非离子表面活性剂在气液界面的分子动力学模拟

采用分子动力学方法研究了十二烷基醇聚氧乙烯醚系列非离子表面活性剂单分子层在气液界面的微观结构,并通过表面张力的计算考察了表面活性剂分子结构与性能的关系.研究结果表明,随着表面活性剂分子乙氧基基团个数的增加,模拟所得的表面张力的变化趋势与实验一致,所有分子的计算误差在5 mN·m-1以内.同时,随着乙氧基基团数目的增加,...     

阴离子表面活性剂在油水界面聚集的分子动力学模拟

用分子动力学方法模拟了油、水和阴离子表面活性剂组成的混合溶液从初始“均相”到“油水两相”分离的动力学过程, 研究了十二烷基苯磺酸钠(SDBS)在界面分离过程中的作用. 模拟发现, 油水两相能够在短时间内分离达到平衡, 形成一个明显的油水界面; 在SDBS存在情况下, 油水界面的分离时间随着SDBS浓度的增加逐渐增加, 达到平衡时SDBS会在界面处形成一个明显的界面膜, 并对油水界面处的水分子有限制作用. 模拟表明, 分子动力学方法可以作为实验的一种补充, 为实验提供必要的微观分子结构信息.     

阴离子Gemini表面活性剂在油/水界面行为的分子动力学模拟

采用分子动力学方法研究了磺酸盐型阴离子Gemini表面活性剂在油/水界面的吸附行为, 考察了不同长度的连接基(Spacer)对表面活性剂在界面的聚集形态及界面性质的影响. 密度分布和微观结构信息显示, Gemini表面活性剂能在油/水界面形成单层膜结构. Gemini表面活性剂能使油/水界面的厚度显著增大, 并使界面形成能降低. 当连接基为6个碳时, 此类磺酸盐型Gemini表面活性剂的界面厚度最大, 形成的界面最稳定. 连接基长度对Gemini表面活性剂单层膜周围的水分子和Na⁺的吸附结构影响不大, 但是能影响水分子的扩散行为.     

Transport Processes at α‐Quartz–Water Interfaces: Insights from First‐Principles Molecular Dynamics Simulations

Car–Parrinello molecular dynamics (CP–MD) simulations are performed at high temperature and pressure to investigate chemical interactions and transport processes at the α‐quartz–water interface. The model system initially consists of a periodically repeated quartz slab with O‐terminated and Si‐terminated (1000) surfaces sandwiching a film of liquid water. At a temperature of 1000 K and a pressure of 0.3 GPa, dissociation of H₂O molecules into H⁺ and OH^? is observed at the Si‐terminated surface. The OH^? fragments immediately bind chemically to the Si‐terminated surface while Grotthus‐type proton diffusion through the water film leads to protonation of the O‐terminated surface. Eventually, both surfaces are fully hydroxylated and no further chemical reactions are observed. Due to the confinement between the two hydroxylated quartz surfaces, water diffusion is reduced by about one third in comparison to bulk water. Diffusion properties of dissolved SiO₂ present as Si(OH)₄ in the water film are also studied. We do not observe strong interactions between the hydroxylated quartz surfaces and the Si(OH)₄ molecule as would have been indicated by a substantial lowering of the Si(OH)₄ diffusion coefficient along the surface. No spontaneous dissolution of quartz is observed. To study the mechanism of dissolution, constrained CP–MD simulations are done. The associated free energy profile is calculated by thermodynamic integration along the reaction coordinate. Dissolution is a stepwise process in which two Si? O bonds are successively broken. Each bond breaking between a silicon atom at the surface and an oxygen atom belonging to the quartz lattice is accompanied by the formation of a new Si? O bond between the silicon atom and a water molecule. The latter loses a proton in the process which eventually leads to protonation of the oxygen atom in the cleaved quartz Si? O bond. The final solute species is Si(OH)₄.     

Dilational Rheology at Air/Water Interface and Molecular Dynamics Simulation Research of Hydroxyl Sulfobetaine Surfactant

Hydroxyl sulfobetaines with hexadecyl-, octadecyl-hydrophobic chain and an industrial product hydroxyl sulfobetaine were synthesized from analytical-grade and industrial-grade tertiary amine, respectively. The dilational properties and surface tension of the three surfactants at the water-air interface were investigated by drop shape analysis and ring method. The influences of oscillating frequency and bulk concentration on dilational properties were explored. The experimental results show that the dilational module of octadecyl-hydroxyl sulfobetaine was higher than hexadecyl hydroxyl sulfobetaine and the dilational elastic component of the three surfactants were higher than dilational viscous component. Furthermore, the dilational elastic component of mixed surfactant system shows two maxima in a lower concentration than that of single surfactant system. As a result, the surface tension of mixed surfactant system reaches to minimum value in a lower concentration compared with single surfactant system. The simulation results show that the hydrophobic chains in the mixed betaine solution were more curled than in single-component betaine solution ascribed to stronger interaction among different hydrophobic chains, resulting to a more compact interfacial film.     

多元体系油水界面上常见表面活性剂行为的分子动力学模拟

采用分子动力学模拟研究了以十二烷基苯磺酸钠(SDBS)为代表的阴离子型表面活性剂,以十二烷基三甲基溴化铵(DTAB)为代表的阳离子型表面活性剂,以壬基酚聚氧乙烯醚(NPE)为代表的非离子型表面活性剂,以十二烷基二甲基甜菜碱(Betaine)为代表的两性表面活性剂及空白实验.模拟了表面活性剂在油水界面上的行为,考察了表面活性剂分子与石油分子之间的径向分布函数(RDF)、石油分子在竖直方向的均方位移(MSD)、油水界面张力(IFT)、石油层与岩石层之间的相互作用能、石油层的相对浓度在竖直方向的分布及石油分子质心位置随模拟时间的变化关系等,讨论了不同表面活性剂的洗油性能.结果表明:(1)SDBS,NPE和Betaine分子初始状态下呈近似的规律排列,非极性端部分插入油相中,极性端延伸进入水相中;随后表面活性剂的极性端表现出聚集趋势,逐渐形成一个外部亲油内部亲水的一个胶束状粒子,粒子随模拟的进行逐渐融入到油层当中;DTAB从开始的近似规则排列逐渐变为无规排列,但是始终保持亲油端插入到油相中,亲水端位于油水界面上.(2)表面活性剂分子与石油分子之间的相互作用强弱顺序为Betaine≈DTABSDBSNPE.(3)由质心高度和动力过程中的图像截图分析,表面活性剂洗油效果的顺序为BetaineSDBSNPEDTABNone.模拟结果与实际的驱油结果一致,从分子层面上解释了不同表面活性剂洗油的规律.     

Complexation of M3+ Lanthanide Cations by Calix[4]arene-CMPO Ligands: A Molecular Dynamics Study in Methanol Solution and at a Water/Chloroform Interface

Abstract

We report a molecular dynamics study on the 1:1 M³⁺ lanthanide (La³⁺, Eu³⁺ and Yb³⁺) inclusion complexes of an important extractant molecule L: a calix[4]arene-tetraalkyl ether substituted at the wide rim by four NH-C(O)-CH₂-P(O)Ph₂ arms. The M(NO₃)₃ and MCl₃ complexes of L are compared in methanol solution and at a water / chloroform interface. In the different environments the coordination sphere of M³⁺ involves the four phosphoryl oxygens and three to four loosely bound carbonyl oxygens of the CMPO-like arms. Based on free energy simulations, we address the question of ion binding selectivity in pure liquid phases and at the liquid-liquid interface where L and the complexes are found to adsorb. According to the simulations, the enhancement of M³⁺ cation extraction in the presence of the calixarene platform, examined by comparing L to the (CMPO)₄ “ligand” at the interface, is related to the fact that (i) the (CMPO)₄Eu(NO₃)₃ complex is more hydrophilic than the LEu(NO₃) one and (ii) the free CMPO ligands spread at the interface, and are therefore less organized for cation capture than L.     

Molecular Structure and Dynamics of Water at the Water–Air Interface Studied with Surface‐Specific Vibrational Spectroscopy

Water interfaces provide the platform for many important biological, chemical, and physical processes. The water–air interface is the most common and simple aqueous interface and serves as a model system for water at a hydrophobic surface. Unveiling the microscopic (<1 nm) structure and dynamics of interfacial water at the water–vapor interface is essential for understanding the processes occurring on the water surface. At the water interface the network of very strong intermolecular interactions, hydrogen‐bonds, is interrupted and the density of water is reduced. A central question regarding water at interfaces is the extent to which the structure and dynamics of water molecules are influenced by the interruption of the hydrogen‐bonded network and thus differ from those of bulk water. Herein, we discuss recent advances in the study of interfacial water at the water–air interface using laser‐based surface‐specific vibrational spectroscopy.     

All-Atom Molecular Dynamics of Pure Water–Methane Gas Hydrate Systems under Pre-Nucleation Conditions: A Direct Comparison between Experiments and Simulations of Transport Properties for the Tip4p/Ice Water Model

(1) Background: New technologies involving gas hydrates under pre-nucleation conditions such as gas separations and storage have become more prominent. This has necessitated the characterization and modeling of the transport properties of such systems. (2) Methodology: This work explored methane hydrate systems under pre-nucleation conditions. All-atom molecular dynamics simulations were used to quantify the performance of the TIP4P/2005 and TIP4P/Ice water models to predict the viscosity, diffusivity, and thermal conductivity using various formulations. (3) Results: Molecular simulation equilibrium was robustly demonstrated using various measures. The Green–Kubo estimation of viscosity outperformed other formulations when combined with TIP4P/Ice, and the same combination outperformed all TIP4P/2005 formulations. The Green–Kubo TIP4P/Ice estimation of viscosity overestimates (by 84% on average) the viscosity of methane hydrate systems under pre-nucleation conditions across all pressures considered (0–5 MPag). The presence of methane was found to increase the average number of hydrogen bonds over time (6.7–7.8%). TIP4P/Ice methane systems were also found to have 16–19% longer hydrogen bond lifetimes over pure water systems. (4) Conclusion: An inherent limitation in the current water force field for its application in the context of transport properties estimations for methane gas hydrate systems. A re-parametrization of the current force field is suggested as a starting point. Until then, this work may serve as a characterization of the deviance in viscosity prediction.     

Developing Efficient Small Molecule Acceptors with sp2-Hybridized Nitrogen at Different Positions by Density Functional Theory Calculations,Molecular Dynamics Simulations and Machine Learning

Chemical structure of small molecule acceptors determines their performance in organic solar cells. Multiscale simulations are necessary to avoid trial-and-error based design, ultimately to save time and resources. In current study, the effect of sp²-hybridized nitrogen substitution at the inner or the outmost position of central core, side chain, and terminal group of small molecule acceptors is investigated using multiscale computational modelling. Quantum chemical analysis is used to study the electronic behavior. Nitrogen substitution at end-capping has significantly decreased the electron-reorganization energy. No big change is observed in transfer integral and excited state behavior. However, nitrogen substitution at terminal group position is good way to improve electron-mobility. Power conversion efficiency (PCE) of newly designed acceptors is predicted using machine learning. Molecular dynamics simulations are also performed to explore the dynamics of acceptor and their blends with PBDB-T polymer donor. Florgy-Huggins parameter is calculated to study the mixing of designed small molecule acceptors with PBDB-T. Radial distribution function has indicated that PBDB-T has a closer packing with N3 and N4. From all analysis, it is found that nitrogen substitution at end-capping group is a better strategy to design efficient small molecule acceptors.