Search results for "Libri"
showing 10 items of 1189 documents
Analytic second derivatives for the spin-free exact two-component theory
2011
The formulation and implementation of the spin-free (SF) exact two-component (X2c) theory at the one-electron level (SFX2c-1e) is extended in the present work to the analytic evaluation of second derivatives of the energy. In the X2c-1e scheme, the four-component one-electron Dirac Hamiltonian is block diagonalized in its matrix representation and the resulting "electrons-only" two-component Hamiltonian is then used together with untransformed two-electron interactions. The derivatives of the two-component Hamiltonian can thus be obtained by means of simple manipulations of the parent four-component Hamiltonian integrals and derivative integrals. The SF version of X2c-1e can furthermore exp…
A theoretical and experimental study of the formation mechanism of 4-X-chalcones by the Claisen–Schmidt reaction
2000
Abstract A theoretical and experimental study on the formation of 4-X-chalcones (X=H, Cl, F, OCH3, N(CH3)2) was carried out by the Claisen–Schmidt reaction. The influence of pH and temperature was analyzed. Under the adopted experimental conditions 4-X-chalcones were the only reaction products. In the theoretical study, the thermodynamic magnitudes and the reactivity indexes calculated with the AM1 method were used. The mechanism of Nayak and Rout was analyzed critically and a new mechanism was proposed, characterized by: (1) rapid nucleophilic attack of the catalyst (OH−) on the carbon atom of the methyl group of acetophenone; (2) attacks of the acetophenonate ion on the carbon atom of the…
Transport coefficients of n-butane into and through the surface of silicalite-1 from non-equilibrium molecular dynamics study
2009
We have studied coupled heat and mass transfer of n-butane through a membrane of silicalite-1. A description of the surface was given using non-equilibrium thermodynamics, and transport coefficients were determined. Three independent coefficients were found for the whole surface: the resistance to heat transfer, the coupling coefficient and the resistance to mass transfer. These coefficients were defined in stationary state. All resistances are significant, and show that the surface acts as a barrier to transport. A new scheme was devised to find the enthalpy of adsorption, from two particular coupling coefficients, namely the measurable heats of transfer. The method yields the enthalpy of …
Activity mediated phase separation: Can we understand phase behavior of the nonequilibrium problem from an equilibrium approach?
2016
We present results for structure and dynamics of mixtures of active and passive particles, from molecular dynamics (MD) simulations and integral equation theory (IET) calculations, for a physically motivated model. The perfectly passive limit of the model corresponds to the phase-separating Asakura-Oosawa model for colloid-polymer mixtures in which, for the present study, the colloids are made self-propelling by introducing activity in accordance with the well known Vicsek model. Such activity facilitates phase separation further, as confirmed by our MD simulations and IET calculations. Depending upon the composition of active and passive particles, the diffusive motion of the active specie…
Aqueous Solubility Diagrams for Cementitious Waste Stabilization Systems: II, End-Member Stoichiometries of Ideal Calcium Silicate Hydrate Solid Solu…
2001
Solubility in the fully hydrated CaO–SiO2–H2O system can be best described using two ideal C-S-H-(I) and C-S-H-(II) binary solid solution phases. The most recent structural ideas about the C-S-H gel permit one to write stoichiometries of polymerized C-S-H-(II) end-members as hydrated precursors of the stable tobermorite and jennite minerals in the form of 5Ca(OH)2·6SiO2·5H2O and 10Ca(OH)2·6SiO2·6H2O, respectively. For thermodynamic modeling purposes, it is more convenient to express the number of basic silica and portlandite units in these stoichiometries using the coefficients nSi and nCa. Thermodynamic solid-solution aqueous-solution equilibrium modeling by applying the Gibbs energy minim…
Static and dynamic screening effects in the electrostatic self-assembly of nano-particles.
2014
In the description of charge screening in the electrostatic self-assembly of nanoparticles (molecules) embedded into a polar solvent, the static screening effects (a contribution associated with the rapid spatial redistribution of small and highly mobile ions of a solvent) are traditionally treated phenomenologically, using the Yukawa short-range potential for describing the interaction between these particles. However, this model has a limited range of applicability being valid only for infinitely diluted systems and high salt concentrations. During a slow self-assembling process with nanoparticle formation, very dense structural elements (aggregates) are formed, in which the distances bet…
Interaction of polyelectrolytes with oppositely charged micelles studied by fluorescence and liquid chromatography
2005
It is studied by spectrofluorimetry the association of ionized cationic micelles (cetyltrimethylammonium bromide, CTAB) with oppositely charged polyelectrolyte [sodium poly(styrenesulfonate), PSSNa]. CTAB provokes a change in the fluorescence intensity emitted by PSSNa. The investigated surfactants form micelle-like aggregates before critical micellar concentration (CMC). Two approaches (binding and partition equilibrium) are used to obtain the association constant, KA, number of CTAB molecules in a binding site, N, and apparent partition coefficient, Γ. Analysis of the parameters as a function of polymer concentration and ionic strength μ is performed. The effect of μ shows an enhancement …
A New Simple Static Method for the Determination of Solubilities of Condensed Compounds in Supercritical Fluids
2002
Abstract A simple static method based on gravimetric measurements has been adopted to measure the solubility of benzenecarboxylic acid, 2-hydroxy-benzenecarboxylic acid, 4-hydroxy-benzenecarboxylic acid and 1,8-dihydroxyanthraquinone along several isotherms in the temperature range 318–357 K and the pressure interval 7–23 MPa. Solubility data have been correlated with the Zieger–Eckert approach, using the Fedor atomic and group contribution method to estimate the solubility parameter and the molar volume of the solutes. A good agreement was obtained between data reported in the literature and those determined using the proposed method which allows one to measure solubility mole fraction as …
Effect of solute association on the apparent adsorption isotherm. A model of the separation of non-racemic mixtures of enantiomers in achiral chromat…
1996
This article discusses the effect of solute association on the apparent adsorption isotherm and on the chromatographic peak profile. The modelling performed has shown that dimerization of a solute in a chromatographic system gives rise to an apparent adsorption isotherm of the non-linear type. This results first of all in peak broadening. Tailing or a fronting can be observed depending on the relative retention of the monomer and the dimer. Secondly, the retention of the solute depends on the amount of sample because of the non-linear character of the adsorption isotherm. As a result, an excess of one of two analytes with absolutely identical adsorption properties i.e. optical isomers, can …