Pressure-Dependent Structure of Methanol-Water Mixtures up to 1.2 GPa: Neutron Diffraction Experiments and Molecular Dynamics Simulations.

Total scattering structure factors of per-deuterated methanol and heavy water, CD3OD and D2O, have been determined across the entire composition range as a function of pressure up to 1.2 GPa, by neutron diffraction. The largest variations due to increasing pressure were observed below a scattering variable value of 5 Å-1, mostly as shifts in terms of the positions of the first and second maxima. Molecular dynamics computer simulations, using combinations of all-atom potentials for methanol and various water force fields, were conducted at the experimental pressures with the aim of interpreting neutron diffraction results. The peak-position shifts mentioned above could be qualitatively reproduced by simulations, although in terms of peak intensities, the accord between neutron diffraction and molecular dynamics was much less satisfactory. However, bearing in mind that increasing pressure must have a profound effect on repulsive forces between neighboring molecules, the agreement between experiment and computer simulation can certainly be termed as satisfactory. In order to reveal the influence of changing pressure on local intermolecular structure in these "simplest of complex" hydrogen-bonded liquid mixtures, simulated structures were analyzed in terms of hydrogen bond-related partial radial distribution functions and size distributions of hydrogen-bonded cyclic entities. Distinct differences between pressure-dependent structures of water-rich and methanol-rich composition regions were revealed.

Chloride ions as integral parts of hydrogen bonded networks in aqueous salt solutions: the appearance of solvent separated anion pairs.

Hydrogen bonding to chloride ions has been frequently discussed over the past 5 decades. Still, the possible role of such secondary intermolecular bonding interactions in hydrogen bonded networks has not been investigated in any detail. Here we consider computer models of concentrated aqueous LiCl solutions and compute the usual hydrogen bond network characteristics, such as distributions of cluster sizes and of cyclic entities, both for models that take and do not take chloride ions into account. During the analysis of hydrogen bonded rings, a significant amount of 'solvent separated anion pairs' have been detected at high LiCl concentrations. It is demonstrated that taking halide anions into account as organic constituents of the hydrogen bonded network does make the interpretation of structural details significantly more meaningful than when considering water molecules only. Finally, we compare simulated structures generated by 'good' and 'bad' potential sets on the basis of the tools developed here, and show that this novel concept is, indeed, also helpful for distinguishing between reasonable and meaningless structural models.

Reverse Monte Carlo modeling of liquid water with the explicit use of the SPC/E interatomic potential.

Reverse Monte Carlo (RMC) modeling of liquid water, based on one neutron and one X-ray diffraction data set, applying also the most popular interatomic potential for water, extended simple point charge (SPC/E), has been performed. The strictly rigid geometry of SPC/E water molecules had to be loosened somewhat, in order to be able to produce a good fit to both sets of experimental data. In the final particle configurations, regularly shaped water molecules and straight hydrogen bonding angles were found to be consistent with diffraction results. It has been demonstrated that the explicit use of interatomic potentials in RMC has a role to play in future structural modeling of water and aqueous solutions.

The structure of PX3 (X = Cl, Br, I) molecular liquids from X-ray diffraction, molecular dynamics simulations, and reverse Monte Carlo modeling.

Synchrotron X-ray diffraction measurements have been conducted on liquid phosphorus trichloride, tribromide, and triiodide. Molecular Dynamics simulations for these molecular liquids were performed with a dual purpose: (1) to establish whether existing intermolecular potential functions can provide a picture that is consistent with diffraction data and (2) to generate reliable starting configurations for subsequent Reverse Monte Carlo modelling. Structural models (i.e., sets of coordinates of thousands of atoms) that were fully consistent with experimental diffraction information, within errors, have been prepared by means of the Reverse Monte Carlo method. Comparison with reference systems, generated by hard sphere-like Monte Carlo simulations, was also carried out to demonstrate the extent to which simple space filling effects determine the structure of the liquids (and thus, also estimating the information content of measured data). Total scattering structure factors, partial radial distribution functions and orientational correlations as a function of distances between the molecular centres have been calculated from the models. In general, more or less antiparallel arrangements of the primary molecular axes that are found to be the most favourable orientation of two neighbouring molecules. In liquid PBr3 electrostatic interactions seem to play a more important role in determining intermolecular correlations than in the other two liquids; molecular arrangements in both PCl3 and PI3 are largely driven by steric effects.

Outstanding Properties of the Hydration Shell around ß-d-Glucose: A Computational Study.

Ab initio molecular dynamics (AIMD) simulations have been performed on aqueous solutions of four simple sugars, α-d-glucose, ß-d-glucose, α-d-mannose, and α-d-galactose. Hydrogen-bonding (HB) properties, such as the number of donor- and acceptor-type HB-s, and the lengths and strengths of hydrogen bonds between sugar and water molecules, have been determined. Related electronic properties, such as the dipole moments of water molecules and partial charges of the sugar O atoms, have also been calculated. The hydrophilic and hydrophobic shells were characterized by means of spatial distribution functions. ß-d-Glucose was found to form the highest number of hydrophilic and the smallest number of hydrophobic connections to neighboring water molecules. The average sugar-water H-bond length was the shortest for ß-d-glucose, which suggests that these are the strongest such H-bonds. Furthermore, ß-d-glucose appears to stand out in terms of the symmetry properties of both its hydrophilic and hydrophobic hydration shells. In summary, in all aspects considered here, there seems to be a correlation between the distinct characteristics of ß-d-glucose reported here and its outstanding solubility in water. Admittedly, our findings represent only some of the important factors that influence the solubility.

The liquid structure of tetrachloroethene: molecular dynamics simulations and reverse Monte Carlo modeling with interatomic potentials.

The liquid structure of tetrachloroethene has been investigated on the basis of measured neutron and X-ray scattering structure factors, applying molecular dynamics simulations and reverse Monte Carlo (RMC) modeling with flexible molecules and interatomic potentials. As no complete all-atom force field parameter set could be found for this planar molecule, the closest matching all-atom Optimized Potentials for Liquid Simulations (OPLS-AA) intra-molecular parameter set was improved by equilibrium bond length and angle parameters coming from electron diffraction experiments [I. L. Karle and J. Karle, J. Chem. Phys. 20, 63 (1952)]. In addition, four different intra-molecular charge distribution sets were tried, so in total, eight different molecular dynamics simulations were performed. The best parameter set was selected by calculating the mean square difference between the calculated total structure factors and the corresponding experimental data. The best parameter set proved to be the one that uses the electron diffraction based intra-molecular parameters and the charges qC = 0.1 and qCl = -0.05. The structure was further successfully refined by applying RMC computer modeling with flexible molecules that were kept together by interatomic potentials. Correlation functions concerning the orientation of molecular axes and planes were also determined. They reveal that the molecules closest to each other exclusively prefer the parallel orientation of both the molecular axes and planes. Molecules forming the first maximum of the center-center distribution have a preference for <30° and >60° axis orientation and >60° molecular plane arrangement. A second coordination sphere at ∼11 Å and a very small third one at ∼16 Å can be found as well, without preference for any axis or plane orientation.

Connecting Diffraction Experiments and Network Analysis Tools for the Study of Hydrogen-Bonded Networks.

A self-consistent scheme is presented that is applicable for revealing details of the microscopic structure of hydrogen-bonded liquids, including the description of the hydrogen-bonded network. The scheme starts with diffraction measurements, followed by molecular dynamics simulations. Computational results are compared with the experimentally accessible information on the structure, which is most frequently the total scattering structure factor. In the case of an at least semiquantitative agreement between experiment and simulation, sets of particle coordinates from the latter may be exploited for revealing nonmeasurable structural details. Calculations of some properties concerning the hydrogen-bonded network are also described, in the order of increasing complexity: starting with the definition of a hydrogen bond, first and second neighborhoods are described via spatial correlations functions. Attention is then turned to cyclic and noncyclic hydrogen-bonded clusters, before cluster size distributions and percolation are discussed. We would like to point out that, as a result of applying the novel protocol, these latter, rather abstract, quantities become consistent with diffraction data: it may thus be argued that the approach reviewed here is the first one that establishes a direct link between measurements and elements of network theories. Applications for liquid water, simple alcohols, and alcohol-water liquid mixtures demonstrate the usefulness of the aforementioned characteristics. The procedure can readily be applied to more complicated hydrogen-bonded networks, like mixtures of polyols (diols, triols, sugars, etc.) and water, and complex aqueous solutions of even larger molecules (even of proteins).

RMC_POT: a computer code for reverse Monte Carlo modeling the structure of disordered systems containing molecules of arbitrary complexity.

An approach has been devised and tested for preserving the molecular dynamics molecular geometry taking into account energetic considerations during Reverse Monte Carlo (RMC) modeling. Instead of the commonly used fixed neighbor constraints, where molecules are held together by constraining distance ranges available for the specified atom pairs, here molecules are kept together via bond, angle, and dihedral potential energies. The scaled total potential energy contributes to the measure of the goodness-of-fit, thus, the atoms can be prevented from drifting apart. In some of the calculations (Lennard-Jones and Coulombic) nonbonding potentials were also applied. The algorithm was successfully tested for the X-ray structure factor-based structure study of liquid dimethyl trisulfide, for which material now significantly more sensible results have been obtained than during previous attempts via any earlier version of RMC modeling. It is envisaged that structural modeling of a large class of materials, primarily liquids and amorphous solids containing molecules of up to about 100 atoms, will make use of the new code in the near future.

Extension of the invariant environment refinement technique + reverse Monte Carlo method of structural modelling for interpreting experimental structure factors: the cases of amorphous silicon, phosphorus, and liquid argon.

The invariant environment refinement technique, as applied to reverse Monte Carlo modelling [invariant environment refinement technique + reverse Monte Carlo (INVERT + RMC); M. J. Cliffe, M. T. Dove, D. A. Drabold, and A. L. Goodwin, Phys. Rev. Lett. 104, 125501 (2010)], is extended so that it is now applicable for interpreting the structure factor (instead of the pair distribution function). The new algorithm, called the local invariance calculation, is presented by the examples of amorphous silicon, phosphorus, and liquid argon. As a measure of the effectiveness of the new algorithm, the ratio of exactly fourfold coordinated Si atoms was larger than obtained previously by the INVERT-RMC scheme.

Detailed intermolecular structure of molecular liquids containing slightly distorted tetrahedral molecules with C(3v) symmetry: chloroform, bromoform, and methyl-iodide.

Analyses of the intermolecular structure of molecular liquids containing slightly distorted tetrahedral molecules of the CXY(3)-type are described. The process is composed of the determination of several different distance-dependent orientational correlation functions, including ones that are introduced here. As a result, a complete structure classification could be provided for CXY(3) molecular liquids, namely for liquid chloroform, bromoform, and methyl-iodide. In the present work, the calculations have been conducted on particle configurations resulting from reverse Monte Carlo computer modeling: these particle arrangements have the advantage that they are fully consistent with structure factors from neutron and x-ray diffraction measurements. It has been established that as the separation between neighboring molecules increases, the dominant mutual orientations change from face-to-face to edge-to-edge, via the edge-to-face arrangements. Depending on the actual liquid, these geometrical elements (edges and faces of the distorted tetrahedra) were found to contain different atoms. From the set of liquids studied here, the structure of methyl-iodide was found to be easiest to describe on the basis of pure steric effects (molecular shape, size, and density) and the structure of liquid chloroform seems to be the furthest away from the corresponding "flexible fused hard spheres" like reference system.

Properties of Hydrogen-Bonded Networks in Ethanol-Water Liquid Mixtures as a Function of Temperature: Diffraction Experiments and Computer Simulations.

New X-ray and neutron diffraction experiments have been performed on ethanol-water mixtures as a function of decreasing temperature, so that such diffraction data are now available over the entire composition range. Extensive molecular dynamics simulations show that the all-atom interatomic potentials applied are adequate for gaining insight into the hydrogen-bonded network structure, as well as into its changes on cooling. Various tools have been exploited for revealing details concerning hydrogen bonding, as a function of decreasing temperature and ethanol concentration, like determining the H-bond acceptor and donor sites, calculating the cluster-size distributions and cluster topologies, and computing the Laplace spectra and fractal dimensions of the networks. It is found that 5-membered hydrogen-bonded cycles are dominant up to an ethanol mole fraction xeth = 0.7 at room temperature, above which the concentrated ring structures nearly disappear. Percolation has been given special attention, so that it could be shown that at low temperatures, close to the freezing point, even the mixture with 90% ethanol (xeth = 0.9) possesses a three-dimensional (3D) percolating network. Moreover, the water subnetwork also percolates even at room temperature, with a percolation transition occurring around xeth = 0.5.

Extended orientational correlation study for molecular liquids containing distorted tetrahedral molecules: application to methylene halides.

The method of Rey [Rey, J. Chem. Phys. 126, 164506 (2007)] for describing how molecules orient toward each other in systems with perfect tetrahedral molecules is extended to the case of distorted tetrahedral molecules of c(2v) symmetry by means of introducing 28 subgroups. Additionally, the original analysis developed for perfect tetrahedral molecules, based on six groups, is adapted for molecules with imperfect tetrahedral shape. Deriving orientational correlation functions have been complemented with detailed analyses of dipole-dipole correlations. This way, (up to now) the most complete structure determination can be carried out for such molecular systems. In the present work, these calculations have been applied for particle configurations resulting from reverse Monte Carlo computer modeling. These particle arrangements are fully consistent with structure factors from neutron and x-ray diffraction measurements. Here we present a complex structural study for methylene halide (chloride, bromide, and iodide) molecular liquids, as possibly the best representative examples. It has been found that the most frequent orientations of molecules are of the 2:2 type over the entire distance range in these liquids. Focusing on the short range orientation, neighboring molecules turn toward each other with there "H,Y"-"H,Y" (Y: Cl, Br, I) edges, apart from CH(2)Cl(2) where the H,H-H,Cl arrangement is the most frequent. In general, the structure of methylene chloride appears to be different from the structure of the other two liquids.

Structural, rheological and dynamic aspects of hydrogen-bonding molecular liquids: Aqueous solutions of hydrotropic tert-butyl alcohol.

HYPOTHESIS: The structural details, viscosity trends and dynamic phenomena in t-butanol/water solutions are closely related on the molecular scales across the entire composition range. Utilizing the experimental small- and wide-angle x-ray scattering (SWAXS) method, molecular dynamics (MD) simulations and the 'complemented-system approach' method developed in our group it is possible to comprehensively describe the structure-viscosity-dynamics relationship in such structurally versatile hydrogen-bonded molecular liquids, as well as in similar, self-assembling systems with pronounced molecular and supramolecular structures at the intra-, inter-, and supra-molecular scales. EXPERIMENTS: The SWAXS and x-ray diffraction experiments and MD simulations were performed for aqueous t-butanol solutions at 25 °C. Literature viscosity and self-diffusion data were also used. FINDINGS: The interpretive power of the proposed scheme was demonstrated by the extensive and diverse results obtained for aqueous t-butanol solutions across the whole concentration range. Four composition ranges with qualitatively different structures and viscosity trends were revealed. The experimental and calculated zero-shear viscosities and molecular self-diffusion coefficients were successfully related to the corresponding structural details. The hydrogen bonds that were, along with hydrophobic effects, recognized as the most important driving force for the formation of t-butanol aggregates, show intriguing lifetime trends and thermodynamic properties of their formation.

Understanding the structure of aqueous cesium chloride solutions by combining diffraction experiments, molecular dynamics simulations, and reverse Monte Carlo modeling.

A detailed study of the microscopic structure of an electrolyte solution, cesium chloride (CsCl) in water, is presented. For revealing the influence of salt concentration on the structure, CsCl solutions at concentrations of 1.5, 7.5, and 15 mol % are investigated. For each concentration, we combine total scattering structure factors from neutron and X-ray diffraction and 10 partial radial distribution functions from molecular dynamics simulations in one single structural model, generated by reverse Monte Carlo modeling. This combination of computer modeling methods is capable of (a) showing the extent to which simulation results are consistent with experimental diffraction data and (b) tracking down distribution functions in computer simulation that are the least comfortable with diffraction data. For the present solutions, we show that the level of consistency between simulations that use simple pair potentials and experimental structure factors is nearly quantitative. Remaining inconsistencies seem to be caused by water-water distribution functions. Changing the pair potentials of water-water interactions from SPC/E to TIP4P-2005 has not had any effect in this respect. As a final result, we obtained particle configurations from reverse Monte Carlo modeling that were in quantitative agreement with both diffraction data and most of the molecular dynamics (MD) simulated partial radial distribution functions (prdf's). From the particle coordinates, the distribution of the number of first neighbors, as well as angular correlation functions, were calculated. The average number of water molecules around cations decreases from about 8 to about 6.5 as concentration increases from 1.5 to 15 mol %, whereas the same quantity for the anions changes from about 7 to about 5. It was also found that the average angle of Cl...H-O particle arrangements, characteristic of anion-water hydrogen bonds, is closer to 180 degrees than that found for O...H-O arrangements (water-water hydrogen bonds). The present combination of experimental and computer simulation methods appears to be promising for the study of other electrolyte solutions.

Benchmarking polarizable molecular dynamics simulations of aqueous sodium hydroxide by diffraction measurements.

Results from molecular dynamics simulations of aqueous hydroxide of varying concentrations have been compared with experimental structural data. First, the polarizable POL3 model was verified against neutron scattering using a reverse Monte Carlo fitting procedure. It was found to be competitive with other simple water models and well suited for combining with hydroxide ions. Second, a set of four polarizable models of OH- were developed by fitting against accurate ab initio calculations for small hydroxide-water clusters. All of these models were found to provide similar results that robustly agree with structural data from X-ray scattering. The present force field thus represents a significant improvement over previously tested nonpolarizable potentials. Although it cannot in principle capture proton hopping and can only approximately describe the charge delocalization within the immediate solvent shell around OH-, it provides structural data that are almost entirely consistent with data obtained from scattering experiments.

Solvation force between surfaces modified by tethered chains: a density functional approach.

The behavior of Lennard-Jones fluid in slitlike pores with walls modified by tethered chain molecules is studied using density functional theory. The effects of confinement and chemical modification of pore walls on the solvation force are investigated. Two models of the pore walls are considered. According to the first model, the chain molecules are chemically bonded by their end segments to opposite walls of the pore, forming flexible pillars. In the second model the chains build up a brush at each wall due to bonding of the first segment at one wall. The nonbonded terminating segment of a molecule is strongly attracted via a short-range potential to any wall of the pore. Then a pillarlike or looplike structure of chains can be formed. In the first model the solvation force at the wall-to-wall is repulsive for narrow pores and strongly attractive for wider pores of the order of the nominal chain length. Oscillations of the solvation force are induced by adsorbed fluid structure and by ordered structure of segments on the fragment of entirely attractive force curve. In the second model, however, the solvation force decays to zero as the pore width increases. Attractive force can be induced at intermediate separation between walls due to modification of the pore walls.

Molecular Dynamics Simulation Studies of the Temperature-Dependent Structure and Dynamics of Isopropanol-Water Liquid Mixtures at Low Alcohol Content.

Series of molecular dynamics simulations for 2-propanol-water mixtures, as a function of temperature (between freezing and room temperature) and composition (xip = 0, 0.5, 0.1, and 0.2), have been performed for temperatures reported in the only available experimental structure study. It is shown that when the all-atom optimized potentials for liquid simulations interatomic potentials for the alcohol are combined with the TIP4P/2005 water model, then the near-quantitative agreement with measured X-ray data, in the reciprocal space, can be achieved. Such an agreement justifies detailed investigations of structural, energetic, and dynamic properties on the basis of simulation trajectories. Here, we focus on characteristics related to hydrogen bonds (HB): cluster-, and in particular, ring formation, energy distributions, and lifetimes of HB-s have been scrutinized for the entire system, as well as for the water and isopropanol subsystems. It is demonstrated that similar to ethanol-water mixtures, the occurrence of 5-membered-hydrogen-bonded rings are significant, particularly at higher alcohol concentrations. Concerning HB energetics, an intriguing double maximum appears on the alcohol-alcohol HB energy distribution function. HB lifetimes have been found significantly longer in the mixtures than they are in pure liquids.

Comparison of interaction potentials of liquid water with respect to their consistency with neutron diffraction data of pure heavy water.

A number of interaction potential models for liquid water are scrutinized from the point of view of their compatibility with results of neutron diffraction experiments on pure heavy water. For the quantitative assessment a protocol developed recently [L. Pusztai et al., Chem. Phys. Lett. 457, 96 (2008)] using the reverse Monte Carlo method has been applied. The approach combines the experimental total scattering structure factor (tssf) and partial radial distribution functions (prdfs) from molecular dynamics simulations in a single structural model (particle configuration). Goodness-of-fit values to the three (O-O, O-H, and H-H) simulated prdfs and to the experimental tssf provided an unbiased measure characterizing the level of consistency between various interaction potentials and diffraction experiments. Out of the sets of prdfs investigated here, corresponding to SPCE, BJH, ST2, POL3, TIP4P, TIP4P-2005, TTMF3, and ENCS interaction potentials, the ones from the TIP4P-2005 potential proved to be the most consistent with the experimental neutron-weighted tssf of heavy water. More importantly, it is shown that none of the above interaction potentials are seriously inconsistent with the measured structure factor at ambient conditions.

Orientational correlations in molecular liquid SnI4.

The total scattering structure factor of liquid tin tetraiodide (SnI(4)) has been interpreted by means of reverse Monte Carlo (RMC) modeling. From the sets of particle coordinates provided by RMC, which are consistent with experimental results within errors, partial radial distribution functions as well as correlation functions characterizing mutual orientations of molecules as a function of distance between molecular centers can be calculated. Interestingly and very much in contrast to liquids of symmetric XCl(4) molecules, the corner-to-face (or "Apollo")-type orientation of neighboring molecules has a significant (about 20%) occurrence in liquid SnI(4). Via comparison with a reference system, obtained by hard sphere Monte Carlo simulation, we demonstrate that intermolecular two-body correlations in liquid SnI(4) are determined largely by excluded volume (steric) effects; that is, intermolecular two-body interactions play only a minor role. On the other hand, as it is manifested in the large difference between the reference and "real" systems in terms of the orientational correlations, higher order interactions are indispensable. This feature can explain the extremely rich phase behavior of SnI(4) at high pressures.