Sodium silver tricobalt bis(diphosphate) and sodium silver copper(II) diphosphate

The crystal structures of two new diphosphates, sodium silver tricobalt bis(diphosphate), (Na_1.42Ag_0.58)Co₃(P₂O₇)₂, and sodium silver copper(II) diphosphate, (Na_1.12Ag_0.88)CuP₂O₇, provide examples of the effect of mixing Na and Ag in the same site of known host phosphate compounds. The small differences in ionic radii of the two monocations do not lead to significant differences in the structural details. In the latter compound, the Cu atom lies on an inversion center.     

Onset of quark-hadron duality in pion electroproduction

A large data set of charged-pion (pi+/-) electroproduction from both hydrogen and deuterium targets has been obtained spanning the low-energy residual-mass region. These data conclusively show the onset of the quark-hadron duality phenomenon, as predicted for high-energy hadron electroproduction. We construct several ratios from these data to exhibit the relation of this phenomenon to the high-energy factorization ansatz of electron-quark scattering and subsequent quark-->pion production mechanisms.     

An Accurate In Vitro Model of the E. coli Envelope

Gram‐negative bacteria are an increasingly serious source of antibiotic‐resistant infections, partly owing to their characteristic protective envelope. This complex, 20 nm thick barrier includes a highly impermeable, asymmetric bilayer outer membrane (OM), which plays a pivotal role in resisting antibacterial chemotherapy. Nevertheless, the OM molecular structure and its dynamics are poorly understood because the structure is difficult to recreate or study in vitro. The successful formation and characterization of a fully asymmetric model envelope using Langmuir–Blodgett and Langmuir–Schaefer methods is now reported. Neutron reflectivity and isotopic labeling confirmed the expected structure and asymmetry and showed that experiments with antibacterial proteins reproduced published in vivo behavior. By closely recreating natural OM behavior, this model provides a much needed robust system for antibiotic development.     

Voltage-induced swelling and deswelling of weak polybase brushes

We have investigated a novel method of remotely switching the conformation of a weak polybase brush using an applied voltage. Surface-grafted polyelectrolyte brushes exhibit rich responsive behavior and show great promise as "smart surfaces", but existing switching methods involve physically or chemically changing the solution in contact with the brush. In this study, high grafting density poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes were grown from silicon surfaces using atom transfer radical polymerization. Optical ellipsometry and neutron reflectivity were used to measure changes in the profiles of the brushes in response to DC voltages applied between the brush substrate and a parallel electrode some distance away in the surrounding liquid (water or D(2)O). Positive voltages were shown to cause swelling, while negative voltages in some cases caused deswelling. Neutron reflectometry experiments were carried out on the INTER reflectometer (ISIS, Rutherford Appleton Laboratory, UK) allowing time-resolved measurements of polymer brush structure. The PDMAEMA brushes were shown to have a polymer volume fraction profile described by a Gaussian-terminated parabola both in the equilibrium and in the partially swollen states. At very high positive voltages (in this study, positive bias means positive voltage to the brush-bearing substrate), the brush chains were shown to be stretched to an extent comparable to their contour length, before being physically removed from the interface. Voltage-induced swelling was shown to exhibit a wider range of brush swelling states in comparison to pH switching, with the additional advantages that the stimulus is remotely controlled and may be fully automated.     

Many Polytopes Meeting the Conjectured Hirsch Bound

The still open Hirsch conjecture asserts that Δ(d,n) ≤n-d for all n > d ≥ 2 , where Δ (d,n) denotes the maximum edge-diameter of (convex) d -polytopes with n facets. This paper adds to the list of pairs (d,n) that are known to be H -sharp in the sense that Δ (d,n) ≥ n-d . In particular, it is proved that Δ(d,n)≥ n-d for all n > d ≥ 14 . Received November 20, 1996, and in revised form March 21, 1997.     

Fine and hyperfine structure in 14N2+

The fine and hyperfine structure of ¹⁴N₂⁺ has been observed in 31 rotational lines of the (0, 1) band of the B²Σ_u⁺-X²Σ_g⁺ system using the method of Doppler-tuned laser-induced fluorescence on a molecular ion beam. The spin-rotation constants (γ′, γ″, γ′_J) are in good agreement with other experiments in which the hyperfine structure was not resolved. The Fermi-contact (b′_F, b″_F) and dipolar (t′, t″) hyperfine coupling constants are in reasonably good agreement with values calculated from ab initio wavefunctions. The least-squares experimental values in MHz are γ″ = 279.1(6), γ′ = 726.4(6), γ′_J = ?0.0460(2), b″_F = 105(4), b′_F = 708(3), t″ = 49(6), and t′ = 26(5).     

Characterisation of Interfaces Between Liquid Tin and Alumina in the Presence of Titanium Alloy Additions

Neutron reflection spectroscopy has been used to characterise the composition of interfaces between liquid Sn-Ti alloys and Al₂O₃. The reflectivity profiles for both 1% and 3% Ti alloys are consistent with the presence of a layer approximately 2 nm thick at the liquid/solid interface, showing extensive segregation of Ti. The composition of this layer is not pure Ti but shows a greater Ti content than any known Ti oxide composition. In all interfaces exposed to liquid metals at elevated temperature (including pure Sn) there is a layer of thickness 20–100 nm on the alumina surface with slightly lower neutron scattering length density than pure alumina. This is interpreted as evidence for surface roughening of the Al₂O₃ surface during exposure to the liquid metal.