Kinetics study of geopolymerization of fly ash using isothermal conduction calorimetry

Isothermal conduction calorimetry has been used to study the reaction kinetics of early geopolymerization of fly ash. Fly ash particles were subjected to react with NaOH solution in 2:1 ratio at isothermal temperatures of 34, 39, 45, 52, and 60 °C. The reaction kinetic parameters such as activation energy, rate of reaction, and pre-exponential function were calculated using the rate of heat evolution data. It is observed that the geopolymerization reaction followed a nucleation and growth mechanism. The activation energy obtained from an Arrhenius plot was around ~100 kJ mol^?1.     

Charge regulation as a stabilization mechanism for shell-like assemblies of polyoxometalates

We show that the equilibrium size of single-layer shells composed of polyoxometalate macroions is inversely proportional to the dielectric constant of the medium in which they are dispersed. This behavior is consistent with a stabilization mechanism based on Coulomb repulsion combined with charge regulation. We estimate the cohesive energy per bond between macroions on the shells to be approximately -6kT. This number is extracted from analysis based on a charge regulation model in combination with a model for defects on a sphere. The value of the cohesive bond energy is in agreement with the model-independent critical aggregate concentration. This observation points to a new class of thermodynamically stable shell-like objects. We point out the possible relevance our findings have for certain surfactant systems.     

Orbital order in ZnV(2)O(4)

In view of recent controversy regarding the orbital order in the frustrated spinel ZnV(2)O(4), we analyze the orbital and magnetic ground state of this system within an ab initio density functional theory approach. While local density approximation+Hubbard U calculations in the presence of a cooperative Jahn-Teller distortion stabilize an A-type staggered orbital order, the consideration of relativistic spin-orbit (SO) effects unquenches the orbital moment and leads to a uniform orbital order with a net magnetic moment close to the experimental one. Our results show that ab initio calculations are able to resolve the existing discrepancies in previous theories and that it is the SO coupling along with electronic correlations which play a significant role in determining the orbital structure in these materials.     

A novel "pro-sensitizer" based sensing of enzymes using Tb(III) luminescence in a hydrogel matrix

Chemically synthesized "pro-sensitizers" release the sensitizer in the presence of lipase or β-glucosidase, triggering a significant luminescence response from a lanthanide based hydrogel.     

Zinc and cadmium 2-pyrazinephosphonates: syntheses, structures and luminescent properties

Three new metal(II) 2-pyrazinephosphonates have been synthesized by hydrothermal reactions based on 2-pyrazinephosphonic acid (C(4)H(3)N(2)PO(3)H(2)1) as ligand, namely, Zn(C(4)H(3)N(2)PO(3)) (2), Cd[(C(4)H(3)N(2)PO(3))(H(2)O)] (3) and Cd[(C(4)H(3)N(2)PO(3)H)Cl]·H(2)O (4). In compound 2, the O-P-O bridged inorganic layers are pillared by pyrazinyl groups into a three-dimensional network. In compound 3, the {CdO(5)N} and {CPO(3)} polyhedra are interconnected via edge and corner-sharing into a metal phosphonate layer. In compound 4, the {Cd(2)Cl(2)} dimers are linked by O-P-O bridges into a one-dimensional double chain, and the chains are joined into a layer by pyrazinyl groups. Here we employ pyrazinephosphonic acids as structure directing motifs to form extended structures and materials with interesting luminescent properties. The luminescent properties studied have also been described.     

Scaling behavior of plasmon coupling in Au and ReO3 nanoparticles incorporated in polymer matrices

Polymer nanocomposites containing different concentrations of Au nanoparticles have been investigated by small angle X‐ray scattering and electronic absorption spectroscopy. The variation in the surface plasmon resonance (SPR) band of Au nanoparticles with concentration is described by a scaling law. The variation in the plasmon band of ReO₃ nanoparticles embedded in polymers also follows a similar scaling law.

     

An operator solution of stochastic games

A class of zero-sum, two-person stochastic games is shown to have a value which can be calculated by transfinite iteration of an operator. The games considered have a countable state space, finite action spaces for each player, and a payoff sufficiently general to include classical stochastic games as well as Blackwell’s infiniteG _δ games of imperfect information. Research supported by National Science Foundation Grants DMS-8801085 and DMS-8911548.     

Continuous-flow potentiometric determination of horseradish peroxidase with a fluoride-selective electrode

A continuous-flow system is reported for the determination of horseradish peroxidase (HRP) with a fluoride-selective electrode. Reaction conditions are optimized for the HRP-catalysed oxidation of p-fluoroaniline (0.104 M) with hydrogen peroxide (4.0 mM) as the oxidant in 0.16 M acetate buffer at pH 4.6. HRP is determined in the concentration range 0.016–0.12 U ml^?1 in the flow system at a sampling rate of 24 h^?1. Interfering effects caused by known HRP inhibitors, including metal ions, cyanide and sulphide, are reported for the range 1 μM–1 mM. Applications of the system for determination of the enzyme in turnip extract and milk are described.