Phase transitions in simple models of rod-like and disc-like micelles

The interplay of interactions between micelles, and the aggregation of these micelles into large, highly anisotropic micelles, is studied. Simple, hard-body, models of rod-like and disc-like micelles are used, which allows us to apply fundamental measure theory to determine the free energy. Then we study the phase transition from the fluid phase to a liquid crystalline phase. We find that aggregation induces a strongly first order transition from a fluid phase of small micelles to a close packed liquid crystalline phase of infinitely large micelles. Received: 3 December 1997     

On a resonant-superlinear elliptic problem

We start by discussing the solvability of the following superlinear problem where , is a smooth bounded domain and f satisfies a one-sided Landesman-Lazer condition. We also consider systems of semilinear elliptic equations with nonlinearities of the above form, so exhibiting superlinearity as and resonance as . A priori bounds for the solutions of the equation and the system are obtained by using Hardy-type inequalities . Existence of solutions is then obtained using topological degree arguments. Received: 18 March 2002 / Accepted: 8 May 2002 / Published online: 5 September 2002     

Evaluation of the combustion behaviour of perhydrous coals by thermal analysis

Summary Perhydrous coals are characterized by a high hydrogen content and exhibit a modified composition and physico-chemical properties in comparison with normal coals. These modifications affect the behaviour of perhydrous coals during pyrolysis and, therefore, may have an influence on the subsequent combustion process. In this work the combustibility behaviour of a series of perhydrous coals was evaluated in order to study the effect of hydrogen enrichment during the thermal treatment of the coals in an oxidant atmosphere. To this end temperature programmed combustion tests for the coals, and air isothermal (500°C) reactivity tests for their chars, were carried out in a thermogravimetric analyser. A clear relationship between the combustion behaviour of the perhydrous coals, and the aromatic to aliphatic hydrogen ratio was found.     

A neutral organometallic fluoro complex can be a good ligand

The reaction of the complex [Mo(OTf)(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)] (1) (OTf = trifluoromethylsulfonate; phen = 1,10-phenanthroline) with tetrabutylammonium fluoride trihydrate afforded the fluoride complex [MoF(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)] (2). The IR spectrum and the oxidation potential of 2 reflect the fact that its metal center is more electron-rich than that of the chloro analogue [MoCl(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)]. Compound 2 reacted with 1 affording the homobinuclear complex [[Mo(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)](2)(mu-F)][OTf] (3), with a fluoride bridge. Compound 2 also reacts with the species generated in situ by triflate abstraction from [M(OTf)(CO)(3)('N-N')] (M = Mn, Re; 'N-N' = 2,2'-bipyridine (bipy), phen) using NaBAr'(4) (Ar' = 3,5-bis(trifluoromethyl)phenyl), affording the heterobinuclear complexes [[Mo(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)](mu-F)[M(CO)(3)('N--N')]][BAr'(4)] (M = Mn, 'N-N' = bipy (4); M = Re, 'N-N' = phen (5)). All new compounds have been characterized by spectroscopic methods (IR and NMR) and, in the case of 1, 2, 3, and 4, also by means of X-ray diffraction analysis.     

Experimental evidence for the existence of non-exo-anomeric conformations in branched oligosaccharides: the neomycin-B case

For the first time in natural O-glycosides, a large amount of non-exo-anomeric conformation is experimentally detected, in solution.     

Synthesis, structure, and physical properties of hybrid nanocomposites for solid-state dye lasers

We report on the synthesis, structural characterization, physical properties, and lasing action of two organic dyes, Rhodamine 6G (Rh6G) and Pyrromethene 597 (PM597), incorporated into new hybrid organic-inorganic materials, where the organic component was either poly(2-hydroxyethyl-methacrylate) (PHEMA) or copolymers of HEMA with methyl methacrylate (MMA), and the inorganic counterpart consisted of silica derived from hydrolysis-condensation of methyltriethoxysilane (TRIEOS) in weight proportion of up to 30%. Lasing efficiencies of up 23% and high photostabilities, with no sign of degradation in the initial laser output after 100 000 pump pulses at 10 Hz, were demonstrated when pumping the samples transversely at 534 nm with 5.5 mJ/pulse. A direct relationship could be established between the structure of the hybrid materials, analyzed by solid-state NMR, and their laser behavior. An inorganic network dominated by di-/tri- substituted silicates in a proportion approximately 35:65, corresponding to samples of HEMA with 15 and 20 wt % proportion of TRIEOS, optimizes the lasing photostability. The thermal properties of these materials, together with the high homogeneity revealed by atomic force microscopy (AFM) images, even in compounds with high silica content, indicate their microstructure to be a continuous phase, corresponding to the polymer matrix, which "traps" the silica components at molecular level via covalent bonding, with few or no silica islands.     

Feuilletages en surfaces, cycles évanouissants et variétés de Poisson

Surface foliations, vanishing cycles and Poisson manifolds. Afoliated cylinder of a foliated manifold (M,F) is a path of integral loopsc _t forF. Such a cylinder defines anon-trivial vanishing cycle c ₀ ifc _t is null-homotopic in its supportF _t for eacht>0, butc ₀ is not null-homotopic in its supportF ₀. Vanishing cycles were introduced by S. P. Novikov to study qualitative aspects of codimension one foliations. In this paper we apply this notion to the study of higher codimensional foliations.The first aim is to show the influence that the triviality of vanishing cycles exerts on the topology of thehomotopy groupoid ofF. It is natural to try to reduce the study of triviality to more regular vanishing cycles, as well as to obtain a nice criterion of triviality. In this way, we introduce the notion ofregular vanishing cycle as the orthogonal version (for a riemannian metric onM) of the classical notion of immersed vanishing cycle and the notion ofcoherent vanishing cycle, i.e. an integral discD ₁ with boundaryc ₁ extends to a global foliated homotopyD _t such thatc _t is the boundary ofD _t for eacht>0. We also prove that the triviality of these vanishing cycles implies the triviality of all vanishing cycles. For compact foliated manifolds, we obtain the following criterion: a regular coherent vanishing cycle is non-trivial if and only if the area of the discsD _t converges to infinity.Finally, we give two applications of these results to surface foliations: we generalize the Reeb stability theorem to higher codimensions and we resolve the problem of the symplectic realization of Poisson structures supported by surface foliations.

     

Current density maps, magnetizability, and nuclear magnetic shielding tensors of bis-heteropentalenes. II. Furo-furan isomers

Magnetic susceptibility and nuclear magnetic shielding at the nuclei of bis-heteropentalenes formed by two furan units ([2,3-b], [3,2-b], [3,4-b], and [3,4-c] isomers) have been computed by several approximated techniques and a large Gaussian basis set to achieve near Hartree-Fock estimates. Ab initio models of the ring currents induced by a magnetic field normal to the molecular plane were obtained for the three isomeric systems of higher symmetry, showing that the pi electrons give rise to intense diamagnetic circulation. The pi currents are responsible for enhanced magnetic anisotropy and strong out-of-plane proton deshielding. The theoretical findings are used to build up a "diatropicity matrix" for two fused five-membered heterocyclic systems.     

Chemisorption and physisorption of CO2 on cation exchanged zeolitesA,X and mor

Calorimetric measurements of the heat of adsorption of CO₂ on zeolites with variable content of mono- and divalent cations lead to common conclusions. High initial heats (up to 120 kJ·mol^–1 for NaA), generally associated with a slow and activated rate of adsorption, are found for high contents of Na⁺, Li⁺ or Ca²⁺. They are attributed to a limited number of chemisorption sites (0.3 per cage in NaA).Physisorption results in lower heats (25–50 kJ·mol^–1). The lowest values are obtained with partially or totally decationized zeolites. Transition metal cations induce frequently weaker interactions than IA or IIA. Finally the stronger the energy of adsorption is, the larger the adsorbed amount is.

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Zusammenfassung Kalorimetrische Messungen der Adsorptionswärme von CO₂ an Zeolithen mit einem unterschiedlichen Gehalt an ein- und zweiwertigen Kationen führen zu allgemeinen Schlußfolgerungen. Für einen hohen Gehalt an Na⁺, Li⁺ oder Ca²⁺ werden hohe Initiierungswärmen (bis zu 120 kJ·mol^–1 für NaA) gefunden, die immer in Verbindung mit langsamen und aktivierten Adsorptionsgeschwindigkeiten auftreten. Dies wird einer begrenzten Anzahl an Stellen für die Chemisorption zugeschrieben (0.3 pro -Cage in NaA). Physisorption verursacht niedrigere Wärmen (25–50 kJ·mol^–1). Die niedrigsten Werte erhält man mit teilweise oder total entkationisierten Zeolithen. Kationen von Übergangs-metallen verursachen häufig schwächere Wechselwirkungen als IA-oder IIA-Kationen. Je höher die Adsorptionsenergie, um so größer ist die adsorbierte Menge.

     

Polarographic study of the coordinated system Zn(II)-glycine-glycinate ion

A polarographic method has been applied to the study of the mixed-ligand system Zn(II)-glycine-glycinate, in aqueous medium, at I= 1.0 M (NaClO(4)) and 25 +/- 0.05 degrees C. The study of the quasi-reversible waves characteristic of Zn(II) has been carried out by means of a new method of analysis proposed by the authors, which is faster and more accurate than the traditional methods. The stabilization of the coordinated species [ZnG](2+) (beta(10) = 2.0), [ZnG(2)](2+) (beta(20) = 0.71), (Zn(G(-))](+) (beta(01) = 7.6 x 10(4)). [Zn(G(-))(2)] (beta(02)= 4.0 x 10(9)) and [ZnG(G(-))](+) (beta(11) = 1.7 x 10(5)) has been established.