Effect of the local hydrogen bonding network on the reorientational and translational dynamics in supercritical water

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 = 1.03T_c 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 3.4ρ_c 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 5.178ρ_c 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 0.8ρ_c, where the local density augmentation is also maximized. Another structural transition has been observed at 0.2ρ_c, 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.

The effect of polymorphism on the structural, dynamic and dielectric properties of plastic crystal water: A molecular dynamics simulation perspective

We have employed molecular dynamics simulations based on the TIP4P/2005 water model to investigate the local structural, dynamical, and dielectric properties of the two recently reported body-centered-cubic and face-centered-cubic plastic crystal phases of water. Our results reveal significant differences in the local orientational structure and rotational dynamics of water molecules for the two polymorphs. The probability distributions of trigonal and tetrahedral order parameters exhibit a multi-modal structure, implying the existence of significant local orientational heterogeneities, particularly in the face-centered-cubic phase. The calculated hydrogen bond statistics and dynamics provide further indications of the existence of a strongly heterogeneous and rapidly interconverting local orientational structural network in both polymorphs. We have observed a hindered molecular rotation, much more pronounced in the body-centered-cubic phase, which is reflected by the decay of the fourth-order Legendre reorientational correlation functions and angular Van Hove functions. Molecular rotation, however, is additionally hindered in the high-pressure liquid compared to the plastic crystal phase. The results obtained also reveal significant differences in the dielectric properties of the polymorphs due to the different dipolar orientational correlation characterizing each phase.

Heterogeneous character of supercritical water at 400 °C and different densities unveiled by simulation

Molecular dynamics simulations are used to examine the molecular microstructures and the “clustering” behavior of supercritical water at 400 °C and different densities.

Study of the microcharacter of ultrastable aqueous foam stabilized by a kind of flexible connecting bipolar-headed surfactant with existence of magnesium ion

In this paper, ultrastable aqueous foam stabilized by a kind of flexible connecting bipolar-headed surfactant alkyl polyoxyethylene sulfate (AE3S) with coexisting Mg(2+) was reported. Detailed molecular behaviors of AE3S in foam film with coexisting divalent cationic Ca(2+) or Mg(2+) were investigated by molecular dynamic simulation, comparing with the traditional surfactant sodium dodecyl sulfate (SDS), to find out how the microcharacter and array behavior of molecules in the foam film determined by molecular interaction effect the foam stability. It was found that the ultrastable foam film obtained by the cooperation of magnesium ions and AE3S was driven from two aspects: one is the favorable arrangement of surfactant molecules, and the other is the increase of capacity of foam films for resolutely holding water molecules deduced by a dipolar pair formed by the flexible connecting head groups of AE3S and hydrated Mg(2+) via intermolecular coactions, both related to the presence of magnesium ions. Foam lamella stability measurement and foam decay method were both used to evaluate the stability of foam. Fourier transform infrared (FT-IR) was used to detect the composition variation of foam film in the drainage process; the vibration peak of OH for water molecule shifted from the 3390 cm(-1) (being assigned to the bulk water integrated by hydrogen bonds) to 3685 cm(-1) (being assigned to the vibration of isolated water molecules) for the ultrastable foam film after complete drainage, which agreed very well with the molecular simulation results.

Density effect on infrared spectrum for supercritical water in the low- and medium-density region studied by molecular dynamics simulation

The origin of the line shape of the O-H stretch vibrational spectrum is analyzed for supercritical water in the low- and medium-density region by using classical molecular dynamics simulation for the flexible point-charge model, SPC/Fw. The spectrum calculated for the water model is in good agreement with the experimental one in the low-density region. The spectral origins in the low-density region of 0.01-0.04 g cm(-3) are assigned to a sharp peak due to the bond oscillation along the O-H vector and two broad bands due to the rotational coupling, by taking an isolated single molecule as a reference in the low-density limit. The bands due to the rotational coupling reduce in intensity with increasing density as the rotations are more hindered by the hydrogen-bonding interactions, and their intensities increase with increasing temperature due to the accelerated rotational motion. The O-H stretch oscillation in the time correlation function attenuates in a timescale comparable with the lifetime of the hydrogen bonds, and the spectra conditioned by the number of hydrogen bonds are dominantly controlled by the local solvation structure.

Hydrogen bond, electron donor-acceptor dimer, and residence dynamics in supercritical CO(2)-ethanol mixtures and the effect of hydrogen bonding on single reorientational and translational dynamics: A molecular dynamics simulation study

The hydrogen bonding and dynamics in a supercritical mixture of carbon dioxide with ethanol as a cosolvent (X(ethanol) approximately 0.1) were investigated using molecular dynamics simulation techniques. The results obtained reveal that the hydrogen bonds formed between ethanol molecules are significantly more in comparison with those between ethanol-CO(2) molecules and also exhibit much larger lifetimes. Furthermore, the residence dynamics in the solvation shells of ethanol and CO(2) have been calculated, revealing much larger residence times for ethanol molecules in the ethanol solvation shell. These results support strongly the ethanol aggregation effects and the slow local environment reorganization inside the ethanol solvation shell, reported in a previous publication of the authors [Skarmoutsos et al., J. Chem. Phys. 126, 224503 (2007)]. The formation of electron donor-acceptor dimers between the ethanol and CO(2) molecules has been also investigated and the calculated lifetimes of these complexes have been found to be similar to those corresponding to ethanol-CO(2) hydrogen bonds, exhibiting a slightly higher intermittent lifetime. However, the average number of these dimers is larger than the number of ethanol-CO(2) hydrogen bonds in the system. Finally, the effect of the hydrogen bonds formed between the individual ethanol molecules on their reorientational and translational dynamics has been carefully explored showing that the characteristic hydrogen bonding microstructure obtained exhibits sufficiently strong influence upon the behavior of them.