ABSTRACT
We present small-angle neutron scattering data proving that, on the insulating side of the metal-insulator transition, the doped perovskite cobaltite La(1-x)Sr(x)CoO(3) phase separates into ferromagnetic metallic clusters embedded in a nonferromagnetic matrix. This induces a hysteretic magnetoresistance, with temperature and field dependence characteristic of intergranular giant magnetoresistance (GMR). We argue that this system is a natural analog to the artificial structures fabricated by depositing nanoscale ferromagnetic particles in a metallic or insulating matrix; i.e., this material displays a GMR effect without the deliberate introduction of chemical interfaces.
ABSTRACT
Using small-angle neutron scattering and dynamic light scattering, we have constructed partial structural phase diagrams of lipid mixtures composed of the phosphatidylcholines dimyristoyl and dihexanoyl doped with calcium ions (Ca2+) and/or the negatively charged lipid, dimyristoyl phosphatidylglycerol (DMPG). For dilute solutions (lipid concentration < or =1 wt %), spontaneously forming unilamellar vesicles (ULVs) were found, and their polydispersity was determined to be approximately 20%. The stability of the Ca2+- or DMPG-doped ULVs was monitored over a period of 4 days and their structural parameters (e.g., average outer radius,
Subject(s)
Calcium/chemistry , Liposomes/chemistry , Micelles , Phosphatidylcholines/chemistry , Phosphatidylglycerols/chemistry , Neutron Diffraction/methodsABSTRACT
In this Letter we present small-angle neutron scattering data from a biomimetic system composed of the phospholipids dimyristoyl and dihexanoyl phosphorylcholine (DMPC and DHPC, respectively). Doping DMPC-DHPC multilamellar vesicles with either the negatively charged lipid dimyristoyl phosphorylglycerol (DMPG, net charge -1) or the divalent cation, calcium (Ca2+), leads to the spontaneous formation of energetically stabilized monodisperse unilamellar vesicles whose radii are concentration independent and in contrast with previous experimental observations.
Subject(s)
Biomimetic Materials/chemistry , Dimyristoylphosphatidylcholine/chemistry , Liposomes/chemistry , Phospholipid Ethers/chemistry , Calcium/chemistry , Cations, Divalent , Kinetics , ThermodynamicsABSTRACT
Small-angle neutron scattering experiments have been performed to investigate orientational ordering of a dispersion of rod-shaped ferromagnetic nanoparticles under the influence of shear flow and static magnetic field. In this experiment, the flow and flow gradient directions are perpendicular to the direction of the applied magnetic field. The scattering intensity is isotropic in zero-shear-rate or zero-applied-field conditions, indicating that the particles are randomly oriented. Anisotropic scattering is observed both in a shear flow and in a static magnetic field, showing that both flow and field induce orientational order in the dispersion. The anisotropy increases with the increase of field and with the increase of shear rate. Three states of order have been observed with the application of both shear flow and magnetic field. At low shear rates, the particles are aligned in the field direction. When increasing shear rate is applied, the particles revert to random orientations at a characteristic shear rate that depends on the strength of the applied magnetic field. Above the characteristic shear rate, the particles align along the flow direction. The experimental results agree qualitatively with the predictions of a mean field model.
ABSTRACT
Using time-resolved small-angle neutron scattering, we have studied the kinetics of the recently observed bilayered-micelle (or so-called "bicelle") to perforated-lamellar transition in phospholipid mixtures. The data suggest that phase-ordering occurs via the early-time coalescence of bicelles into stacks of lamellae that then swell. Our measurements on this biomimetic system highlight the ubiquitous role of transient metastable states in the phase ordering of complex fluids.
Subject(s)
Biophysics/methods , Lipid Bilayers/chemistry , Micelles , Kinetics , Lipids/chemistry , Models, Chemical , Neutrons , Normal Distribution , Phospholipids/chemistry , Scattering, Radiation , Temperature , Time FactorsABSTRACT
Bilayered micelles, or bicelles, which consist of a mixture of long- and short-chain phospholipids, are a popular model membrane system. Depending on composition, concentration, and temperature, bicelle mixtures may adopt an isotropic phase or form an aligned phase in magnetic fields. Well-resolved (1)H NMR spectra are observed in the isotropic or so-called fast-tumbling bicelle phase, over the range of temperatures investigated (10-40 degrees C), for molar ratios of long-chain lipid to short-chain lipid between 0.20 and 1.0. Small angle neutron scattering data of this phase are consistent with the model in which bicelles were proposed to be disk-shaped. The experimentally determined dimensions are roughly consistent with the predictions of R.R. Vold and R.S. Prosser (J. Magn. Reson. B 113 (1996)). Differential paramagnetic shifts of head group resonances of dimyristoylphosphatidylcholine (DMPC) and dihexanoylphosphatidylcholine (DHPC), induced by the addition of Eu(3+), are also consistent with the bicelle model in which DHPC is believed to be primarily sequestered to bicelle rims. Selective irradiation of the DHPC aliphatic methyl resonances results in no detectable magnetization transfer to the corresponding DMPC methyl resonances (and vice versa) in bicelles, which also suggests that DHPC and DMPC are largely sequestered in the bicelle. Finally, (1)H spectra of the antibacterial peptide indolicidin (ILPWKWPWWPWRR-NH(2)) are compared, in a DPC micellar phase and the above fast-tumbling bicellar phases for a variety of compositions. The spectra exhibit adequate resolution and improved dispersion of amide and aromatic resonances in certain bicelle mixtures.
Subject(s)
Magnetic Resonance Spectroscopy/methods , Membrane Proteins/chemistry , Micelles , Neutrons , Antimicrobial Cationic Peptides/chemistry , Dimyristoylphosphatidylcholine , Lipid Bilayers/chemistry , Models, Theoretical , Phospholipid Ethers , Scattering, RadiationABSTRACT
The NIST Materials Science and Engineering Laboratory works with industry, standards bodies, universities, and other government laboratories to improve the nation's measurements and standards infrastructure for materials. An increasingly important component of this effort is carried out at the NIST Center for Neutron Research (NCNR), at present the most productive center of its kind in the United States. This article gives a brief historical account of the growth and activities of the Center with examples of its work in major materials research areas and describes the key role the Center can expect to play in future developments.
ABSTRACT
A model that makes use of the cooperative organization of inorganic and organic molecular species into three dimensionally structured arrays is generalized for the synthesis of nanocomposite materials. In this model, the properties and structure of a system are determined by dynamic interplay among ion-pair inorganic and organic species, so that different phases can be readily obtained through small variations of controllable synthesis parameters, including mixture composition and temperature. Nucleation, growth, and phase transitions may be directed by the charge density, coordination, and steric requirements of the inorganic and organic species at the interface and not necessarily by a preformed structure. A specific example is presented in which organic molecules in the presence of multiply charged silicate oligomers self-assemble into silicatropic liquid crystals. The organization of these silicate-surfactant mesophases is investigated with and without interfacial silicate condensation to separate the effects of self-assembly from the kinetics of silicate polymerization.
Subject(s)
Cetrimonium Compounds/chemistry , Silicates/chemistry , Surface-Active Agents/chemistry , Benzene Derivatives/chemistry , Cetrimonium , Crystallization , Crystallography, X-Ray , Freeze Fracturing , Hydrogen-Ion Concentration , Magnetic Resonance Spectroscopy , Methylamines/chemistry , Micelles , Microscopy, Electron , Molecular Structure , Temperature , ThermodynamicsABSTRACT
The small angle neutron scattering technique is a valuable method for the characterization of morphology of various materials. It can probe inhomogeneities in the sample (whether occurring naturally or introduced through isotopic substitution) at a length scale from the atomic size (nanometers) to the macroscopic (micrometers) size. This work provides an overview of the small angle neutron scattering facilities at the National Institute of Standards and Technology and a review of the technique as it has been applied to polymer systems, biological macromolecules, ceramic, and metallic materials. Specific examples have been included.
ABSTRACT
In order to combine the dynamic hip screw with a plate that anchors the greater trochanter, detailed measurements of the greater trochanter are necessary and its relation to the femoral head and neck must be studied. The hips in 200 X-ray films in the AP view were measured. The radiographs were obtained from 46 males and 87 females (69.2 +/- 16.9 years old) without hip disease. Concerning the neck shaft angle, no selection was done. The axis of the femoral head and neck was drawn; a second horizontal line passed through the apex of the lesser trochanter. Both lines intersected the lateral cortex of the femur. The distance between those two intersections was measured: d = 0.41 +/- 0.28 cm. In the next step, 74 human femora were obtained from 21 females and 17 males (79.9 +/- 9.0 years old). A special gauge was fixed at the lateral site of the femur. Using this gauge, the size and shape of the greater trochanter were measured: (1) the apex of the greater trochanter lay exactly on the line, which was determined by the lateral cortex of the femoral shaft (+/- 0.4 cm); (2) the maximum lateral extension of the greater trochanter was measured half-way from the lesser trochanter niveau to the apex of the greater trochanter (minor-major distance: 6.09 +/- 0.82 cm; minor-lateral maximum distance: 3.03 +/- 0.59 cm); (3) the maximum lateral extension of the greater trochanter measured 11.4 +/- 3 mm.