattering pattern for fully restructured aggregates was about −2.1, indicating an increase in fractal dimension. Partial restructuring at large length scales produced an upward turn in the scattering pattern at low qro, while at high qro the fractal section of the pure DLCA aggregate was retained. This last result was expected and is consistent with the results and postulations of several other workers. This simulation shows that the type of scattering pattern often obtained from orthokinetic or sheared aggregation can be produced by restructuring of aggregates at large length scales.

Structure and catalytic activity of palladium–platinum aggregates obtained by laser vaporisation of bulk alloys

The European emission legislation concerning the pollution abatement of the exhaust gas from light cars imposes a reduction of the concentration of aromatics in gasoline, down to 2%. The hydrogenation of these hydrocarbons can be performed on Pd or Pt catalysts. However, these catalysts are poisoned by traces of sulphur — 100–300 ppm — remaining in the fuels. It is claimed in the literature that bimetallic Pd–Pt catalysts are less affected by this poisoning than the pure metals. To verify this point, both Pd–Pt and pure metal aggregates were synthesised by laser vaporisation of bulk alloys and deposited on alumina. Analytical microscopy and EXAFS have shown that these clusters have a very narrow composition distribution and form alloys. The study of their activity in the hydrogenation of Tetralin, a model molecule, in the presence of variable amounts of H 2S, has shown that Pt is more active than Pd at low sulphur concentration, whereas Pd becomes more active for the highest H 2S content. Contrary to what is claimed in the literature, no synergetic effect has been found found by alloying these two metals; actually their activity is the simple additivity of that of the two metals.

Acid-induced gelation of heat-treated milk studied by diffusing wave spectroscopy

Fresh skim milk is a stable colloidal system containing casein micelles and whey proteins. By decreasing the pH, the casein micelles become unstable and a gel is formed. During heat treatment at temperatures higher than 70°C, the major whey proteins, e.g. α-lactalbumin and β-lactoglobulin denature and start to interact with each other and with casein micelles. This changes the colloidal properties of the casein micelles. In this article, the pH-induced gel formation of heat-treated milk and the role of whey proteins was studied. Heat treatment in the range 70–90°C induced a shift in gelation pH of skim milk to more alkaline pH values. This shift was directly related to whey protein denaturation. By using WPF milk it was shown that β-lactoglobulin is principally responsible for the shift in gelation pH. α-lactalbumin caused neither alone nor in combination with β-lg, an effect on the gelation pH. Heat treatment of milk for 10 min at 90°C resulted in complete denaturation of the β-lg present in skim milk but it is estimated that the casein micelles are coated only up to 40% by whey proteins when compared with pure whey protein aggregates.

Deformation partitioning during folding of banded iron formation

Domainal electron and optical microscopy and c-axis fabric analysis are utilized to document microstructures and the associated deformation partitioning between crystal plastic, brittle and fluid-assisted deformation mechanisms during folding of Proterozoic banded iron formation (a hematite–quartz–calcite multilayer) from the Quadrilátero Ferrı́fero granite–greenstone terrain (southeastern Brazil). The operation of different mechanisms was partially determined by the contrasting rheologic response of these three minerals at greenschist facies metamorphic conditions. Fracturing was the main deformation process in hematite, while solution-transfer accounted for part of the deformation in both calcite and quartz. Notably, there is strong interaction among the different deformation mechanisms, which have influenced each other in various ways. Fracturing of hematite in hinge zones of folds caused opening of gaps, which were sealed by direct precipitation of silica from the fluid phase. As a consequence, a chemical potential gradient between crystals and fluid phase was produced, and quartz was dissolved to restore the thermodynamical crystal–fluid equilibrium. Thus, brittle deformation of hematite partially controlled solution-precipitation creep in quartz. Heterogeneous access of fluid into the deforming medium also affected the deformation processes. Inhomogeneous deformation in the quartz–calcite aggregates generated intergranular porosity and increased fluid access, with solution-transfer becoming dominant in these domains. In contrast, the relatively more homogeneous deformation in the pure quartz aggregates served to maintain well-adjusted grain boundaries and reduced fluid access into the intergranular space, such that these domains deformed uniformly by crystal plastic processes, at relatively dry conditions.

RuNAway Disease: A two cycle model for transmissible spongiform encephalopathies (TSEs) wherein SINE proliferation drives PrP overproduction.

Despite decades of research, the agent responsible for transmitting spongiform encephalopathies (TSEs) has not been identified. The Prion hypothesis, which dominates the field, supposes that modified host PrP protein, termed PrPSc, acts as the transmissible agent. This model fits the observation that TSE diseases elicit almost no immune reaction. Prion transmission has not been verified, however, as it has not been possible to produce pure PrPSc aggregates. One long-standing objection to the Prion model is the observation that TSE disease agents show classical genetic behaviours, such as reproducible strain variation, while also responding to selection for novel traits such as adaptation to new hosts. Moreover, evidence has been steadily accumulating that infectious titre is decoupled from the quantity (or even the presence) of PrPSc deposits. Rather awkwardly for the Prion hypothesis, PrP0/0 knockout mice have been found to incubate and transmit TSE agents (despite themselves being refractory to TSE disease). In this article, a new scheme, RuNAway, is proposed whereby uncontrolled proliferation of a type of parasitic gene, the small dispersed repeat sequences (SINEs), in somatic cells induces overproduction of PrP with pathogenic consequences. The RuNAway scheme involves twin tandem positive feedback loops: triggering the second loop leads to the pathogenic disease. This model is consistent with the long latency period and much shorter visible disease progression typical of TSEs.

Micro–macro approach for the rock salt behaviour

Rock salt is considered as a pure aggregate of halite (mineral NaCl) crystals and its behaviour is investigated by a micro–macro approach. The behaviour of the polycrystalline aggregate is deduced from the properties of the constituent halite crystals. A model for the elastoplastic behaviour of halite crystal has been deduced from experimental data available in the literature. The basic equations of the micro–macro model for the polycrystalline medium and the calculation method are then presented and the elastoplastic behaviour of rock salt is investigated by this method. The hardening effects obtained for the polycrystal are found to be very different from those obtained for FCC metal polycrystals. The differences are explained as a consequence of differences of families of glide systems in these crystals. Finally, the internal stresses in the polycrystal are studied in order to elucidate the origin of cracking and damage of the rock salt.

A computer-assisted thin-section study of Lake Baikal sediments: a tool for understanding sedimentary processes and deciphering their climatic signal

A freeze-drying technique for cutting thin-sections of soft sediments without disturbance is used to study several Lake Baikal sedimentary microstructures. Image analysis methodology is applied to selected thin-sections. This new technique provides quantification of the size, shape, orientation and packing of the objects forming the sedimentary structures. Sedimentary processes, which were previously poorly documented, have been identified, and others are better understood. Spheroidal lens-like pure aggregates of the diatom genus Synedra are found in hemipelagic sediments, providing a new insight into their traditional paleoecological interpretation. They are possibly related to a transportation mechanism from the littoral zone or to lacustrine snow. Laminae of Aulacoseira have also been recorded. Evidence of rapid sedimentation suggests they are due to massive algal blooms. The depositional mechanism that was suggested by other studies for explaining the laminations at the Buguldeika uplift is confirmed: the hemipelagic sedimentation is interrupted by terrigenous pulses due to discharge events. The sedimentation rate appears to be increasing during these pulses. Preliminary results from the Academician Ridge show stronger microbioturbation during cold periods. This observation strengthens the hypothesis of intense water circulation during colder times. Thin-section image analysis provides crucial information for deciphering lacustrine records and their regional and palaeoclimatic significance.

High‐temperature plastic deformation of quartz‐plagioclase multilayers by layer‐normal compression

It is a common practice in rheological modeling to take the extrapolated compressive flow strengths of quartzite and plagioclase rock as the bulk flow strengths of the upper and lower continental crusts, respectively. Such a practice implies that the bulk flow strength of polyphase rocks is identical to that of the pure weak phase. To test this assumption, we performed deformation experiments on synthetic layered and particulate quartz‐anorthite (50∶50) composites and aggregates of the end‐members in a Paterson gas‐medium apparatus at a confining pressure of 300 MPa, a constant strain rate of 10−5 s−1 and temperatures from 1273 to 1473 K. Under these conditions, anorthite deforms by recrystallization‐accommodated dislocation creep, while quartz is semibrittle. Electron backscatter diffraction measurements show a strong lattice‐preferred orientation (LPO) of anorthite developed in deformed specimens, and the LPO pattern is interpreted as a result of dislocation slip on the (010)[100] system. Our experimental results show that the bulk flow strength of layered composites increases with decreasing thickness of the layers. Thinlayered composites are significantly stronger than pure anorthite aggregates and a homogeneous quartz‐anorthite mixture (i.e., particulate composites) with the same modal composition but weaker than pure quartz aggregates. Our experimental results together with the theoretical overview presented previously in the materials science literature demonstrate that (1) compressive flow strength of a polyphase rock cannot be represented by that of the weak phase and (2) thin‐layered rocks compressed normal to the layering are rheologically stronger than homogeneous, isotropic mixtures under the same deformation conditions. Thus weak‐phase‐based rheology will lead to an underestimation of the bulk flow strength of the continental crust in which polyphase rocks dominate.

Structural coherence between phases in Ni0.35Ag0.65 thin films

Ni0.35Ag0.65 thin films were studied by means of both x-ray absorption spectroscopy and x-ray diffraction, in the as-deposited and annealed states. Short-range-order investigation has established that nickel atoms are either clustered in small pure Ni aggregates or dispersed in an Ag-rich solid solution. For the as-deposited and 250 °C annealed samples, an accurate interpretation of the diffraction spectra requires one to take into account a degree of structural coherence between these two phases, which are preferentially oriented with the dense planes parallel to the surface. Local order and long-range-order characterizations then give coherent results. The mean size of the pure Ni aggregates increases from approximately 1 nm in the as-deposited state to 3-5 nm after a 400 °C anneal. The maximum Ni solubility in the Ag matrix was estimated to be around 7 at.% for these samples. All of the reported results - the natures of the various phases, and mean sizes, shapes and orientations of the aggregates - are discussed with regard to the other structural (extended x-ray absorption fine-structure, anomalous small-angle x-ray scattering) and magnetic information.

From Molecular Diversity to Template-Directed Self-Assembly – New Trends in Metallo-Supramolecular Chemistry

Molecular diversity can easily be generated in metallo-supramolecular systems by simple mixing of oligodentate ligands and appropriate metal ions. In this reaction either a defined coordination compound is formed in a selective self-assembly process or a mixture is obtained. Depending on the system such a mixture can possess a statistical distribution of components or the formation of some species is thermodynamically favored leading to only a few out of several possible compounds (or in the extreme to only one). Simple well-defined mixtures containing only a few components or pure supramolecular aggregates can be generated from sequential or directional ligands, from mixtures of ligands and/or metals, and by introducing templates which support the formation of defined metallo-supramolecular aggregates. In the latter case it is possible first to generate a mixture of components which are in dynamic equilibrium (dynamic combinatorial library). In a second step, a template can be added, which in a dynamic process transforms such a library into one well-defined species. Thus, the initial generation of molecular diversity allows in a subsequent selection step in an evolutionary process the formation of the most favored receptor/substrate adduct (``dynamic combinatorial chemistry'').

Post-mortem spermatozoa recovery and freezing in a cantabric brown bear ( ursus arctos): A preliminary report

At present the Cantabric Brown Bear (Ursus arctos), which probably constitutes the last pure breed aggregate of brown bear in the world, is a seriously endangered population (~80 animals are living in a fragmentary area in the North of Spain). The harvesting of gametes obtained post-mortem could be a useful tool in creating a genetic resource bank. The present work is a preliminary report about post-mortem spermatozoa (spz) recovery in a Cantabric Brown Bear (7 years old, 170 kg), died 8 days after an accident in the wilds (3 rd May 1998). The testis was extracted 70 min post-mortem and the epididymis was dissected at 5°C. A sample of gametes was obtained from each epididymis region and deferens ductus (reference of the preejaculatory spermatic cells). The percentage and position of the cytoplasmic droplets (CD) were evaluated as a gamete maturity index (viability prediction).

Influence of micaceous impurity on dynamically recrystallized quartz c-axis fabric in L- S tectonites from the Singhbhum Shear Zone and its footwall, Eastern India

When temperature and strain rate remain constant the quartz c-axis fabric in deformed pure quartz aggregates, is largely dependent on deformation kinematics. Asymmetry of the fabric, e.g. in type-I asymmetric crossed girdle pattern in natural quartz tectonites, simulated fabric or experimentally deformed quartz aggregates is generally related to sense of vorticity for a non-coaxial flow. Natural quartz tectonites, however, often contain micaceous impurities. Measurement on a sample of 59 quartz tectonites with mesoscopic L- S fabric and representing low T T m , deformation under non-coaxial flow, from the Singhbhum Shear Zone and Dhanjori quartzites. Eastern India provides the basic data to quantitatively assess the influence of mica on (i) asymmetry of quartx c-axis fabric and (ii) degree of crystallographic preferred orientation, i.e. fabric intensity, taking c-axes of dynamically recrystallized quartz grains as a fabric element. A fabric intensity parameter (κ) is defined as the ratio of the greatest eigenvalue to the least eigenvalue of the orientation tensor matrix corresponding to c-axis orientations in each measured specimen. The modal percent of mica (μ) in the sample varies from 2 to 35; that of recrystallized quartz grains (ν), as opposed to relict clasts. from 45 to 98. The asymmetry of the fabrics in the above sample, measured either as the Am statistic or as the angle between the central segment of the fabric skeleton and direction of mineral elongation lineation is independent of mica content. Correlation-regression analysis of the variables κ, μ, and ν. demonstrate a negative correlation between micaceous impurity and the fabric intensity parameter. The regression equation is of the form κ = 0.13 ν 1.11 μ −0.617.

Water Dynamics in Glycosphingolipid Aggregates Studied by LAURDAN Fluorescence

We have characterized the fluorescence properties of 6-dodecanoyl-2-dimethylamine-naphthalene (LAURDAN) in pure interfaces formed by sphingomyelin and 10 chemically related glycosphingolipids (GSLs). 1 The GSLs contain neutral and anionic carbohydrate residues in their oligosaccharide chain. These systems were studied at temperatures below, at, or above the main phase transition temperature of the pure lipid aggregates. The extent of solvent dipolar relaxation around the excited fluorescence probe in the GSLs series increases with the magnitude of the glycosphingolipid polar headgroup below the transition temperature. This conclusion is based on LAURDAN’s excitation generalized polarization (GP ex) and fluorescence lifetime values found in the different interfaces. A linear dependence between the LAURDAN GP ex and the intermolecular spacing among the lipid molecules was found for both neutral and anionic lipids in the GSLs series. This relationship was also followed by phospholipids. We conclude that LAURDAN in these lipid aggregates resides in sites containing different amounts of water. The dimension of these sites increases with the size of the GSLs polar headgroup. The GP function reports on the concentration and dynamics of water molecules in these sites. Upon addition of cholesterol to Gg 4Cer, the fluorescence behavior of LAURDAN was similar to that of pure cerebrosides and sphingomyelin vesicles. This observation was attributed to a change in the interfacial hydration as well as changes in the shape and size of the Gg 4Cer aggregates in the presence of cholesterol. After the addition of cholesterol to gangliosides, the changes in the LAURDAN’s spectral parameters decrease progressively as the polar headgroup of these lipids becomes more complex. This finding suggests that the dehydration effect of cholesterol depends strongly on the curvature radius and the extent of hydration of these lipid aggregates. In the gel phase of phrenosine, GalCer, Gg 3Cer, sulfatide, and sphingomyelin, the excitation red band (410 nm) of LAURDAN was reduced with respect to that of LAURDAN in the gel phase of pure phospholipids. This observation indicates a local environment that interacts differently with the ground state of LAURDAN in GSLs when compared with LAURDAN in phospholipids.

Interaction of colistin with lipids in liposomes and monolayers

Colistin is an antibiotic member of the polymixin family, showing a high amphipathic character. Its mechanism of action has been related to its ability to disrupt phospholipid bilayers of the bacterial membrane. Due to its hydrophobic properties colistin can interact both with the polar heads and the alkyl chains of the phospholipids. Moreover, it has a polycationic structure, so its interactions with phospholipids should be highly dependent on the electric charge of the lipid and the ionisation state of polymixin molecule. In the present paper we report on physicochemical studies to define the type and characteristics of these interactions. The surface activity of colistin has been shown to be highly dependent on the pH of the medium, the less protonated forms being more stable at the air–water interface. Interaction with phospholipid monolayers shows the same tendency. Colistin is also able to form mixed monolayers with phospholipids with small deviations from ideality. Physicochemical studies carried out with fluorescent probes indicate the presence of pure colistin aggregates that can coexist with mixed micelles composed of colistin/phospholipids. In no case did the interaction cause significative changes in the transition temperature of phospholipids.