Ion and water transport reasonably involves rotation and pseudorotation: measurement and modeling the temperature dependence of small-angle neutron scattering from aqueous SrI2.

X-ray and neutron scattering have provided insight into the short range (<8 Å) structures of ionic solutions for over a century. For longer distances, single scattering bands have, however, been seen. For the non-hydrolyzing salt SrI2 in aqueous (D2O) solution, a structure sufficient to scatter slow neutrons has been seen to persist down to a concentration of 0.1 mol L-1 where the measured average spacing between scatterers is over 20 Å. Theoretical studies of such long distance solution structures are difficult, and these difficulties are discussed. The width of the distribution in distances between the scatterers (ions, ion pairs, etc.) remains less than 10 Å, which approximates the average size of the ions and their first hydration shell. Here, we measure the temperature dependence from 10 °C to 90 °C of the small angle neutron scattering (SANS) by a 0.5 molar SrI2 solution in D2O and find that this surprisingly narrow distribution of the distances remains constant within experimental uncertainty. This structure of the ions in the solution appears to endure because changes in interion distances along any single spatial dimension require displacements near the size of a water molecule. Together, the experimental measurements support a rotatory mechanism for simultaneous ion transport and water countertransport. Since rotation minimizes displacement of the solution framework, it is suggested that water transport alone also involves rotation of multimolecular structures, and that the interpretation of single-molecule water rotation is confounded by pseudorotation that results from paired picosecond proton exchanges. It is pointed out that NMR-determined millisecond to microsecond proton exchange times of chelated-metal-ion bound waters and the much faster chelate rotational correlation times around 10 picoseconds, both of which require making and breaking of hydrogen bonds, are difficult to impossible to reconcile.

The Ninth Industrial Fluid Properties Simulation Challenge.

The Ninth Industrial Fluid Properties Simulation Challenge aimed to test the ability of molecular modeling approaches to predict water/oil interfacial tension (IFT) at conditions of high temperature and pressure. In particular, the challenge featured water/oil IFT where the oil was n-dodecane, toluene, or a 50:50 n-dodecane/toluene blend at 1.825 MPa and temperatures in the range of 383 K to 443 K. Seven entries were received including approaches such as molecular dynamics (MD) and Monte Carlo (MC) simulations, COSMO-RS, and iSAFT, and they were judged by comparison to pendant drop tensiometer benchmark data. The quality of predictions varied among the entries between approximately 20 % and 70 % of the total points possible with the entries based on MD and MC having the highest scores in most cases. As is often the case in molecular modeling, predictions of the relative trends tended to be reliable even if the absolute values were not.

Molecular Dynamics Simulation of Trimer Self-Assembly Under Shear.

The self-assembly of patchy trimer particles consisting of one attractive site and two repulsive sites is investigated with nonequilibrium molecular dynamics simulations in the presence of a velocity gradient, as would be produced by the application of a shear stress on the system. As shear is increased, globular-shaped micellar clusters increase in size and become more elongated. The globular clusters are also more stable at higher temperatures in the presence of shear than at equilibrium. These results help to increase our understanding of the effect of shear on self-assembly for a variety of applications.

Correlations for the Dielectric Constants of H2S, SO2, and SF6.

A new method is developed for correlating the static dielectric constant of polar fluids over wide ranges of conditions where few experimental data exist. Molecular dynamics simulations are used to establish the temperature and density dependence of the Kirkwood g-factor, and also the functional form for the increase of the effective dipole moment with density. Most parameters in the model are obtained entirely from simulation; a single proportionality constant is adjusted to obtain agreement with the limited experimental data. The method is applied to hydrogen sulfide (H2S) and sulfur dioxide (SO2), both of which are important in geochemistry but have only a few dielectric data available. The resulting correlations agree well with the available liquid data, obey physical boundary conditions at low density and at high temperature, and interpolate in density and temperature in a physically reasonable manner. In addition, we present a more conventional correlation for the dielectric constant of sulfur hexafluoride, SF6, where more data are available.

The Eighth Industrial Fluids Properties Simulation Challenge.

The goal of the eighth industrial fluid properties simulation challenge was to test the ability of molecular simulation methods to predict the adsorption of organic adsorbates in activated carbon materials. In particular, the eighth challenge focused on the adsorption of perfluorohexane in the activated carbon BAM-109. Entrants were challenged to predict the adsorption in the carbon at 273 K and relative pressures of 0.1, 0.3, and 0.6. The predictions were judged by comparison to a benchmark set of experimentally determined values. Overall good agreement and consistency were found between the predictions of most entrants.

Adsorption, X-ray Diffraction, Photoelectron, and Atomic Emission Spectroscopy Benchmark Studies for the Eighth Industrial Fluid Properties Simulation Challenge.

The primary goal of the eighth industrial fluid properties simulation challenge was to test the ability of molecular simulation methods to predict the adsorption of organic adsorbates in activated carbon materials. The challenge focused on the adsorption of perfluorohexane in the activated carbon standard BAM-P109 (Panne and Thünemann 2010). Entrants were challenged to predict the adsorption of perfluorohexane in the activated carbon at a temperature of 273 K and at relative pressures of 0.1, 0.3, and 0.6. The relative pressure (P/Po) is defined as that relative to the bulk saturation pressure predicted by the fluid model at a given temperature (273 K in this case). The predictions were judged by comparison to a set of experimentally determined values, which are published here for the first time and were not disclosed to the entrants prior to the challenge. Benchmark experimental studies, described herein, were also carried out and provided to entrants in order to aid in the development of new force fields and simulation methods to be employed in the challenge. These studies included argon, carbon dioxide, and water adsorption in the BAM-P109 activated carbon as well as X-ray diffraction, X-ray microtomography, photoelectron spectroscopy, and atomic emission spectroscopy studies of BAM-P109. Several concurrent studies were carried out for the BAM-P108 activated carbon (Panne and Thünemann 2010). These are included in the current manuscript for comparison.

Molecular Dynamics Evaluation of Dielectric-Constant Mixing Rules for H2O-CO2 at Geologic Conditions.

Modeling of mineral reaction equilibria and aqueous-phase speciation of C-O-H fluids requires the dielectric constant of the fluid mixture, which is not known from experiment and is typically estimated by some rule for mixing pure-component values. In order to evaluate different proposed mixing rules, we use molecular dynamics simulation to calculate the dielectric constant of a model H2O-CO2 mixture at temperatures of 700 K and 1000 K at pressures up to 3 GPa. We find that theoretically based mixing rules that depend on combining the molar polarizations of the pure fluids systematically overestimate the dielectric constant of the mixture, as would be expected for mixtures of nonpolar and strongly polar components. The commonly used semiempirical mixing rule due to Looyenga works well for this system at the lower pressures studied, but somewhat underestimates the dielectric constant at higher pressures and densities, especially at the water-rich end of the composition range.

Microstructure and hydrogen bonding in water-acetonitrile mixtures.

The connection of hydrogen bonding between water and acetonitrile in determining the microheterogeneity of the liquid mixture is examined using NPT molecular dynamics simulations. Mixtures for six, rigid, three-site models for acetonitrile and one water model (SPC/E) were simulated to determine the amount of water-acetonitrile hydrogen bonding. Only one of the six acetonitrile models (TraPPE-UA) was able to reproduce both the liquid density and the experimental estimates of hydrogen bonding derived from Raman scattering of the CN stretch band or from NMR quadrupole relaxation measurements. A simple modification of the acetonitrile model parameters for the models that provided poor estimates produced hydrogen-bonding results consistent with experiments for two of the models. Of these, only one of the modified models also accurately determined the density of the mixtures. The self-diffusion coefficient of liquid acetonitrile provided a final winnowing of the modified model and the successful, unmodified model. The unmodified model is provisionally recommended for simulations of water-acetonitrile mixtures.

An internally consistent method for the molecular dynamics simulation of the surface tension: application to some TIP4P-type models of water.

A set of molecular dynamics simulations of three members of the TIP4P model family for water (TIP4P, TIP4P/2005, TIP4P-i) are performed to illustrate the appropriate procedures to employ when simulating the planar interface between coexisting liquid and vapor phases. A method for efficiently including the long-range parts of the intermolecular dispersion interactions introduced by Janecek (J. Phys. Chem. B 2006, 110, 6264) is used. This method produces an interfacial region that is internally consistent with the dynamics of the molecules. A set of procedures are presented to verify that the surface tension estimate has converged and to obtain the underlying uncertainty of the surface tension estimate. The surface tension for the models is determined for temperatures between 300 and 550 K with an uncertainty of about 2 mN/m for this temperature range. The uncertainty is not sensitive to the temperature, but the relative uncertainty increases strongly with temperature. The surface tension is not sensitive to the system size in this temperature range provided at least 500 molecules are used, and it is shown that a thermostat does not bias the computed values.

Solvation of perfluorooctane and octane in water, methanol, acetonitrile, and aqueous mixtures of methanol and acetonitrile.

Molecular dynamics simulations are used to examine the local solvation structure of single octane and perfluorooctane molecules in liquid water, methanol, acetonitrile, and aqueous mixtures of methanol and acetonitrile. The motivation is to obtain baseline information about the solvation of perfluorooctane by liquids used as the mobile phase in liquid chromatography and how it differs from the solvation of octane. While octane is uniformly solvated by both water and the second component, perfluorooctane is solvated by methanol and acetonitrile with the exclusion of water from the first solvation layer when the solvent is a mixture.

Comparative study of the effect of tail corrections on surface tension determined by molecular simulation.

We report results from a comparative study of the influence of tail corrections on the surface tension of the Lennard-Jones fluid. We find that cutoff-independent surface tensions can be obtained by applying a set of tail corrections recently introduced by Janecek at each step of an interfacial Monte Carlo (MC) or molecular dynamics (MD) simulation. The effect of tail corrections on an alternative methodology for calculating surface tension, the combination of finite-size scaling and grand-canonical transition-matrix Monte Carlo (FSS/GC-TMMC), was also investigated. Using this indirect method, surface tensions were calculated with standard (bulk-fluid) tail corrections and lattice sums, the latter usually considered more accurate but computationally more intensive than the former. With standard tail corrections, we find that the surface tension decreases with increasing cutoff distance, reaching a limiting value corresponding to the maximum cutoff possible, namely half the simulation box length. In contrast, surface tension values obtained with the lattice summation were cutoff-independent. More importantly, these values were equivalent to those surface tension values obtained using standard tail corrections and a cutoff distance of half the box length. We also find that the surface tension values obtained here are in agreement with those found in the literature. Last, we find that surface tension values obtained by MD and FSS/GC-TMMC are in decent agreement so long as the appropriate tail correction schemes are used, and that the relative uncertainties in the surface tensions calculated by MD are generally an order of magnitude greater than those calculated by FSS/GC-TMMC. However, the time required by MD on a single central processing unit is less than that required by FSS/GC-TMMC.

System size and control parameter effects in reverse perturbation nonequilibrium molecular dynamics.

The issue of system size effects in the reverse perturbation nonequilibrium molecular dynamics method for determining transport coefficients of fluids is examined for the case of the Lennard-Jones model. It is found that when adequate precautions are observed in obtaining linear temperature or momentum profiles, a 250 atom system is adequate for determining the thermal conductivity and the shear viscosity. Also, a means of determining the uncertainties in the transport coefficients is described. The conclusion is that this method is computationally competitive with other simulation methods for estimating transport coefficients.

Molecular dynamics simulations of alkylsilane stationary-phase order and disorder. 1. Effects of surface coverage and bonding chemistry.

"Shape-selective" polymeric alkylsilane stationary phases are routinely employed over the more common monomeric phases in reversed-phase liquid chromatography (RPLC) to improve the separation of geometric isomers of shape-constrained solutes. We have investigated the molecular dynamics of chromatographic models that represent both monomeric and polymeric stationary phases with alkylsilane surface coverages and bonding chemistries typical of actual materials in an effort to elucidate the molecular-level structural features that control shape-selective separations. The structural characterization of these models is consistent with previous experimental observations of alkyl chain order and disorder: (1) alkyl chain order increases with increased surface coverage; and (2) monomeric and polymeric phases with similar surface coverages yield similar alkyl chain order (although subtle differences exist). In addition, a significant portion of the alkyl chain proximal to the silica surface is disordered (primarily gauche conformations) and the distal end is most ordered. Models that represent shape-selective RPLC phases possess a significant region of distal end chain order with primarily trans dihedral angle conformations. This is consistent with the view that the alkyl chains comprising polymeric stationary phases contain a series of well-defined and rigid voids in which shape-constrained solutes can penetrate and hence be selectively retained.

Molecular dynamics simulations of alkylsilane stationary-phase order and disorder. 2. Effects of temperature and chain length.

In an effort to elucidate the molecular-level structural features that control shape-selective separations, we have investigated the molecular dynamics of chromatographic models that represent both monomeric and polymeric stationary phases with alkylsilane length and temperature conditions analogous to actual materials of low to high shape selectivity. The structural characterization of these models is consistent with previous experimental observations of alkyl chain order and disorder: alkyl chain order increases both with alkyl chain length and with reduced temperature. Models that represent shape-selective reversed-phase liquid chromatography (RPLC) phases possess a significant region of distal end chain order with primarily trans dihedral angle conformations; the extension of these ordered regions into the phase increases with an increase in chain length. Models with extended chain length (C30) possess a higher degree of conformational order and are relatively insensitive to changes in surface coverage, bonding chemistry, and temperature. Chromatography models of various chain lengths and over a temperature range that represents highly shape-selective RPLC stationary phases all contain a series of well-defined and rigid cavities; the size and depth of these "slots" increase for the C30 models, which may promote the enhanced separations of larger size shape-constrained solutes, such as carotenoids.

A polarizable model for ethylene oxide.

A series of interaction models for ethylene oxide are developed for use in molecular simulation of the thermal properties of both the gas and liquid phases. While it is possible to develop nonpolarizable models to accurately generate either the gas or liquid properties separately, it was not possible to do so using a single model for both phases. A polarizable, rigid all-atom model was developed that reproduces the temperature dependence of the second virial coefficient B(T) and the pressure of the liquid at ambient conditions. The model consists of Lennard-Jones and Coulomb interactions between intermolecular atomic sites plus a scalar polarizability located at the midpoint of the line joining the carbon sites. The electrostatic charges and the polarizability are set to match the experimentally determined dipole and quadrupole moments and the molecular polarizability.

Molecular dynamics simulations of end-to-end contact formation in hydrocarbon chains in water and aqueous urea solution.

We probe the urea-denaturation mechanism using molecular dynamics simulations of an elementary "folding" event, namely, the formation of end-to-end contact in the linear hydrocarbon chain (HC) CH(3)(CH(2))(18)CH(3). Electrostatic effects are examined using a model HC in which one end of the chain is positively charged (+0.2e) and the other contains a negative charge (-0.2e). For these systems multiple transitions between "folded" (conformations in which the chain ends are in contact) and "unfolded" (end-to-end contact is broken) can be observed during 4 ns molecular dynamics simulations. In water and 6 M aqueous urea solution HC and the charged HC fluctuate between collapsed globular conformations and a set of expanded structures. The collapsed conformation adopted by the HC in water is slightly destablized in 6 M urea. In contrast, the end-to-end contact is disrupted in the charged HC only in aqueous urea solution. Despite the presence of a large hydrophobic patch, on length scales on the order of approximately 8-10 A "denaturation" (transition to the expanded unfolded state) occurs by a direct interaction of urea with charges on the chain ends. The contiguous patch of hydrophobic moieties leads to "mild dewetting", which becomes more pronounced in the charged HC in 6 M aqueous urea solution. Our simulations establish that the urea denaturation mechanism is most likely electrostatic in origin.

High Temperature Elastic Constants and the Evaluation of Effective Pair Potentials.

It is shown by example that the predicted temperature dependence of the elastic constants is a useful measure of the ability of an effective pair potential to estimate the high temperature thermal properties of a metal. Our example is based on a model pair potential constructed for aluminum. This potential predicts the low temperature elastic constants and phonon dispersion relations with good accuracy (± a few percent). The high temperature elastic constants for this model potential are determined using the Monte Carlo method and are found to be approximately independent of temperature. Since the elastic constants of aluminum are strongly decreasing functions of temperature, this potential is seen to be a poor one for determining the properties of aluminum. We conclude that the temperature dependence of the elastic constants is a useful further test of pair potentials which satisfy the low temperature tests currently employed.

Molecular Dynamics Study of Liquid Rubidium and the Lennard-Jones Fluid.

Molecular dynamics calculations were made for three thermodynamic states of a model of liquid rubidium and for two states of the Lennard-Jones fluid in order to investigate the influence of density, temperature and interatomic potential on the spectra of density fluctuations in these fluids. Here the results for the intermediate scattering function, the radial distribution function, the velocity autocorrelation function and the transverse momentum autocorrelation function are presented in tabular form. The procedures employed in this study are discussed and the major features of these functions are described.

Single Particle Motions in Liquids: Qualitative Features of Memory Functions.

Memory functions, which enter into the equations of motion for time correlation functions, are constructed from neutron scattering, infrared absorption and light scattering data involving single particle motions in liquids. The qualitative features of these memory functions are related to the shape of the corresponding time correlation functions. It is found that a negative portion to the memory function is indicative of a rapid loss of correlation in time while strong temporal correlations imply a memory function which does not go negative. The mathematical structure of a memory function is examined for the case of the ideal gas by expanding and evaluating the projection operator representation of the function. The resulting expression has a rich mathematical structure and can be expressed in a closed form only for its Laplace transform.

The Pair Correlation Function in Liquid 4He.

The pair correlation function obtained from the neutron diffraction data of Mozer, De Graaf, and Le Neindre, is given for liquid 4He at several thermodynamic states above and below the superfluid transition. A method for smoothly and accurately extrapolating the pair function into the limit of zero internuclear separation is considered. The pair function is computed from an approximation integral equation for the ground state wave function and compared to the experimental results.