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Atomistic simulations provide a meaningful way to determine the physicochemical properties of liquids in a consistent theoretical framework. This approach takes on a particular usefulness for the study of molten carbonates, in a context where thermodynamic and transport data are crucially needed over a large domain of temperatures and pressures (to ascertain the role of these melts in geochemical processes) but are very scarce in the literature, especially for the calcomagnesian compositions prevailing in the Earth's mantle. Following our work on Li2CO3–Na2CO3–K2CO3 melts, we extend our force field to incorporate Ca and Mg components. The empirical interaction potentials are benchmarked on the density data available in the experimental literature [for the crystals and the K2Ca(CO3)2 melt] and on the liquid structure issued from ab initio molecular dynamics simulations. Molecular dynamics simulations are then performed to study the thermodynamics, the microscopic structure, the diffusion coefficients, the electrical conductivity, and the viscosity of molten Ca,Mg-bearing carbonates up to 2073 K and 15 GPa. Additionally, the equation of state of a Na–Ca–K mixture representative of the lavas emitted at Ol Doinyo Lengai (Tanzania) is evaluated. The overall agreement between the MD results and the existing experimental data is very satisfactory and provides evidence for the ability of the force field to accurately model any MgCO3–CaCO3–Li2CO3–Na2CO3–K2CO3 melt over a large T–P range. Moreover, it is the first report of a force field allowing us to study the transport properties of molten magnesite (MgCO3) and molten dolomite [CaMg(CO3)2]. [ABSTRACT FROM AUTHOR]

Although molten carbonates only represent, at most, a very minor phase in the Earth's mantle, they are thought to be implied in anomalous high-conductivity zones in its upper part (70–350 km). Besides, the high electrical conductivity of these molten salts is also exploitable in fuel cells. Here, we report quantitative calculations of their properties, over a large range of thermodynamic conditions and chemical compositions, which are a requisite to develop technological devices and to provide a better understanding of a number of geochemical processes. To model molten carbonates by atomistic simulations, we have developed an optimized classical force field based on experimental data of the literature and on the liquid structure issued from ab initio molecular dynamics simulations performed by ourselves. In implementing this force field into a molecular dynamics simulation code, we have evaluated the thermodynamics (equation of state and surface tension), the microscopic liquid structure and the transport properties (diffusion coefficients, electrical conductivity, and viscosity) of molten alkali carbonates (Li2CO3, Na2CO3, K2CO3, and some of their binary and ternary mixtures) from the melting point up to the thermodynamic conditions prevailing in the Earth's upper mantle (∼1100–2100 K, 0–15 GPa). Our results are in very good agreement with the data available in the literature. To our knowledge, a reliable molecular model for molten alkali carbonates covering such a large domain of thermodynamic conditions, chemical compositions, and physicochemical properties has never been published yet. [ABSTRACT FROM AUTHOR]

Author(s): Philippe Sarda (corresponding author) [1, 3]; Bertrand Guillot [2] Degassing of the Earth is still poorly understood, as is the large scatter in He/Ar ratios observed in mid-ocean ridge [...]

With the purpose of clarifying a number of points raised in the experimental literature, we investigate by molecular dynamics simulation the thermodynamics, the structure and the vibrational properties of vapor-deposited amorphous ice (ASW) as well as the phase transformations experienced by crystalline and vitreous ice under ion bombardment. Concerning ASW, we have shown that by changing the conditions of the deposition process, it is possible to form either a nonmicroporous amorphous deposit whose density (∼1.0 g/cm3) is essentially invariant with the temperature of deposition, or a microporous sample whose density varies drastically upon temperature annealing. We find that ASW is energetically different from glassy water except at the glass transition temperature and above. Moreover, the molecular dynamics simulation shows no evidence for the formation of a high-density phase when depositing water molecules at very low temperature. In order to model the processing of interstellar ices by cosmic ray protons and heavy ions coming from the magnetospheric radiation environment around the giant planets, we bombarded samples of vitreous ice and cubic ice with 35 eV water molecules. After irradiation the recovered samples were found to be densified, the lower the temperature, the higher the density of the recovered sample. The analysis of the structure and vibrational properties of this new high-density phase of amorphous ice shows a close relationship with those of high-density amorphous ice obtained by pressure-induced amorphization. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

By performing molecular dynamics simulations we have generated several forms of high density amorphous (HDA) water (ρ>=1.1 g/cm3), either by pressurizing cubic ice or by quenching liquid water maintained under pressure. It is shown that the temperature of amorphization is a key variable in understanding the thermodynamic and structural properties of the resulting amorphous phase. Thus, the higher the temperature of amorphization the denser the amorphous ice recovered at low temperature. Two limiting forms of high density are identified, a less dense one obtained by pressure-induced amorphization of cubic ice at low temperature which can be assigned to the HDA phase commonly investigated experimentally, and a denser form obtained by vitrifying the liquid under pressure whose structure is analogous to the VHDA phase recently discovered. Upon heating, both phases transform into low density phases (LDA[sub I] and LDA[sub II]), respectively, with slightly different properties. In particular, it is found that the LDA[sub I] phase exhibits thermodynamic, structural, and dynamical properties very close to that of hyperquenched glassy water. These findings can be understood by analyzing the relative displacements of the molecules with respect to their original positions in the sample at the beginning of the amorphization process. This study supports the idea of a continuity of metastable amorphous phases in low temperature water. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

To quantify the role played by the proton transfer reaction (NH[sub 4][sup +]+Cl[sup -][=]NH[sub 3]+HCl) on the phase behavior of ammonium chloride (NH[sub 4]Cl) we have developed a simple interaction potential (independent atom model) to implement in a molecular dynamics simulation (MD). In using one phase and two phase MD simulations, the solid–solid and solid–fluid coexistence lines have been located. When the proton transfer reaction is found to play no role in the stability and coexistence of the solid phases (ionic), the occurrence of covalent species (NH[sub 3], HCl) in the fluid phase has an important influence on the melting process and on the solid–fluid equilibria. In particular, a solid–fluid coexistence is found between a purely ionic solid and a fluid mixture composed of ionic and covalent species in nearly equal proportions. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In order to shed some light on the peculiar liquid–vapor phase transition of ammonium chloride (NH[sub 4]Cl), we make use of a recently developed model potential (see paper I) accounting for the proton transfer reaction (NH[sub 4][sup +]+Cl[sup -][=]NH[sub 3]+HCl) to investigate by classical molecular dynamics simulation the phase diagram of NH[sub 4]Cl in the fluid range. A key result of the simulation is that a phase coexistence is found between a liquid mixture composed of ionic (NH[sub 4][sup +],Cl[sup -]) and covalent (NH[sub 3],HCl) species in nearly equal proportion and a vapor composed exclusively of molecules (NH[sub 3]), findings which explain the conductivity data of the literature. Although the agreement with the experimental coexistence curve is only semiquantitative, the simulation leads to a decisive improvement compared with current theoretical approaches. In addition, the evolution of the reaction kinetics and of the composition of the fluid with the thermodynamic conditions are evaluated, and the sequence of events which accompany the proton transfer is analyzed in detail. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates the quantum effects in water by implementing the Feynman-Hibbs effective potential in a molecular dynamics code. Description of reference pair potential and its thermodynamic properties; Temperature and density of thermodynamics; Comparison of the structure and properties of two isotopic species.

The solubility of rare gases in a simulated model of fused silica is evaluated by the test particle method. It is shown that the order of magnitude of the rare gas solubility in liquid silica is mainly governed by the entropy of cavity formation and only marginally by the solvation energy of the solute in the melt. Hence, the hierarchy of solubilities is dictated by the size of the noble gases: The smaller the atom the higher the solubility. Moreover, the solubility exhibits only a moderate temperature dependence at fixed density. In silica glass the same solubility hierarchy is found (in agreement with the experimental data) although the energetic contribution to the absorption process may become significant, particularly when the temperature decreases. These results in silica are in strong contrast with those obtained in liquid water where the energetic contribution to the rare gas solubility is large enough to overcompensate the entropic loss of cavity formation, the net result being that light noble gases (e.g., He) are less soluble than heavier ones (e.g., Xe). These contrasting behaviors are explained by pointing out that the liquid phase occurs in very different temperature ranges for these two systems (SiO2 and H2O) while the rare gas–solvent interaction energy is essentially of the same order of magnitude. Finally, the structure of silica around helium and neon atoms is found to be interstitial (cristobalitelike) while in the case of heavier rare gases, the structure of cavities is more reminiscent of the polyhedral arrangement encountered with clathrasils. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In using a modified version of the interionic potential of Tsuneyuki, Tsukada, Aoki, and Matsui [Phys. Rev. Lett. 61, 869 (1988)] for silica we have evaluated by molecular dynamics simulation the liquid–vapor coexistence curve of this model fluid. Although the critical point is located at a very high temperature (Tc=11 976 K, ρc=0.58 g/cm3, and Pc∼2 kbar) as expected for an ionic fluid, the overall shape of the coexistence curve is found to be very similar to that of a molecular fluid and especially close to that of water in the low temperature region (i.e., near and below the boiling point). A detailed comparison with available experimental data on silica melts and glasses is also presented. An analysis of the evolution of the silica structure with temperature shows that the tetrahedral SiO4 units subsist into the simulated melt up to approximately 8000 K and that the network formed by corner-sharing SiO4 tetrahedra is disrupted by a considerable extent only above this temperature. It is stressed that similar behavior has been recently reported in another network former liquid (i.e., water). Finally it is shown that the nature of the structural transformations exhibited by the simulated silica under very high pressure (up to the megabar range) may provide some clues to improve our knowledge about the constitution and the electrical conductivity of the earth’s mantle. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The liquid–vapor coexistence curve of NaCl is evaluated by molecular dynamics simulations in using the well-known Born–Huggins–Mayer–Fumi–Tosi (BHMFT) interionic potential. Due to the limited size of our sample (Nions=512), the calculated isotherms present van der Waals loops which can be described by an empirical equation of state whose critical parameters are Tc=3068 K, ρc=0.174 g/cm3, and Pc=105.4 bar. By comparing with the available experimental data on molten NaCl and according to corresponding states arguments, we are able to deduce from our simulation data an estimation of the critical parameters of real NaCl, namely, Tc=3300 K, ρc=0.18 g/cm3, and Pc=325 bar. The electrical conductivity of our simulated molten salt is then evaluated along the coexistence curve, in the highly compressed liquid and in the dilute gas. No evidence for an insulator-ionic conductor transition is found. Instead a continuous transition between a highly conducting fluid at high density and a low conducting gas at low temperature is pointed out. The degree of dissociation is obtained from the conductivity data and from the knowledge of the self-diffusion coefficients through a generalized Nernst–Einstein relation. The evolution of the degree of dissociation with density and temperature is corroborated by the analysis of the charge–charge distribution functions which show the occurrence of ion pairing at low density and low temperature. Finally, the mean field type behavior exhibited by our simulated molten salt is discussed in perspective with the available experimental data for other fused salts as well as with the current state of the theory concerning the criticality in ionic fluids. [ABSTRACT FROM AUTHOR]

The liquid–vapor coexistence curve of a model water (the extended simple point charge model, SPCE) is evaluated by molecular dynamics simulation in the (N,V,E) ensemble. It is shown that the simulated system (N=256 water molecules) is too small to present a spinodal decomposition and, hence, can be described by a classical equation of state whose the critical parameters (Tc=651.7 K, ρc=0.326 g/cm3, and Pc=189 bar) are found to be very close to that of real water (Tc=647.13 K, ρc=0.322 g/cm3, and Pc=220.55 bar). The critical parameters for SPCE water in the thermodynamic limit are deduced from the simulation data employing Wegner type expansions for the order parameter and the coexistence curve diameter; here also the values of the critical parameters (Tc=640 K, ρc=0.29 g/cm3, and Pc=160 bar) are close to that of real water. The temperature dependence of the dielectric constant for water and steam at orthobaric densities is next evaluated between ambient and Tc; the agreement with the experimental data is quite remarkable (e.g., εSPCE=81.0 at 300 K and εSPCE=6. at Tc instead of 78.0 and 5.3, respectively, in real water). The modifications experienced by water’s architecture with the temperature are deduced from the evaluation of the atom–atom correlation functions. It is shown that a structural change occurs in the temperature range 423–473 K. This important reorganization is characterized by a shift of the second shell of neighbors from 4.5 to 5.5 A and the loss of almost all angular correlations beyond the first solvation shell. Moreover, it is observed that the average number of hydrogen bonds per molecule nHB scales with the density all along the saturation curve. In the same way the values of nHB for orthobaric densities seems to follow a law analogous to the law of rectilinear diameter for orthobaric densities. [ABSTRACT FROM AUTHOR]

The far infrared spectrum of liquid water at room temperature is calculated by molecular dynamics simulation over the spectral range 0.5–1000 cm-1. It is shown that the experimental absorption intensity can be reproduced satisfactorily provided that the dipole induced dipole mechanism is conveniently implemented in the calculation and the classical profile corrected for quantum effects. The contribution due to exchange overlap dipoles between O and H atoms is also investigated but its role in the genesis of the far infrared (FIR) spectrum is negligible. Although the dipole induced dipole (DID) mechanism is found to be responsible for the peculiar band shape near 200 cm-1 by revealing the intermolecular oscillations of the hydrogen bond network, no other translational band is detected in the region 10–60 cm-1, a result in contradistinction with data put forward recently. Moreover, it is shown that the absorption spectrum is the seat of various cancellation effects between permanent and induced dipoles, effects which are described in detail. [ABSTRACT FROM AUTHOR]

The far infrared spectra of solutions of a number of alkali halide salts in protic (methanol) and dipolar aprotic solvents (acetonitrile, acetone, dimethyl sulfoxide) have been investigated in the frequency range 25–650 cm-1. Special attention has been paid to detect weak bands in making the difference between the absorption spectrum of the solution and that of the pure solvent. These spectra are characterized by several absorption peaks spread all over the FIR spectral range. Introducing a chemical model of the electrolyte solution, it is shown that the time correlation function at the origin of the absorption phenomenon is governed essentially by three processes, namely, (i) the ion-induced dipole mechanism, (ii) the ionic association, and (iii) the relaxation of the solvent molecules within the ionic solvation shell. A band shape analysis, based upon the Mori theory, permits us to identify the molecular motions which give rise to specific frequency modes on the absorption spectrum. In particular, the role played by the time scale separation between the fast dynamics occuring within the first solvation shell of ionic species and the time spent to interconvert inner and outer solvation shells, is emphasized. [ABSTRACT FROM AUTHOR]

A model calculation is presented to evaluate the long range dispersion dipole occurring when three nonoverlapping dissimilar atoms interact. Deviation from pairwise additivity is taken into account by a simplified theory based on electrostatic arguments. By comparing the long range triplet dispersion dipole with the one resulting from exchange overlap effects previously investigated [J. Chem. Phys. 90, 650 (1989)], it is shown that a cancellation between both contributions occurs in the vicinity of the van der Waals separation. The far infrared spectra of dense rare gas fluids are next generated by molecular dynamics simulation including long range dispersion and short range overlap dipoles. An estimate of the expected absorption in liquid krypton is given, αmax =3.6×10-9 cm-1 am-2, which might give some hope to experimentalists. As far as rare gas mixtures are concerned, the model calculation reproduces surprisingly well the experimental absolute spectral density of a Kr–Ar liquid mixture (15% Kr, 135 K) over a large domain of frequency but fails to reproduce the low frequency dip. Nevertheless, irreducible three-body dipoles contribute significantly to the spectrum. The difficulty of obtaining accurate results from an approximate model is emphasized and a guideline for further improvements is sketched. [ABSTRACT FROM AUTHOR]

A theory is proposed to evaluate the induced-dipole moment occuring when three dissimilar atoms are mutually interacting. Based on the one-effective electron model, the theory predicts that, at short and intermediate distances, the triatomic dipole moment originates from three different processes, namely, overlap, quadrupole-induced, and dipole-induced. These three-body dipoles have then been implemented into a molecular dynamics simulation in order to generate the collision-induced absorption spectra by rare-gas mixtures. For Ar–Kr liquid mixtures, the irreducible three-body contributions to the spectral density are found so important that they exceed two-body contributions. This is due to a profound cancellation between two-body dipoles which does not occur between irreducible three-body dipoles. However, the comparison with experimental data is poor because of a shortcoming of the model calculation which does not take into account long-range dispersion interactions. On the contrary, a quantitative agreement is obtained for the fundamental vibration band of compressed H2 –He mixtures, indicating that exchange overlap, three-body dipoles play the leading role for such light systems. [ABSTRACT FROM AUTHOR]

The far infrared absorption spectrum of dense rare gas mixtures is analyzed in terms of two-, three-, and four-body contributions. It is argued that pair, triplet, and quadruplet terms must be accurately known at short times, in order to describe the overall spectral shape, since the long time region, where diffusion processes can occur, is drastically reduced by the range of the interaction and the cancellation effect. A model calculation is presented where each piece of the total correlation function C(t) is expressed as a Gram–Charlier series. The parameters are then estimated with the help of a lattice gas model which includes a correction to the Kirkwood superposition approximation in the case of three-body correlations. The dip in the reduced spectral density is interpreted as a dynamical cancellation effect between two-, three-, and four-body correlation functions, the role played by the cage effect at liquid density being exacerbated by the cancellation. The width and the depth of the dip are found to be strongly dependent on the mass ratio, density, and concentrations, and some comparisons with experimental data are made for Ne–Ar and Kr–Ar liquid mixtures. Finally, the phenomenological interpretation of the dip given by Van Kranendonk and Lewis in the 1970’s (intercollisional interference effect at low density) is discussed in detail and the possible relationship with the present theory is examined. [ABSTRACT FROM AUTHOR]