22 research outputs found
Local motifs in GeS-GaS glasses
The structure of (GeS)(GaS) and
(GeS)(GaS) glasses was investigated by Raman
scattering, high energy X-ray diffraction and extended X-ray absorption fine
structure (EXAFS) measurements at the Ga and Ge K-edges. The reverse Monte
Carlo simulation technique (RMC) was used to obtain structural models
compatible with diffraction and EXAFS datasets. It was found that the
coordination number of Ga is close to four. While Ge atoms have only S
neighbors, Ga binds to S as well as to Ga atoms showing a violation of chemical
ordering in GeS-GaS glasses. Analysis of the corner- and
edge-sharing between [GeS] units revealed that about 30% of germanium
atoms participate in the edge-shared tetrahedra.Comment: 23 pages, 7 figures, accepted for publication in Journal of Alloys
and Compound
Reverse Monte Carlo modeling of liquid water with the explicit use of the SPC/E interatomic potential
Reverse Monte Carlo modeling of liquid water, based on one neutron and one
X-ray diffraction data set, applying also the most popular interatomic
potential for water, 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 explicit use of interatomic potentials in RMC has a role to
play in future structural modeling of water and aqueous solutions.Comment: 8 pages, 5 figures, submitted to The Journal of Chemical Physic
Temperature-dependent structure of methanol-water mixtures on cooling: X-ray and neutron diffraction and molecular dynamics simulations
Methanol-water liquid mixtures have been investigated by high-energy synchrotron X-ray and neutron diffraction at low temperatures. We are thus able to report the first complete sets of both X-ray and neutron weighted total scattering structure factors over the entire composition range (at 12 different methanol concentrations (xM) from 10 to 100 mol%) and at temperatures from ambient down to the freezing points of the mixtures. The new diffraction data may later be used as reference in future theoretical and simulation studies. The measured data are interpreted by molecular dynamics simulations, in which the all atom OPLS/AA force field model for methanol is combined with both the SPC/E and TIP4P/2005 water potentials. Although the TIP4P/2005 water model was found to be somewhat more successful, both combinations provide at least semi-quantitative agreement with measured diffraction data. From the simulated particle configurations, partial radial distribution functions, as well as various distributions of the number of hydrogen bonds have been determined. As a general trend, the average number of hydrogen bonds increases upon cooling. However, the number of hydrogen bonds between methanol molecules slightly decreases with lowering temperatures in the concentration range between ca. 30 and 60 mol% alcohol content. The same is valid for water-water hydrogen bonds above 70 mol% of methanol content, from room temperature down to 193 K
Properties of hydrogen bonded network 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 of the hydrogen bonded network
structure, as well as of its changes on cooling. Various tools have been
exploited for revealing details concerning hydrogen bonding, like determining
H-bond acceptor and donor sites, calculating cluster size distributions and
cluster topologies, as well as 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 content of 70% at room temperature, above which
concentration ring structures nearly disappear. Percolation has been given
special attention, so that it could be shown that at low temperature, close to
the freezing point even the mixture with 90% ethanol possesses a 3D percolating
network. Moreover, the water sub-network also percolates even at room
temperature, with a percolation transition occurring around 50% ethanol
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
2 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
Temperature dependent network stability in simple alcohols and pure water: The evolution of Laplace spectra
A number of computer-generated models of water, methanol and ethanol are considered at room temperature and ambient pressure, and also as a function of temperature (for water and ethanol), and the potential model (for water only). The Laplace matrices are determined, and various characteristics of them, such as eigenvalues and eigenvectors, as well as the corresponding Laplace spectra are calculated. It is revealed how the width of the spectral gap in the Laplace matrix of H-bonded networks may be applied for characterising the stability of the network. A novel method for detecting the presence percolated network in these systems is also introduced