A Novel Fractional Brownian Dynamics Method for Simulating the Dynamics of Confined Bottle-Brush Polymers in Viscoelastic Solution.

In crowded fluids, polymer segments can exhibit anomalous subdiffusion due to the viscoelasticity of the surrounding environment. Previous single-particle tracking experiments revealed that such anomalous diffusion in complex fluids (e.g., in bacterial cytoplasm) can be described by fractional Brownian motion (fBm). To investigate how the viscoelastic media affects the diffusive behaviors of polymer segments without resolving single crowders, we developed a novel fractional Brownian dynamics method to simulate the dynamics of polymers under confinement. In this work, instead of using Gaussian random numbers ("white Gaussian noise") to model the Brownian force as in the standard Brownian dynamics simulations, we introduce fractional Gaussian noise (fGn) in our homemade fractional Brownian dynamics simulation code to investigate the anomalous diffusion of polymer segments by using a simple "bottle-brush"-type polymer model. The experimental results of the velocity autocorrelation function and the exponent that characterizes the subdiffusion of the confined polymer segments can be reproduced by this simple polymer model in combination with fractional Gaussian noise (fGn), which mimics the viscoelastic media.

Anomalous Diffusion of Polyelectrolyte Segments on Supported Charged Lipid Bilayers.

This work provides mesoscale models for the anomalous diffusion of a polymer chain on a heterogeneous surface with rearranging randomly distributed adsorption sites. Both the "bead-spring" model and oxDNA model were simulated on supported lipid bilayer membranes with various molar fractions of charged lipids, using Brownian dynamics method. Our simulation results demonstrate that "bead-spring" chains exhibit sub-diffusion on charged lipid bilayers which agrees with previous experimental observations for short-time dynamics of DNA segments on membranes. In addition, the non-Gaussian diffusive behaviors of DNA segments have not been observed in our simulations. However, a simulated 17 base pairs double stranded DNA, using oxDNA model, performs normal diffusion on supported cationic lipid bilayers. Due to the number of positively charged lipids attracted by short DNA is small, the energy landscape that the short DNA experiences during diffusion is not as heterogeneous as that experienced by long DNA chains, which results in normal diffusion rather than sub-diffusion for short DNA.

Conversion mechanism of thermal plasma-enhanced CH4-CO2 reforming system to syngas under the non-catalytic conditions.

Thermal plasma activation of CH4-CO2 reforming (CRM) to syngas under non-catalytic conditions is an efficient and clean technology for the large-scale utilization of hydrocarbon resources and the conversion of greenhouse gases. This study investigates the equilibrium state and transformation mechanism of a CRM reaction system activated by thermal plasma through experimental, thermodynamic, and kinetic analyses. The experimental results illustrated that the CO2 conversion rate and H2 selectivity showed a downward trend with an increase in the CO2/CH4 molar ratio, whereas the CH4 conversion rate and CO selectivity showed the opposite trend. When CO2/CH4 molar ratio was 6/4, the selectivity for CO and H2 increased to 87.0 % and 80.8 %, respectively. Excess CO2 promotes the partial oxidation of CH4 to eliminate carbon deposition, resulting in an H2/CO molar ratio value closer to 1. Thermodynamic results show that the thermal-plasma-initiated CRM reaction can reach thermodynamic equilibrium more easily than the conventional catalyzed reactions, achieving much higher feedstock gas conversion without carbon deposition. The kinetic results obtained from the PSR model revealed that CH4 and CO2 were cleaved to form free radicals at the instant of contact with the plasma flame. O, H, and other particles generated in the form of free radicals rapidly collided with each other and transformed into CO and H2, accelerating the reaction process. The results presented in this study will help reveal the transformation mechanism of the CRM reaction activated by thermal plasma under non-catalytic conditions and provide a new perspective for studying CRM reactions.

Slurryability and Influencing Mechanism of Hydrophobic Structures in Additive-Coal-Water Ternary System.

This study investigates the effects of additive adsorption onto coal particles on surface properties, hydrophobic groups on the slurryability, and the moisture occurrence form on the performance of coal water slurry (CWS). Mechanisms related to the different hydrophobic structures of the additives are proposed. The adsorption method of sulfonated acetone formaldehyde enhances the adsorption capacity of coal surfaces but is not conducive to slurrying. Sodium lignin sulfonate has hydrophobic ends with nonpolar aromatic groups, three-dimensional macromolecular structures, and complex branched chains, which provide CWS with good stability and slurryability. Naphthalenesulfonate formaldehyde has a double benzene ring structure and provides the thick but nonuniform adsorption layers on coal surfaces. The many amorphous structures and low molecular weights of sodium humic sulfonate lead to nonuniform hydration films and poor slurryability. The results of this paper provide guidance for improving synergism in coal-water-additive systems and enhancing slurry performance.

Mesoscale Simulation of Bacterial Chromosome and Cytoplasmic Nanoparticles in Confinement.

In this study we investigated, using a simple polymer model of bacterial chromosome, the subdiffusive behaviors of both cytoplasmic particles and various loci in different cell wall confinements. Non-Gaussian subdiffusion of cytoplasmic particles as well as loci were obtained in our Langevin dynamic simulations, which agrees with fluorescence microscope observations. The effects of cytoplasmic particle size, locus position, confinement geometry, and density on motions of particles and loci were examined systematically. It is demonstrated that the cytoplasmic subdiffusion can largely be attributed to the mechanical properties of bacterial chromosomes rather than the viscoelasticity of cytoplasm. Due to the randomly positioned bacterial chromosome segments, the surrounding environment for both particle and loci is heterogeneous. Therefore, the exponent characterizing the subdiffusion of cytoplasmic particle/loci as well as Laplace displacement distributions of particle/loci can be reproduced by this simple model. Nevertheless, this bacterial chromosome model cannot explain the different responses of cytoplasmic particles and loci to external compression exerted on the bacterial cell wall, which suggests that the nonequilibrium activity, e.g., metabolic reactions, play an important role in cytoplasmic subdiffusion.

Combined ReaxFF and Ab Initio MD Simulations of Brown Coal Oxidation and Coal-Water Interactions.

In this manuscript, we use a combination of Car-Parrinello molecular dynamics (CPMD) and ReaxFF reactive molecular dynamics (ReaxFF-MD) simulations to study the brown coal-water interactions and coal oxidation. Our Car-Parrinello molecular dynamics simulation results reveal that hydrogen bonds dominate the water adsorption process, and oxygen-containing functional groups such as carboxyl play an important role in the interaction between brown coal and water. The discrepancy in hydrogen bonds formation between our simulation results by ab initio molecular dynamics (CPMD) and that by ReaxFF-MD indicates that the ReaxFF force field is not capable of accurately describing the diffusive behaviors of water on lignite at low temperatures. The oxidations of brown coal for both fuel rich and fuel lean conditions at various temperatures were investigated using ReaxFF-MD simulations through which the generation rates of major products were obtained. In addition, it was observed that the density decrease significantly enhances the generation of gaseous products due to the entropy gain by reducing system density. Although the ReaxFF-MD simulation of complete coal combustion process is limited to high temperatures, the combined CPMD and ReaxFF-MD simulations allow us to examine the correlation between water adsorption on brown coal and the initial stage of coal oxidation.

Molecular Dynamics Simulation of the Implantation of b-Oriented ZSM-5 Film Modified α-Quartz Substrate Surface With Different Modifiers.

In this study, we investigated the structure-absorption relationship of common surface modifiers of chitosan (CTS), polyvinyl acetate (PVA), and titanium dioxide (TiO2) with α-quartz surface using molecular dynamics simulation. And the orientations and combinations derived from structures between modified α-quartz and ZSM-5 crystallites were also investigated. The results show that PVA is a non-linear organic macromolecule with a large amount of hydroxyl groups on its surface, which easily adhere to the surface of the substrate and agglomerate. CTS is a straight-chain structure containing a large number of hydroxyl and amino groups, which easily accumulate and spread on the surface of the substrate. TiO2 not only forms hydrogen bonds and complexes with the substrate but also interacts with each other to form a dense modifier layer. We observed that a large number of stable Ti-O-Si chemical bonds formed between the modified layer of inorganic small-molecule TiO2 and the surface of α-quartz, which compacted and stabilized the attached ZSM-5 film. Moreover, the orientation angle of the ZSM-5 nanocrystalline nucleus on the modified α-quartz was computed, which confirmed that the b-axis orientation of the ZSM-5 nanocrystalline nucleus was the highest on the surface of the substrate modified by TiO2. We discussed the influence of the modified temperature of modifiers in the constructed materials, and we have observed the adsorption state differences of TiO2 at different modified temperatures. We also discussed the catalytic properties of the materials prepared by the corresponding methods in conversion of methanol-to-aromatics (MTA) reaction. These results agree with our previous experimental data. By employing molecular dynamics simulation, we have obtained more precise conclusive information of the b-oriented growth of ZSM-5 crystallites, which highly depends on the surface modifiers.

Effect of Surface Structure and Adsorption Activity on Implanting of b-Oriented ZSM-5 Zeolite Film on Modified α-Quartz Substrate.

b-oriented ZSM-5 zeolite film was synthesized on the macropore α-quartz substrate modified with titanium dioxide (TiO2), polyvinyl acetate (PVA), and chitosan (CTS) by hydrothermal crystallization. By comparing the binding energy and b-oriented angle of zeolite film on each modified α-quartz substrate, the orientations, and combinations derived from structure-adsorption relationship were investigated with Material Studio simulation. Furthermore, the effects of calcination temperature and ultraviolet (UV) irradiation time on the surface structure and adsorption activity of TiO2 coating were studied. The increase adsorption potential energy and the formation of Ti-O-Si bind between zeolite crystal phase and substrate facilitate the continuous and uniform zeolite film growth. The TiO2 interlayer with anatase phase after UV irradiation presents a smooth surface with high Ti-OH density, consequently to high selectivity of b-orientation growth for the ZSM-5 crystals. Compared with the traditional ZSM-5, the higher stability has been exhibited with b-oriented ZSM-5 film /TiO2/α-quartz in the MTA reaction, and the methanol conversion and BTX selectivity remained higher than 90 and 70%, after 6 h reaction.