Local Density Inhomogeneities and Dynamics in Supercritical Water:  A Molecular Dynamics Simulation Approach

Abstract

Molecular dynamics atomistic simulations in the canonical ensemble (NVT-MD) have been used to investigate the "Local Density Inhomogeneities and their Dynamics" in pure supercritical water. The simulations were carried out along a near-critical isotherm (Tr = T/Tc = 1.03) and for a wide range of densities below and above the critical one (0.2 rho(c) - 2.0 rho(c)). The results obtained reveal the existence of significant local density augmentation effects, which are found to be sufficiently larger in comparison to those reported for nonassociated fluids. The time evolution of the local density distribution around each molecule was studied in terms of the appropriate time correlation functions C(Delta)rhol(t). It is found that the shape of these functions changes significantly by increasing the density of the fluid. Finally, the local density reorganization times for the first and second coordination shell derived from these correlations exhibit a decreasing behavior by increasing the density of the system, signifying the density effect upon the dynamics of the local environment around each molecule.

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

“…Song and Maroncelli 62 systematically examined the disagreement between their simulation and experiment in the case of supercritical fluoroform. In that paper Song and Maroncelli pointed out that a possible reason for this discrepancy could be the assumption of a linear proportionality between spectral shifts and local densities as well as the methodology employed to derive local densities from the Raman shifts observed by Saitow et al 67 In general, several recent studies have provided information about the density dependence of spectroscopic observables 24,33,34,62,[71][72][73] in supercritical fluids. From these studies we may see that, in general, there is a nonlinear bulk density dependence; however the relation between these spectroscopic observables with the local density is still not very clear.…”

“…The use of effective local densities to investigate local density inhomogeneities in supercritical fluids has been extensively discussed and justified in previous publications. 33,34,61,62 To calculate the effective local densities in SCW, we initially calculated the average coordination number N c corresponding to the first solvation shell of SCW at each state point,…”

“…One other characteristic property of SCW is the existence of significant local density augmentation effects. One of the authors revealed in quite recent publications 24,33 that the local density augmentation and enhancement factors are much more pronounced in the case of SCW in comparison with other HB and non-HB fluids. 24,[34][35][36] This behavior has been related to the nonlinear density dependence of the mean number of hydrogen bonds per water molecule, which exhibits much greater deviations form linearity in comparison with other HB fluids such as alcohols and ammonia.…”

Hydrogen bond dynamics of superheated water and methanol by ultrafast IR-pump and EUV-photoelectron probe spectroscopy

“…Previous studies have already shown the ability of employed SPC (water) and OPLS (methanol) models to describe correctly the structure and thermodynamics of these liquids under supercritical conditions. 38,42,77 In order to compare the dynamics in water and methanol, simulations were performed at temperatures of T H 2 O = 775 K and T MeOH = 650 K. For both liquids, these temperatures are about 120 K and 140 K higher than their critical temperatures, respectively. To match the fast experimental laser heating of the liquids a rectangular box of each liquid at normal densities were heated to the respective temperatures (T H 2 O = 775 K; T MeOH = 650 K), neglecting molecules possibly present in the vapor phase during the heating in the experiment (due to the dimensions of the simulation box only a few of them would be present at the liquid-vapor interface).…”

Modeling Solvation in Supercritical CO2

“…[71] As equential molecular dynamics (MD) andq uantum mechanics approach was used by Millot et al to compute the UV/Vis spectrum of organic dyes, [72] and more recently Born-Oppenheimer MD simulations were performed by Canuto's group to study the electronic spectrum of para-nitroaniline. [73,74] The organization of the solvent around the solute and its influence on the solution structural and dynamic properties were investigated by means of MD for small solutes, [75][76][77][78][79][80][81][82][83][84] polymers [85,86] and proteins, [87] and passivated gold nanoparticles. [88] Some work based on Car-Parrinello MD was also performed.…”