Thermogravimetric study of the kinetics of structural relaxation of amorphous silica

To determine the role of the forms of fixed water in the relaxation of the silica structure, silica as treated with hot water vapour in an autoclave, and the kinetics of its dehydration were studied by thermogravimetry. Five successive stages of relaxation were established:

1. Flocculation of globules, accompanied by evaporation of the polymolecular water layer situated in the interglobular space.
2. Formation of a glassy structure, accompanied by further water evaporation.
3. Transition of the glassy structure to cristobalite with the formation of large particles.
4. Formation of quartz seeds within each particle.
5. Aggregation of oriented seeds into monocrystals of quartz.

The crystallization of amorphous silica in all stages depends on the interaction of different water forms with fragments of the silicon-oxygen skeleton. The activation energy of water evaporation was calculated; it varies from 21 to 38 kJ/mol for the different stages of the process. The activation energy of silica relaxation was evaluated to be about 125 kJ/mol.     

Penetration von Polyethylenglycoldodecylethern in gespreitete Methylstearatmonoschichten

1. The penetration isotherms of octaethyleneglycoldodecyl ether and tetraethyleneglycoldodecyl ether in methyl stearate monolayers at different film areas A_M were measured at T=295 K.
2. The penetration isotherms of the system methyl stearate/octaethyleneglycoldodecyl ether depended only weakly on the area of methyl stearate. Also, the relative shift of the penetration isotherms, related to the adsorption isotherm, was small.
3. The general penetration behaviour of both the systems methyl stearate/tetraethyleneglycoldodecyl ether and methyl stearate/octaethyleneglycoldodecyl ether was nearly identical.
4. The thermodynamical analysis of the system methyl stearate/octaethyleneglycoldodecyl ether indicated that the components are only partially miscible.

     

Limits of cyclodextrin application in oral drug preparations

Cyclodextrins are applied to facilitate formulation problems, to improve stability and bioavailability. Following factors are determining whether or not cyclodextrins can be applied in oral pharmaceutical preparations:

- properties of the selected CD: solubility, price, specific catalytic properties,

- the drug to be complexed: molecular weight, polarity, solubility,

- drug dose

- solubility properties of the complex and the “super solubility” /temporary over-saturation/

- complex stability and possibility to shift the dissociation equilibrium toward the appropriate direction

- legislative procedures

     

Hippurathydrolase,ein neues Enzym aus Mikroorganismen, 2. Mitt.: Enzymeigenschaften

Characteristic properties of the new enzymehippurate hydrolase are reported:

1. The Michaelis constant for hippurate hydrolysis is 5.5·10^?4 mole/l at 5° and pH 7.5. The specific activity of the purified enzyme preparation is 7.5 μmole/minute · mg enzyme protein at 37° and pH 7.5. The pH optimum is 7.5.
2. The effects of temperature on the reaction rate and on the stability of the enzyme were determined.
3. The effects of various inhibitors (HgCl₂, pCMB, EDTA, 8-hydroxyquinoline) and cations were studied. Participation of metal ions in the catalytic function ofhippurate hydrolase was not detectable.

     

Thermisches Phasenverhalten homologer α- und β-methylverzweigtkettiger Lecithine im wassergesättigten Zweiphasengebiet

1. By means of differential scanning calorimetry the phase transition temperatures (T _u) and phase transition enthalpies (ΔH _u) ofα- andβ-branched lecithins were determined. The lecithin samples contained 50% water per weight.
2. The occurrence of the pre-transition was dependent on the position of the methyl group in the acid residue as well as on the length of the carbon chains. TheΔH _u values of the pre-transition were about 2.5 kJ·mol^?1.
3. The hydration number (HZ) (number of water molecules per lecithin headgroup) was independent on the position of the methyl group and the length of the acyl group. The average HZ-values for all the lecithins studied is about HZ=14+-1.
4. The influence of the chain length of theα- andβ- branched acyl groups affects the thermodynamical parameters of the main transitions (gel→L _α). TheΔH _m andT _m-values increased as the chain length increases.

     

Kompressionsverhalten und Gleichgewichtsspreitungsdrucke binärer monomolekularer Dipalmitoylkephalin/Dimyristoylkephalin-Mischungen

1. The concentration dependence of theF/A-isotherms (premixed spreading, separate spreading) and the equilibrium spreading pressuresF ^e of the system dipalmitoylcephaline/dimyristoyl-cephaline were studied atT=298 K. The results were analysed and compared by means of thermodynamics with respect to miscibility of the components.
2. The spreading technique (pre-mixed spreading, separate spreading (with and without barrier), does not essentially influence the run of theF/A-isotherm. Alone, the collaps pressuresF _K differ.
3. The concentration dependence of theF/A-isotherms of premixed spread monolayers is analysed in terms of theA-x ^σ-diagram. The additivity of molecular areas at all surface pressures are assumed to indicate complete miscibility of the cephalins.
4. The phase rule according to Crisp was applied to analyse the spreading diagram in connection with the phase diagram of the mixtures in bulk. The results show that the cephalins are completely miscible in the monolayer.
5. The experimental collaps pressuresF _K of pre-mixed spread monolayers are very much larger than the equilibrium collapse presuresF _K calculated according to Garrett. The difference indicate the strong supercompression of mixed film during the compression.

     

Relaxation dynamics of salt‐free polyelectrolyte solutions using flow birefringence and rheometry

Relaxation dynamics of salt‐free, aqueous solutions of sodium poly(styrene sulfonate) (NaPSS) were investigated by mechanical rheometry and flow birefringence measurements. Two semidilute concentration regimes were studied in detail for a range of polymer molecular weights. At solution concentrations c < 10 mg mL, limiting shear viscosity η₀ was found to scale with molecular weight and concentration as η₀ ∼ c^0.5M_w over nearly two decades in concentration. At higher solution concentrations, c > 10 mg mL, a change in viscosity scaling was observed η₀ ∼ c^1.5M, consistent with a change from simple Rouse dynamics for unentangled polyions to near‐perfect reptation dynamics for entangled chains. Characteristic relaxation times τ deduced from shear stress and birefringence relaxation measurements following start‐up of steady shearing at high rates reveal very different physics. For c < 10 mg mL, both methods yield τ ∼ c^−0.42M and τ ∼ c⁰M for c > 10 mg mL. Curiously, the concentration scalings seen in both regimes are consistent with theoretical expectations for salt‐free polyelectrolyte solutions undergoing Rouse and reptation dynamics, respectively, but the molecular weight scalings are not. Based on earlier light scattering studies using salt‐free NaPSS solutions, we contend that the unusual relaxation behavior is likely due to aggregation and/or coupled polyion diffusion. Simultaneous stress and birefringence measurements suggest that in concentrated solution, NaPSS aggregates are likely well permeated by solvent, supporting a loose collective of aggregated chains rather than the dense polymer aggregates previously supposed. Nonetheless, polyion aggregates of either variety cannot account for the inverse dependence of relaxation time on polymer molecular weight for c < 10 mg mL. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 825–835, 1999     

Hamiltonian circuits,Hamiltonian paths and branching graphs of benzenoid systems

A benzenoid systemH is a finite connected subgraph of the infinite hexagonal lattice with out cut bonds and non-hexagonal interior faces. The branching graphG ofH consists of all vertices ofH of degree 3 and bonds among them. In this paper, the following results are obtained:

1. A necessary condition for a benzenoid system to have a Hamiltonian circuit.
2. A necessary and sufficient condition for a benzenoid system to have a Hamiltonian path.
3. A characterization of connected subgraphs of the infinite hexagonal lattice which are branching graphs of benzenoid systems.
4. A proof that if a disconnected subgraph G of the infinite hexagonal lattice given along with the positions of its vertices is the branching graph of a benzenoid system H, then H is unique.

     

Röntgenkristallstrukturanalyse und Organische Chemie

An introduction into X-ray crystal structure analysis as method to determine the structure of organic compounds is given. In this connection two new methods for phase determination of X-ray reflections of macromolecules are reported. A few specially selected samples of structures are given to demonstrate the efficiency of X-ray crystal structure analysis with regard to the elucidation of stereochemical problems in the field of organic chemistry. The examples of application are heterocyclic organic compounds (cycloaddition products of nitrile-ylides, a stable azomethinimine, a cyanine dyestuff) and biologically relevant molecules (aminoacids of the amatoxins, the main poison of the green deathcup toadstool). The mentioned molecular structures determined by X-ray crystal structure analysis are discussed under the following aspects.

1. Elucidation of constitution, configuration and conformation.
2. Identification and verification of chemical pathways for new classes of compounds.
3. Bond relationships, influence of ring strain and other intramolecular interactions upon bond lengths and angles.
4. The attempt to find general stereochemical principles among related molecules or molecular parts points to the fundamental aspects of these experiments. Generally, the 1,4-diaza-s-trans-1,3-butadiene system, the steric influence of a free electron pair at a double bonded nitrogen and the CF₃-group are studied.
5. Obtaining of initial parameters for quantum mechanical calculations and comparison with theoretical results.
6. With regard to the structure analysis of two amino acid fragments of the amatoxins the configuration of the hydroxylated isoleucines of the amatoxins and the configuration at the chiral SO center of the natural amatoxins and of the synthetic phalloidin sulphoxides are fixed. The configuration at the SO center is decisive for the poisonous effect.

     

Charakterisierung der Mischbarkeitseigenschaften binärer Monoschichten durch Messung der Gleichgewichtsspreitungsdrucke

1. The equilibrium spreading pressures were studied as a function of composition of the following binary systems by use of the Wilhelmy-method atT=295 K on aqueous substrate:

octadecyldiethylphosphine oxide/1-hexadecanol (I)

octadecyldiethylphosphine oxide/1-eicosanol (II)

octadecyldiethylphosphine oxide/stearic acid (III)

1. The equilibrium spreading pressures of the pure components of the systems I und II, respectively, decrease due to addition of the corresponding components of mixture. A minimum occurs at the mole fractionx=0,50. The equilibrium spreading pressures of mixtures of system III rise with increasing mole fraction of octadecyldiethylphosphine oxide.
2. The three spreading diagrams are based on one general type. The phase rule is applied and shows that the components of the 3 systems must be immiscible in the monolayer but miscible in the bulk phase.

     

Über die Synthese von substituierten 2-Amino-1-pyrrolinen, 1. Mitt.

Three methods for the synthesis of various substituted 2-amino-1-pyrrolines are described:

1. Cyclization of 2.2-disubstituted 4-bromobutyronitriles with ammonia to yield 3.3-disubstituted 2-amino-1-pyrrolines.
2. Reduction of 2-aminopyrrolin-5-ones or 2-iminosuccinimides with LiAlH₄, a route leading especially to 3.3.4-substituted 2-aminopyrrolines.
3. Chlorination of 4.4-disubstituted pyrrolidones with POCl₃ followed by treatment with NH₃ gives 4.4-disubstituted 2-amino-1-pyrrolines.

     

Molecular structure of paramagnetic <Emphasis Type="Italic">bis</Emphasis>-diisobutyl dithiophosphinate complexes of lanthanides(III) with 1,10-phenanthroline in solution by the data of NMR relaxation spectroscopy

NMR relaxation spectroscopy (RS) is used to examine the molecular structure of paramagnetic bis-diisobutyl dithiophosphinate complexes of lanthanides (by the example of europium and lutetium) with 1,10-phenanthroline in CDCl₃ solution. The results obtained are consistent with the results of studying the molecular structure of bis-diisobutyl dithiophosphinate complexes of yttrium with 1,10-phenanthroline in the crystalline phase by XRD.     

Effects of chain length distribution on viscoelastic properties of entangled linear polymers: Blending laws for binary blends

The reptation idea of de Gennes and the tube model theory of Doi and Edwards are extended to explain the terminal viscoelastic properties of binary blends in the highly entangled state of two linear monodisperse polymers with different molecular weights M₁ and M₂. A modified tube model is proposed that considers the significance of the constraint release by local tube renewal in accounting for the relaxation process of the higher molecular weight chain. Its relaxation by both reptation and the constraint release is remodeled as the disengagement by pure reptation of an equivalent primitive chain. From knowledge of the longest relaxation times of the blend components, the stress equation is formulated from which blending laws of viscoelastic properties for the binary blends are derived. To force better agreement between theory and experiment at the pure monodisperse limits of the blends, a crude treatment to include the effect of contour-length fluctuation in the equivalent-chain model is also discussed. Theoretical predictions of the zero-shear viscosity and steady-state shear compliance are shown to be in good agreement with literature data on undiluted polystyrenes and polybutadienes over a wide range of the blend composition and M₂/M₁ ratio. The asymptotic of the laws for blends with M₂/M₁ → 1 and 0 are comparable to those from the relaxation spectrum proposed by others earlier on the basis of the tube model.     

Molecular model potentials: Combination of atomic boxes

A new Combination of Atomic Boxes (CAB) molecular orbital model is introduced, having the following characteristics:

1. Atomic model potentials are one-dimensional potential boxes of finite depth U _A and of length L _A, the box-parameters being chosen to give valence electron ionization energies.
2. Explicit molecular model potentials are constructed by combining all the model potentials of the atoms in the molecule.
3. A minimum computational effort leads to rigorous solutions of the resulting Schroedinger equation.

The model is tested on a large variety of π-electron systems containing atoms of four rows of the periodic system. Branched and cyclic molecules are also treated. The comparison of the calculated first and higher ionization and first excitation energies with the observed data gives the mean deviations 0.540 eV and 0.388 eV resp. The common physical basis of the CAB and the model-pseudopotential methods is discussed. The constant potential within the box is a consequence of the partial cancellation of the electrostatic potential by a repulsive term representing the Pauli principle. It is shown that CAB is not restricted to π-electrons but can be extended to σ-electron systems as well.     

Neue enzymatische Analysenmethoden zur Bestimmung von Maltose,Stärke und Lactat in der Lebensmittelchemie

1. Determination of Maltose. Maltose is hydrolyzed by the enzyme α-glucosidase into glucose, which is determined by the enzymes hexokinase and glucose-6-phosphate-dehydrogenase. α-Glucosidase is specific for oligosaccharides with α-1,4 and α-1,2 bonds.
2. Determination of Starch and Glycogen. Starch and glycogen are splitted to glucose by the enzyme amylo-glucosidase. Starch has to be dissolved before enzymatic cleavage. A comparison of different methods for preparing starch solutions is given.
3. Determination of d- and l-Lactate. It is possible to determine d-lactate and l-lactate with the specific enzymes d-lactate-dehydrogenase and l-lactate-dehydrogenase. By different samples it is shown that no equal quantities of d- and l-lactate were found in the analyzed foods.

     

Frequency and molecular-mass-dependent nonexponential proton NMR relaxation in polymer melts

A theoretical treatment of the nonexponential relaxation behavior of the different proton nuclear magnetic resonance (NMR) relaxation processes in polymer melts is presented. Formulas are derived for a three-component model given by two versions and a homogeneous distribution of correlation times. The theoretical results were tested with measurements of T₁, T_2e, and T₂ as functions of frequency and molecular mass in linear fractionated polyethylene samples. While the T₁ relaxation always yields exponential magnetization decays, the T_2e and T₂ measurements show biexponential relaxation behavior. From the calculations it was found that the correlation time of the local motion is independent of the molecular mass, whereas the correlation time of the slowest motional process increases with M^2.8_w for the three-component model and with M^2.2_w for the distribution of correlation times, respectively. © 1992 John Wiley & Sons, Inc.     

Organically-Modified Silica Xerogels for Adsorption of CH4 at 298 K

Organic tethers have been incorporated into acid- and base-catalysed silica sol–gels. The surface area is modified by this insertion suggesting an interaction at a molecular level. The HCl-catalysed SiO₂ sol–gel had its relative extent of adsorption at 298 K of CH₄ relative to H₂O:

1. depressed by the addition of 5% bis(triethoxysilyl)methane
2. increased by the addition of 5% bis(trimethoxysilyl)hexane

which attributed the tether hydrophilic-lipophilic balance (HLB). The reasons for this unexpected physical adsorption of CH₄ at a temperature far above its boiling point are now being explored, along with the potential of these materials for selective hydrocarbon sensors and separators.     

Klassifizierung und Isomorphie von Vanadium-, Niob- und Tantal-Legierungen

The vanadium, niobium, and tantalum phases have been classified and their isomorphous behaviour has been studied.

1. It has been found that many vanadium and niobium compounds crystallize with the Cr₃Si structure, but just 3 compounds of tantalum have this structure.
2. The sigma phase occurs in many compounds, and commonly is homogeneous over a wide compositional range.
3. TheLaves phases mentioned in this work have a radius ratio of 1.66–1.30, which lies in the range mentioned byLaves (1956)²³ andDwight (1961)²². One could expect some moreLaves phases in these systems, such as VCo₂, VCr₂, HfNb₂, ZrNb₂, HfTa₂, TaZn2, and ZrTa₂.
4. The following structure types have more than 10 representatives: Cr₃Si, MgZn₂, W, sigma, CrSi₂, TiAl₃, MgCu₂, Mn₅Si₃, CFe₃W₃, α-Mn, E-NiSiti, AuCu₃, AlCu₂Mn, W₅Si₃, Cu₁₆Mg₆Si₇, and NaCl.

     

Imidazo- und Diazino-Anellierungen an das [1]Benzothieno[2,3—d]pyrimidin-System

Derivatives of the following six ring systems were synthesized:

1. 3,10-Dihydro-[1]benzothieno[2,3-d]imidazo[1,5-a]-pyrimidine (I)
2. 6H-[1]Benzothieno[2,3-d]pyrazino[1,2-a]pyrimidine (II)
3. 1,5-Dihydro-[1]benzothieno[2,3-d]imidazo[1,2-a]-pyrimidine (III)
4. 6H-[1]Benzothieno[2,3-d]pyrimido[1,2-a]pyrimidine (IV)
5. 1,5-Dihydro-imidazo[1,2-a]thieno[2,3-d]pyrimidine (V)
6. 4H-Pyrimido[1,2-a]thieno[2,3-d]pyrimidine (VI)

The first four types are new heterocyclic systems. 2-Aminomethyl-5,6,7,8-tetrahydro-[1]benzothieno[2,3-d]pyrimidin-4(3H)-one (5), which was used as intermediate for typesI andII, was synthesized by various methods. TypesIII andIV were prepared from 2-methylthio-5,6,7,8-tetrahydro-[1]-benzothieno[2,3-d]pyrimidin-4(3H)-one via the corresponding 2-benzylamino derivatives, followed by ring closure.     

Heats of chemisorption ofn-Aliphatic alcohols on α-Al2O3 at 25–200°C

A differential microcalorimeter (E. Calvet) was used to study the processes of adsorption of five aliphatic alcohols (C₁-C₅) on α-Al₂O₃ at 25, 50, 100, 150 and 200°C. In particular, the importance of the thermokinetic study of the chemisorption of such alcohols at different experimental temperatures was demonstrated, with regard to the variations in the thermokinetic parameters (t_max, t_1/2 and t₀) and the evolution of the alcohol vapor pressure on the adsorbent during the adsorption process. It was concluded that:

1. all the heat emission peaks of alcohol adsorption have the same rounded shape at 25°C;
2. on passing from methanol to 1-pentanol, the area of the adsorption peaks increases as the chain length or molecular weight increases;
3. on passing from 25 to 200°C, the shape of the adsorption peaks becomes more pointed.