过冷液体钾形核初期微观动力学机理的模拟研究

采用分子动力学方法对液态金属钾凝固过程进行了模拟,根据凝固过程体系平均原子能量、原子成键类型和成团类型,以及均方位移和非Gauss参数等动力学参数的演化特征,对过冷熔体形核初期微观动力学机理进行了研究.结果表明:根据过冷液体钾结晶形核过程热力学、动力学和结构特性的演化规律, 其过冷温度区间可以分为两个明显不同的阶段,潜在结晶核心出现在过冷液体较低温区.过冷熔体钾在形核初期,二十面体团簇结构在α-弛豫阶段逐渐解体,同时具有体心立方(bcc)结构的潜在结晶核心逐步形成,其临界晶核包含约300个原子.     

Dynamics of hydrogen bonds in water and consequences for the unusual behaviour of supercooled water

The dynamics of liquid water is evaluated by the coherent quasi-elastic scattering at two different momentum transfers, in order to discriminate hydrogen bond lifetime from molecular dynamics. The results indicate a possible issue for the puzzle of the behaviour of supercooled water.      

Density and bond-orientational relaxations in supercooled water

ABSTRACT

Recent computational studies have reported evidence of a metastable liquid–liquid phase transition (LLPT) in molecular models of water under deeply supercooled conditions. A competing hypothesis suggests, however, that non-equilibrium artefacts associated with coarsening of the stable crystal phase have been mistaken for an LLPT in these models. Such artefacts are posited to arise due to a separation of time scales in which density fluctuations in the supercooled liquid relax orders of magnitude faster than those associated with bond-orientational order. Here, we use molecular simulation to investigate the relaxation of density and bond-orientational fluctuations in three molecular models of water (ST2, TIP5P and TIP4P/2005) in the vicinity of their reported LLPT. For each model, we find that density is the slowly relaxing variable under such conditions. We also observe similar behaviour in the coarse-grained mW model of water. Our findings, therefore, challenge the key physical assumption underlying the competing hypothesis.     

A molecular dynamics study of the swelling patterns of Na/Cs-montmorillonites and the hydration of interlayer cations

We report on a molecular dynamics study of the swelling patterns of an Na-rich/Cs-poor montomorillonite and a Csmontomorillonite.The recently developed CLAYFF force field is used to predict the basal spacing as a function of the water content in the interlayer.The simulations reproduce the swelling patterns of the Na and Cs-montomorillonite,suggesting a mechanism of its hydration different from that of the montomorillonite.In addition,we find that the differences in size and hydration energy of Na and Cs ions have strong implications for the structure and the internal energy of interlayer water.In particular,our results indicate that the hydrate difference in the presence of coexistent Na and Cs has a larger influence on the behavior of a clay-water system.For Na-rich/Cs-poor montomorillonite,the hydration energy values of Na ions and water molecules each have a dramatic increase compared with those in Na-montomorillonite on the interlayer spacing,and the hydration energy values of Cs ions and water molecules decrease somewhat compared with those in Cs-montomorillonite.     

水溶液中结合水的定义与量化

水溶液中溶质的结合水具有不同于远离溶质的自由水的结构和性质.结合水的存在对水和溶质结构和动力学性质均具有显著甚至决定性的影响.然而,对结合水动力学和热力学性质的定量理解在诸多方面一直存在争议甚至严重分歧,其中重点包括如何定义和量化结合水,如何表征结合水和自由水的动力学差别,结合水如何参与生物大分子各种生物功能过程,以及溶质或界面影响结合水结构与性质的途径等.给出结合水定义的物理学依据和量化方法,是深入理解上述问题的第一步.本文简述了各种不同谱学方法定义结合水的基本原理及量化的困难,强调具有不同时间和空间响应尺度的测试方法所得结合水数不必完全可比.此外,系列水溶液物性随浓度升高会明显改变其浓度依赖关系,相应拐点浓度常被用于量化稀溶液中的溶质结合水数.我们近期研究的水溶液玻璃化转变温度-浓度关系,为结合水的定义、量化和水溶液的三区划分提供了物理依据,同时揭示了上述利用性质-浓度关系拐点浓度量化结合水方法的不足.     

Molecular dynamics study on water self-diffusion in aqueous mixtures of methanol,ethylene glycol and glycerol: investigations from the point of view of hydrogen bonding

Molecular dynamics simulations have been performed to investigate the aqueous binary mixtures of alcohols, including methanol, ethylene glycol (EG) and glycerol of molalities ranging from 1 to 5 m at the temperatures of 273, 288 and 298 K, respectively. The primary purpose of this paper is to investigate the mechanism of water self-diffusion in water-alcohol mixtures from the point of view of hydrogen bonding. The effects of temperature and concentration on water self-diffusion coefficient are evaluated quantitatively in this work. Temperature and concentration to some extent affect the hydrogen bonding statistics and dynamics of the binary mixtures. It is shown that the self-diffusion coefficient of water molecules decreases as the concentration increases or the temperature decreases. Moreover, calculations of mean square displacements of water molecules initially with different number n of H-bonds indicate that the water self-diffusion coefficient decreases as n increases. We also studied the aggregation of alcohol molecules by the hydrophobic alkyl groups. The largest cluster size of the alkyl groups clearly increases as the concentration increases, implying the emergence of a closely connected network of water and alcohols. The clusters of water and alcohol that interacted could block the movement of water molecules in binary mixtures. These findings provide insight into the mechanisms of water self-diffusion in aqueous binary mixtures of methanol, EG and glycerol.     

Molecular dynamics study of swelling patterns of Na/Cs-montmorillonites and hydration of interlayer cations

We report on a molecular dynamics study of swelling patterns of an Na-rich/Cs-poor montomorillonite and a Cs-montomorillonite. The recently developed CLAYFF force field is used to predict the basal spacing as a function of the water content in the interlayer. The simulations reproduce the swelling patterns of the Na and Cs-montomorillonite, suggesting a mechanism of its hydration different from that of the montomorillonite. In the meanwhile, we find that the differences in size and hydration energy of Na and Cs ions have strong implications for the structure and the internal energy of interlayer water. In particular, our results indicate that the hydrate difference in the presence of coexistent Na and Cs has a larger influence on the behavior of clay-water system. For Na-rich/Cs-poor montomorillonite, the hydration energy values of Na ions and water molecules each have a dramatic increase compared with those in Na-montomorillonite on the interlayer spacing, and the hydration energy values of Cs ions and water molecules decrease somewhat compared with those in Cs-montomorillonite.     

The structure and dynamics of water inside armchair carbon nanotube

In this paper we present some simulation results about the behaviour of water molecules inside a single wall carbon nanotube (SWNT). We find that the confinement of water in an SWNT can induce a wave-like pattern distribution along the channel axis, similar phenomena are also observed in biological water channels. Carbon nanotubes(CNTs) can serve as simple nonpolar water channels. Molecular transport through narrow CNTs is highly collective because of tight hydrogen bonds in the protective environment of the pore. The hydrogen bond net is important for proton and other signal transports. The average dipoles of water molecules inside CNTs (7,7), (8,8) and (9,9) are discussed in detail. Simulation results indicate that the states of dipole are affected by the diameter of SWNT. The number of hydrogen bonds, the water--water interaction and water--CNT interaction are also studied in this paper.     

High-temperature dynamic behavior in bulk liquid water: A molecular dynamics simulation study using the OPC and TIP4P-Ew potentials

Classical molecular dynamics simulations were performed to study the high-temperature (above 300 K) dynamic behavior of bulk water, specifically the behavior of the diffusion coefficient, hydrogen bond, and nearest-neighbor lifetimes. Two water potentials were compared: the recently proposed “globally optimal” point charge (OPC) model and the well-known TIP4P-Ew model. By considering the Arrhenius plots of the computed inverse diffusion coefficient and rotational relaxation constants, a crossover from Vogel–Fulcher–Tammann behavior to a linear trend with increasing temperature was detected at T* ≈ 309 and T* ≈ 285 K for the OPC and TIP4P-Ew models, respectively. Experimentally, the crossover point was previously observed at T* ± 315–5 K. We also verified that for the coefficient of thermal expansion α _P (T, P), the isobaric α _P (T) curves cross at about the same T* as in the experiment. The lifetimes of water hydrogen bonds and of the nearest neighbors were evaluated and were found to cross near T*, where the lifetimes are about 1 ps. For T < T*, hydrogen bonds persist longer than nearest neighbors, suggesting that the hydrogen bonding network dominates the water structure at T < T*, whereas for T > T*, water behaves more like a simple liquid. The fact that T* falls within the biologically relevant temperature range is a strong motivation for further analysis of the phenomenon and its possible consequences for biomolecular systems.     

A study of cavitation nucleation in pure water using molecular dynamics simulation

To discover the microscopic mechanism responsible for cavitation nucleation in pure water, nucleation processes in pure water are simulated using the molecular dynamics method. Cavitation nucleation is generated by uniformly stretching the system under isothermal conditions, and the formation and development of cavitation nuclei are simulated and discussed at the molecular level. The processes of energy, pressure, and density are analyzed, and the tensile strength of the pure water and the critical volume of the bubble nuclei are investigated. The results show that critical states exist in the process of cavitation nucleation. In the critical state, the energy, density, and pressure of the system change abruptly, and a stable cavitation nucleus is produced if the energy barrier is broken and the critical volume is exceeded. System pressure and water density are the key factors in the generation of cavitation nuclei. When the critical state is surpassed, the liquid is completely ruptured, and the volume of the cavitation nucleus rapidly increases to larger than 100 nm³; at this point, the surface tension of the bubble dominates the cavitation nucleus, instead of intermolecular forces. The negative critical pressure for bubble nucleation is -198.6 MPa, the corresponding critical volume is 13.84 nm³, and the nucleation rate is 2.42×10³² m^-3·^-1 in pure water at 300 K. Temperature has a significant effect on nucleation: as the temperature rises, nucleation thresholds decrease, and cavitation nucleation occurs earlier.     

Swelling of K+, Na+ and Ca2+-montmorillonites and hydration of interlayer cations: a molecular dynamics simulation

This paper performs molecular dynamics simulations to investigate the role of the monovalent cations K, Na and the divalent cation Ca on the stability and swelling of montmorillonite. The recently developed CLAYFF force field is used to predict the basal spacing as a function of the water content in the interlayer. The simulations reproduced the swelling pattern of these montmorillonites, suggesting a mechanism of their hydration different (K+ 相似文献   

Peptide friction in water nanofilm on mica surface

Peptide frictions in water nanofilms of various thicknesses on a mica surface are studied via molecular dynamics simulations. We find that the forced lateral motion of the peptide exhibits stick-slip behaviour at low water coverage; in contrast, the smooth gliding motion is observed at higher water coverage. The adsorbed peptide can form direct peptide-surface hydrogen bonds as well as indirect peptide-water-surface hydrogen bonds with the substrate. We propose that the stick-slip phenomenon is attributed to the overall effects of direct and indirect hydrogen bonds formed between the surface and the peptide.     

Structure and dynamics of ordered water in a thick nanofilm on ionic surfaces

The structure and dynamics of water in a thick film on an ionic surface are studied by molecular dynamic simulations. We find that there is a dense monolayer of water molecules in the vicinity of the surface. Water molecules within this layer not only show an upright hydrogen-down orientation, but also an upright hydrogen-up orientation. Thus, water molecules in this layer can form hydrogen bonds with water molecules in the next layer. Therefore, the two-dimensional hydrogen bond network of the first layer is disrupted, mainly due to the O atoms in this layer, which are affected by the next layer and are unstable. Moreover, these water molecules exhibit delayed dynamic behavior with relatively long residence time compared with those bulk-like molecules in the other layers. Our study should be helpful to further understand the influence of water film thickness on the interfacial water at the solid-liquid interface.     

A comparative study of optic phonon-like excitations in liquid mixtures and molten salts

We report a study of collective excitations in an equimolar Lennard–Jones liquid mixture KrAr and a molten salt NaCl within the parameter-free generalized collective modes (GCM) approach. It is shown that the high-frequency propagating modes in liquid KrAr and molten NaCl correspond to optic phonon-like excitations, caused by fast mass-concentration (charge in NaCl) fluctuations. Dispersion curves for optic collective excitations are discussed.     

A classical molecular dynamics study of a Diels Alder cycloaddition in supercritical water

Molecular dynamics simulations were performed to investigate the Diels Alder cycloaddition of cyclopentadiene and methyl vinyl ketone in high pressure, high temperature water. It was found that the reaction was favoured by high temperatures at 1000?atm due to increasing entropy. Similarly, at 400?atm, the entropy caused both the equilibrium and rate constant to increase to a peak at 698?K before rapidly falling once more with increasing temperature. At a constant temperature of either 598?K or 898?K, increasing pressure resulted in a lowering of the equilibrium constant. This effect was significantly more pronounced for 898?K, caused by less favourable solvation of the products and an increasing amount of work required for reaction.     

Molecular dynamics and quantum chemical approaches in the study of the hydration of protonated cyclohexyldiamines

Explicit hydration of the neutral and charged cyclohexylamine and of the cyclohexyldiamine isomers in their mono- or diprotonated forms is investigated through classical molecular dynamics (MD) simulations in aqueous solutions combined with DFT calculations in amine–water complexes. The MD studies performed in the monoamines reveal that the structure of the hydration shell around the neutral amino group (NH₂) is quite distinct from that around the charged one (NH₃⁺). On average, the number of water molecules surrounding the two groups is calculated to be ～2 and 3–4, respectively. The variation of the hydration structure prompted by the groups’ proximity is discussed based on the data found for the mono- and diprotonated diamines. To have a more detailed picture of the water molecules’ arrangement around the amino groups and of the amine–water hydrogen bonds, geometry optimisations in hydrates with up to six water molecules are carried out at the B3LYP/aug-cc-pVDZ level. Complexation energies are also computed. The main findings emerging from these calculations are found to be very helpful to rationalise the mutual influence of the amino groups and therefore to better elucidate the MD findings. The complementary nature of the two research methods is emphasised as an excellent tool in order to closely examine the hydration of polyamines, as exemplified for the cyclohexyldiamines.     

A molecular dynamics simulation study on the cavitation inception of water with dissolved gases

The promotion/prevention mechanism of dissolved gases on cavitation inception is essential for many high-tech industries and research. In the present study, large-scale molecular dynamics simulations are performed to investigate the effects of water cavitation caused by different gas types by using nitrogen and oxygen gases with TIP4P/2005 water. The cavitation inception behaviour is analyzed via Mean First Passage Time method. Water with dissolved gases has a higher nucleation rate and is easier to cavitate than pure water. At the same gas concentration, the cavitation of water with nitrogen is promoted to a greater extent than that with oxygen. The number and energy of hydrogen bond (HB) are further calculated by the Acceptor-Hydrogen-Donor method to explain this promotion mechanism. The number and energy of HB in water with gases decrease compared with those in pure water. The introduction of gases weakens the HB network and promotes cavitation inception because of weaker interactions between gas and water molecules. A model is developed to describe the relationship between nucleation rate and HB energy. Gas molecules assemble on the surface of bubbles during water cavitation, which may decrease the free energy of bubble surface, maintain the existing bubble, and contribute to the growth process.     

Aqueous electrolyte surfaces in strong electric fields: molecular insight into nanoscale jets and bridges

Exposing aqueous surfaces to a strong electric field gives rise to interesting phenomena, such as formation of a floating water bridge or an eruption of a jet in electrospinning. In an effort to account for the phenomena at the molecular level, we performed molecular dynamics simulations using several protocols on both pure water and aqueous solutions of sodium chloride subjected to an electrostatic field. All simulations consistently point to the same mechanisms which govern the rearrangement of the originally planar surface. The results show that the phenomena are primarily governed by an orientational reordering of the water molecules driven by the applied field. It is demonstrated that, for pure water, a sufficiently strong field yields a columnar structure parallel to the field with an anisotropic arrangement of the water molecules with their dipole moments aligned along the applied field not only in the surface layer but over the entire cross section of the column. Nonetheless, the number of hydrogen bonds per molecule does not seem to be affected by the field regardless of its strength and molecule’s orientation. In the electrolyte solutions, the ionic charge is able to overcome the effect of the external field tending to arrange the water molecules radially in the first coordination shell of an ion. The ion–water interaction interferes thus with the water–electric field interaction, and the competition between these two forces (i.e., strength of the field versus concentration) provides the key mechanism determining the stability of the observed structures.     

Relaxation dynamics and power spectra of liquid water: a molecular dynamics simulation study

We present molecular dynamics simulations of liquid water at normal and supercooled conditions. Autocorrelation functions (ACFs) of several structural quantities and their fourier transforms are obtained and analysed. Structural correlations and relaxation times increase linearly with degree of supercooling. Power spectra of ACFs show increase in librational motion of liquid water with cooling. These modes intensify with supercooling because of structuring and ordering of water molecules. Overall, liquid water structure is homogenous over the temperatures and pressures studied and undergoes fluctuation–dissipation in its local-density variations [English and Tse, Phys. Rev. Lett. 106, 037801 (2011)].