Guang-Jun Guo

Can the dodecahedral water cluster naturally form in methane aqueous solutions? A molecular dynamics study on the hydrate nucleation mechanisms

By performing a large scale of molecular dynamics simulations, we analyze 60 x 10(6) hydration shells of methane to examine whether the dodecahedral water cluster (DWC) can naturally form in methane aqueous solutions--a fundamental question relevant to the nucleation mechanisms of methane hydrate. The analyzing method is based on identifying the incomplete cages (ICs) from the hydration shells and quantifying their cagelike degrees (zetaC=0-1). Here, the zetaC is calculated according to the H-bond topological network of IC and reflects how the IC resembles the complete polyhedral cage. In this study, we obtain the zetaC distributions of ICs in methane solutions and find the occurrence probabilities of ICs reduce with zetaC very rapidly. The ICs with zetaC>or=0.65 are studied, which can be regarded as the acceptable cagelike structures in appearance. Both increasing the methane concentration and lowering the temperature can increase their occurrence probabilities through slowing down the water molecules. Their shapes, cage-maker numbers, and average radii are also discussed. About 13-14 of these ICs are face saturated, meaning that every edges are shared by two faces. The face-saturated ICs have the potential to act as precursors of hydrate nucleus because they can prevent the encaged methane from directly contacting other dissolved methane when an event of methane aggregation occurs. The complete cages, i.e., the ICs with zetaC=1, form only in the solutions with high methane concentration, and their occurrence probabilities are about 10(-6). Most of their shapes are different from the known hydrate cages, but we indeed observe a standard 5(12)6(2) hydrate cage. We do not find the expected DWC, and its occurrence probability is estimated to be far less than 10(-7). Additionally, the IC analysis proposed in this work is also very useful in other studies not only on the formation, dissociation, and structural transition of hydrates but also on the hydrophobic hydration of apolar solutes.

Lifetimes of cagelike water clusters immersed in bulk liquid water: A molecular dynamics study on gas hydrate nucleation mechanisms

Molecular dynamics simulations were performed to observe the evolution of cagelike water clusters immersed in bulk liquid water at 250 and 230 K. Totally, we considered four types of clusters--dodecahedral (5(12)) and tetrakaidecahedral (5(12)6(2)) cagelike water clusters filled with or without a methane molecule, respectively. The lifetimes of these clusters were calculated according to their Lindemann index (delta) using the criterion of delta> or =0.07. The lifetimes of the clusters at 230 K are longer than that at 250 K, and their ratios are the same as the ratio of structure relaxation times of bulk water at these temperatures. For both the filled and empty clusters, the lifetimes of 5(12)6(2) cagelike clusters are similar to that of 5(12) cagelike clusters. Although the methane molecules indeed make the filled cagelike water clusters live longer than the empty ones, the empty cagelike water clusters still have the chance of being long lived. These observations support the cluster nucleation hypothesis for the formation mechanisms of gas hydrates.

METALLOGENIC GEODYNAMIC BACKGROUND OF MESOZOIC GOLD DEPOSITS IN GRANITE-GREENSTONE TERRAINS OF NORTH CHINA CRATON

The spatial distribution map of 65 mid-large gold-deposits hosted in the granite-greenstone terrains of the North China Craton is first drawn. These gold deposits mainly concentrate in the Mesozoic remobilized Yinshan-Yan-shan-Liaoning-Jilin intracontinental collisional orogenic belt, the northern Qinling and the Jiaodong Mesozoic collisional orogenic belts, and the Mesozoic intracontinental fault-magmatic belts developed along the Taihangshan and the Tan-Lu faults; their mineralizing time is predominantly Jurassic-Cretaceous, i. e. the Yanshanian. The metallogenic geodynamic background is exactly the compression-to-extension transition regime during continental collision.

Equilibrium molecular dynamics calculation of the bulk viscosity of liquid water

Twenty independent equilibrium molecular dynamics simulations were performed in NVE ensemble to calculate the bulk viscosity of water at a temperature of 303 K and a density of 0.999 gcm−3. The energy of each simulation with a production time of 200ps was conserved within 1 part in 104. By stopping the velocity-scaling procedure at a proper step, the energies of independent simulations were specified precisely. This caused the simulations of different start configurations to sample the same NVE ensemble. The shear viscosity of SPC/E water obtained in the present study was 6.5±0.4 × 10−4 Pas, which is in close agreement with a previous calculation in the NVT ensemble (Balasubramanian, S., Mundy, C. J., and Klein, M. L., 1996, J. clzern. Phys., 105, 11 190). The bulk viscosity was 15.5 ± 1.6 × 10−4 Pas, which is 27% smaller than the experimental value. Thus, like its behaviour in predicting the shear viscosity, the SPC/E model also underestimates the bulk viscosity of real water.

Solubility of aqueous methane under metastable conditions : implications for gas hydrate nucleation

To understand the prenucleation stage of methane hydrate formation, we measured methane solubility under metastable conditions using molecular dynamics simulations. Three factors that influence solubility are considered: temperature, pressure, and the strength of the modeled van der Waals attraction between methane and water. Moreover, the naturally formed water cages and methane clusters in the methane solutions are analyzed. We find that both lowering the temperature and increasing the pressure increase methane solubility, but lowering the temperature is more effective than increasing the pressure in promoting hydrate nucleation because the former induces more water cages to form while the latter makes them less prevalent. With an increase in methane solubility, the chance of forming large methane clusters increases, with the distribution of cluster sizes being exponential. The critical solubility, beyond which the metastable solutions spontaneously form hydrate, is estimated to be ~0.05 mole fraction in this work, corresponding to the concentration of 1.7 methane molecules/nm(3). This value agrees well with the cage adsorption hypothesis of hydrate nucleation.

Finite-size effect at both high and low temperatures in molecular dynamics calculations of the self-diffusion coefficient and viscosity of liquid silica

Molecular dynamics simulations are performed at 6543 and 3310 K to investigate how the viscosity and self-diffusion coefficient scale with system size in the liquid BKS silica system. We find that at high temperature the finite-size effect on shear viscosity is negligible. However, the size effect on the diffusion coefficient still exists and scales linearly with 1/N1/3, where N is the total number of particles in the system. At low temperature, the size effect on the viscosity becomes stronger than that on diffusion, and the logarithm of the viscosity and the diffusion coefficient scale linearly with 1/N. These results are consistent with previous theoretical developments, and demonstrate that the finite-size effect should be considered in both high- and low-temperature molecular dynamics simulations of liquid silica.

Progress and records in the study of endogenetic mineralization during collisional orogenesis

To develop and perfect the theory of plate tectonics and regional metallogeny, metallogenesis during collisional orogenesis should be thoroughly studied and will attract increasing attention of more and more scientists. This paper presents the main aspects of research and discussions on metallogenesis during collisional orogenesis after the development of plate tectonics, and accordingly divides the study history into two stages, i.e. the junior stage during 1971–1990 and the senior stage after 1990. Beginning with the negation of mineralization in the collision regime by Guild (1971), the focus of study was put on whether there occurred any mineralization during collisional orogenesis at the junior stage. At the senior stage, which is initiated by the advance of metallogenic and petrogenic model for collisional orogenesis, scientists begin to pay their attention to the geodynamic mechanism of metallogenesis, spatial and temporal distribution of ore deposits, ore-forming fluidization, relationship between petrogenesis and mineralization in collisional orogenesis, etc. Abundance of typical collisional orogens such as Himalayan, China has best natural conditions to study collisional metallogenesis. Great progress in the study of metallogenesis during collisional orogenesis has been made by Chinese geologists. Therefore, we hope that the’ Chinese geologists and Chinese governments at various levels to pay more attention to the study of collisional metallogenesis. Some urgent problems are suggested to be solved so as to bring about breakthroughs in the aspects concerned.

Viscosity and stress autocorrelation function in supercooled water: a molecular dynamics study

Following Guo, G.-J., and Zhang, Y.-G., 2001, Molec. Phys., 99, 283, which calculates the bulk and shear viscosities of SPC/E water at 30°C and 0.999 g cm−3, further molecular dynamics simulations have been performed at state points of 0°C, −20°C, −40°C, and −60°C along an approximate isobar with the previous state point. SACF and BACF (stress autocorrelation functions related to shear and bulk viscosities, respectively) of high precision have been obtained and compared for their similarities and differences. Shear and bulk viscosities calculated from them showed an increased deviation from real water with decreasing temperature. These correlation functions were then fitted using a uniform two-step relaxation function including a fast oscillatory Kohlrausch law and a slow straightforward Kohlrausch law. The fitting parameters of SACF and BACF have been analysed in detail, and several interesting dynamic phenomena were observed. (1) The oscillation frequency of SACF (44 ~ 48ps−1) for short time intervals agrees with the stretching mode of hydrogen bonds, while that of BACF (7 ~ 12ps−1) agrees with the bending mode of hydrogen bonds. (2) With decreasing temperature, the slow relaxation fraction of the BACF increases, while that of the SACF remains constant. (3) The exponents β in the Kohlrausch laws with values greater than 1 are obtained for BACF at ambient temperatures. (4) With regard to both shear and bulk viscosities, the slow relaxation time largely increases with decreasing temperature, while the fast relaxation time slightly decreases. These phenomena are qualitatively explained and discussed.

Partitioning of Si and O between liquid iron and silicate melt: A two-phase ab-initio molecular dynamics study

[1] The magma ocean process in the early history of the Earth has a great influence on the light element identities and contents of the core which subsequently affect the energy of the geodynamo provided by the compositional convection and the inner core growth through their effect on the phase diagram of iron alloy. In the present work, a twophase ab-initio molecular dynamics method is established to study the solubility of silicon and oxygen in liquid iron in equilibrium with silicate melt. The ab-initio results are found to be in close agreement with experimental data. At the base of a deep magma ocean (39 GPa and 3116 K), liquid iron contains 2.7 wt% silicon and 0.5 wt% oxygen at the current bulk Earth composition. The oxygen content is low compared with its current estimate in the core, indicating a deeper magma ocean may need to be invoked. Citation: Zhang, Y., and G. Guo (2009), Partitioning of Si and O between liquid iron and silicate melt: A two-phase ab-initio molecular dynamics study, Geophys. Res. Lett., 36, L18305,

Effect of guests on the adsorption interaction between a hydrate cage and guests

The adsorption interaction between a water cage and a guest molecule, shown by the potential of mean force (PMF), is key to understanding hydrate formation mechanisms. In this work, we performed a series of PMF calculations to investigate how the guests affect adsorption interactions. We considered spherical guests for five species of inert gases (He, Ne, Ar, Kr, and Xe), and non-spherical guests for carbon dioxide, hydrogen, hydrogen sulfide, ethane, and propane. The results show that the location of the first well of PMF mainly depends on the guest size (i.e., the kinetic diameter of the guest) rather than the guest structure; and the activation energy (the free energy difference between the first well and the first barrier) mainly depends on the water–guest cross interaction and the size of the adsorption face. An empirical criterion is suggested to distinguish the adsorption face (dring/dguest 1.72) for a specific guest molecule, where dring is the diameter of the circumcircle of a water ring in a cage and dguest is the kinetic diameter of the guest. Additionally, some interesting phenomena are observed. For example, CO2 has the smallest activation energy and a predominant orientation near the adsorption face. H2S and Kr have almost the same cage–guest adsorption interaction owing to their similar size. C3H8 has the largest distance for the first barrier of PMF so as to require the smallest critical concentration to trigger hydrate nucleation according to the cage adsorption hypothesis. The present observations are very helpful to understand hydrate formation mechanisms.