Plateau's problem for minimal surfaces with a catenoidal end

Dedicated to Dieter Puppe on the occasion of his sixtieth birthday.     

Differential scanning calorimetry applied to cross-linking of a filled epoxy resin: Accuracy of the Borchardt and Daniels equation for describing the curing process kinetics

From the point of view of the chemical reactions describing the curing process of a mixture epoxy resin-dicarboxylic acid anhydride-tertiary amine-polyol and silica filler, it is difficult to understand the network formation: several different reactions are involved, so that the interaction between the epoxy resin and the other components of the mixture is unquestionably complex. However, a linear plot is obtained when Kissinger's method is applied to dynamic DSC results (four different scanning rates), which confirms the calculation assumptions. In particular, the Borchardt and Daniels equation, where the specific rate constant is assumed to be of Arrhenius form appears to be a good mathematical model for describing the curing process under dynamic DSC conditions. The apparent activation energy determined by means of this analytical method is in good agreement with those obtained by other methods in the literature.

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Zusammenfassung Die bei der Härtung eines Epoxidharz, Dicarboxylsäure-Anhydrid, tertiäres Amin, Polyol und einen SiO₂-Füllstoff enthaltenden Gemisches vor sich gehende Vernetzung ist schwer zu überblicken; es verlaufen mehrere unterschiedliche Reaktionen, so daß die Wechselwirkung zwischen dem Epoxidharz und den anderen Komponenten außerordentlich komplex ist. Eine lineare Abhängigkeit wird jedoch erhalten, wenn dynamische DSC-Ergebnisse erhalten bei 4 verschiedenen Scanning-Geschwindigkeiten nach der Kissinger-Methode aufgearbeitet werden, was die Richtigkeit der für die Berechnung gemachten Annahmen bestätigt. Besonders die Gleichung von Borchardt und Daniel, bei deren Ableitung für die spezifische Geschwindigkeitskonstante eine Arrhenius-Form angenommen wird, scheint ein gutes mathematisches Modell zur Beschreibung des Härteprozesses unter DSC-Bedingungen zu sein. Die nach dieser Methode bestimmte scheinbare Aktivierungsenergie stimmt gut mit den nach anderen in der Literatur beschriebenen Methoden bestimmten Werten überein.

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Anion-/cation-directed reaction routes to polymorphic forms of a pyrazole-type ligand and its coordination compounds with zinc. Key structural differences between polymorphs’

Synthetic paths toward the two polymorphs of a monohydrate, one anhydrous polymorph of 1-carboxamidino-5-hydroxy-3-methylpyrazole (hcmp) and two polymorphs of zinc complexes containing hcmp ligand are presented. By choosing ions which are not part of the final product, it is possible to direct the synthesis toward the particular polymorph. In all three modifications of hcmp, the same hydrogen bonding motif appears, leading to formation of similar molecular chains. Differences arise due to different modes of chain aggregation and the presence of solvent water. Analysis of the crystal packing and the energetic features of hcmp polymorphs is made using the PIXEL model. The thermal decomposition processes are examined using differential scanning calorimetry and thermogravimetry. Analysis of crystal packing in the two polymorphs of zinc complex suggests the key role of the hydrogen bonding capacity of the aqua ligand for the appearance of the two polymorphic forms. In both polymorphs of zinc complex, stacking interactions have an important role. However, the enhanced hydrogen bonding capacity of the aqua ligand influences the formation of multistacking arrangement.     

An efficient strategy for the synthesis of 1-chloroethyl phosphates and phosphoramidates

[reaction: see text] A versatile, efficient, and simple method for the preparation of various 1-chloroethyl phosphates and phosphoramidates is described. The protected chlorophosphates or phosphoramidates are synthesized to the vinyl derivative under mild conditions, followed by conversion to the chloroethylidene phosphate or phosphoramidate by dry HCl gas, resulting in good to excellent yields. 1-Chloroethyl phosphates and phosphoramidates are excellent building blocks for the synthesis of novel ethylidene-linked phosphate prodrugs.     

Two new irregular acyclic sesquiterpenes aldehydes from Santolina corsica essential oil

Two isomeric irregular sesquiterpene aldehydes, namely 3,9-dimethyl-6-isopropyl-2(E),7(E),9-decatrienal and 3,9-dimethyl-6-isopropyl-2(Z),7(E),9-decatrienal, were isolated from the essential oil of Santolina corsica and their structures were elucidated by 1D and 2D NMR spectroscopy.     

Anisotropy and field-dependence of the spin-density-wave dynamics in the quasi one-dimensional conductor (TMTSF)<Subscript>2</Subscript>PF<Subscript>6</Subscript>

Exchange biasing was studied in an exchange-spring system consisting of two ferrimagnetic films with different coercivity. Magnetite and Co-Fe ferrite were chosen as the soft and hard magnetic bilayer components, respectively. The samples were epitaxially grown on MgO single crystal substrates by pulsed laser deposition. The exchange-bias field was investigated as a function of system size and shape, magnetic field direction and magnetization reversal in the hard layer. A clear dependence of the exchange-bias field on the sample size and shape was found. This was attributed to an interplay between exchange and dipolar energies. Micromagnetic simulations agree with the experimental results.     

Recognition of homo-polynucleotides containing adenine by a phenanthridinium bis-uracil conjugate in aqueous media

Among novel bis-nucleobase-phenanthridinium conjugates bis-uracil analogue stabilized significantly more effective poly-dA-poly-dT and poly-AH(+)-poly-AH(+) than adenine analogue and reference compound . For the alternating poly-dAdT-poly-dAdT however, the binding preference is lost, pointing to the importance of specific interactions of uracils of with homopolynucleotides containing consecutive adenines.     

Thermochemistry of dissolution of sodium alkylsulfonates

The effect of temperature on the solubility of ionic surfactants was interpreted in terms of standard enthalpy and entropy of dissolution at reference temperature by considering the change in the heat capacity. The significant value of the latter quantity causes the curvature of the function logarithm of equilibrium constant (or solubility) vs. the reciprocal thermodynamic temperature. The solubility data for several sodium n-alkylsulfonates, published by Saito, Moroi, and Matuura, were interpreted by nonlinear regression analysis. It was found that both the enthalpy and entropy of dissolution decrease with the chain length. The heat capacity increases in the course of the dissolution process.