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At least 1,171 records · Page 65Linked to original sources
PubMed2004

Extraction of zirconium nitrate by TBP in n-octane: influence of cation type on third phase formation according to the "sticky spheres" model.

Small-angle neutron scattering (SANS) data for the tri-n-butyl phosphate (TBP)-n-octane, HNO(3)-Zr(NO(3))(4) solvent extraction system, obtained under a variety of experimental conditions, have been interpreted using the Baxter model for hard spheres with surface adhesion. The increase in scattering intensity in the low Q range observed when increasing amounts of Zr(NO(3))(4) were extracted into the organic phase was interpreted as arising from interactions between small reverse micelle-like particles containing two to three TBP molecules. Upon extraction of Zr(NO(3))(4), the particles interact through attractive forces between their polar cores with a potential energy that exceeds 2 k(B)T. The interparticle attraction, under suitable conditions, leads to third phase formation. A linear relationship exists between the derivative of the potential energy of attraction with respect to the concentration of nitrate ions in the organic phase and the ionization potential or the hydration enthalpy of the extracted metal cations.

Cations↗
PubMed2004

Adsorption of aromatic counterions at the surfactant/water interface: a neutron reflectivity study of hydroxybenzoate and chlorobenzoate counterions at the hexadecyl trimethylammonium surfactant/water interface.

Specular neutron reflectivity has been used to investigate the adsorption of the aromatic counterions hydroxybenzoate and chlorobenzoate at the hexadecyl trimethylammonium bromide surfactant monolayer/water interface. The degree of counterion binding and the location of the counterions at the interface are shown to depend on the isomeric form of the counterion. For hydroxybenzoate, the para-substituted counterion is located within the headgroup region of the surfactant monolayer, and there is of order one counterion for every two surfactant ions. For the ortho-substituted counterion, the degree of counterion binding is higher. There is of order 0.85 counterions for each surfactant ion, and the counterion is located within the hydrophobic region of the monolayer, some 5 A from the center of the headgroup distribution. Similar results were found for the chlorobenzoate counterion, but in that case it was the para-substituted counterion that was more tightly bound and located within the hydrophobic region of the surfactant monolayer. The results for the ortho-substituted hydroxybenzoate and for the para-substituted chlorobenzoate are consistent with those previously reported for the para-tosylate. The results are discussed in the context of the ability of the specific aromatic counterion isomer to promote massive micellar growth, and the results shed light on that mechanism.

Adsorption↗
PubMed2004

Hydrophilic chain length dependence of the ionic amphiphilic polymer monolayer structure at the air/water interface.

Detailed analysis of an interesting poly(methacrylic acid) (PMAA) brush structure in water of a diblock copolymer [(Et(2)SB(m)-b-(MMA)(n), where Et(2)SB is diethylsilacyclobutane] monolayer reported previously was performed by X-ray and neutron reflectometry and indicated that the hydrophilic layer formed with a relatively long PMAA chain is not a simple layer but is divided into two layers, that is, a "carpet"-like dense PMAA layer near the hydrophobic layer and a polyelectrolyte brush layer. The hydrophilic chain length dependence of the diblock copolymer monolayer was analyzed using m:n = 30:x polymer samples, especially of the PMAA double layer structure. With the increase in PMAA chain length in polymer samples, a carpet layer is mainly formed up to n approximately 50. With further increase in the PMAA chain length beyond n approximately 50, a well-defined brush layer appears. On the other hand, the variation in hydrophobic layer thickness with methacrylic acid unit is minimum at the critical PMAA length, that is, n approximately 50 under a constant surface pressure condition. It is strongly suggested that the two discrete layers contribute differently to surface pressure. Furthermore, from the comprehensive viewpoint, the major factor for brush formation was clarified not to be the absolute length of the PMAA chain, but the ratio of PEt(2)SB and PMAA chain length is an important factor.

Air↗
PubMed2004

Effect of the hydrophilic size on the structural phases of aqueous nonionic gemini surfactant solutions.

Aggregate structures of aqueous nonionic Gemini surfactant solutions, alpha,alpha'-[2,4,7,9-tetramethyl-5-decyne-4,7-diyl]bis[omega-hydroxyl-polyoxyethylene] with three different length polyoxyethylenes (i.e., 10, 20, and 30 ethylene oxide monomers, denoted from now on as S-10, S-20, and S-30, respectively), are investigated using small angle neutron scattering, dynamic light scattering, and fluorescence spectroscopy. For S-10 at low surfactant concentrations (Cs < 0.9 wt %), large "clusters", with an average hydrodynamic radius (RH) > 40 nm, are found to coexist with monomers. At intermediate Cs (0.9 < Cs < 2 wt %), some clusters break down forming micelles, with an (RH) approximately 2-3 nm, while the remaining clusters coexist with micelles. Increasing Cs further (>2 wt %) results in a pure micellar phase with little or no clusters present. S-20 and S-30 mixtures, on the other hand, differ from S-10 in that irrespective of surfactant concentration, large clusters and small monomers/dimers are found to coexist, while there is no direct evidence for the presence of micelles.

Light↗
PubMed2004

Interactions of poly(amidoamine) dendrimers with the surfactants SDS, DTAB, and C12EO6: an equilibrium and structural study using a SDS selective electrode, isothermal titration calorimetry, and small angle neutron scattering.

Interactions in aqueous solutions of different generations of poly(amidoamine) (PAMAM) dendrimers containing amine, hydroxyl, or delta-glucolactone functional groups at the periphery with the anionic surfactant sodium dodecyl sulfate (SDS) were investigated. We used a SDS-specific electrode (EMF) for SDS monomer concentration monitoring, isothermal titration calorimetry (ITC) for binding information, and small angle neutron scattering (SANS) for structural studies. ITC experiments monitoring the interaction of the dendrimers with cationic dodecyltrimethylammonium bromide (DTAB) and nonionic hexaethylene glycol mono-n-dodecyl ether (C12EO6) showed no significant binding effects. In contrast, SDS binds to all of the above dendrimers. EMF and ITC data demonstrated a regular trend for both the onset of binding and binding saturation as the generation in each family of dendrimers increased. In addition, generation G6 exhibited a noncooperative binding process at very low SDS concentrations. Furthermore, the onset of cooperative binding in the EMF experiments started at lower concentrations as the weight % (w/v), the size, and the numbers of the internal or surface groups increased. On the other hand, the binding capacity of the dendrimers showed only a small dependence on the above parameters. At SDS concentrations approaching the binding limit and also at selective concentrations within the binding range, SANS measurements indicated that in all cases the bound surfactant is in the micellar form. From the electromotive force (EMF) measurements, ITC data, and SANS data, the stoichiometry of the supramolecular complexes was determined.

Calorimetry↗
PubMed2006

Incorporating intermicellar interactions in the fitting of SANS data from cationic wormlike micelles.

Small-angle neutron scattering (SANS) from cationic wormlike micellar solutions composed of hexadecyltrimethylammonium bromide (CTABr) and hexadecylpyridinium bromide (CPyBr) in deuterated water was studied at 40 degrees C as a function of surfactant and salt concentrations. Two scattering functions of semiflexible chains incorporating excluded volume effects, with and without the intermicellar interactions, were used in SANS data model fitting. Two needed changes were made in the well-accepted models. Extensive and systematic SANS data analysis suggests the robustness of these corrected scattering functions when the intermicellar interactions are included. The influence of the headgroups and ionic strength on the contour length and micellar flexibility of these two systems was demonstrated on the basis of the quantitative structural information obtained from the model fitting. Micellar flexibility was found to depend on surfactant concentration, even when intermicellar interactions were taken into account, despite predictions to the contrary.

Cations↗
PubMed2006

Supramolecular aggregates of amphiphilic gadolinium complexes as blood pool MRI/MRA contrast agents: physicochemical characterization.

In this paper, we present the development of a new potential blood pool contrast agent for magnetic resonance imaging applications (MRA/MRI) based on gadolinium complexes containing amphiphilic supramolecular aggregates. A novel amphiphilic unimer, containing the DTPAGlu chelating agent covalently bound to two C18 alkylic chains, has been synthesized. DTPAGlu is a well-known chelating agent for a wide number of ions such as the paramagnetic metal ion Gd3+ used as contrast agent in MRA/MRI. The wide aggregation behavior of this surfactant, as free base or as gadolinium complex, has been studied and compared by means of dynamic light scattering, small-angle neutron scattering and cryogenic transmission electron microscopy techniques. Near neutral pH in both cases, the dominant aggregates are micelles. The high negative actual charge of the surfactant headgroup causes a strong headgroups repulsion, promoting the formation of large and high curvature aggregates. By decreasing pH and less markedly increasing the ionic strength, we observe a micelle-to-vesicle transition driven by a decreased electrostatic repulsion. A straightforward switch between different aggregation states can be particularly useful in the development of pH-responsive MRA/MRI contrast agents.

Chemical Phenomena↗
PubMed2006

pH-induced release from P2VP-PEO block copolymer vesicles.

The pH-induced release of hydrophilic dyes from poly(2-vinylpyridine-b-ethylene oxide) (P2VP-PEO) block copolymer vesicles is investigated. The structure of the vesicles is characterized using small-angle neutron scattering (SANS) and cryo-electron microscopy (cryo-TEM). A decrease of the pH below 5 leads to protonation and dissolution of the poly-2-vinylpyridine blocks which induces rupture and dissolution of the vesicle membrane. Details of the rupture, dissolution, and release process are studied by fluorescence video microscopy, gel electrophoresis, and high-performance ultrafiltration.

Cryoelectron Microscopy↗
PubMed2004

Equilibrium cluster formation in concentrated protein solutions and colloids.

Controlling interparticle interactions, aggregation and cluster formation is of central importance in a number of areas, ranging from cluster formation in various disease processes to protein crystallography and the production of photonic crystals. Recent developments in the description of the interaction of colloidal particles with short-range attractive potentials have led to interesting findings including metastable liquid-liquid phase separation and the formation of dynamically arrested states (such as the existence of attractive and repulsive glasses, and transient gels). The emerging glass paradigm has been successfully applied to complex soft-matter systems, such as colloid-polymer systems and concentrated protein solutions. However, intriguing problems like the frequent occurrence of cluster phases remain. Here we report small-angle scattering and confocal microscopy investigations of two model systems: protein solutions and colloid-polymer mixtures. We demonstrate that in both systems, a combination of short-range attraction and long-range repulsion results in the formation of small equilibrium clusters. We discuss the relevance of this finding for nucleation processes during protein crystallization, protein or DNA self-assembly and the previously observed formation of cluster and gel phases in colloidal suspensions.

Colloids↗
PubMed2006

Magnetic behaviour of layered Ag(II) fluorides.

Fluoride phases that contain the spin-1/2 4d9 Ag(II) ion have recently been predicted to have interesting or unusual magnetochemistry, owing to their structural similarity to the 3d9 Cu(II) cuprates and the covalence associated with this unusual oxidation state of silver. Here we present a comprehensive study of structure and magnetism in the layered Ag(II) fluoride Cs2AgF4, using magnetic susceptometry, inelastic neutron scattering techniques and both X-ray and neutron powder diffraction. We find that this material is well described as a two-dimensional ferromagnet, in sharp contrast to the high-T(C) cuprates and a previous report in the literature. Analyses of the structural data show that Cs2AgF4 is orbitally ordered at all temperatures of measurement. Therefore, we suggest that orbital ordering may be the origin of the ferromagnetism we observe in this material.

Journal Article↗
PubMed2003

Atomic resolution of lithium ions in LiCoO2.

LiCoO2 is the most common lithium storage material for lithium rechargeable batteries, used widely to power portable electronic devices such as laptop computers. Operation of lithium rechargeable batteries is dependent on reversible lithium insertion and extraction processes into and from the host materials of lithium storage. Ordering of lithium and vacancies has a profound effect on the physical properties of the host materials and the electrochemical performance of lithium batteries. However, probing lithium ions has been difficult when using traditional X-ray and neutron powder diffraction techniques due to lithium's relatively low scattering power when compared with those of oxygen and transition metals. In the work presented here, we have succeeded in simultaneously resolving columns of cobalt, oxygen and lithium atoms in layered LiCoO2 battery material, using experimental focal series of LiCoO2 images obtained at sub-ångstrom resolution in a mid-voltage transmission electron microscope. Lithium atoms are the smallest and lightest metal atoms, and scatter electrons only very weakly. We believe our observations of lithium to be the first by electron microscopy, and that they show promise for direct visualization of the ordering of lithium and vacancies in transition metal oxides.

Lithium↗
PubMed2003

Mechanism of formation of DNA-cationic vesicle complexes.

Cationic vesicles and DNA form complexes that are promising gene delivery systems. Despite the increasing number of publications on their morphology and structure, the mechanism leading to their formation is not yet understood due to a lack of kinetic data. In the present study the kinetics of the interaction between DNA and cationic vesicles were followed using stopped-flow turbidity and small-angle neutron scattering techniques. The neutron real-time experiments were performed on a high-flux diffractometer, the D22 at the ILL, using a stopped-flow set-up. Extruded mixed vesicles of dimethyldioctadecylammonium bromide (DODAB) with various amounts of dioleoylphosphatidylethanolamine (DOPE) were investigated at 25 degrees C. The results show that the transition from unilamellar vesicles to a multilamellar structure upon DNA addition occurs in three steps. The first step, on the millisecond time scale, is currently not accessible to neutron scattering but was observed by stopped-flow turbidity and fluorescence experiments. The second step, on a time scale of seconds, corresponds to the formation of an intermediate with a locally cylindrical structure. As time progresses this unstable intermediate evolves to a multilamellar structure, on a time scale of minutes. An understanding of the mechanisms behind the DNA-cationic vesicle complex formation event will allow the production of more homogeneous, efficient delivery systems in pharmaceutically acceptable forms.

DNA↗
PubMed2003

The dynamic transition in proteins may have a simple explanation.

The transition that has been observed in the dynamics of hydrated proteins at low temperatures (180-230 K) is normally interpreted as a change from vibrational, harmonic motion at low temperatures to anharmonic motions as the temperature is raised. It is taken to be an intrinsic property of proteins and has been associated with the onset of protein functions. Examination of the dynamic behaviour of proteins in solution within a defined timescale window suggests that certain observations can be explained without the need to invoke a discontinuity in the dynamics of proteins with temperature, i.e. the existence of a dynamical transition is not required. This is discussed in the context of recent evidence that enzyme activity is independent of the activation of anharmonic picosecond dynamics and declines steadily with temperature through the apparent dynamic transition, in accordance with the Arrhenius relationship. That similar timescale dependent dynamical behaviour has been observed experimentally in chain polymers, and seen also in computer simulations of silica glasses, suggests that the phenomenon may be of wide general relevance in both simple glassy and more complex polymeric systems.

Enzymes↗
PubMed2004

Preparation and topotactical oxidation of ScVO3 with bixbyte structure: a low-temperature route to stabilize the new defect fluorite ScVO(3.5) metastable phase.

ScVO3 has been prepared by controlled reduction of a ScVO4 precursor under an H2/N2 flow at 1250 degrees C. The crystal structure of this material has been studied at room temperature by Rietveld refinement of high-resolution neutron powder diffraction (NPD) data. Sc3+ and V3+ are distributed at random over the metal sites of a C-M2O3 bixbyite-type structure, space group Ia3, a = 9.6182(1) Angstroms. The thermal analysis of ScVO3 in an air flow shows two subsequent oxidation processes, with a final reversal to ScVO4 above 600 degrees C. An intermediate phase of composition ScVO(3.5), containing V4+ cations, can be isolated by isothermal annealing at 350 degrees C in air. This metastable phase has been identified by X-ray diffraction (XRD) as a fluorite-type oxide (space group Fm3m, a = 4.947(2) Angstroms), also showing a random distribution of Sc and V cations over the metal positions. The Rietveld refinement of the ScVO(3.5) structure from powder XRD data in a fluorite structural model yields abnormally high thermal factors for the oxygen atoms, suggesting oxygen mobility in this metastable material.

Journal Article↗
PubMed2005

Location of H+ sites in the fast proton-conductor (H3O)SbTeO6 pyrochlore.

The defect pyrochlore (H3O)SbTeO6 oxide is an excellent proton conductor, showing a conductivity value of 10(-1) S cm(-1) at 30 degrees C under saturated water vapor partial pressure. It can be prepared by ion exchange from KTeSbO6 pyrochlore in sulfuric acid at 453 K for 12 h. The full characterization of the structure of the (H3O)SbTeO6 pyrochlore, including the location of the H3O+ units within the three-dimensional framework, has been carried out by neutron powder diffraction. A first Rietveld refinement of the [SbTeO6]- framework was performed in the Fd3m space group (a= 10.1510(1) A); a difference Fourier map enabled the unambiguous location of the O2 atoms from the H3O+ ions at 32e (x,x,x) positions, and subsequently the H atoms at 96g (x,x,z). The (H3O)SbTeO6 crystal structure is constituted by a network of randomly distributed Sb(V)O6 and Te(VI)O6 octahedra linked by their corners with (Sb,Te)-O1-(Sb,Te) angles of 136.2 degrees. Hydronium ions are located off-center around the large 8a cages of the pyrochlore. The geometry of the (O2)-H3 units is that of an almost regular tetrahedron, with O2 atoms at the center and the three H atoms in three of the vertices; the fourth vertex is supposed to be occupied by the O2 lone pair. The three O2-H bonds have equal distances of 1.020(8)A. The H3O+ units are linked to the O1 framework oxygens by weaker hydrogen bonds, with O1-H bond lengths of 1.649(7) A. The relatively large thermal factors of O2 and H, of 2.5 and 3.7 A2, respectively, suggest that both kinds of atoms are not static at fixed positions but could be dynamically fluctuating between crystallographically equivalent sites.

Antimony↗