Birefringence induite dans les directions remarquables d'une suspension de particules rigides en ecoulement

A theoretical formula is derived for the flow birefringence exhibited along the streamlines and in the direction of the shear rate (G) by dilute suspensions of rigid spheroid-like macromolecules; numerical values are computed up to σ = G/D = 900 (D = rotary diffusion coefficient). These results provide two tests for identifying prolate and oblate particles; when combined with the data for observation perpendicular to the plane of flow, they confirm the connection between non-Newtonian viscosity and the Maxwell effect for monodisperse samples; for vanishing rate of shear, they allow a mixture of particles to be characterized by means of a mean rotary diffusion constant ($\text{D}$) and a mean shape coefficient ($\text{R}$).     

Theoretical treatment of intermediate electrolysis electrochemical detectors

A simple approach for deriving expressions for the limiting steady-state response of the open tubular electrode, under conditions of intermediate electrolytic yields, is described. This approach couples the principles of laminar fluid dynamics with the theory of the porous electrode reactor. The dependence of the limiting current, I₁, on flow rate, V_f, has the form of I₁ α V^m_f , with m ? $\text{1}\text{3}$, depending upon the electrolytic yield. The effects of electroactive species depletion upon the detector response are discussed. Experimental results are incorporated to support the theoretical conclusions. For a tubular electrode (4-cm long) the degree of conversion decreases from 0.21 to 0.03 and m decreases from 0.42 to 0.32 as the flow rate increases from 0.15 to 3.5 ml min^-1.     

The behaviour of an electrochemical detector used in liquid chromatography and continuous flow voltammetry: Part 1. Mass transport-limited current

A simple expression relating the current at a rectangular channel-type flow-through electrode to the volume flow rate of solution, cell dimensions and physical constants is derived. The expression for steady-state current is valid for laminar flow with neglect of longitudinal depolarizer diffusion. The equation is derived to extract information on the analytical utility of these cells, i.e. signal-to-noise ratio (SNR). For standard amperometric detection the optimum cell design is one in which the width of the cell is equal to the width of the electrode and the length and thickness of the cell are as small as possible, the limit of these dimensions either being physical or dictated by amplifier noise in the output. There is no optimum shape for an electrode of a given size. For standard amperometric detection with a constant cell volume, V, the optimum dimensions are given by b = ($\text{VD}\text{0.42}$U)^{$\text{1}\text{2}$}, (b = thickness, D = diffusion coefficient, -U= average volume flow rate). For flow rate-modulated operation, the optimum thickness is vanishingly small, and the electrode area (shape not critical) is given by A = 0.52 UbD^-1.     

Coagulation and flocculation measurements by photon correlation spectroscopy — colloidal SiO2 bare and covered by polyethylene oxide

With photon correlation spectrometry (PCS) the diffusion coefficients, average diameters and polydispersities of colloidal particles can be determined in dilute aqueous suspensions. In this study PCS is used to follow the coagulation and flocculation of silica particles. Electrolyte solution added to suspensions of bare particles and of particles covered with adsorbed polyethylene oxide layers induces aggregation. The rate constants of aggregation are evaluated by the second-order Smoluchowski theory with the assumptions of spherical aggregated particles and volume proportional light-scattering amplitude. Adsorbed PEO layers of molar mass lower thanM _w=160000 decrease the critical flocculation concentration and the flocculation states and rate constants for bare and covered particles are the same at high electrolyte concentrations. Polymer layers of high molar mass (M _w=325000, 900000) reducved at full coverage the rate constants and stabilize the suspensions even at high electrolyte concentrations. At low coverage adsorption of high molar mass polymers results in the same values as of low molar mass PEO. The correlation between rate constants and hydrodynamic PEO layer thicknesses demonstrates the steric influence of the tails of the adsorbed macromolecules on stability and flocculation.Dedicated to Prof. Dr. Joachim Klein on the occasion of his 60th birthday     

Flocculation kinetics and aggregate structure of kaolinite mixtures in laminar tube flow

Flocculation is commonly used in various solid-liquid separation processes in chemical and mineral industries to separate desired products or to treat waste streams. This paper presents an experimental technique to study flocculation processes in laminar tube flow. This approach allows for more realistic estimation of the shear rate to which an aggregate is exposed, as compared to more complicated shear fields (e.g. stirred tanks). A direct sampling method is used to minimize the effect of sampling on the aggregate structure. A combination of aggregate settling velocity and image analysis was used to quantify the structure of the aggregate. Aggregate size, density, and fractal dimension were found to be the most important aggregate structural parameters. The two methods used to determine aggregate fractal dimension were in good agreement. The effects of advective flow through an aggregate's porous structure and transition-regime drag coefficient on the evaluation of aggregate density were considered. The technique was applied to investigate the flocculation kinetics and the evolution of the aggregate structure of kaolin particles with an anionic flocculant under conditions similar to those of oil sands fine tailings. Aggregates were formed using a well controlled two-stage aggregation process. Detailed statistical analysis was performed to investigate the establishment of dynamic equilibrium condition in terms of aggregate size and density evolution. An equilibrium steady state condition was obtained within 90 s of the start of flocculation; after which no further change in aggregate structure was observed. Although longer flocculation times inside the shear field could conceivably cause aggregate structure conformation, statistical analysis indicated that this did not occur for the studied conditions. The results show that the technique and experimental conditions employed here produce aggregates having a well-defined, reproducible structure.     

A model of macroradical recombination in dilute solutions

The dependence of exchange rate constants and diffusion coefficients of stable macroradicals (modified polydimethylsiloxane ($\text{P}\text{= 15, 29, 97}$) and polyethylenoxide ($\text{P}\text{= 25, 47, 140}$) on the length of polymer chain in various solvents is investigated. The results are compared with the rate constants for diffusive bimolecular recombination of macroradicals of various lengths. The dependence of rate constants of exchange and combination for macroradicals of different lengths obey the same law.The rate of combination for macroradical with P = 15-10⁵ is shown to depend mainly on the mobility of an active centre, limited by diffusion of the whole macromolecule. The diffusion rate constant of combination can be described by the Smoluchowski equation in which the radius of an active centre is used as the effective radius of interaction.A model of bimolecular termination of macroradicals, a model of “freely penetrating coils” (FPC), is proposed. It implies that k_ter ～ P^{?0.9 ± 0.1} (for “good” solvents). The reactivity of an active centre in recombination is independent of the length of a macroradical chain for P > 15.The exchange interaction in a radical pair broadens the ESR lines of radicals and recombination with equal effectiveness.     

Extra-column dispersion of macromolecular solutes in aqueous-phase size-exclusion chromatography

A set of dextran standards was used to study the extra-column dispersion in conventional chromatographic equipment at a broad range of molecular weights, different mobile phase flow rates and connecting tube lengths and diameters. All known correlations for the tube dispersion at laminar flow, including those for short tubes, overestimated the values of the variance of the outlet concentration signal. The difference increased with the solute molecular weight and the flow rate. It was assumed that the discrepancy was due to the effect of natural convection invoked by the density differences of the injected dextran solutions and water. A suitable approximation of the relative band spreading was suggested in a form of a power function of the Reynolds and Schmidt numbers. A significant decrease of the dispersion was observed when the chromatography tubing was coiled into a circle. This decrease was successfully predicted combining the existing correlations for long coiled tubes and short straight tubes.     

The amperometric response of tubular electrodes applied to flow-injection determinations

The classical treatment by Taylor for the dispersion of mass injected into a fluid stream in a linear, tubular channel is applied for amperometric detection in flow-injection systems with a tubular, flowthrough electrode. Peak current is predicted to be dependent upon the $\text{l}\text{3}$rd root of flow rate for low dispersion and the —$\text{1}\text{6}$th root of flow rate for high dispersion. Agreement between experimental results and theory is excellent for flow rates of 0.5 ml min^-1 or less in a linear, tubular channel with an inner radius of 0.0483 cm. As examples, experimental and predicted currents agree to better than 0.1 μA throughout a peak with a maximum value near 2 μA, and effects of retention volume, sample volume, and flow rate on the ratio of peak current to steady-state current are predicted with errors of 9% or less.     

Bestimmung von blei und zink in stäuben durch atomabsorption

The contents of lead and zinc in particles of ambient air, in particulate samples of waste gases and in particles of automobile exhaust gases are determined by atomic absorption spectrometry. The samples are dissolved by constantly boiling hydrochloric acid; commonly some nitric acid is added. Unsolvable components are separated by filtration. The concentrations of lead and zinc in the hydrochloric solutions are estimated by use of the acetylene-air-flame.The measuring ranges are between 1 and 10 mg $\text{Pb}\text{l}$, 5 and 50 mg $\text{Pb}\text{l}$ and 50 to 250 mg $\text{Pb}\text{l}$, and between 0,05 and 1,0 mg $\text{Zn}\text{l}$ and 0,5 and 5,0 mg $\text{Zn}\text{l}$. In the hydrochloric solutions of particles sampled in exhaust gases of automobiles the concentrations of lead were determined between 1 and 250 mg $\text{Pb}\text{l}$. The percentage of different particulate samples was found between 0,1 and 1,2 % for Pb and between 0,1 and 74% for Zn.     

Note: Aris-Taylor dispersion from single-particle point of view

When a point Brownian particle diffuses in a straight circular tube in the presence of a laminar stationary flow of the liquid, its effective diffusion coefficient along the tube axis increases compared to its value in the absence of flow. The effective diffusion coefficient as a function of the average fluid velocity and the tube radius is given by the Aris-Taylor formula. We give a new derivation of this formula, which is based on consideration of the axial displacement of the particle that moves in the plane normal to the tube axis along a given trajectory. The result is obtained by averaging the displacement and its square over different realizations of the particle trajectory and analyzing the long-time asymptotic behavior of the two moments.     

On rheological properties of polydisperse polymers

An investigation has been carried out into the effect of the fractional composition on the rheological (flow and elastic) properties of a system, using mixtures of polybutadienes with narrow molecularweight distribution (MWD). For mixtures of high-molecular-weight components, the initial Newtonian viscosity is determined by the weight-average molecular weight: $\text{η}{}_0\text{～}\text{M}{}^{\mathrm{\alpha }}{}_{\mathrm{<mtext>w</mtext>}}$; when low-molecular weight components are introduced, it is also determined by the MWD moment ratio. The characteristic relaxation time of a system is determined by the z-average molecular weight: , and in the general case α₁ ≠ α. A new model has been proposed to explain the non-Newtonian phenomenon as a consequence of the existence of a molecular-weight distribution. According to this model, as the shear rate increases the high-molecular-weight components gradually (at their critical rates) pass over to the high-elastic state. Therefore, at high shear rates, their contribution to viscous losses of a polydisperse polymer is associated with their behaviour as a viscoelastic filler in a viscous liquid.     

Analysis of cycloheptanone conformation from temperature dependent circular dichroism measurements of (+)-(3R)-methylcycloheptanone and (−)-(4R)-methylcycloheptanone

Conformational analysis of methylcycloheptanones was achieved by relating CD rotational strengths to conformational free energies. Twist conformers with C₁ on the symmetry axis (TC₁) were found to be more stable than those with C₂ or C₇ on the symmetry axis (TC₂ or TC₇). The free energy differences were found to range from 0.2 to 1.2$\text{kcal}\text{mole}$. A similar analysis of (+)(3R)-methylcyclohexanone showed the equatorial chair conformer to be most stable, by 0.50–0.75 $\text{kcal}\text{mole}$.     

Resolution of N-benzyl-N-(1,2-dihydro-2-oxoquinolin-1-yl)glycine

The $\text{N}$$\text{N}$ chiral axis in the title glycine derivative has allowed its resolution into enantiomers: the barrier Δ G^≠ for racemisation (rotation around the $\text{N}$$\text{N}$ bond) is 26.2 Kcal mol^?1.     

Template molecular weight effects on the polymerization of acrylic acid

Acrylic acid was polymerized in dilute aqueous solution in the presence of a series of polyvinylpyrrolidones (PVP) of different molecular weight. Both inhibition time and rate of polymerization increased with PVP molecular weight. Purification of the PVP had no effect on these results. The complex produced by the polymerization was separated by methylation of the acrylic acid. The degree of polymerization ($\text{DP}$) of the polyacrylic acid formed in template polymerizaton was found to be remarkably similar to the $\text{DP}$ of the template PVP. A significant degree of isotacticity had also been induced in the PAA.     

Détermination de l'aptitude à l'extrusion de polyéthylène basse densité pour le gainage des cables—I. Influence des caractéristiques physico-chimiques sur les propriétés rhéologiques de polyéthylène basse densité

For determining the capacity of low density polyethylene for wire coating, we have studied the influence of physical-chemical characteristics on the rheological properties of IdPE in the molten state. The viscous flow, characterized by the average relaxation time $\text{τ}$ and zero shear viscosity η₀ is affected by the weight-average molecular weight and the width of the molecular weight distribution. A correlation with the behaviour of these polymers during industrial processing has been established.     

The X-ray crystal structure of a 1:1 complex between 1,3:1′, 3′:4,6:4′,6′-tetra-O-methylene-2,2′:5,5′-bis-O- oxidiethylenedi-D-mannitol and water

In the crystalline complex referred to in the title, the host molecule $\text{DD}$-$\text{1}$ has crystallographic C₂ symmetry with the hydrogen bonded guest water molecule straddling the two-fold axis.     

Synthesis and some structural data of nylon 18

The new polyamide nylon 18 was synthesized, mainly to investigate whether a rather large number of methylene groups in the monomeric unit might change the chain packing in comparison with that of lower even nylons. By X-ray diffraction measurements on a mono-oriented fibre, the following crystallographic parameters were found: $\text{a = 4.76}\text{A}\text{?}$; $\text{b = 9.52}\text{A}\text{?}$; $\text{c = 46.9}\text{A}\text{?}$ (fibre axis); γ = 120°. A γ-type packing for the chains was found, similar to that shown by other polyamides of the series.     

Improved dewatering behavior of clay minerals dispersions via interfacial chemistry and particle interactions optimization

Orthokinetic flocculation of clay dispersions at pH 7.5 and 22 degrees C has been investigated to determine the influence of interfacial chemistry and shear on dewatering and particle interactions behavior. Modification of pulp chemistry and behavior was achieved by using kaolinite and Na-exchanged (swelling) smectite clay minerals, divalent metal ions (Ca(II), Mn(II)) as coagulants and anionic polyacrylamide copolymer (PAM A) and non-ionic polyacrylamide homopolymer (PAM N) as flocculants. The pivotal role of shear, provided by a two-blade paddle impeller, was probed as a function of agitation rate (100-500 rpm) and time (15/60 s). Particle zeta potential and adsorption isotherms were measured to quantify the interfacial chemistry, whilst rheology and cryogenic SEM were used to investigate particle interactions and floc structure and aggregate network, respectively. Osmotic swelling, accompanied by the formation of "honeycomb" particle network structure and high yield stress, was produced by the Na-exchanged smectite, but not kaolinite, dispersions. Dispersion of the clay particles in 0.05 M Ca(II) or Mn(II) solution led to a marked reduction in particle zeta potential, complete suppression of swelling, honeycomb network structure collapse and a concomitant reduction in shear yield stress of smectite pulps. Optimum conditions for improved, orthokinetic flocculation performance of negatively charged clay particles, reflecting faster settling flocs comprised (i) coagulation, (ii) moderate agitation rate, (iii) shorter agitation time, and (iv) anionic rather than non-ionic PAM. The optimum dewatering rates were significantly higher than those produced by standard, manual-mixing flocculation techniques (plunging and cylinder inversion) commonly used in industry for flocculant trials. The optimum flocculation conditions did not, however, have a significant impact on the final sediment solid content of 20-22 wt%. Further application of shear to pre-sedimented pulps improved consolidation by 5-7 wt% solid. Higher shear yield stresses and greater settling rates were displayed by PAM A based than PAM N based pulps and this is attributed to the former's more expanded interfacial conformation and greater clay particles bridging ability. It appears that the intrinsic clay particles' physico-chemical properties and interactions limit compact pulp consolidation.     

Ternary chalcogenide compounds AB2X4: The crystal structures of SiPb2S4 and SiPb2Se4

The crystal structures of SiPb₂S₄ and SiPb₂Se₄ were determined from three dimensional X-ray diffraction data collected with Mo radiation. Both structures are monoclinic with space group $\text{P2}{}_1\text{c}$ and 4 formula units per unit cell. Lattice dimensions for SiPb₂S₄ are a = 6.4721(5) Å, b = 6.6344(9) Å, c = 16.832(1) Å, and β = 108.805(7)°. For SiPb₂Se₄, a = 8.5670(2) Å, b = 7.0745(3) Å, c = 13.6160(3) Å, and β = 108.355(3)°. The Si is tetrahedrally coordinated to S and Se with SiS about 2.10 Å and SiSe about 2.27 Å. The structural framework can be described as consisting of trigonal prisms of S or Se atoms which form a prismatic tube by sharing the triangular faces. These tubes in turn share edges to form corrugated sheets, with the unshared edges projecting alternately on each side of the sheet. The structures are very similar but not identical. In the sulfide one Pb is in sevenfold coordination and the other crystallographically independent Pb is in eightfold coordination. The PbS distances range from 2.82–3.50 Å. In SiPb₂Se₄ both Pb atoms are in sevenfold coordination. PbSe distances range from 2.97 to 3.54 Å. In the sulfide the Pb atoms form a zig-zag chain within the channels formed by the prismatic tubes while in the selenide they are in a straight line.