Deuterium NMR studies of inositol liquid crystals [1]

Deuterium NMR investigations are presented on members of two new mesogenic series derived from the naturally occurring stereoisomers myo- and scW/o-inositol. Tetraethers of these two series exhibit thermotropic columnar phases in which the columns are apparently formed by stacked hydrogen bonded dimers of these molecules which chemically are vicinal diols. Deuterium NMR measurements were performed on the tetraoctyl homologues 2e (a cis diol) and 3e (a trans diol) of these series. We have investigated mixtures of these diols with small amounts of benzene-d₆ as probe molecules as well as samples of the neat diol compounds deuteriated at their hydroxyl groups. The results obtained show that the mesophases of both compounds are uniaxial and align partially in a magnetic field upon slow cooling from their isotropic liquids. The alignment is with the director parallel to the field direction indicating that the anisotropic magnetic susceptibility of this mesophase is positive. The deuterium quadrupole splitting of the benzene-d₆ probe in both systems is temperature dependent and in the trans diol 3e it even changes sign. This is interpreted in terms of a model in which the benzene-d₆ probe equilibrates rapidly between two (or more) solvation sites with quadrupole splittings of opposite signs The deuterium spectra of the neat deuterium labelled cis diol 2e exhibit two different signals due to the two deuterons which are located at the axial and equatorial hydroxyl groups. This indicates that there is no fast intra- or intermolecular exchange of the hydroxyl hydrogens. The overall quadrupole splittings of the hydroxyl deuterons in this compound are highly reduced compared to their static values and this is interpreted in terms of motional modes involving both reorientation of the hydroxyl deuterons about their C-O axis and overall reorientation of the molecules (or pairs of molecules) around the columnar axes. The corresponding spectra of the neat deuteriated trans diol 3e exhibit a single spectrum indicating that both hydroxyl deuterons in this compound are equivalent, or very nearly so. Within the mesophase region the spectrum undergoes gradual changes due to the increase in the molecular mobility, but the overall motional narrowing is less than in the cis isomer 2e. Apparently due to stronger hydrogen bonding in the trans isomer 3e the precession of the hydroxyl groups is hindered and a fast molecular reorientation is only possible at high temperatures.     

Differential determination of arsenic(III) and total arsenic using flow injection on-line separation and preconcentration for graphite furnace atomic absorption spectrometry

Arsenic(III) can be quantitatively extracted using sodium diethyldithiocarbamate (NaDDTC) as the complexing agent and C18 reversed phase packing as the column material for solid phase extraction. Arsenic(V) must be reduced to its trivalent oxidation state prior to extraction. A mixture of sodium sulphite, hydrochloric acid, sodium thiosulphate and potassium iodide was found to be optimum for on-line reduction. When the sorbent extraction is carried out without and with the addition of the reduction mixture, arsenic(III) and total arsenic can be determined sequentially by graphite furnace atomic absorption spectrometry with detection limits (3 σ) of 0.32 ng for As(III) and 0.43 ng for total arsenic. A 7.6-fold enhancement in peak area compared to direct injection of 40 μl samples was obtained after 60 s preconcentration. Results obtained for sea water standard reference materials, using aqueous standards for calibration, agree well with certified values. A precision of 5.5% RSD was obtained for total arsenic in a sea water sample (1.65 As). Results obtained for synthetic mixtures of trivalent and pentavalent arsenic agreed well with expected values.     

Transport properties of a side-chain liquid-crystalline polymer. Poly(4-acryloyloxy biphenyl)

The transport properties of a side-chain liquid-crystalline polymer forming a smectic A phase have been investigated using dichloromethane as a permeant. Samples differing in the isotropization enthaoly were analyzed. A. Correlation between this thermal parameter and sorption has been found, while the diffusion coefficient is substantially the same in all samples. The results obtained can be explained in terms of a biphasis model in which a disordered permeable phase is present within the liquid-crystalline phase, which is impermeable to the diffusant molecules.     

Measurements of the perpendicular width of ionization-produceddouble layers

Measurements of the perpendicular (to B) double-layer widths of ionization-produced (anode) double layers are presented. These widths were determined from measurements of the double-layer charge separations. The variations of the transverse-to-B widths with plasma density and magnetic field strength are given     

Effects of saturation on optical bistability with coupled surface plasmons

We present exact numerical results for a symmetric layered medium (prism/Ag film/nonlinear dielectric/Ag film/prism) where the middle dielectric slab is assumed to have a saturation-type nonlinearity. We show bistable behaviour in the power dependence of the reflectivity ofp-polarized light under the condition when coupled surface modes are excited in the structure. Moreover, we study the effect of saturation on the bistable behaviour to show that multivalued character is inhibited by saturation effects. The field distributions corresponding to the minimum reflectivity states of the nonlinear structure are also presented.     

Formation of Diol Divinyl Diethers in the Synthesis of 1,2-Epoxy-3-(vinyloxyalkoxy)propanes

Russian Journal of Organic Chemistry -     

A Parabolic Spline on a Nonuniform Grid

We obtain estimates for errors of interpolation on a nonuniform grid for a parabolic nonperiodic spline of defect 1.     

Mixing mechanisms of vortex ring formed by gravity slumping motion

 This paper is concerned with an experimental investigation of the mixing inside the vortex ring formed by the gravity slumping motion of a dense cloud in a less dense atmosphere. The dynamics of the spreading and instantaneous structures of the turbulent flow were examined by visualization, single and multi-point measurements of velocity and concentration for two heavy gases, carbondioxide (CO₂) and dichlorodifluoromethane (CCl₂F₂), in a configuration in that heavy gas, initially trapped in a reservoir, was released with the rise of a shutter into calm air of a sector-shaped dispersion channel. Visualization of the cloud as a whole showed a spreading motion in which an advancing frontal structure was followed by a stratified flow with a layer of dense fluid of higher velocities near the wall and, on top of it, a layer of dilute fluid whose concentration is controlled by the mixing mechanisms within the head. During the course of spreading, there was always a phase in which the head attained to a constant speed of advance, which occurred as 0.13 m/s for CO₂ and 0.48 m/s for CCl₂F₂. It was interesting to observe for CO₂ that the phase of constant speed took place in between two acceleration phases; the former was due to the initial slumping of the cloud at the exit of the reservoir, and the latter was attributed to the collapse of the head on the transition to the passive dispersion phase. Instantaneous two-dimensional velocity field, measured with particle image velocimeter (PIV), showed that the cloud overran the ambient air which caused the approaching dense fluid deflected away from the wall with significant vertical velocities and downstream-moving separation, and the air trapped under the head resulted in the density inversion which introduced further intricacy to the turbulent structure of the head. Instabilities at the upper free shear layer due to density and velocity discontinuity rolled into periodic array of vortices which engulfed a considerable amount of air as they were convected backwards over the head, but the incorporation of heavy and light fluids was completed with the appearance of microscales after the collapse on the stratified layer. Analyses of the cloud head at different downstream locations also revealed that its size remained unchanged when the speed of advance was constant, allowing the rate of change of the cloud volume being modeled with the rate of spreading. Contours of concentration obtained from digitized PIV pictures confirmed the kinematic features of the mixing revealed by the velocity field and that the concentration values within the large structures were higher than those at the upper part of the stratified layer. Motivated by the experimental observations, a semi-empirical analysis was presented to describe the results and based on local values of the Richardson and Reynolds numbers. Received: 4 October 1995 / Accepted: 4 July 1996     

Oblique reflection of shock waves from rigid porous materials

A study to determine the general gas dynamic behaviour associated with the impact of a shock wave on a porous wedge has been undertaken. A number of interesting features are noted. The pattern of wave reflection is shown to be significantly affected by the inflow of gas into the wedge. This has the effect of reducing the triple point trajectory angle for cases of Mach reflection and for strongly reducing the reflection angle in regular reflection. The permeability of the wedge has a significant effect on the strength of the reflected wave and in some cases this wave can be attenuated to the extent that it is almost eradicated. Pressure measurements taken under the wedge are characterized by oscillations which are of similar shape, for a given wedge, over a range of shock wave Mach numbers. It is shown that the wave transmitted into the wedge is attenuated to varying degrees depending on the material properties, and that for weak incident waves the mean propagation velocity can be less than the sound speed in the pore fluid. Photographs taken using a specially constructed wedge which allows the transmitted wave to be visualised, show that the transmitted wave is nearly plane.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.     

Stationary soliton on a charged surface of liquid helium and hydrogen films

A change in the shape of a charged surface of liquid hydrogen and helium — the formation of a solitary wave (a positively charged hump for hydrogen and a negatively charged dimple for helium)-is observed in an electric field exceeding a critical value under conditions of total compensation of the applied field by the surface charge. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 7, 547–552 (10 April 1997)