Structural, electronic, and bonding properties of zeolite Sn-beta: a periodic density functional theory study

The structural, electronic, and the bonding properties of the zeolite Sn-beta (Sn-BEA) have been investigated by using the periodic density functional theory. Each of the nine different T-sites in BEA were substituted by Sn atoms and all the nine geometries were completely optimized by using the plane-wave basis set in conjunction with the ultra-soft pseudopotential. On the basis of the structural and the electronic properties, it has been demonstrated that the substitution of Sn atoms in the BEA framework is an endothermic process and hence the incorporation of Sn in the BEA is limited. The lowest unoccupied molecular orbitals (LUMO) energies have been used to characterize the Lewis acidity of each T-site. On the basis of the relative cohesive energy and the LUMO energy, the T2 site is shown to be the most favorable site for the substitution Sn atoms in the BEA framework.     

Indirect spectrofluorimetric determination of arsenic at nanotrace levels in alloys, underground water, industrial waters and sewage sludge

An inexpensive and virtually specific nonextractive Spectrofluorimetric method for the determination of As(III) (1 ng/ml-1 g/ml) in 0.18–1.08N H₂SO₄ has been developed. A known excess of Cr(VI) is allowed to quantitatively oxidize As(III) in the essential presence of a mixture of iodide and chloride as catalyst, followed by the addition of excess 2-(-pyridyl) thioquinaldinamide and measurement of the fluorescence intensity (_em(max) = 488 nm _ex(max) = 360 nm) caused by Cr(VI) remaining after As(III) oxidation. The measurement is repeated without As(III) addition and the decrease in fluorescence intensity gives the As(III) concentration with the help of a calibration graph. The method is very precise and accurate (S. D. = ± 0.474, R. S. D. = 0.95%, for the analysis of 50ng/ml As(III), 11 replicates). The detection limit is 0.3ng/ml As(III). Large excesses of over fifty cations, anions and some complexing agents do not interfere. The method has been successfully applied for arsenic determination in various complex matrices: environmental waters, sewage sludges and several certified reference materials (alloys). This method has also been extended to determine As(V), after its reduction to As(III), in water samples, waste discharge water and sewage sludges.Presented at International Conference Arsenic in Ground Water: Cause, Effect and Remedy, Calcutta, India, February, 1995     

Spectrophotometric determination of arsenic via arsine generation and in-situ colour bleaching of methylene blue (MB) in micellar medium

A new spectrophotometric method has been developed to determine arsenic in parts-per-million (ppm) level. It is based on the colour bleaching of methylene blue (MB) in anionic micellar medium. Arsine gas was formed by borohydride reduction of arsenite/arsenate. Arsine generation and colour bleaching (quantification of arsenic) could be done in one-pot. The presence of silver or gold nanoparticles makes the determination faster. Different calibration graphs at the three different ranges of arsenic concentration such as 0-8.63, 0-1.11 and 0-0.11 ppm were constructed and limit of detection (LODs) were found to be 1.3, 0.53 and 0.03 ppm, respectively. The method is simple, rapid, reproducible (relative standard deviations lies within +/-5%) and eco-friendly. It is free from phosphate and silicate interferences and applicable for real sample analysis.     

Fine-powder Al2O3 and SiO2 for preparation of multielement standards for rare-earth element analysis

The present paper based on experimental results contains discussions and suggestions on the possible use of fine-powder Al₂O₃ and SiO₂ with their original content of microimpurities of up to 40 elements, as multielement standards for neutron activation analysis. For example, activation analysis of As, Au, Ba, Cr, Cs, Fe, Ga, K, Ni, Sb, Sc, Se, Sr, Ta, Th, Ti, U, W, Zn, Zr and the REE La, Ce, Nd, Sm, Eu, Tb, Tm, Yb contained in SiO₂ powder off MERCK reagents showed their concentrations to be 0.1 to 5% of those in IAEA standard SL-1. In Al₂O₃ this level is even lower, approximately 10 times and more for the majority of the above-mentioned elements. As Al₂O₃ and SiO₂ are good sorbents for the majority of elements, additional introduction of some elements may allow more methods of analysis. The homogeneity of Al₂O₃ and SiO₂ samples both in the original state and after introduction of some elements was determined by neutron activation analysis, and the SD did not exceed 1% for an Al₂O₃ sample weight of 0.1 g, and 2% for SiO₂.     

Thermodynamic behavior of poly(3-alkyl thiophene) blends: Equilibrium cocrystal formation and phase segregation

The equilibrium cocrystal formation of poly(3-alkyl thiophene) (P3AT) blends has been studied by isothermal cocrystallization in a differential scanning calorimeter (DSC-7). The equilibrium melting points (T(m)0) of the cocrystals are measured using the Hoffman-Weeks extrapolation procedure. The equilibrium phase diagrams are of three different types: (a) concave upward, (b) linear, and (c) linear with phase separation at higher content of lower melting component. The phase diagram nature depends on the regioregularity difference and also on the difference in the number of carbon atoms in the pendent alkyl group of the components. The origin of biphasic nature of type "c" phase diagram has been explored from the glass transition temperature (Tg) measurement using a dynamic mechanical analyzer. The biphasic compositions show two glass transition temperatures (Tg) as well as two beta transition temperatures (T beta). The T(g)s of phase-separated regions correspond to almost the component values but the T(beta)s correspond to that of a lower (T beta) component value, and the other is higher than that of the higher (T beta) component value. Possible reasons are discussed from the interchain lamella thickness in the P3AT blends and molecular modeling using molecular mechanics program.     

Emission behavior of 1-methylaminopyrene in aqueous solution of anionic surfactants

A new fluorescent probe, methylamino derivative of pyrene, has been considered to characterize the concentration dependent emission behavior of an aqueous solution of anionic surfactants, viz., SDS, DSS, and SDBS. It was found that the emission of the probe is uniquely sensitive to the changes in surfactant (anionic) concentration due to the functional group effect of the probe over the parent moiety, pyrene. Here, 1-methylaminopyrene (MAP) showed significant quenching of emission well below the critical micellar concentration (cmc) of the surfactant. Excimer emission of the probe due to the formation of premicellar aggregates of the surfactant solutions at a concentration close to but below the cmc and again an enhanced emission of the probe above the cmc were observed as a consequence of definite MAP-surfactant interactions. These observations assisted the possible quantification ofsurfactant concentrations and their chain length dependent premicellar aggregate formations. Significant monomer emission in relation to probe distribution in micelle was analytically authenticated. Dynamic light scattering (DLS) studies revealed the incorporation of the probe molecules in the micellar core. The fluorophore emission showed nonlinear behavior when the surfactant concentration was far above the cmc. Abrupt changes in the emission characteristics in relation to the micellar concentration led to the determination of the cmc of the surfactants.     

Mineral and microelement compositions of urinary stones

The mineral and microelement compositions of urinary stones from patients in various districts of the Novosibirsk region are analyzed. The mineral composition is determined using X-ray powder diffraction and vibrational spectroscopy. The microelement composition is identified using synchrotron radiation X-ray fluorescence analysis. Calcium oxalates (whewellite CaC₂O₄ · H₂O and weddellite CaC₂O₄ · 2H₂O) are the most frequent components of the urinary stones. Oxalate uroliths contain a variety of microelements in significant amounts. Phosphate uroliths, represented by hydroxylapatite Ca₅(PO₄)₃(OH) and struvite MgNH₄PO₄ · 6H₂O, account for about one-fifth of the collection. Apatite urinary stones contain maximal strontium amounts. The struvite uroliths have higher rubidium levels. Uric acid uroliths (C₅H₄N₄O₃) account for about 11% of the collection. Their strontium concentrations are minimal. The element composition of the urinary stones is a function of their mineral constituents, the environmental surroundings, and the metabolism specifics of the patient.     

Hydration layer of a cationic micelle, C(10)TAB: structure, rigidity, slow reorientation, hydrogen bond lifetime, and solvation dynamics

We report a theoretical study of the structure and dynamics of the water layer (the hydration layer) present at the surface of the cationic micelle decyltrimethylammonium bromide (DeTAB) by using atomistic molecular dynamics simulations. The simulated micelle consisted of 47 surfactant molecules (and an equal number of bromide ions), in good agreement with the pioneering light scattering experiments by Debye which found an aggregation number of 50. In this micelle, three partially positively charged methyl groups of each surfactant headgroup face the surrounding water. The nature of the cationic micellar surface is found to play an important role in determining the arrangement of water which is quite different from that in the bulk or on the surface of an anionic micelle, like cesium perfluorooctanoate. Water molecules present in the hydration layer are found to be preferentially distributed in the region between the three partially charged methyl headgroups. It is found that both the translational and rotational motions of water exhibit appreciably slower dynamics in the layer than those in the bulk. The solvation time correlation function (TCF) of bromide ions exhibits a long time component which is found to originate primarily from the interaction of the probe with the micellar headgroups. Thus, the decay of the solvation TCF is controlled largely by the residence time of the probe in the surface. The residence time distribution of the water molecules also exhibits a slow time component. We also calculate the collective number density fluctuation in the layer and find a prominent slow component compared to the similar quantity in the bulk. This slow component demonstrates that water structure in the hydration layer is more rigid than that in the bulk. These results demonstrate that the slow dynamics of hydration layer water is generic to macromolecular surfaces of either polarity.     

Inorganic ion-exchangers in radioactive waste management</p> </p>

Summary The uptake of indigenously synthesized amorphous stannic and zirconium phosphate was assessed for, one of the important fission fragment, cesium from aqueous solutions using a radiotracer technique. A virtual increase in sorptive concentration (from 1.0 ^. 10^-8 to 1.0 ^. 10^-2 mol ^. dm^-3) and pH (from 2.4 to 10.2) and temperature (from 303 to 333 K) enhanced the uptake of cesium on stannic phosphate. However, the extremely high degree of uptake of cesium on zirconium phosphate was almost unaffected with the dilution beyond 10^-5 mol ^. dm^-3 and pH (i.e., from 2.4 to 10.2) and temperature (from 303 to 323 K). Irreversible uptake occurring for these solids follow the Freundlich adsorption isotherm and the presence of several complexing agents viz., sulphate, phosphate, glycine and EDTA did not affect appreciably the uptake of cesium on zirconium phosphate but it did affect for stannic phosphate system. Both these solids showed good radiation stability towards a 11.1 GBq Ra-Be neutron source having neutron flux ca. 3.2 ^. 10⁶ n ^. cm^{-2 .} s^-1 and associated with a nominalg-dose of ca. 1.72 Gy/h, at least for the uptake of cesium.</p> </p>     

New routes to condensed polynuclear compounds—VIII : Scope of phenanthridine synthesis through cyclisation of haloanils

The new phenanthridine synthesis based on reaction of haloanils with metal amides is of wide application. The cyclisation succeeds in presence of alkyl, alkoxy, dialkylamino, cyano, carboxy, halogeno and carbonyl groups but fails in case of nitro substituents.