Effect of various factors influencing the delamination behavior of surfactant intercalated layered double hydroxides

The delamination-restacking behavior of a number of layered double hydroxides (LDHs) differing in [M-II]/[M-III] ratio, constituent metal ions and intercalated surfactant anions in different organic solvents has been studied. Colloidal dispersion due to delamination and the stability of the colloid obtained have been found to be not affected much by the nature of the constituent metal ions but increase with increase in the size of the surfactant anion. LDHs with low [M-II]/[M-III] ratio delaminate better than the ones with high [M-II]/[M-III] ratio. Delamination is best in alcohols such as 1-butanol, 1-hexanol, 1-octanol and I-decanol, while a little delamination occurs in nonpolar solvents such as hexane. In all the cases, the original layered solid could be obtained through restacking of layers from the colloidal dispersion.

Diagnosis of dominant forcing factors for large-scale vertical velocities during active and break phases of the monsoon

An attempt to diagnose the dominant forcings which drive the large-scale vertical velocities over the monsoon region has been made by computing the forcings like diabatic heating fields,etc. and the large-scale vertical velocities driven by these forcings for the contrasting periods of active and break monsoon situations; in order to understand the rainfall variability associated with them. Computation of diabatic heating fields show us that among different components of diabatic heating it is the convective heating that dominates at mid-tropospheric levels during an active monsoon period; whereas it is the sensible heating at the surface that is important during a break period. From vertical velocity calculations we infer that the prime differences in the large-scale vertical velocities seen throughout the depth of the atmosphere are due to the differences in the orders of convective heating; the maximum rate of latent heating being more than 10 degrees Kelvin per day during an active monsoon period; whereas during a break monsoon period it is of the order of 2 degrees Kelvin per day at mid-tropospheric levels. At low levels of the atmosphere, computations show that there is large-scale ascent occurring over a large spatial region, driven only by the dynamic forcing associated with vorticity and temperature advection during an active monsoon period. However, during a break monsoon period such large-scale spatial organization in rising motion is not seen. It is speculated that these differences in the low-level large-scale ascent might be causing differences in convective heating because the weaker the low level ascent, the lesser the convective instability which produces deep cumulus clouds and hence lesser the associated latent heat release. The forcings due to other components of diabatic heating, namely, the sensible heating and long wave radiative cooling do not influence the large-scale vertical velocities significantly.

Recycling of the Posttermination Complexes of Mycobacterium smegmatis and Escherichia coli Ribosomes Using Heterologous Factors

In eubacteria, ribosome recycling factor (RRF) and elongation factor G (EFG) function together to dissociate posttermination ribosomal complexes. Earlier studies, using heterologous factors from Mycobacterium tuberculosis in Escherichia coli revealed that specific interactions between RRF and EFG are crucial for their function in ribosome recycling. Here, we used translation factors from E.coli,Mycobacterium smegmatis and M. tuberculosis, and polysomes from E. coli and M. smegmatis, and employed in vivo and in vitro experiments to further understand the role of EFG in ribosome recycling. We show thatE. coli EFG (EcoEFG) recycles E. coli ribosomes with E. coli REF (EcoRRF), but not with mycobacterial RRFs. Also, EcoEFG fails to recycle M. smegmatis ribosomes with either EcoRRF or mycobacterial RRFs. On the other hand, mycobacterial EFGs recycle both E. coli and M. smegmatis ribosomes with either of the RRFs. These observations suggest that EFG establishes distinct interactions with REF and the ribosome to carry out ribosome recycling. Furthermore, the EFG chimeras generated by swapping domains betweenmycobacterial EFGs and EcoEFG suggest that while the residues needed to specify the EFG interaction with REF arelocated in domains IV and V. those required to specify its interaction with the ribosome are located throughout the molecule. (C) 2010 Elsevier Ltd. All rights reserved.

Coherence factors in excitonic insulators

Results of a theoretical study on ultrasonic attenuation and NMR relaxation in excitonic insulators are reported. The transition rates derived have anomalous temperature dependence owing to the occurrence of coherence factors analogous to the case of superconductors. It is found that these coherence factors are characteristically different for the interband and the intraband scattering processes. It is suggested that experimental observation of these temperature-dependent coherence factors may help identify the existence of an excitonic phase.

Influence of Geometrical Factors on the Physico-Chemical Properties of Perovskite and Layered Perovskite Oxides

A brief qualitative comparison is made of perovskite ABO sub 3 and layered perovskite ABO sub 3 and layered perovskite A sub 2 BO sub 4 oxides with special emphasis on the influence of geometrical factors on certain physico-chemical properties. The layered perovskite oxides are distinguished from three-dimensional oxides by a looser packing, frustration in three-dimensional interactions, more internal pressure on B--O bonds for small tolerance factors, and by different values of site-percolation thresholds. Their influence on electronic configurations of metal ions, stabilities and syntheses of compounds is discussed. The influence of increased anisotropy in layered oxides on localisation of charge carriers and in suppressing the onset of long-range ferromagnetic ordering is also discussed.

Theory of unitarity bounds and low-energy form factors

We present a general formalism for deriving bounds on the shape parameters of the weak and electromagnetic form factors using as input correlators calculated from perturbative QCD, and exploiting analyticity and unitarily. The values resulting from the symmetries of QCD at low energies or from lattice calculations at special points inside the analyticity domain can be included in an exact way. We write down the general solution of the corresponding Meiman problem for an arbitrary number of interior constraints and the integral equations that allow one to include the phase of the form factor along a part of the unitarity cut. A formalism that includes the phase and some information on the modulus along a part of the cut is also given. For illustration we present constraints on the slope and curvature of the K-l3 scalar form factor and discuss our findings in some detail. The techniques are useful for checking the consistency of various inputs and for controlling the parameterizations of the form factors entering precision predictions in flavor physics.

Dominant role of the Cu---O charge-transfer energy, electronic polarizability and associated factors in the superconductivity of cuprates

Although it is believed that there is strong hybridization between the Cu(3d) and O(2p) orbitals in the layered cuprates and that the parent compounds such as La2CuO4 are charge-transfer gap insulators, very few models consider the Cu---O charge-transfer energy, Δ, or the hybridization strength, tpd, to be the important factors responsible for the superconductivity of these materials. Based on the crucial experimental observation that the relative intensity of the features in Cu(2p) photoemission of several families of cuprates varies systematically with the hole concentration, nh, we have been able to show that both these properties vary smoothly with Δ /tpd. More importantly, we show that the electronic polarizability of the CuO2 sheets, α , is sufficiently large to favour hole pairing and that the value α also depends on Δ/tpd. Both nh and α increase smoothly with decreasing Δ /tpd. Considering that the maximum Tc in the various cuprate families containing the same number of CuO2 sheets occurs around the same nh value (e.g., nh≈ 0.2 in cuprates with two CuO2 sheets). The present study demonstrates how Δ /tpd, α and such chemical bonding characteristics have an important bearing on the superconducting properties of the cuprates.

Improving the phenomenology of Kl3 form factors with analyticity and unitarity

The shape of the vector and scalar K-l3 form factors is investigated by exploiting analyticity and unitarity in a model-independent formalism. The method uses as input dispersion relations for certain correlators computed in perturbative QCD in the deep Euclidean region, soft-meson theorems, and experimental information on the phase and modulus of the form factors along the elastic part of the unitarity cut. We derive constraints on the coefficients of the parameterizations valid in the semileptonic range and on the truncation error. The method also predicts low-energy domains in the complex t plane where zeros of the form factors are excluded. The results are useful for K-l3 data analyses and provide theoretical underpinning for recent phenomenological dispersive representations for the form factors.

Drainage and separation factors for static foams containing agglomerates of microbial cells

The presence of cell agglomerates has been found to influence significantly the rates of liquid drainage from static foams. The process of drainage has been modelled by considering the foam to be made of pentagonal dodecahedral bubbles yielding films, nearly horizontal and nearly vertical Plateau borders. The films are assumed to drain into both kinds of Plateau borders equally. The horizontal Plateau borders are assumed to receive liquid from the films and drain into the vertical Plateau borders, which, in turn, form the main flow paths for gravity drainage. The drainage process is assumed to be similar to that for pure liquid until a stage is reached where the size of the cell agglomerates become equivalent to those of films and Plateau borders. Thereafter, a squeezing flow mechanism has been formulated where the aggromerates deform and flow. The model based on the above assumptions has been verified against experimental results and has been found to predict not only drainage data but also the separation of cell agglomerates from broths.

Influence of non-geometrical factors on intracrystalline diffusion -- Role of sorbate-zeolite interactions

Molecular dynamics simulations on Xe in NaY and Ar in NaCaA zeolite are reported. Rates of cage-to-cage crossovers in the two zeolites exhibit trends which are contrary to that expected from geometrical considerations. The results suggest the important role of the sorbate-zeolite interactions in determining the molecular sieve properties of zeolites for small sized sorbates. The results are explained in terms of the barrier height for cage-to-cage crossover in the two zeolites.

Bearing capacity factors of circular foundations for a general c-phi soil using lower bound finite elements limit analysis

By using the lower bound limit analysis in conjunction with finite elements and linear programming, the bearing capacity factors due to cohesion, surcharge and unit weight, respectively, have been computed for a circular footing with different values of phi. The recent axisymmetric formulation proposed by the authors under phi = 0 condition, which is based on the concept that the magnitude of the hoop stress (sigma(theta)) remains closer to the least compressive normal stress (sigma(3)), is extended for a general c-phi soil. The computational results are found to compare quite well with the available numerical results from literature. It is expected that the study will be useful for solving various axisymmetric geotechnical stability problems. Copyright (C) 2010 John Wiley & Sons, Ltd.

Dynamic structure factors of a dense mixture

We compute the dynamic structure factors of a dense binary liquid mixture. These describe dynamics on molecular length scales, where structural relaxation is important. We find that the presence of a few large particles in a dense fluid of small particles slows down the dynamics considerably. We also observe a deep narrowing of the spectrum for a disordered mixture composed of a nearly equal packing of the two species. In contrast, a few small particles diffuse easily in the background of a dense fluid of large particles. We expect our results to describe neutron scattering from a dense mixture.

Factors contributing to phantom bonds in inorganic molecules: Interpretations and predictions

The validity of various qualitative proposals for interpreting and predicting the existence of short contacts between formally non-bonded atoms, as in cyclodisiloxane and related inorganic ring systems, is critically evaluated. The models range from simple considerations of geometric constraints, lone pair repulsions and pi-complex formation to proposals such as the unsupported pi-bond model and the sigma-bridged-pi bond concept. It is pointed out that a unified description based on a combination of closed and open 3-centre 2-electron bonds is possible. The role of hybridisation is emphasized in the short phantom bond computed in an earlier model system. These insights are used to predict structures with exceptionally short Si..Si and B..B phantom bonds. The proposals are confirmed by ab initio calculations.