Coupled convection of solution near the surface of ion-exchange membranes in intensive current regimes

Mechanisms responsible for the overlimiting ion transfer in membranes systems are discussed. The overlimiting transfer is shown to be due largely to the action of four effects coupled with the concentration polarization of the system. Two of these are connected with the water dissociation near the membrane/solution interface: the emergence of additional charge carriers (ions H⁺ or OH^?) in the depleted solution layer and the exaltation of transfer of salt counterions. The latter effect is connected with the perturbation of electric field caused by the water dissociation products. The other two effects are two versions of coupled convection, which leads to partial destruction of the depleted diffusion layer. These include gravitational convection and electroconvection. The former is caused by the emergence of the solution’s density gradient. The latter develops via a mechanism of electroosmotic slip. In this work, methods of voltammetry and chronopotentiometry and pH measurements are used to study the transfer of ions through homogeneous membranes Nafion-117 and AMX as a function of the concentration of sodium chloride solutions in the underlimiting and overlimiting current regimes. In a 0.1 M NaCl solution, gravitational convection makes a considerable contribution to the transfer of salt ions near the membrane surface in intensive current regimes. The influence of this effect on the electrochemical behavior of membrane systems weakens with the solution dilution and with increasing relative transfer of the H⁺ and OH^? ions that are generated at the membrane/solution interface. In conditions where gravitational convection is suppressed and the water dissociation near the membrane/solution interface is not great, the major contribution to the overlimiting growth of current is made by electroconvection. Topics for discussion in the paper include the mutual influence of effects on one another, in particular, the effect the rate of generation of the H⁺ and OH^? ions exerts on the gravitational convection and electroconvection and the reasons for the different behavior of cation-and anion-exchange membranes in intensive current regimes.     

Influence of extra-framework cations on the adsorption properties of X-faujasite systems: microcalorimetry and molecular simulations

Isotherms and differential enthalpies of adsorption are obtained for nitrogen at ambient temperature on monovalent (Li(+), Na(+), K(+)) and divalent (Ca(2+), Ba(2+), Sr(2+), Mn(2+)) substituted X-faujasite systems by microcalorimetry measurements. These experimental data are compared with those obtained by combining grand canonical Monte Carlo simulations and newly derived force fields for describing the interactions between the extra-framework cations and the adsorbates obtained from a simple model based only on the intrinsic properties of the cations. It is the first time that such good qualitative agreement is reported between experiment and simulation for a series of both monovalent and divalent cations.     

Morphological analysis of photocured polyurethane acrylate networks. Correlation with viscoelastic properties

In this paper, we investigate the final morphology of photocured polyurethane acrylates based on polypropylene oxide by means of Transmission Electron Microscopy (TEM), Small Angle X‐ray Scattering (SAXS), and dynamic mechanical measurements. Two interrelated structural features on two different size scales can occur in these systems. TEM analysis demonstrates the presence of inhomogeneities on the length scale of 10–200 manometers, mostly constituted by clusters of small hard units (diacrylated diisocyanate) connected by polyacrylate chains. The bimodal shape of the dynamic mechanical relaxation spectra corroborates this two‐phase structure. Besides, a suborganization of the reacted diisocyanate hard segments inside the polyurethane acrylate matrix is revealed by SAXS measurements, depending on the nature of the hydroxylacrylate used for the synthesis of the precursor. Finally, UV‐exposure time is found to induce modifications on the viscoelastic properties of the final network, even at high double‐bond conversion: this effect can be due to a postreaction and to an increase of the crosslinking density inside the hard segments domains. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 919–937, 1999     

The chemistry of naphthazarine derivatives

Based on IR and¹H and¹³C NMR spectroscopic studies, the oxidation product of echinochrome with Ag₂O was assigned the structure of 2,3-epoxy-7-ethyl-2,3-dihydro-2,3,5,6,8-pentahydroxy-1,4-naphthoquinone. For part 4, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1607–1609, August, 1999.     

Synthesis and characterization of biodegradable homopolymers and block copolymers based on adipic anhydride

Homopolymers of adipic anhydride (AA) and block copolymers of ϵ-caprolactone (ϵ-CL) and AA have been synthesized with aluminum triisopropoxide as an initiator. Homopolymerization was studied at 20°C in toluene and methylene chloride (CH₂Cl₂). The end-group analysis agrees with a coordination insertion mechanism based on the acyl-oxygen cleavage of the AA ring. Living poly(ϵ-caprolactone) (PCL) chains are very efficient macro-initiators for the polymerization of AA, with formation of diblock copolymers of a narrow molecular weight distribution. At our best knowledge, low molecular weight ω-aluminum alkoxide PCL macroinitiators (M_n < 1000) allow the first valuable synthesis of PAA with a molecular weight as high as 58,000 and a quite narrow polydispersity (M_w/M_n = 1.2). Size-exclusion chromatography (SEC) and ¹³C NMR confirm the blocky structure of the copolymers, in agreement with DSC that shows two melting endotherms and two glass transitions characteristic of the crystalline and amorphous phases of PCL and PAA, respectively. Block copolymers of ϵ-CL and AA are also sensitive to hydrolysis, which makes them possible candidates for biomedical applications. Initiation of the AA polymerization in bulk with aluminum triisopropoxide in the presence of various ligands is also discussed. © 1997 John Wiley & Sons, Inc.     

The application of novel extraction chromatographic materials to the characterization of radioactive waste solutions

A simple method for the separation and preconcentration of radiostrontium from acidic nuclear waste solutions for subsequent determination is described. The method involves passage of the waste solution, acidified to at least 2M with nitric acid, through an extraction chromatographic column consisting of a 1M solution of bis-4,4(5)[(t-butyl)cyclohexano]-18-crown-6 in 1-octanol sorbed on an inert polymeric substrate, which preferentially retains strontium. The strontium may then be stripped from the column with a small volume of either dilute (0.05M) nitric acid or water. Actinides present are removed quantitatively prior to strontium separation by passage of the sample through an actinide-specific extraction chromatographic column.Work performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, U. S. Department of Energy, under contract number W-31-109-ENG-38.     

Some classes of non general type codimension two subvarieties inP n

We classify smooth subvarieties of codimension twoX⊂P ⁿ , 4≤n≤5, which are arithmetically Cohen-Macaulay and of non general type. By the way we exhibit some classes of non extendable subvarieties. Then we give new proofs of the classification of scrolls inP ⁴; finally we consider smooth surfaces of non general type inP ⁴ arising from rank three vector bundles.

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Sunto Classifichiamo le sottovarietà lisce di codimensione dueX⊂P ⁿ , 4≤n≤5, aritmeticamente Cohen-Macaulay e non di tipo generale. Nel contempo descriviamo alcune classi di sottovarietà non estendibili. Diamo poi due nuove dimostrazioni della classificazione degli scrolls inP ⁴; infine consideriamo superfici lisce inP ⁴, non di tipo generale, associate a fibrati di rango tre.

     

Tail Calculus with Remainder,Applications to Tail Expansions for Infinite Order Moving Averages,Randomly Stopped Sums,and Related Topics

We derive asymptotic expansions for tails of infinite weighted convolutions of some heavy-tailed distributions. Applications are given to tail expansion of the marginal distribution of ARMA processes, randomly stopped sums, as well as limiting waiting time distribution. AMS 2000 Subject Classifications. Primary—62E99, Secondary—41A60, 44A35, 60G50, 60K25     

Analysis of the M/Gi/1 →/M/1 queueing model

An M/GI/1 queueing system is in series with a unit with negative exponential service times and infinite waiting room capacity. We determine a closed form expression for the generating function of the joint queue length distribution in steady state. This result is obtained via the solution of a new type of functional equation in two variables.     

On the reconstruction index of permutation groups: semiregular groups

Summary. The reconstruction index of all semiregular permutation groups is determined. We show that this index satisfies 3 ￡ r(G, W) ￡ 5 3 \leq \rho(G, \Omega) \leq 5 and we classify the groups in each case.