Prediction of minimum water amount to stop thermal decomposition of forest fuel

Presented are results of experimental studies of the heat transfer processes in suppression of thermal decomposition of typical forest fuels (FFs) (spruce needles, birch leaves, aspen twigs, or amixture thereof) due to the effect of aerosol water flow (drop radii: 0.01–0.12 mm; concentration: 3.8 · 10^?5 m³ drop/m³ of gas). The experiments have been carried out with FF samples in the form of cylinders of a thickness of 40–100 mm and a diameter of 20–150 mm. The times required to stop the thermal decomposition of FF have been found, as well as amounts of water necessary to lower in a given time the temperature in a layer of material to the point of decomposition beginning. A dimensionless complex has been derived for prediction of water spraying parameters (amount and time of supply) that ensure sustainable termination of FF decomposition within a specified time interval.     

Magnetoviscous properties of Fe3O4 silicon oil based ferrofluid

A Fe₃O₄ silicon oil-based ferrofluid (FF) was prepared and the viscosity properties of the FFs were investigated by a rotating viscometer and a torsional oscillation cup viscometer, respectively. Experimental results show that the viscosity of the FFs decreases with increasing temperature, and increases with increasing magnetic field intensity due to the existence of the magnetic particles. The hysteresis curve of the viscosity–magnetic field shows that the formation and destruction of chain-like or drop-like structures has obvious effect on the viscosity of the FFs. When the field is relatively strong, the viscosity at the decreasing stage is higher than that at the increasing stage. In contrast, when the field is relatively weak, the viscosity at the decreasing stage is slightly lower than that at the increasing stage. In addition, the relation between viscosity of the FFs and time under the magnetic field shows that time is an effective factor in the evolution of the magnetically induced structures.     

Predicting the solubility of xenon in n-hexane and n-perfluorohexane: a simulation and theoretical study

The solubility of xenon in n-hexane and n-perfluorohexane has been studied using both molecular simulation and a version of the SAFT approach (SAFT-VR). The calculations were performed close to the saturation line of each solvent, between 200 K and 450 K, which exceeds the smaller temperature range where experimental data are available in the literature. Molecular dynamics simulations, associated with Widom's test particle insertion method, were used to calculate the residual chemical potential of xenon in n-hexane and n-perfluorohexane and the corresponding Henry's law coefficients. The simulation results overestimate the solubility of xenon in both solvents when simple geometric combining rules are used, but are in good agreement if a binary interaction parameter is included. With the SAFT-VR approach we are able to reproduce the experimental solubility for xenon in n-hexane, using simple Lorentz-Berthelot rules to describe the unlike interaction. In the case of n-perfluorohexane as a solvent, a binary interaction parameter was introduced, taken from previous work on (xe + C₂F₆) mixtures. Overall, good agreement is obtained between the simulation, theoretical and experimental data.     

Development of the reaction time accelerating molecular dynamics method for simulation of chemical reaction

We present a novel and efficient method to integrate chemical reactions into molecular dynamics to simulate chemical reaction systems. We have dubbed this method RTAMD, an acronym for reaction time accelerating molecular dynamics. The methodology we propose here requires no more than the knowledge of the empirical intermolecular potentials for the species at play as well as the elementary reaction path among them. Bond formation during the simulation is performed by changing the inter-atomic potentials from those of the non-bonded species to those of the bonded ones, and a reaction is deemed to occur by the distance separating the bond forming atoms. In this way the energy barrier for a reaction is no longer considered; the estimation of the reaction rate, however, is possible by introducing the principles of the transition state theory. The simplicity of the present scheme to simulate chemical reactions enables it to be used in large-scale MD simulations involving a large number of simultaneous chemical reactions and to evaluate kinetic parameters. In this paper, the basic theory of the method is presented and application to simple equiatomic reaction system where the reaction rates were estimated was illustrated.     

Study of effective hardness and condensed Fukui functions using AIM,ab initio,and DFT methods

The effective chemical hardness for some triatomic molecules has been studied using the ‘atom in a molecule’ approach (AIM). Molecular chemical hardness values calculated from the effective chemical hardness of individual atoms agree with experimental and theoretically calculated molecular hardness values. Condensed Fukui functions have been calculated using Mulliken, NPA and AIM charges calculated by HF and B3LYP methods using the 6-31+G** basis set. All population schemes have predicted few negative Fukui functions, which correlate well with effective chemical hardness values.     

Hydrophilic Labeling Reagents of Dipyrrylmethene-BF2 Dyes for Two-Photon Excited Fluorometry: Syntheses and Photophysical Characterization

Recently introduced bioaffinity assay technology, ArcDia TPX, is based on two-photon excited fluorescence (TPE) and it enables separation-free ultra-sensitive immunoassays from microvolumes. Here we present syntheses of novel two-photon excitable fluorescent labeling reagents which have been specially designed to be used as label molecules in the ArcDia TPX assay technique. The labeling reagents are based on dipyrrylmetheneboron difluoride (dipyrrylmethene-BF2) chromophore, which have been substituted with aryl, heteroaryl or arylalkenyl chemical groups to extend the pi-electron conjugation. These substitutions results in a series of dipyrrylmethene-BF2 fluorophores with different photophysical properties. Dipyrrylmethene-BF2 fluorophores have been further substituted with a dipeptide linker unit and finally activated as succinimidyl esters to enable specific coupling with primary amino groups. The dipeptide linker serves as a spacer arm between the label and a target, and enhances the solubility of the label in aqueous solutions. Study of the chemical and photophysical performance of the new labeling reagents is described. The new labeling reagents exhibit high fluorescence quantum yields, and molar absorption coefficients. The results show that the new labels with the hydrophilic dipeptide linker unit provide large two-photon excitation cross-sections, high fluorescence quantum efficiency and good solubility in aqueous solutions. The results suggest that the novel dipyrrylmethene-BF2 labels are highly applicable to bioaffinity assays based on two-photon excitation of fluorescence.     

Thermal quantum time-correlation functions from classical-like dynamics

Thermal quantum time-correlation functions are of fundamental importance in quantum dynamics, allowing experimentally measurable properties such as reaction rates, diffusion constants and vibrational spectra to be computed from first principles. Since the exact quantum solution scales exponentially with system size, there has been considerable effort in formulating reliable linear-scaling methods involving exact quantum statistics and approximate quantum dynamics modelled with classical-like trajectories. Here, we review recent progress in the field with the development of methods including centroid molecular dynamics , ring polymer molecular dynamics (RPMD) and thermostatted RPMD (TRPMD). We show how these methods have recently been obtained from ‘Matsubara dynamics’, a form of semiclassical dynamics which conserves the quantum Boltzmann distribution. We also apply the Matsubara formalism to reaction rate theory, rederiving t → 0₊ quantum transition-state theory (QTST) and showing that Matsubara-TST, like RPMD-TST, is equivalent to QTST. We end by surveying areas for future progress.     

Physics of suppression of thermal decomposition of forest fuel using surface water film

The paper presents results of an experimental study of the suppression of thermal decomposition of forest fuel (FF) due to formation of a thin surface water film (3 mm thick). The investigations were carried out with birch leaves, as well as a mixture of birch leaves, pine needles, and twigs of aspen. The experiments involved model bases of fire of the above FFs in the form of cylinders. The cylinders were 20 mm to 60 mm in diameter and 40 mm to 100 mm high. The effect of water on the FFs was monitored via high-speed (up to 105 frames per second) video recording and fast (a thermal lag of less than 1 s) thermal transducers. The times of termination of FF thermal decomposition and the minimum (required) volumes of water were determined. The feasibility of complete suppression of the FF thermal decomposition process due to formation of a thin surface water film (10–15 times thicker than the reacted material) was experimentally supported. The foundations of the physical model of a complex of processes that occur owing to the water film effect on the forest fuel heated to a high temperature (well above the point of thermal decomposition) were formulated.     

Modelling nucleation from solution with the string method in the osmotic ensemble

ABSTRACT

Direct molecular simulation of nucleation from solution is a challenging task, requiring a combination of ‘rare events’ techniques and methods to control the chemical potential. Rare event methods usually keep the total number of molecules fixed, resulting in artificial free energy profiles due to the depletion of solute from the solution phase. In order to address this issue, we present a new approach that uses the string method in collective variables in the osmotic ensemble to obtain minimum free energy pathways for nucleation at constant supersaturation. Our method does not require using an explicit reservoir of solute molecules, or making additional assumptions about the activity coefficients in the solution. We apply the new method to the crystallisation of sulfamerazine from acetonitrile and methanol solutions, and compare the resulting potential of mean force profiles to those obtained using analytical corrections previously employed in the literature.     

纳米结构泡沫金冲击响应的分子动力学模拟

采用分子动力学计算程序对纳米结构泡沫金(Au)的冲击响应进行了模拟,得到了不同疏松度条件下泡沫Au的冲击压缩特性。通过获取不同势函数条件下实密Au的冲击Hugoniot关系以及泡沫结构稳定性测试选取适合描述Au泡沫冲击过程中原子的相互作用势。采用密堆积球壳的方式建立泡沫Au的初始构型。通过改变空心球壳的尺寸得到不同疏松度的稳定的泡沫Au结构。对泡沫Au的冲击过程进行分子动力学模拟,获得了不同疏松度泡沫Au在不同冲击压缩强度下的热力学状态参数。将模拟结果与已有的状态方程数据库以及疏松物质冲击压缩模型进行比较,结果表明,计算和理论模型给出的结果仍然存在明显的差异性,亟需通过进一步实验研究来验证模拟计算和理论模型结果的可靠性。     

Switching kinetics of ferroelastic ferroelectrics

Switching kinetics of uniaxial ferroelastic ferroelectrics (FFs) in external electric and stress fields is studied using classical theory of nucleation and growth. The stage in which the polarization and deformation reversal involves the main body of the FF and the final stage (Ostwald ripening) of the FF switching are studied with allowance for the change in the repolarization and redeformation during the phase transition. The time dependences of the repolarization and redeformation are found, and equations are derived from which the polarization current and the deformation flux, as well as their time dependence, can be calculated. The calculated main characteristics of the FF switching are compared with the experimental data for switching of Rochelle salt single crystals.     

Effect of quantization and positron concentration on shielding potentials in electron-positron-ion plasma

The kinetic theory of plasma has been employed to compute the test-charge potential distributions accounting for quantization effects in magnetized electron-positron-ion (EPI) plasmas. In this regard, the degenerate positrons and electrons are assumed to follow the Fermi-Dirac distribution, while inertial ions are modelled by Maxwellian velocity distribution. By solving the Fourier-transformed Vlasov–Poisson equations, a modified dielectric function and electrostatic potential is obtained. By imposing various constraints on the test-charge speed, the potential profile has been analysed in terms of Debye–Hückel (DH), far-field (FF), and wake-field (WF) potentials. It has been found that the amplitude of DH and FF potentials increases by the inclusion of quantization effects, and it becomes the opposite for the WF potential profile. Furthermore, the variation of positron concentration significantly affects the DH, FF, and WF potentials. The present findings are important to understand the shielding phenomenon in degenerate multi-species plasmas.     

Molecular dynamics simulation of C8E5 micelle in explicit water: structure and hydrophobic solvation thermodynamics

Results are presented from a large scale molecular dynamics (MD) simulation of a non-ionic micelle comprising 80 C₈E₅ surfactant molecules in 9798 explicit TIP3P waters. The results are consistent with the conventional static picture of a simple spherical micelle (‘hydrophilic heads out, hydrophobic tails in’). In addition, the MD simulation reveals structural details that show clearly the dynamic nature of the micellar aggregate. The micelle is roughly spherical with thermal fluctuations leading to instantaneous shapes that are significantly non-spherical. The individual surfactant molecules adopt various nonlinear conformations, predominately in the hydrophilic segment, thereby leading to the overall compact globular shape of the micelle aggregate. Atomic distribution functions and dihedral angle distributions show that the micelle interior is similar to liquid n-octane. Although no water penetration in the micelle hydrophobic core is observed, there is considerable exposure of the hydrophobic tails of the surfactant molecules to water in the interfacial region. The excess chemical potentials of hydrophobic Lennard-Jones (LJ) solutes and their WCA analogs show a preference for the michelle core relative to bulk water. Interestingly, the solvation energies of LJ solutes show a minimum in the interfacial region. A qualitative explanation for this behaviour based on different packing tendencies of water and chain-like molecules is presented.     

Prediction of aqueous solubility of compounds based on neural network

Neural network algorithms are gradually applied to the field of chemical informatics. In this paper, the neural network models are used to predict the properties of compounds based on Molecular Information. We predict the aqueous solubility of compounds, and evaluate the prediction results of the Neural Networks including CNN, RNN, DNN, SNN. The performance of the models in predicting the solubility is able to meet or exceed the predicted effect of the method based on the molecular structure (ESOL). DNN model performance is of more accuracy, and RNN performance is of better stability. This method can directly avoid complex molecular structure characterisation, and provide a convenient and flexible way to predict properties of compounds.     

Molecular dynamics study of hydrogenated silicon clusters at high temperatures

This paper reports on a study of the stability of silicon clusters of intermediate size at a high temperature. The temperature dependence of the physicochemical properties of 60- and 73-atom silicon nanoparticles are investigated using the molecular dynamics method. The 73-atom particles have a crystal structure, a random atomic packing, and a packing formed by inserting a 13-atom icosahedron into a 60-atom fullerene. They are surrounded by a ‘coat’ from 60 atoms of hydrogen. The nanoassembled particle at the presence of a hydrogen ‘coat’ has the most stable number (close to four) of Si–Si bonds per atom. The structure and kinetic properties of a hollow single-layer fullerene-structured Si₆₀ cluster are considered in the temperature range 10 K ≤ T ≤ 1760 K. Five series of calculations are conducted, with a simulation of several media inside and outside the Si₆₀ cluster, specifically, the vacuum and interior spaces filled with 30 and 60 hydrogen atoms with and without the exterior hydrogen environment of 60 atoms. Fullerene surrounded by a hydrogen ‘coat’ and containing 60 hydrogen atoms in the interior space has a higher stability. Such clusters have smaller self-diffusion coefficients at high temperatures. The fullerene stabilized with hydrogen is stable to the formation of linear atomic chains up to the temperatures 270–280 K.     

Quantification of cross-conjugation

The effects of ‘through-conjugation’ (TC) vs. ‘cross-conjugation’ (CC) have been studied in a series of [n]dendralene analogues with the aid of high-level molecular orbital (MO) calculations using composite G3MP2 method. The energy differences between TC and the corresponding CC alkenes have been determined and found to be small and comparable to the conformational barrier for rotation around the C–C single bond. The destabilisation in CC vs. TC alkenes per C = C unit does not show different trends within the even and odd series of [n]dendralenes. The observed differences in UV–vis and nuclear magnetic resonance (NMR) spectra and chemical reactivities between even and odd [n]dendralenes are thus solely due to conformational preferences which of course affect C = C bond conjugation (π-delocalisation). We also propose new partially hydrogenated graphene materials (‘dendraphenes’) which were introduced in the isodesmic reactions designed to study CC.     

The relation of chemical potentials and reactivity studied by a state path sum

In order to study the relation between chemical potentials and reactivity we divide the interaction region into physically decisive potential ‘boxes’. Each box is considered to be a small interaction region for a virtual chemical reaction with corresponding virtual transition probability amplitudes. These amplitudes are linked together and superimposed in a ‘state path sum’ which relates local potential properties with exact reaction probabilities. The state path sum is carried out by the supermatrix technique of the statistical theory of chain molecules.