Nematic colloidal suspensions of V2O5 in water—or Zocher phases revisited

Abstract

Vanadium pentoxide gels and sols, V₂O₅, nH₂O, exhibit a ribbon-like structure. They form colloidal suspensions (also called Zocher phases or ‘tactophases’) which are clearly shown here to present a homogeneous lyotropic nematic phase for concentrations larger than ≈0·12mol 1^?1. Classical threaded textures were observed in polarized light and small angle X-ray scattering patterns of oriented samples displayed a diffuse spot quite comparable to that of the Tobacco Mosaic Virus. For a sample of volume fraction φ ≈ 5 per cent, the V₂O₅ ribbons are separated by about 160 Å, so that the cross sectional area of the ribbons is about 1300 Ų, in agreement with previous estimations by electron microscopy.     

Atomic structure of the (3 × 2) Si–GaAs (0 0 1) reconstructed surface: A clue to δ doping mechanism derived from in situ grazing incidence X-ray diffraction data

In view of understanding silicon incorporation in the δ doping process of GaAs (0 0 1), Si atoms have been deposited, under UHV, on a α(2 × 4) arsenic terminated substrate. In the low coverage regime, a transition to a less As rich (3 × 2) reconstructed Si–GaAs (0 0 1) surface was observed whose atomic structure has been investigated by grazing incidence X-ray diffraction performed in situ. Silicon is found to occupy not only a Ga substitutional site, precursor of a donor dopant but also to form nuclei for neutral clusters, on a template made by the (3 × 2) GaAs (0 0 1) reconstructed surface observed by Martrou et al. [Phys. Rev. B 72 (2005) 241307®]. The maximum surface concentration of donor-like silicon is estimated at 1.04 × 10¹⁴ cm^?2 (1/6th monolayer).     

Photochemical Functionalization of Heterocycles with EBX Reagents: C−H Alkynylation versus Deconstructive Ring Cleavage**

The development of novel methodologies for the functionalization of saturated heterocycles is highly desirable. Herein, we report a cheap and efficient photochemical method for the C−H functionalization of saturated O-heterocycles, as well as the deconstructive ring-cleavage of S-heterocycles, employing hypervalent iodine alkynylation reagents (ethynylbenziodoxolones, EBX). This photochemical alkynylation is performed utilizing phenylglyoxylic acid as the photoinitiator, leading to the corresponding products in good to high yields, under household fluorescent light bulb irradiation. When O-heterocycles were employed, the expected α-C−H alkynylation took place. In contrast, oxidative ring-opening to form a thioalkyne and an aldehyde was observed with S-heterocycles. Preliminary mechanistic experiments are presented to give first insights into this puzzling divergent reactivity.     

Structure of ferromagnetic CrAs epilayers grown on GaAs(001)

Magnetic and structural properties of CrAs epilayers grown on GaAs(001) by molecular beam epitaxy have been studied. CrAs epilayers are orthorhombic for all thicknesses investigated but show a structural transition from a metastable phase for very thin films, to the usual bulk MnP-type orthorhombic phase at higher thicknesses. At intermediate thicknesses, there is a predominance of the new phase, although a contribution from the usual CrAs bulk phase remains clearly present. These results strongly suggest that the ferromagnetic signal measured at room temperature comes from the new metastable orthorhombic structure with an expanded b-axis induced by the substrate strain.     

Reversible destruction of dynamical localization

Dynamical localization is a localization phenomenon taking place, for example, in the quantum periodically driven kicked rotor. It is due to subtle quantum destructive interferences and is thus of intrinsic quantum origin. It has been shown that deviation from strict periodicity in the driving rapidly destroys dynamical localization. We report experimental results showing that this destruction is partially reversible when the deterministic perturbation that destroyed it is slowly reversed. We also provide an explanation for the partial character of the reversibility.     

Experimental test of universality of the Anderson transition

We experimentally test the universality of the Anderson three dimensional metal-insulator transition, using a quasiperiodic atomic kicked rotor. Nine sets of parameters controlling the microscopic details have been tested. Our observation indicates that the transition is of second order, with a critical exponent independent of the microscopic details; the average value 1.63±0.05 agrees very well with the numerically predicted value ν=1.58.     

Quantum scaling laws in the onset of dynamical delocalization

We study the destruction of dynamical localization experimentally observed in an atomic realization of the kicked rotor by a deterministic Hamiltonian perturbation, with a temporal periodicity incommensurate with the principal driving. We show that the destruction is gradual, with well-defined scaling laws for the various classical and quantum parameters, in sharp contrast to predictions based on the analogy with Anderson localization.     

Native ESI Mass Spectrometry Can Help to Avoid Wrong Interpretations from Isothermal Titration Calorimetry in Difficult Situations

We studied by native ESI-MS the binding of various DNA-polymerase-derived peptides onto DNA-polymerase processivity rings from Escherichia coli, Pseudomonas aeruginosa, and Mycobacterium tuberculosis. These homodimeric rings present two equivalent specific binding sites, which leads to successive formation during a titration experiment of singly- and doubly occupied rings. By using the ESI-MS free-ring spectrum as a ruler, we derived by robust linear regression the fractions of the different ring species at each step of a titration experiment. These results led to accurate K_d values (from 0.03 to 0.5 μM) along with the probability of peptide loss due to gas phase dissociation (GPD). We show that this good quality is due to the increased information content of a titration experiment with a homodimer. Isothermal titration calorimetry (ITC) led with the same binding model to K_d(ITC) values systematically higher than their ESI-MS counterparts and, often, to poor fit of the ITC curves. A processing with two competing modes of binding on the same site requiring determination of two (K_d, ΔH) pairs greatly improved the fits and yielded a second K_d(ITC) close to K_d(ESI-MS). The striking features are: (1) ITC detected a minor binding mode (~20%) of ‘low-affinity’ that did not appear with ESI-MS; (2) the simplest processing of ITC data with only one (K_d, ΔH) pair led wrongly to the Kd of the low-affinity binding mode but to the ΔH of the high-affinity binding mode. Analogous misleading results might well exist in published data based on ITC experiments.

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Graphical Abstract ?

     

Radiation and Chemical Grafting of Styrene to Polybutadiene Latex

Earlier studies of the gamma radiation and potassium persulfate grafting of styrene to uncrosslinked small particle size polybutadiene latices have been extended to a commercial cross-linked large particle size latex. The larger size particles and high gel content of the substrate latex was found to lead to a more complicated pattern of behavior. The conversion curves and the molecular weights were found to be complex functions of the initial monomer concentration, number of particles and temperature with both methods of initiation. The simple Smith-Ewart theory did not, in general, apply to these systems, and the k_p and E_p varied with conditions and were not in agreement with the generally accepted literature values. The molecular weights of the extracted polystyrene homopolymer were lower in the case of radiation initiation, in agreement with the previous work. This suggests that shorter but numerous grafted side chains are possible with radiation presumably due to the higher radical fluxes. (Some parallel experiments indicated that the molecular weights of the extracted homopolystyrene are similar to those of the grafted side chains.) This phenomenon also leads to somewhat lower graft efficiency with radiation initiation. Nevertheless, radiation was found to give grafting efficiencies of more than 80% under the best conditions. The conversions were also quite efficient with economical yields per radiation dose. These results, coupled with the ease of control and other features of radiation, make it a viable alternative method of initiation for industrial use.