Brownian dynamics simulation of dense binary colloidal mixtures. I. Structural evolution and dynamics

We have carried out Brownian dynamics simulations of binary mixtures of charged colloidal suspensions of two different diameter particles with varying volume fractions phi and charged impurity concentrations n(i). For a given phi, the effective temperature is lowered in many steps by reducing n(i) to see how structure and dynamics evolve. The structural quantities studied are the partial and total pair distribution functions g(tau), the static structure factors, the time average g(<(tau)over bar>), and the Wendt-Abraham parameter. The dynamic quantity is the temporal evolution of the total meansquared displacement (MSD). All these parameters show that by lowering the effective temperature at phi = 0.2, liquid freezes into a body-centered-cubic crystal whereas at phi = 0.3, a glassy state is formed. The MSD at intermediate times shows significant subdiffusive behavior whose time span increases with a reduction in the effective temperature. The mean-squared displacements for the supercooled liquid with phi = 0.3 show staircase behavior indicating a strongly cooperative jump motion of the particles.

Structural, optical and EPR studies on ZnO:Cu nanopowders prepared via low temperature solution combustion synthesis

Cu (0.1 mol%) doped ZnO nanopowders have been successfully synthesized by a wet chemical method at a relatively low temperature (300 degrees C). Powder X-ray diffraction (PXRD) analysis, scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Fourier transformed infrared (FTIR) spectroscopy, UV-Visible spectroscopy, Photoluminescence (PL) and Electron Paramagnetic Resonance (EPR) measurements were used for characterization. PXRD results confirm that the nanopowders exhibit hexagonal wurtzite structure of ZnO without any secondary phase. The particle size of as-formed product has been calculated by Williamson-Hall (W-H) plots and Scherrer's formula is found to be in the range of similar to 40 nm. TEM image confirms the nano size crystalline nature of Cu doped ZnO. SEM micrographs of undoped and Cu doped ZnO show highly porous with large voids. UV-Vis spectrum showed a red shift in the absorption edge in Cu doped ZnO. PL spectra show prominent peaks corresponding to near band edge UV emission and defect related green emission in the visible region at room temperature and their possible mechanisms have been discussed. The EPR spectrum exhibits a broad resonance signal at g similar to 2.049, and two narrow resonances one at g similar to 1.990 and other at g similar to 1.950. The broad resonance signal at g similar to 2.049 is a characteristic of Cu2+ ion whereas the signal at g similar to 1.990 and g similar to 1.950 can be attributed to ionized oxygen vacancies and shallow donors respectively. The spin concentration (N) and paramagnetic susceptibility (X) have been evaluated and discussed. (C) 2011 Elsevier B. V. All rights reserved.

Monte Carlo simulation of network formation based on structural fragments in epoxy-anhydride systems

A method combining the Monte Carlo technique and the simple fragment approach has been developed for simulating network formation in amine-catalysed epoxy-anhydride systems. The method affords a detailed insight into the nature and composition of the network, showing the distribution of various fragments. It has been used to characterize the network formation in the reaction of the diglycidyl ester of isophthalic acid with hexahydrophthalic anhydride, catalysed by benzyldimethylamine. Pre-gel properties like number and weight distributions and average molecular weights have been calculated as a function of epoxy conversion, leading to a prediction of the gel-point conversion. Analysis of the simulated network further yields other characteristic properties such as concentration of crosslink points, distribution and concentration of elastically active chains, average molecular weight between crosslinks, sol content and mass fraction of pendent chains. A comparison has been made of the properties obtained through simulation with those predicted by the fragment approach alone, which, however, gives only average properties. The Monte Carlo simulation results clearly show that loops and other cyclic structures occur in the gel. This may account for the differences observed between the results of the simulation and the fragment model in the post-gel phase. Copyright (C) 1996 Elsevier Science Ltd.

Role of spacer chain length in dimeric micellar organization. Small angle neutron scattering and fluorescence studies

Micelles of different dimeric amphiphiles Br-, n-C(16)H(33)NMe(2)(+) -(CH)(m)-N(+)Me(2)-n-C16H33, Br- (where m = 3, 4, 5, 6, 8, 10, and 12) adapt different morphologies and internal packing arrangements in aqueous media depending on their spacer chain length (m). Detailed measurements of small angle neutron scattering (SANS) cross sections from different bis-cationic, dimeric surfactant micelles in aqueous media (D2O) are reported. The data have been analyzed using the Hayter and Penfold model for macro ion solution to compute the interparticle structure factor S(Q) taking into account the screened Coulomb interactions between the dimeric micelles. The SANS analysis clearly indicated that the extent of aggregate growth and the variations of shapes of the dimeric micelles depend primarily on the spacer chain length. With spacer chain length, m less than or equal to 4, the propensity of micellar growth was particularly pronounced. The effects of the variation of the concentration of dimeric surfactants with m = 5 and 10 on the SANS spectra and the effects of the temperature variation for the micellar system with m = 10 were also examined. The critical micelle concentrations (cmc) and their microenvironmental feature, namely, the microviscosities that the dimeric micellar aggregates offer to a solubilized, extrinsic fluorescence probe, 1,6-diphenyl-1,3,5-hexatriene, were also determined. The changes of cmcs and microviscosities as a function of spacer chain length have been explained in terms of conformational variations and progressive looping of the spacer in micellar core upon increasing m values.

Raman study of pressure-induced structural transitions in CsIO4 to 12 GPa

High pressure Raman scattering studies have been carried out on cesium periodate (CsIO4) using the diamond anvil cell. Three pressure-induced phase transitions occur in the range 0.1�12 GPa as indicated by abrupt changes in the Raman spectra, and pressure dependence of the phonon frequencies. The transitions are observed at 1.5, 4.5 and 6.2 GPa in the increasing pressure cycle. A large hysteresis is noticed for the reverse transition when releasing the pressure. The high pressure phase is nearly quenchable to ambient pressure. The nature of the pressure-induced transitions are discussed in terms of the sequence of pressure-induced transitions expected for scheelite-pseudoscheelite structure ABO4 compounds from crystal chemical considerations. For the softening of the two high frequency internal modes, a pressure-induced electronic change involving the 5 d states of cesium and 5 p states of iodine is invoked.

Structural and dielectric properties of Bi2NbxV1-xO5.5 ceramics

Bi2NbxV1-xO5.5 ceramics with x ranging from 0.01 to 0.5 have been prepared. The crystal system transforms from an orthorhombic to tetragonal at x greater than or equal to 0.1 and it persists until x = 0.5. Scanning electron microscopic (SEM) investigations carried out on thermally etched Bi2NbxV1-xO5.5 ceramics confirm that the grain size decreases markedly (18 mu m to 4 mu m) with increasing x. The shift in the Curie temperature (725 K) toward lower temperatures, with increasing x, is established by Differential Scanning Calorimetry (DSC). The dielectric constants as well as the loss tangent (tan delta) decrease with increasing x at room temperature.

Structural and optical properties of RF sputtered Bi2VO5.5 thin films

Thin films of Bismuth Vanadate Bi2VO5.5 (BiV) have been deposited on amorphous quartz and polycrystalline silicon substrates by r.f. sputtering technique and characterised for their structural and optical properties. The os-deposited films at room temperature are found to be amorphous and transparent over the spectral range of 0.55 mu m to 12 mu m. Post-deposition annealing at 400 degrees C in air shows the formation of the BiV crystalline phase. The optical constants namely refractive index. extinction coefficient and optical bandgap of both amorphous and crystalline films have been determined. The refractive index of the as-deposited film is around 2.4 at 0.7 mu m and drops to 2.26 at 1.56 mu m. The optical bandgap of the material has been determined from the computed values of the absorption coefficients.

Structural variants of hyperbranched polyesters

Hyperbranched polyesters based on 3,5-dihydroxybenzoic acid and its derivatives were prepared by self-condensation of the corresponding ester under standard trans-esterification conditions. The spacer segment length that connects the branching points was systematically varied by starting from the appropriate ethyl 3,5-bis(omega-hydroxyoligo(ethyleneoxy))benzoate. The thermal properties of the hyperbranched polyesters were studied using DSC, and they have been compared with those of the linear analogues prepared from the corresponding p-hydroxybenzoic acid derivatives and also with the molecularly ''kinked'' analogues prepared from the meta isomers. These hyperbranched polyesters were also terminally functionalized by using a potentially mesogenic 4-butoxybiphenylcarboxylic acid derivative in an attempt to prepare novel hyperbranched liquid crystalline polyesters. This was achieved by copolymerization of the AB(2) monomer with the mesogenic A-type capping unit. These polymers were found to be amorphous and did not exhibit any liquid crystalline phases, probably due to the random distribution of the mesogenic segments on the polymer framework, making it difficult to both crystallize and form mesophases.

Simple Structural And Bonding Representations Of Fullerenes

Bonding in buckminsterfullerene, C60, can be described in terms of a unique canonical representation in which all six membered rings have a benzenoid Kekule structure while the pentagons are all made of exclusively single bonds. The corresponding valence bond structure reflects the full symmetry of the molecule and is consistent with the observed bond length variations. Computational support for the bonding description is provided using localized MO's obtained at the MNDO level. The requirement of benzenoid structures for all the hexagons can be used as a criterion of stability of fullerenes which complements the pentagon isolation rule. A convenient two-dimensional representation of the fullerene structures incorporating the above bonding description is suggested, especially for use in mechanistic discussions.

The structural theory and physical organic chemistry

Woolley's revolutionary proposal that quantum mechanics does not sanction the concept of ''molecular structure'' - which is but only a ''metaphor'' - has fundamental implications for physical organic chemistry. On the one hand, the Uncertainty Principle limits the precision with which transition state structures may be defined; on the other, extension of the structure concept to the transition state may be unviable. Attempts to define transition states have indeed caused controversy. Consequences for molecular recognition, and a mechanistic classification, are also discussed.

Tribridged diruthenium complexes: A new structural motif

Dinuclear complexes containing a (mu-oxo)bis(mu-carboxylato) diruthenium (III) core have been prepared by a novel synthetic route using metal-metal bonded diruthenium(II,III) tetracarboxylates as precursors. The complexes have been structurally characterized and they are redox active. The terminal ligands play an important role in tuning the electronic structure of the core. The stability of the core is found to be dependent on the size and pi-acidic nature of the terminal ligand cis- to the mu-oxo ligand. The chemistry of such tribridged complexes is relatively new. The rapid growth of this chemistry is based on the discovery of similar core structures in several non-heme iron- and manganese-containing metalloproteins. The tribridged core presents a new structural motif in coordination chemistry. The chemistry of diruthenium complexes with a [Ru-2(mu-O) (mu-O(2)CR)(2)(2+)] core has been reviewed.

Structural and vibrational studies of the layered structure solid Fe1 ? xNixPS3 (1 greater-or-equal, slanted x greater-or-equal, slanted 0)

We report here the results of structural and vibrational studies on the solid solution Fe1 ? xNixPS3 (1 greater-or-equal, slanted x greater-or-equal, slanted 0) systems. From the structural analysis, we show that there is a lattice compaction as the composition x is varied from 0 to 1, the basic lattice symmetry being maintained. We find that the compaction is more in the basal plane. These subtle structural changes are also reflected in the vibrational bands. We observed splitting of certain bands due to these small changes in the lattice constants, which we explained as arising from a correlation splitting. These changes in the vibrational bands have also been seen on cooling where there is a preferential thermal compaction in the basal plane compared to that perpendicular to the plane.

Structural dependence of density in high molecular weight esters

The effect of molecular structure on density has been examined in high molecular weight esters (molecular weight 300-900), having varying degrees of branching. Densities were calculated from an empirical equation, which agrees well with the experimental values (error +/-1.5%), irrespective of branching. Since density is related to molecular packing and hence to the molecular rotation, in n-alkanes, the glass transition temperature (T(g)) and density both increase with molecular weight, and hence T(g) is directly related to the density. The esters exhibit a complex behavior. In linear esters the T(g) decreases with molecular weight which is explained from group contribution and molecular interactions. In the +-branched esters, however, T(g) decreases with molecular weight until the molecular weight reaches 600 and increases sharply thereafter. The Y-branched esters show an intermediate behavior. The dependence of T(g) on molecular weight has been explained from the segmental motion.

Structural Hypotheses And Raman-Spectroscopy Investigations Of Glasses Belonging To The B2s3-Li2s-Lii System

A previous B-11 nuclear magnetic resonance investigation of glasses belonging to the B2S3-Li2S-LiI system had allowed the authors to determine the variation of the number of three and four coordinated boron atoms with composition. These results, in addition to the observation that vitreous B2S3 quite easily forms fibres during casting, have led us to propose structural hypotheses for B2S3 based glasses, which are supported by the present Raman spectroscopy study. For vitreous B2S3 the spectra were accounted for on the basis of the various types of BS3/2 triangles proposed by the model. Molecular orbital considerations allowed us to assign the most significant lines for the binary glasses by assuming that BS3/2 triangles (with or without nonbridging sulphur atoms) and BS4 tetrahedra were present. In the ternary system, lithium iodide has been found to interact slightly on the structural entities, altering their vibrational characteristics without fundamentally modifying their nature.

Prediction of nonlocal scaling parameter for armchair and zigzag single-walled carbon nanotubes based on molecular structural mechanics, nonlocal elasticity and wave propagation

Atomic vibration in the Carbon Nanotubes (CNTs) gives rise to non-local interactions. In this paper, an expression for the non-local scaling parameter is derived as a function of the geometric and electronic properties of the rolled graphene sheet in single-walled CNTs. A self-consistent method is developed for the linearization of the problem of ultrasonic wave propagation in CNTs. We show that (i) the general three-dimensional elastic problem leads to a single non-local scaling parameter (e(0)), (ii) e(0) is almost constant irrespective of chirality of CNT in the case of longitudinal wave propagation, (iii) e(0) is a linear function of diameter of CNT for the case of torsional mode of wave propagation, (iv) e(0) in the case of coupled longitudinal-torsional modes of wave propagation, is a function which exponentially converges to that of axial mode at large diameters and to torsional mode at smaller diameters. These results are valid in the long-wavelength limit. (C) 2011 Elsevier Ltd. All rights reserved.