Testing and validation of a self-diffusion coefficient model based on molecular dynamics simulations

Analysis of the Distribution and Influencing Factors of Diffusion Coefficient Model Parameters Based on Molecular Dynamics Simulations

The establishment of mathematical models to predict the diffusion coefficients of gas and liquid systems have important theoretical significance and practical value. In this work, based on the previously proposed diffusion coefficient model D_LV, the distribution and influencing factors of the model parameters characteristic length (L) and diffusion velocity (V) were further investigated using molecular dynamics simulations. The statistical analysis of L and V for 10 gas systems and 10 liquid systems was presented in the paper. New distribution functions were established to describe the probability distributions of molecular motion L and V. The mean values of the correlation coefficients were 0.98 and 0.99, respectively. Meanwhile, the effects of molecular molar mass and system temperature on the molecular diffusion coefficients were discussed. The results show that the effect of molecular molar mass on the diffusion coefficient mainly affects the molecular motion L, and the effect of system temperature on the diffusion coefficient mainly affects V. For the gas system, the average relative deviation of D_LV and D_MSD is 10.73% and that of D_LV and experimental value is 12.63%; for the solution system, the average relative deviation of D_LV and D_MSD is 12.93% and that of D_LV and experimental value is 18.86%, which indicates the accuracy of the new model results. The new model reveals the potential mechanism of molecular motion and provides a theoretical basis for further study of the diffusion process.

Application Process of Coating Agent and the Coating Effect Evaluation Based on Molecular Dynamics

In the field of energy-containing materials, the modification of nanoaluminum powders has been widely studied. However, in modified experimental design, the lack of theoretical prediction usually leads to long experimental cycles and high resource consumption. To this end, this study evaluated the process and effect of dopamine (PDA)- and polytetrafluoroethylene (PTFE)-modified nanoaluminum powders based on molecular dynamics (MD). Through the calculation of the coating stability, compatibility, and oxygen barrier performance of the modified material, the modification process and effect were explored from a microscopic point of view. The results showed that the adsorption of PDA on the nanoaluminum was the most stable, and the binding energy was 463.03 kcal·mol^-1. PDA and PTFE with different ratios are compatible systems at 350 K, and the best compatibility ratio is 10 wt % PTFE/90 wt % PDA. The 90 wt % PTFE/10 wt % PDA bilayer model has the best barrier performance for oxygen molecules in a wide temperature range. The calculated results of the coating stability agree with the experiments, and it is pointed out that it is feasible to evaluate the modification effect in advance by MD simulation. In addition, the simulation results concluded that the double-layered PDA and PTFE have better oxygen barrier properties. Compatibility can be used only to determine whether phase separation occurs between mixtures and is not directly related to the dense mixing of polymers and the barrier properties of small gas molecules. The simulation provided in this article can predict the experimental results and provide theoretical guidance for coating modification experiments in order to reduce unnecessary experiments, shorten the experimental cycle, and reduce costs.

Simulation Study on the Screening of Hydrophobic Surface Materials for Pipeline Drag Reduction Based on Adsorption Properties

Hydrophobic surface drag reduction techniques are effective in reducing the frictional resistance of fluids, the adsorption of liquid molecules on hydrophobic surfaces can reflect the resistance to fluid flow through such solid surfaces. Based on molecular simulation technology, we investigate the adsorption characteristics of water molecules on hydrophobic surfaces to achieve rapid screening of hydrophobic materials in fire-fighting water supply systems. The Monte Carlo method was used to simulate the adsorption process of polymers and to analyze the effects of temperature and fixed adsorption quantity. Contact angle tests were also done to verify polymer hydrophobicity. The isothermal adsorption heat, water molecule distribution, and energy distribution were studied by molecular mechanics and molecular dynamics methods. Then, adsorption localization simulations and electrostatic potential distributions were used to predict possible adsorption sites on hydrophobic surfaces and single-molecule chains. Finally, the interaction energy, diffusion coefficient, and free volume were investigated to explain the adsorption mechanism at the molecular level. Simulation results show that, overall, PTFE was more hydrophobic and PES was more hydrophilic and at 298 K, the number of adsorbed water molecules was ranked as follows: PTFE < PVDF < PVC < PMMA < PPS < CSM < BD-HDI < BD-MDI < BD-TDI < PES. Furthermore, PTFE, PVDF, PVC, PES, and PPS have more stable adsorption configurations on the (0 -1 0) surface. According to the findings, hydrogen bonding dominates the interaction between water molecules and hydrophilic polymers, whereas π-π interactions increase water molecules' diffusion resistance in polymers with benzene rings. In addition, PES contains many sulfone groups and ether bonds, which disorganize the chain arrangement to provide more free volume, whereas the water adsorption rate of PTFE is reduced because its molecular chains are less convoluted and more organized.

Optimal design of a multi-dimensional validated synergistic extraction process for the treatment of atmosphere-vacuum distillation wastewater

The wastewater discharged from atmosphere-vacuum distillation of oil refining process contains a high concentration of phenolic compounds, which are toxic and not eco-friendly. Direct discharge of the untreated wastewater will have an adverse impact on the surrounding environment. This paper proposes a multi-dimensional synergistic extraction solution to realize the effective disposal of atmosphere-vacuum distillation wastewater. Firstly, extraction experiments are conducted to select the optimal extractant. Secondly, the microscopic mechanism of separating phenolic compounds from wastewater with synergistic extractant of methyl isobutyl ketone and n-pentanol is investigated by molecular dynamics simulation. Finally, the synergistic extraction process is modeled and optimized based on above multi-dimensional analyses. The optimization is performed through sensitivity analysis from three aspects: operating parameters, synergistic extractant cycling, and waste heat recovery. A control scheme is then designed to maintain the smooth operation of synergistic extraction process. Feed disturbances are specifically added to test the anti-interference capability of the control scheme. With the novel treatment process proposed in this paper, the removal rate of phenolic compounds from atmosphere-vacuum distillation wastewater reaches 93.02%.