Influence of molecular association on the solubility of methylxanthines in supercritical solvent CO<Subscript>2</Subscript>–methanol

Analysis of the experimental data on the solubility of methylxanthines (theophylline and theobromine) in supercritical (SC) solvent CO₂–methanol at various concentrations of methanol within the framework of the ASL model (Associated Solution + Lattice) based on molecular association theory and the simple lattice model is presented. Hetero-association of methylxanthines with the molecules of methanol is studied by means of ¹H NMR spectroscopy at 313 K. The contribution of molecular association to the solubility of methylxanthines in the mixed SC solvent CO₂–methanol is analyzed. It is shown that the presence of NH group in the molecule of methylxanthine, when passing from caffeine to theophylline or theobromine, leads to an increase of the contribution of the component related to molecular association to solubility.     

Adsorption of water—methanol mixtures in carbon and aluminosilicate pores: a molecular simulation study

A theoretical study is reported of the adsorption behaviour of water—methanol mixtures in slit carbon and in uncharged alumino-silicate micropores. The adsorption isotherms are obtained for a pore of width of 2 nm and at a temperature of 298 K from grand canonical ensemble Monte Carlo simulations. The results show that the graphite and uncharged silicate surfaces are covered by a dense layer of flatly adsorbed water and methanol molecules having weaker hydrogen bonding. In the interior of the pore, the fluid exhibits bulk-like behaviour with a stronger hydrogen bonded structure. Solvation forces are also calculated as a function of pore size. The positive values found for the solvation force for all pore sizes reflect the hydrophobic interactions of the mixture with the carbon and uncharged alumino-silicate walls.     

A molecular level study of liquid water with a flexible water model

The structural properties of water at different temperatures and pressures have been investigated by using a flexible water model and the inherent structure mechanism. The presence of 60$^\circ$ peak in the O--O--O angle distribution function and the behaviours of the hydrogen bonds in the first shell indicate that some water molecules in the second shell move toward the central molecules through the bending (not breaking) of hydrogen bonds and even become first-shell molecules of the central molecule on the basis of the O--O cutoff distance but not first-shell molecules by means of the hydrogen-bond criterion. The inherent-structure analysis of the O--O radial distribution functions at different pressures shows that the first peak is almost independent of the pressure; the position of second peak moves from 0.45 to 0.32nm as the pressure increases from $1\times 10^5$Pa to $1\times 10^9$Pa. This particularly evident pressure effect, i.e. the constant nearest-neighbours and the transformation of outer-neighbours on the basis of O--O distance, together with the results at different temperatures, gives a positive evidence for the two-state outer-neighbour mixture model: liquid water is a mixture of Ice-Ih-type-bonding and Ice-II-type bonding structures.     

Solvation properties of Li+ and CI− in water: molecular dynamics simulation with a non-rigid model

Molecular dynamics (MD) simulations were carried out to investigate the solvation properties of Li⁺ and C1^? ions in water with a relatively accurate but rarely used non-rigid model, RWK2, in this study. A new set of ion-water interaction parameters was evaluated from the experimental data and first principles calculation results of stable clusters, Li⁺(H₂O) _n (n = 1- 6) and CI^?(H₂O) _n (n = 1–4). With the ion-water potential parameters evaluated from the data of the clusters and the water-water potential predetermined from the non-rigid RWK2 model, the structural (radial distribution functions, angular distribution functions, spatial distribution functions, coordination number), dynamical (residence time) and energetic properties of the ionic salvations in bulk water were studied through a comprehensive analysis of our MD simulation outputs. These results not only agree well with experimental data and first principles calculations, but also reveal some new insights into the microscopic ionic salvation processes.     

Numerical study of the effect of water content on OH production in a pulsed-dc atmospheric pressure helium–air plasma jet

A numerical study of the effect of water content on OH production in a pulsed-dc atmospheric pressure helium–air plasma jet is presented. The generation and loss mechanisms of the OH radicals in a positive half-cycle of the applied voltage are studied and discussed. It is found that the peak OH density increases with water content in air(varying from0% to 1%) and reaches 6.3×1018m-3when the water content is 1%. Besides, as the water content increases from 0.01%to 1%, the space-averaged reaction rate of three-body recombination increases dramatically and is comparable to those of main OH generation reactions.     

Effect of a localized impurity on soliton dynamics in the Bose-Einstein condensates

By using a multiple-scale method,we analytically study the effect of a localized impurity on the soliton dynamics in the Bose-Einstein condensates.It is shown that a dark soliton can be transmitted through a repulsive (or attractive) impurity,while at the position of the localized impurity the soliton can be quasitrapped by the impurity.Additionally,we find that the strength of the localized impurity has an important effect on the dark soliton dynamics.With increasing strength of the localized impurity,the amplitude of the dark soliton becomes bigger,while its width is narrower,and the soliton propagates slower.     

Effect of phosphorus and copper additions on the structure of Pt and Pt–Cu nanoparticles in a radiation-induced reduction method

A facile chemical process has been developed for the preparation of magnetic FeCo nanoparticles. The FeCo nanoparticles were mono-dispersed, obtained by the safe and ecofriendly method, possessed saturation magnetization up to 187 emu/g, and demonstrated excellent chemical stability. In this work, we have studied how to control Fe/Co ratio by variation of precursor ratio, and how to vary particle size from 9.3 to 12.3 nm by surfactant amount used. The cytotoxicity of as-synthesized nanoparticles was investigated after coating with the poly(methyl methacrylate-co-butyl acrylate) by the emulsion process and the results demonstrated high biocompatibility. Similarly, the same synthesis method was used with the single precursor FeO(OH) or Co₃O₄. The results showed that this method can also fabricate 10 nm mono-dispersed spherical Fe₃O₄ particles and self-assembly Co nanoneedles.     

Influence of solvent and intramolecular relaxation on dynamics of luminescence spectra in donor–acceptor complexes

The oscillator strengths for Rydberg states of a NaHe molecule are calculated using a semianalytic procedure with the l-coupling effect taken into account (due to the dipole potential of a core). This effect gives rise to nonzero oscillator strengths for transitions forbidden in the atomic model of molecular Rydberg states. The difference between calculations in terms of the atomic model and calculations with consideration of the dipole moment of a core is shown for allowed transitions.     

Solubility of caffeine in the supercritical CO<Subscript>2</Subscript>–methanol binary solvent

The caffeine–methanol association constant at 313 K has been determined by ¹H NMR spectroscopy. The caffeine solubility in the supercritical carbon dioxide (SC-CO₂)–methanol mixed solvent has been calculated using the association constant experimentally measured by NMR in the framework of the associated solution + lattice (ASL) model, which is based on the theory of molecular association and a simple lattice model. Individual contributions to the solubility have been determined, and the relative role of various factors determining the solubility of caffeine in the mixed solvent has been analyzed. The caffeine solubility as a function of the methanol content of the SC-CO₂–methanol system is predicted to pass through a maximum.     

Effect of copper on the structure–phase transformations and the properties of quasi-binary TiNi–TiCu alloys

The effect of copper alloying up to 25 at % on the structure–phase transformations and the physicomechanical properties of ternary alloys from the quasi-binary TiNi–TiCu section is studied by measuring the physicomechanical properties, transmission electron microscopy, scanning electron microscopy, electron diffraction, and X-ray diffraction (XRD). The data of temperature measurements of the electrical resistivity and the magnetic susceptibility and XRD data are used to plot a general diagram for the thermoelastic B2 ? B19', B2 ? B19 ? B19', and B2 ? B19 martensitic transformations, which occur in the alloys upon cooling as the copper content increases in the ranges 0–8, 8–15, and 15–25 at % Cu, respectively. The experimental results are compared to the well-known data, including differential scanning calorimetry data, obtained for these alloys. The changes in the mechanical properties and the microstructure of the alloys in the state of B19 or B19' martensite are discussed.     

Force-field dependence on the interfacial structure of oil–water interfaces

We investigate the performance of different force-fields for alkanes, united (TraPPE) and all atom (OPLS-AA) models, and water (SPC/E and TIP4P-2005), in the prediction of the interfacial structure of alkane (n-octane, and n-dodecane)–water interfaces. We report an extensive comparison of the interfacial thermodynamic properties as well as the interfacial structure (translational and orientational). We use the recently introduced intrinsic sampling method, which removes the averaging effect of the interfacial capillary waves and provides a clear view of the interface structure. The alkane interfacial structure is sensitive to the environment, i.e. alkane–vapour or alkane–water interfaces, showing a stronger structure when it is in contact with the water phase. We find that this structure is fairly independent of the level of detail, full or united atom, employed to describe the alkane phase. The water surface properties show a small dependence on the water model. The dipole moment of the SPC/E model shows asymmetric fluctuations, with a tendency to point both towards the alkane and water phases. On the other hand the dipole moment of the TIP4P-2005 model shows a tendency to point towards the water phase only. Analysis of the intrinsic electrostatic field indicates that the surface water potential is confined to an interfacial region of about 8 Å. Overall we find that the intrinsic structure of alkane–water interfaces is a robust interfacial property, which is independent of the details of the force-field employed. Hence, it should provide a good reference to interpret experimental data.     

Effect of the shape of an organic water–coal fuel particle on the condition and characteristics of its ignition in a hot air flow

The results of experimental studies of the effect of the shape of an organic water–coal fuel (OCWF) particle on its ignition delay time and the time of its complete burnout in a hot air flow are reported. Three most common shapes of real particles, such as spherical, ellipsoidal, and irregular-polyhedron-like, are considered. It is shown that the shortest ignition delay time and the time of complete burnout correspond to polyhedron- shaped OCWF particles. Conditions are identified under which this factor significantly influences the ignition characteristics. The experiments were carried out at initial particle sizes (averaged maximum values) of 0.5–5 mm and temperatures and velocities of the oxidant flow of 600–900 K and 0.5–5 m/s, respectively. The main components of the studied fuels were coal processing wastes and waste motor, turbine, and transformer oils.     

Effect of aluminum content on the mechanochemical synthesis of in-situ TiN in the Al–Ti–AlN system and subsequent shock consolidation

To determine the effect of aluminum content on the formation of in-situ TiN in the Al–Ti–AlN system, a mixture of aluminum, titanium and aluminum nitride powders was subjected to high energy milling. Al content of the mixture was changed according to the following stoichiometric reaction: Ti+AlN+XAl→TiN+(1+X)Al. The value of X was varied from 5.35 to 19.65 based on the stoichiometric calculation of the molar mass of each component expected to result in aluminum matrix composite with TiN weights of 30%, 20% and 10%, respectively, in addition to reaction corresponding to X=0(Ti+AlN→TiN+Al). Thermodynamic factors determine that the amount of Al in the mixture plays a key role in the formation of in-situ TiN. XRD and EPMA results showed that at lower Al content (X=0, 5.35), reaction proceed through a gradual mode. By increasing Al content (X=19.65), no mechanochemical reaction occurred between Ti and AlN. Continuation of the milling process allowed acquisition of in-situ TiN in the designed compositions of AlN–TiN, Al–Ti–AlN–30%TiN, and to some extent, of Al–Ti–AlN–20%TiN. A nanocrystalline solid solution evolved by mechanical alloying (MA) was sustained for prolonged milling time. The mean TiN crystallite size obtained was 10 nm for the AlN–TiN composition. The end product milled powder after 40 h of milling time, equating to the Al–Ti–AlN–30%TiN composition was consolidated into bulk compact using the underwater shock compaction method. The milled specimens were characterized by XRD, scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and microhardness testing. The sample had a uniform and fine-grained composite structure with 99% theoretical density and average microhardness of 434 HV0.1. The results confirmed the possibility of fabricating reliable bulk nanostructured materials by imposing shock compaction on submicron sized powders.     

Effect of Ni and vacancy concentration on initial formation of Cu precipitate in Fe–Cu–Ni ternary alloys by molecular dynamics simulation

In the present work, the effects of Ni atoms and vacancy concentrations(0.1%, 0.5%, 1.0%) on the formation process of Cu solute clusters are investigated for Fe–1.24%Cu–0.62%Ni alloys by molecular dynamics(MD) simulations. The presence of Ni is beneficial to the nucleation of Cu precipitates and has little effect on coarsening rate in the later stage of aging. This result is caused by reducing the diffusion coefficient of Cu clusters and the dynamic migration of Ni atoms. Additionally, there are little effects of Ni on Cu precipitates as the vacancy concentration reaches up to 1.0%,thereby explaining the embrittlement for reactor pressure vessel(RPV) steel. As a result, the findings can hopefully provide the important information about the essential mechanism of Cu cluster formation and a better understanding of ageing phenomenon of RPV steel. Furthermore, these original results are analyzed with a simple model of Cu diffusion, which suggests that the same behavior could be observed in Cu-containing alloys.     

Reaction intermediates of methanol synthesis and the water–gas-shift reaction on the ZnO(0001) surface

The polar Zn-ZnO(0001) surface is involved in the catalysis of methanol synthesis and the water–gas-shift reaction. We use density functional theory calculations to explore the favorable binding geometries and energies of adsorption of several molecular species relevant to these reactions, namely carbon monoxide (CO), carbon dioxide (CO₂), water (H₂O) and methanol (CH₃OH). We also consider several proposed reaction intermediates, including hydroxymethyl (CH₂OH), methoxyl (CH₃), formaldehyde (CH₂O), methyl (CH₃), methylene (CH₂), formic acid (HCOOH), formate (HCOO), formyl (HCO), hydroxyl (OH), oxygen (O) and hydrogen (H). For each, we identify the preferred binding geometry at a coverage of 1/4 monolayers (ML), and report calculated vibrational frequencies that could aid in the identification of these species in experiment. We further explore the effects on the binding energy when the adsorbate coverage is lowered to 1/9 and 1/16 ML.     

Diffusion activation energy versus the favourable energy in two-order-parameter model:A molecular dynamics study of liquid Al

In the present work, we find that both diffusion activation energy E_a(D) and E_a(S^ex) increase linearly with pressure and have the same slope (0.022±0.001 eV/GPa) for liquid Al. The temperature and pressure dependence of excess entropy is well fitted by the expression -S^ex(T,P)/k_B=a(P)+b(P)T+c(P)exp(E_f/k_BT), which together with the small ratio of E_f/k_BT leads to the relationship of excess entropy to temperature and pressure, i.e. S^ex≈-cE_f/T, where c is about 12 and E_f (=Δ E-PΔV) is the favourable energy. Therefore, there exists a simple relation between E_a(S^ex) and E_f, i.e. E_a(S^ex)≈cE_f.     

Molecular dynamics simulation of dielectric constant and cluster structure of liquid methanol: the role of cluster–cluster dipole correlations

A molecular dynamics simulation of liquid methanol at ambient conditions with two different three-site potential models was performed and the evaluated dielectric constant was discussed in the light of the cluster structure of the liquid. The distribution of the pair interaction energy of molecules and the cluster size distribution were calculated. An aggregation contribution to dielectric constant was defined and calculated as a function of the threshold H-bond energy using energetic criterion of H-bond. The structural information on dipole–dipole correlations of molecules incorporated in the size and structure distribution of aggregates proved to cover about 80% of the calculated dielectric constant of methanol. The other 20% should be attributed to the cluster–cluster dipole correlations.     

Effect of atomic disorder and temperature on incommensurate helical spin waves in the Anderson–Hubbard model

Based on the single-band t–t' Anderson–Hubbard model, the effect of disorder on the parameters and ranges of existence of incommensurate helical spin waves is studied. The problem is solved within the functional integration theory in static approximation, taking into account longitudinal fluctuations of the magnetic moment. Magnetic phase diagrams and parameters of incommensurate helical spin waves are obtained as functions of temperatures and electron and impurity concentrations. It is shown that disorder can lead to the first-order transition from the antiferromagnetic phase to the (Q, π) phase and the metal–dielectric transition from antiferromagnetic metal to antiferromagnetic dielectric far from the half-filled band. The results obtained are used to explain the incommensurate magnetic order observed in cuprates in the overdoped mode.     

Structure and transport properties of AgI–AgCl–CsCl glasses: molecular dynamics study

Molecular dynamics (MD) simulation have been performed to investigate the structure and transport properties of molten and glass state of silver alkali halide, AgI–AgCl–CsCl. The screened Born-Mayer type potentials including the polarizability of ions have been used in MD. The temperature dependence of the structure, conductivity, share viscosity, and relaxation times have been discussed by MD results. Electrical density of states for molten and glass states have also been examined using ab initio calculation.