Study of the peak effect phenomenon in single crystals of 2H-NbSe2

The weakly pinned single crystals of the hexagonal 2H-NbSe₂ compound have emerged as prototypes for determining and characterizing the phase boundaries of the possible order-disorder transformations in the vortex matter. We present here a status report based on the ac and dc magnetization measurements of the peak effect phenomenon in three crystals of 2H-NbSe₂, in which the critical current densities vary over two orders of magnitude. We sketch the generic vortex phase diagram of a weakly pinned superconductor, which also utilizes theoretical proposals. We also establish the connection between the metastability effects and pinning.     

Constraints on Contact Angles for Multiple Phases in Thermodynamic Equilibrium

For three or more fluid phases in thermodynamic equilibrium and in contact with a solid surface, the Young equation can be used to find relations between the contact angles for different pairs of fluids. For an n-fluid-phase system, n(n-1)/2 contact angles can be defined, but there are (n-1)(n-2)/2 constraints between them, leaving only n-1 independent values of the contact angle. These constraints are very powerful in limiting and determining possible types of wetting behavior. The consequences are discussed for three- and four-phase flow. They have important applications for the understanding of gas injection processes in petroleum reservoirs. Copyright 2001 Academic Press.     

Shaping Solid-State Supramolecular Cavities: Chemically Induced Accordionlike Movement of gamma-Zirconium Phosphate Containing Polyethylenoxide Pillars

The hydrophilic oxygen atoms of polyethylenoxide chains inserted as pillars in gamma-zirconium phosphate form hydrogen bonds with the acid groups of the host. As a result the pillars are almost perpendicular to the gamma layers. Upon changing the pH level of the supernatant solution the hydrogen bonds are broken and the pillars become almost perpendicular to the layers (shown schematically). Thus there is a reversible enlargement-shortening of the interlayer space.     

General synthesis of mono-, di-, and tri-acetylated indoles from indolin-2-ones

Having developed the one-pot triacetylation of indolin-3-ones, we have now devised a simple two-step reaction sequences to produce di- and mono-acetylated indoles from indolin-2-ones. The indolin-2-ones were first subjected to acetylation in the presence of acetic anhydride and a catalytic amount of N,N-dimethylaminopyridine to give 2-acetoxy-1,3-diacetylindoles. Subsequently, an enzyme-assisted deacetylation resulted in the chemoselective deprotection of the acetoxy group to produce 1,3-diacetyl-2-hydroxyindoles. However, a chemical deacetylation of 2-acetoxy-1,3-diacetylinoles under mild basic or acidic conditions resulted in the formation of 3-acetyl-2-hydroxyindoles.     

Determination of Water-Oil Interfacial Area during 3-Phase Gravity Drainage in Porous Media

Water-oil interfacial area in porous media was determined in laboratory experiments using sand columns consisting of either 2 (water and oil) or 3 (water, oil, and air) fluid phases. Surfactant sorption at the water-oil interface was directly measured for a wide range of water, oil, and air saturations undergoing gravity drainage. Differing values of the water-oil interfacial tension were also examined. The Gibbs adsorption equation was then used to obtain values for the water-oil interfacial area. Both 2- and 3-phase water-oil experiments showed a linear increase in interfacial area with decreasing water saturation. Results also showed that interfacial areas were not affected by changes in interfacial tension. The interfacial areas in the 3-phase experiments were less than half the calculated values of the corresponding 2-phase experiments, which contradicts predictions from a conventional pore level analysis of 3-phase flow. Copyright 2000 Academic Press.     

Carbon-13 NMR spectra of some tetra- and pentacyclic triterpene methyl ethers

The carbon-13 NMR spectra have been recorded for representatives of three previously unexamined classes of pentacyclic triterpenes—lupanes, fernanes and arboranes. In addition to the spectra of lupeol methyl ether, cylindrin and arundoin, the spectra of a number of other triterpene methyl ethers are also recorded. The effect of methyl ether formation on the shilding values for carbons in the 3β-hydroxy-4,4-dimethyl- ring A system is defined.