摘要:

Monte Carlo molecular modelling studies of smectite–water interactions at elevated pressures and temperatures are reported. These simulations are the first to mimic conditions encountered by clays in real sedimentary basins, down to a burial depth of 9 km. To achieve this, geostatic and geothermal gradients of 1·5 x 107Pa km-1and 30°C km-1respectively are applied. The density of interlayer pore water exceeds that of the bulk, except at very low water content. Hydrated smectite is therefore enthalpically stable at burial depths of up to 1·5 km. Below this depth the remaining fluid prefers the bulk phase. This prediction is consistent with the observation that diagenetic release of interlayer water, and consequent conversion of hydrated smectite to anhydrous illite, occurs at burial depths greater than 1·5 km.

ISSN:0026-8976    

DOI:10.1080/002689797170545

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Molecular dynamics simulations have been performed on bilayers of dioctadecyldimethyl-ammonium chloride adsorbed between two solid surfaces. The separation of the layers is reduced so that the alkyl chains overlap to produce a repulsive normal pressurePzz. The layers are then sheared at a constant velocity of between 1 ms-1and 100 ms-1and the resulting pressurePxzacross the boundary and in the direction of the shear, is calculated. This method provides a measure of the friction coefficient μ = |Pxz|/Pzz. The friction coefficient has been calculated as a function of the surface density of the amphiphiles, the relative shearing velocity and the applied normal force. This friction coefficient decreases with increasing normal force, increases with decreasing amphiphile density, and increases with increasing shearing velocity. Results are given for the translational, orientational and conformational structure of the bilayer under shear.

ISSN:0026-8976    

DOI:10.1080/002689797170554

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A generalized implementation is presented of the Lekner method (Lekner, J., 1991,Physica A,176, 485) of calculating Coulomb interactions in two-dimensionally periodic systems. The generalized scheme eliminates the restriction on the shape of the primary cell, so it is now applicable to systems with non-rectangular shape. The generalized scheme retains the simplicity of the original method so it is easy to incorporate it into molecular dynamics and Monte Carlo codes. Comparisons are made with two other methods for calculating long range interactions in two-dimensionally periodic systems which are based on a Ewald type approach (Heyes, D. M., Barber, M., and Clarke, J. H. R., 1977,J. chem. Soc. Faraday Trans ii,73, 1485; Hautman, J., and Klein, M. L., 1992,Molec. Phys.,75, 379). The results show that the generalized Lekner method is superior in terms of accuracy, and that the speed of the new implementation is comparable with the method devised by Hautman and Klein.

ISSN:0026-8976    

DOI:10.1080/002689797170563

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The υ4+ υ11(Eu) infrared band of C2D6has been rotationally analysed under high resolution, between 1195 cm-1and 1335 cm-1, taking into account thex,yCoriolis coupling with υ2+ υ4(A1u. The rotational structure is affected by strongl-type interactions, with both Δl= Δk= ±2 and Δl= plusmn;2, Δk= ±1. Torsional splittings are not resolved, as expected. Several Q branches of the hot band (2υ4+ υ11)- υ4have been identified, and some of them occur as narrow features with unresolvedJstructure, due to thel-interaction mechanisms. The hotRQ1andRQ2are so sharp that the maxima of the torsional components can be resolved, and a splitting of about 0·015 cm-1is measured. This value, apparently too small, is in agreement with the fact that υ11in 2υ4+ υ11is not a pure E2dvibrational mode, due to the γ Coriolis coupling mechanisms in the CD3deformation vibration–torsion system.

ISSN:0026-8976    

DOI:10.1080/002689797170572

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Anisotropic ion–ion distribution functionsgij(R1,R2), whereR1andR2are the positions of ions of speciesiandjrespectively, and various other properties of electric double layer systems have been calculated by simulation and integral equation methods. The system is composed of a 1 : 1 or 2 : 1 electrolyte solution between two planar walls with rather high surface charge density (0·267 Cm-2) and in equilibrium with a bulk electrolyte solution with a certain concentration (1·0 M or 2·0 M). In the integral equation calculations, the hypernetted chain (HNC) or the reference hypernetted chain (RHNC) closure has been applied for the ion-ion distribution functions in the inhomogeneous electrolyte (the so-called anisotropic HNC or RHNC approximations). The Widom test particle technique has been used to calculate the ion–ion distribution functions in the simulations. The results of the anisotropic RHNC calculations and the simulations are in almost perfect agreement throughout (they virtually coincide). The HNC approximation is almost as good as the RHNC approximation for 2 : 1 electrolytes, but it is not as good for 1 : 1 electrolytes due to the higher packing density of the monovalent counterions near the surfaces. The excellent results for the pair distributions ensures that thermodynamic properties calculated in the RHNC approximation will be very good. The double layer interaction pressure, which is sensitive to details of the distributions, is indeed in excellent agreement with the simulations. The results also show that the Widom technique is successful for calculating pair distributions for inhomogeneous electrolytes by simulation.

ISSN:0026-8976    

DOI:10.1080/002689797170581

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

We calculate the bulk viscosity of a dilute molecular gas from a kinetic model which considers the internal degrees of freedom in a semiclassical way. Grad's method is implemented to solve the Wang Chang–Uhlenbeck–de Boer kinetic equations in a 12-moment approximation. Such a kinetic calculation allows us to obtain an expression for the bulk viscosity in the framework of generalized hydrodynamics. As a consequence we observe the role played by the temperature inhomogeneities and we give an estimation of the region in which the diffusion of excited molecules is the leading contribution to the bulk viscosity.

ISSN:0026-8976    

DOI:10.1080/002689797170590

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The Ewald sum method is commonly used in equilibrium simulations of polar fluids to enhance convergence of long-range Coulombic forces within modest-sized cubic simulation cells. In this work, we derive a form of the standard Ewald sum technique for use with non-equilibrium molecular dynamics (NEMD) simulations of viscosity that make use of the Lees–Edwards boundary conditions. This generalized Ewald sum can be used for any parallelepiped simulation cell. The method was tested by performing NEMD simulations at various temperatures and densities for simulated liquid methanol. The results were in excellent agreement with experimental data for methanol and Green–Kubo simulations of the viscosity using the standard cubic-cell Ewald sum. A simple truncation of the polar interactions at 10 Å was found to produce errors of over 200% in the simulated viscosities. Values obtained with the polar interactions turned off (i.e. using only dispersion forces) were generally 40–60% below the experimental values. These results show that long-range Coulombic interactions may be significant in simulated liquid viscosities and that they can be accurately handled in NEMD simulations with the proposed extension of the Ewald sum.

ISSN:0026-8976    

DOI:10.1080/002689797170608

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A 2- and 3-body model potential for the Be3cluster has been constructed. The parameters of the 2- and 3-body terms were fitted separately to the correspondingab initiomany-body energies calculated at the MP4 level. The study of the 3-body component in different triangle conformations reveals that the 3-body forces determine the magnitude of the interaction energy in a relatively large region of distances and angles. The analysis of exchange and dispersion contributions makes it possible to elucidate the nature of binding in the Be3cluster.

ISSN:0026-8976    

DOI:10.1080/002689797170617

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The reaction Cl + CH2I→ICl + CH2I has been studied at a relative collision energy of 53·2kJ mol-1using the crossed-beam technique. Angular and translational energy distributions of reactive scattered ICl products have been measured. The angular distribution is consistent with reaction occurring through a short-lived complex with lifetime about 0·8 of the rotational period. The kinetic energy distributions measured for this system show strong coupling to scattering angles. The forward- and backward-scattered products have higher translational energy than those that are sideways scattered. A phase-space model is developed which explains the experimental energy distributions well.

ISSN:0026-8976    

DOI:10.1080/002689797170626

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Newab initiosymmetry-coordinate carbon–proton and proton–proton spin–spin coupling surfaces for the methane molecule have been computed. Calculations have been performed at the SOPPA(CCSD) level using a large-basis set and a grid of 55 geometries on the two surfaces. The nuclear motion in the isotopomers CH4, CH3D, CH2D2, CHD3and CD4has been averaged over these surfaces to give values ofJ(13C, H) andJ(13C, D) for the13C isotopomers andJ(H, D) for the12C isotopomers at selected temperatures. Calculated isotope effects onJ(C, H) andJ(C, D) are very close to the values observed experimentally. This validates analysis into the various nuclear motion contributions to show that bond stretching at first order dominates and that angle bending at second order is also significant. However, the bond-stretching terms stem from the ‘other’ three bonds and there is surprisingly very little contribution from the bond containing the coupled nuclei. For the proton–deuteron coupling it is second-order bending which plays the major part but here all contributions are very small which explains why no change inJ(H, D) has ever been observed along the series CH3D, CH2D2and CHD3. The source of non-additivity in the isotope effects onJ(C, H),J(C, D) andJ(H, D) is located. It is believed that the present results are the most accurateab initioresults for the spin–spin coupling in any polyatomic molecule, possibly the most accurate for the magnetic property of any polyatomic molecule and among the most accurate for a molecular property of a pentatomic molecule.

ISSN:0026-8976    

DOI:10.1080/002689797170635

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor