15 research outputs found
Fingerprints of the crossing of the Frenkel and melting line on the properties of high-pressure supercritical water
Using molecular dynamics simulations in combination with the two-phase thermodynamic model, we reveal novel characteristic fingerprints of the crossing of the Frenkel and melting line on the properties of high-pressure water at a near-critical temperature (1.03Tc). The crossing of the Frenkel line at about 1.17 GPa is characterized by a crossover in the rotational and translational entropy ratio Srot/Strans, indicating a change in the coupling between translational and rotational motions which is also reflected in the shape of the rotational density of states. The observed isosbestic points in the translational and rotational density of states are also blue-shifted at density and pressure conditions higher than the ones corresponding to the Frenkel line. The first-order phase transition from a rigid liquid to a face-centered cubic plastic crystal phase at about 8.5 GPa is reflected in the discontinuous changes in the translational and rotational entropy, particularly in the significant increase of the ratio Srot/Strans. A noticeable discontinuous increase of the dielectric constant has also been revealed when crossing this melting line, which is attributed to the different arrangement of the water molecules in the plastic crystal phase. The reorientational dynamics in the plastic crystal phase is faster in comparison with the ârigidâ liquid-like phase, but it remains unchanged upon a further pressure increase in the range of 8.5â11 GPa.Postprint (author's final draft
On the different faces of the supercritical phase of water at a near-critical temperature: pressure-induced structural transitions ranging from a gaslike fluid to a plastic crystal polymorph
The present study reports a systematic analysis of a wide variety of structural, thermodynamic, and dynamic properties of supercritical water along the near-critical isotherm of T = 1.03Tc and up to extreme pressures, using molecular dynamics and Monte Carlo simulations. The methodology employed provides solid evidence about the existence of a structural transition from a liquidlike fluid to a compressed, tightly packed liquid, in the density and pressure region around 3.4Âżc and 1.17 GPa, introducing an alternative approach to locate the crossing of the Frenkel line. Around 8.5 GPa another transition to a face-centered-cubic plastic crystal polymorph with density 5.178Âżc is also observed, further confirmed by Gibbs free energy calculations using the two-phase thermodynamic model. The isobaric heat capacity maximum, closely related to the crossing of the Widom line, has also been observed around 0.8Âżc, where the local density augmentation is also maximized. Another structural transition has been observed at 0.2Âżc, related to the transformation of the fluid to a dilute gas at lower densities. These findings indicate that a near-critical isotherm can be divided into different domains where supercritical water exhibits distinct behavior, ranging from a gaslike one to a plastic crystal one.Postprint (author's final draft
Local structural fluctuations, hydrogen bonding and structural transitions in supercritical water
The contribution of hydrogen bonding interactions to the formation of local density inhomogeneities in supercritical water at near-critical conditions has been extensively studied by means of molecular dynamics simulations. The results obtained have revealed the strong effect of water molecules forming one and two hydrogen bonds on the determination of the local density augmentation in the fluid. The local structural order has also been studied in terms of the trigonal and tetrahedral order parameters, revealing the correlation between local orientational order and hydrogen bonding. The dynamics of the structural order parameters exhibit similarities with local density ones. The local structural analysis performed in terms of nearest neighbors around the individual molecules provides additional significant evidence about the existence of a liquid-like to gas-like structural transition in supercritical water at the density range close to 0.2 Âżc, further supporting previous suggestions based on the interpretation of experimental thermodynamic data.Postprint (author's final draft
Local density augmentation and dynamic properties of hydrogen-and non-hydrogen-bonded supercritical fluids: A molecular dynamics study
The local density inhomogeneities in neat supercritical fluids were
investigated via canonical molecular dynamics simulations. The selected
systems under investigation were the polar and hydrogen-bonded fluid
methanol as well as the quadrupolar non-hydrogen-bonded carbon dioxide
one. Effective local densities, local density augmentation, and
enhancement factors were calculated at state points along an isotherm
close to the critical temperature of each system (T-r=1.03). The results
obtained reveal strong influence of the polarity and hydrogen bonding
upon the intensity of the local density augmentation. It is found that
this effect is sufficiently larger in the case of the polar and
associated methanol in comparison to those predicted for carbon dioxide.
For both fluids the local density augmentation values are maximized in
the bulk density region near 0.7 rho(c), a result that is in agreement
with experiment. In addition, the local density dynamics of each fluid
were investigated in terms of the appropriate time correlation
functions. The behavior of these functions reveals that the bulk density
dependence of the local density reorganization times is very sensitive
to the specific intermolecular interactions and to the size of the local
region. Also, the estimated local density reorganization time as a
function of bulk density of each fluid was further analyzed and
successfully related to two different time-scale relaxation mechanisms.
Finally, the results obtained indicate a possible relationship between
the single-molecule reorientational dynamics and the local density
reorganization ones. (c) 2007 American Institute of Physics
Novel approaches to the structure and dynamics of liquids
Proceedings of the NATO Advanced Study Institute, Rhodes, Greece from 6 to 15 September 2002
Estimation of the Interaction-Induced Effects on the Far-Infrared and Infrared Correlation Functions of HCl Dissolved in CCl 4
Translational component of the interactionâinduced dipole correlation functions in dense fluids
Gburski Z, Samios J, DorfmĂŒller T. Translational component of the interactionâinduced dipole correlation functions in dense fluids. The Journal of Chemical Physics. 1987;87(12):7348-7349.The correlation functions associated with the translational motion of a dipole induced by the permanent multipoles of an order l=1,2,3,... in dense media have been calculated analytically assuming the isotropic diffusion mechanism. The reflecting wall boundary condition at the distance of the closest approach of two molecules has been taken into account. The translational correlation functions obtained from this model have been compared with the results of molecular dynamics simulations in liquids N2 and CS2, and good agreement has been obtained
Translational correlation function of interactionâinduced dipoles in dense media
Gburski Z, Samios J, DorfmĂŒller T. Translational correlation function of interactionâinduced dipoles in dense media. The Journal of Chemical Physics. 1987;86(1):383-390
Special issue - Novel Approaches to the Structure and Dynamics of Liquids: Experiments, Theories and Simulations - Meeting of the European and Japanese Molecular Liquid Groups (EMLG/JMLG) September 7-15, 2002, Rhodos, Greece - Preface
Local intermolecular structure, hydrogen bonding and related dynamics in the liquid cis/trans N-methylformamide mixture: A density functional theory based Born-Oppenheimer molecular dynamics study
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