Effect of temperature and pressure on the structural (alpha-) and the true Johari Goldstein (beta-) relaxation in binary mixtures (original) (raw)
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The Journal of Chemical Physics, 2003
Molecular-dynamics computer simulations have been carried out on liquid methanol, methylamine, and methanethiol. The local structure of the liquids was studied based on radial distribution functions and the density projections of the neighboring molecules obtained on the basis of simulated molecular configurations. The extent of hydrogen bonding was investigated by direct analysis of the connectivity of molecules forming hydrogen-bonded clusters in these liquids. By this analysis, the methanol molecules were found to form linear chainlike structures. The local structure of hydrogen-bonded molecules of methylamine proved to be rather space filling due to the great extent of chain branching. Methanethiol molecules also proved to form hydrogen bonds forming small compact clusters. No evidence was found, however, for the clustering of hydrophobic methyl groups in any of the liquids. The quality of simulations was checked by derivation of neutron total and composite radial distribution functions and by comparison of those with available experimental data.
Magnetic effects on the solvent properties investigated by molecular dynamics simulation
2014
This paper investigates how an external constant magnetic field in the Z-direction affects the performance of a solvent. The molecular dynamics simulation comprised common inorganic and organic solvents including water, acetone, acetonitrile, toluene, and n-hexane at the ambient temperature and pressure. A static magnetic field applied in the simulation process is able to reduce the solvent mobility in the solution in order to enhance the solvent-solute reaction. Simulation results show that the diffusivity decreases because of increasing the effective interactions. Besides, magnetic field reduces the volume of the solvent and increases the strength of the hydrogen bonds by maximizing attractive electrostatic and vdW interactions caused by changes in the radial distribution function of the solvents. Hydrogen-bonding characteristics of solvents investigated by molecular dynamics simulations were evidence for the hydrogen bonding strength of O Á Á Á H that is a more efficient intermolecular hydrogenbonding in comparison with N Á Á Á H.
Mutal interactions between hydrogen bonds in alcohols - nuclear magnetic relaxation study
1993
Proton NMR relaxation times Ti and T2 were measured in methonal, ethanol, propanol and butanol at a temperature of 37°C and at an RF frequency of 20 MHz. The hydrogen bond energies of the above alcohols are in the range 2.33-4.56 KcaVmole. The relaxation time Ti is found to decrease with increase of hydrogen bond energy in these alcohols and this is explained by postulating mutal interactions between adjacent hydrogen bonds.
Journal of Molecular Structure, 2008
This article examines the effect of a static magnetic field on liquid water. Experimental measure of viscosity, enthalpies and surface tension of water submitted to a magnetic field suggests that intra cluster hydrogen bonds were broken. From our theoretical and experimental data, we may conclude that the competition between the different hydrogen bonds networks (intra-and intermolecular) gives rise to the weakening of the hydrogen bonds intra cluster forming smaller cluster with stronger inter cluster hydrogen bonds.
Molecular dynamics simulations of the properties of water-methanol mixtures. Effects of force fields
Condensed Matter Physics, 2019
Isothermal-isobaric molecular dynamics simulations are used to examine the microscopic structure and some properties of water-methanol liquid mixture. The TIP4P/2005 and SPC/E water models are combined with the united atom TraPPE and the all-atom force field model for methanol. Our principal focus is to evaluate the quality of predictions of different combinations of model force fields concerning the composition dependence of basic properties of this system. Specifically, we explored the composition effects on density, excess molar volume and excess entropy, as well as on the surface tension and static dielectric constant. In addition, the structural properties are described in terms of the coordination numbers and the average number of hydrogen bonds between molecules of constituent species. Finally, the composition dependence of self-diffusion coefficients of the species is evaluated. All theoretical predictions are tested with respect to experimental data.
Effect of Hydrogen Bond Cooperativity on the Behavior of Water
Four scenarios have been proposed for the low-temperature phase behavior of liquid water, each predicting different thermodynamics. The physical mechanism that leads to each is debated. Moreover, it is still unclear which of the scenarios best describes water, because there is no definitive experimental test. Here we address both open issues within the framework of a microscopic cell model by performing a study combining mean-field calculations and Monte Carlo simulations. We show that a common physical mechanism underlies each of the four scenarios, and that two key physical quantities determine which of the four scenarios describes water: (i) the strength of the directional component of the hydrogen bond and (ii) the strength of the cooperative component of the hydrogen bond. The four scenarios may be mapped in the space of these two quantities. We argue that our conclusions are model independent. Using estimates from experimental data for H-bond properties the model predicts that the low-temperature phase diagram of water exhibits a liquid-liquid critical point at positive pressure.
Variations of the Hydrogen Bonding and Hydrogen-Bonded Network in Ethanol–Water Mixtures on Cooling
The Journal of Physical Chemistry B, 2018
Extensive molecular dynamics computer simulations have been conducted for ethanol-water liquid mixtures in the water-rich side of the composition range, with 10, 20 and 30 mol % of the alcohol, at temperatures between room temperature and the experimental freezing point of the given mixture. All-atom type (OPLS) interatomic potentials have been assumed for ethanol, in combination with two kinds of rigid water models (SPC/E and TIP4P/2005). Both combinations have provided excellent reproductions of the experimental X-ray total structure factors at each temperature; this provided a strong basis for further structural analyses. Beyond partial radial distribution functions, various descriptors of hydrogen bonded assemblies, as well as of the hydrogen bonded network have been determined from the simulated particle configurations. A clear tendency was observed towards that an increasing proportion of water molecules participate in hydrogen bonding with exactly 2 donor-and 2 acceptor sites as temperature decreases. Concerning larger assemblies held together by hydrogen bonding, the main focus was put on the properties of cyclic entities: it was found that, similarly to methanolwater mixtures, the number of hydrogen bonded rings has increased with lowering temperature. However, for ethanol-water mixtures the dominance of not the six-, but of the five-fold rings could be observed.