A study of Ar-N₂ supercritical mixtures using neutron scattering, molecular dynamics simulations and quantum mechanical scattering calculations

The microscopic structure of Ar-N₂ supercritical mixtures was obtained using neutron scattering experiments at temperatures between 128.4 - 154.1 K, pressures between 48.7 - 97.8 bar and various mole fractions. Molecular Dynamics simulations (MD) were used to study the thermodynamics, microscopic structure and single molecule dynamics at the same conditions. The agreement between experimental and theoretical results on the intermolecular structure was very good. Furthermore, a new explicitly-correlated coupled cluster potential energy surface was obtained for the Ar-N₂ van der Waals complex. The ab initio potential energy surface (PES) was found in agreement with the MD interaction potential. The global minimum of the ab initio PES Dₑ = 98.66 cm⁻¹ was located at the T-shaped geometry and at the intermolecular equilibrium distance of Rₑ = 7.00a₀. The dissociation energy of the complex was determined to be D₀ = 76.86 cm⁻¹. Quantum mechanical (QM) calculations on the newly obtained PES were used to provide the bound levels of the complex. Finally, integral and differential QM cross sections in Ar + N₂ collisions were calculated at collision energy corresponding to the average temperature of the experiments and at room temperature

Local Density Inhomogeneities and Dynamics in supercritical water: A molecular dynamics simulation Approach

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 ρc - 2.0 ρc). 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 Δρl(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 k the system, signifying the density effect upon the dynamics of the local environment around each molecule. © 2006 American Chemical Society

Local intermolecular structure and dynamics in binary supercritical solutions. A molecular dynamics simulation study of methane in carbon dioxide

The binary supercritical mixture methane-carbon dioxide (SC CH4-CO2) with mole fraction of methane X = 0.2 was investigated at temperature T = 323.15 K and density ρ = 0.23814 g/cm3 corresponding to a pressure 9.94 MPa, by using the molecular dynamics (MD) simulation technique in the canonical (NVT) ensemble. The local intermolecular structure of the fluid was investigated in terms of the appropriate pair radial distribution functions, which have been calculated and analyzed. The estimated average local coordination numbers and mole fractions around the species in the mixture have been used to explore the repulsive or attractive nature of the fluid. The results obtained have led to the conclusion that the SC CH4-CO2 mixture at these conditions might be characterized as a repulsive one. This result supports the conclusions of a previously reported experimental work by Baglin et al. [F.G. Baglin, S.K. Murray, J.E. Daugherty, T.E. Palmer, W. Stanbery, Mol. Phys. 98 (2000) 409.]. The single reorientational dynamics of both the solute and solvent species in the mixture were also investigated by calculating the appropriate time correlation functions. The observed time dependence of these functions are discussed in relation with the local environment around the species, which in the case of the solute molecules seems to be strongly affected. © 2005 Elsevier B.V. All rights reserved

Molecular simulations of benzene and hexafluorobenzene using new optimized effective potential models: Investigation of the liquid, vapor-liquid coexistence and supercritical fluid phases

The development of new effective intermolecular potential models of benzene and hexafluororbenzene, capable in reproducing the thermodynamic and structural properties of molecular systems in a wide range of thermodynamic state points has been presented and discussed. Subsequently, the properties of the fluids have been investigated by employing molecular dynamics and Monte Carlo simulation techniques. The main purpose of this study was to reveal information concerning the liquid state, vapor-liquid equilibrium and supercritical phase properties of these fluids. In the case of the supercritical phase, we mainly focused on the behavior of local density inhomogeneities and related properties. Our calculations reveal that the local density augmentation is much more pronounced in the case of hexafluorobenzene. The origins of possible resemblances and discrepancies with available experimental data have been also systematically discussed and related to our conclusions reported in previous publications. The local density reorganization dynamics as a function of the bulk density and the size of the local region have been also studied, revealing a significant density and length scale dependence similar to the ones presented for other pure supercritical fluids in previous publication of our group. © 2009 Elsevier B.V. All rights reserved

Investigation of the local composition enhancement and related dynamics in supercritical CO2 -cosolvent mixtures via computer simulation: The case of ethanol in CO2

The supercritical mixture ethanol-carbon dioxide (EtOH-C O2) with mole fraction of ethanol XEtOH 0.1 was investigated at 348 K, by employing the molecular dynamics simulation technique in the canonical ensemble. The local intermolecular structure of the fluid was studied in terms of the calculated appropriate pair radial distribution functions. The estimated average local coordination numbers and mole fractions around the species in the mixture reveal the existence of local composition enhancement of ethanol around the ethanol molecules. This finding indicates the nonideal mixing behavior of the mixture due to the existence of aggregation between the ethanol molecules. Furthermore, the local environment redistribution dynamics have been explored by analyzing the time correlation functions (TCFs) of the total local coordination number (solvent, cosolvent) around the cosolvent molecules in appropriate parts. The analysis of these total TCFs in the auto-(solvent-solvent, cosolvent-cosolvent) and cross-(solvent-cosolvent, cosolvent-solvent) TCFs has shown that the time dependent redistribution process of the first solvation shell of ethanol is mainly determined by the redistribution of the C O2 solvent molecules. These results might be explained on the basis of the C O2 -C O2 and EtOH-C O2 intermolecular forces, which are sufficiently weaker in comparison to the EtOH-EtOH hydrogen bonding interactions, creating in this way a significantly faster redistribution of the C O2 molecules in comparison with EtOH. Finally, the self-diffusion coefficients and the single reorientational dynamics of both the cosolvent and solvent species in the mixture have been predicted and discussed in relationship with the local environment around the species, which in the case of the EtOH molecules seem to be strongly affected. © 2007 American Institute of Physics

Solvation structure and dynamics of cis - And trans -1,2 dichloroethene isomers in supercritical carbon dioxide. A molecular dynamics simulation study.

Molecular dynamics simulation techniques have been employed to investigate the solvation structure and dynamics in dilute mixtures of cis- and trans-1,2-dichloroethene in supercritical carbon dioxide. The calculations were performed for state points along a near-critical isotherm (1.02 Tc) over a wide range of densities, using new developed optimized potential models for both isomers. The similarities and differences in the solvation structures around each isomer have been presented and discussed. The local density augmentation and enhancement factors of CO2 around the isomers have been found significantly larger than the corresponding values for pure supercritical CO2. The dynamic local density reorganization has been investigated and related to previously proposed relaxation mechanisms. The density dependence of the calculated self-diffusion coefficients has revealed the existence of a plateau in the region of 0.7-1.1 ρc, where the local density augmentation exhibits the maximum value. The reorientational dynamics of the C≡C bond vector have been also studied, exhibiting significant differences between the two isomers in the case of the second-order Legendre time correlation functions. © 2011 American Chemical Society

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 ρc, further supporting previous suggestions based on the interpretation of experimental thermodynamic data. © 2017 Elsevier B.V

The Polar Cosolvent Effect on Caffeine Solvation in Supercritical CO2-Ethanol Mixtures: A Molecular Modeling Approach

The effect of the addition of a small amount of ethanol cosolvent in supercritical CO2 on the solvation structure and dynamics of caffeine in a mixed supercritical solvent has been investigated using a systematic multiscale molecular modeling approach. An effective interaction potential model has been employed for caffeine, using the intramolecular geometry and charge distribution from quantum chemical calculations performed in the present treatment and adopting well-established Lennard-Jones parameters from the literature. The solvation structure and related dynamics have been further investigated by means of classical molecular dynamics simulations. The results obtained have revealed an enhancement of the local mole fraction of ethanol around caffeine due to the formation of hydrogen bonds between caffeine and its nearest ethanol molecules. This effect becomes less pronounced as the pressure of the system increases due to the denser packing of CO2 molecules in the first solvation shell of caffeine. The reorientational dynamics of caffeine is controlled by the intermittent hydrogen-bond dynamics, and its translational diffusion has been found to be significantly lower in comparison with the values obtained for ethanol and CO2. The pressure effects on the self-diffusion have also been found to be more pronounced in the cases of CO2 and EtOH in comparison with caffeine. The findings of the present study confirm a previous hypothesis in the literature, according to which polar solutes approach the polar domains formed by the alcohol aggregates and become more easily dissolved in the mixed CO2-ethanol solvent than in pure supercritical CO2. © 2021 American Chemical Society

Structure and dynamics of liquid CS2: Going from ambient to elevated pressure conditions

Molecular dynamics simulation studies were performed to investigate the structural and dynamic properties of liquid carbon disulfide (CS2) from ambient to elevated pressure conditions. The results obtained have revealed structural changes at high pressures, which are related to the more dense packing of the molecules inside the first solvation shell. The calculated neutron and X-ray structure factors have been compared with available experimental diffraction data, also revealing the pressure effects on the short-range structure of the liquid. The pressure effects on the translational, reorientational, and residence dynamics are very strong, revealing a significant slowing down when going from ambient pressure to 1.2 GPa. The translational dynamics of the linear CS2 molecules have been found to be more anisotropic at elevated pressures, where cage effects and librational motions are reflected on the shape of the calculated time correlation functions and their corresponding spectral densities. © 2016 Author(s)

The effect of intermolecular interactions on local density inhomogeneities and related dynamics in pure supercritical fluids. A comparative molecular dynamics simulation study

The effect of intermolecular interactions of different strength on the local density inhomogeneities in pure supercritical fluids (scfs), with different intramolecular structure, was investigated by employing molecular dynamics (MD) simulation techniques. The simulations were performed at state points along an isotherm close to the critical temperature of each system (Tr) T/Tc = 1.03). The molecular fluids under study have been chosen on the basis of the electrostatic character of their intermolecular interactions as follows: monatomic, dipolar and hydrogen bonding (HB), quadrupolar, and octupolar. In the case of dipolar scfs, their HB nature when present was systematically explored and related to the behavior of the created local density inhomogeneities at all densities. The results obtained reveal strong influence of the dipolar and HB interactions of the investigated systems upon the local density augmentation. We found that this effect is fairly larger in the case of the dipolar and HB fluids (H2O, CH3OH, and NH3) compared to those for the non-dipolar ones (Xe, CH4, CO2, and N2). In the case of sc CO2, the dependence of the local density augmentation on the bulk density is in agreement with available experimental data as also reported previously. The estimated average number of hydrogen bonds per molecule (nHB) in these HB fluids shows an analogue nonlinear trend compared to the behavior of the average coordination numbers Nco(ρ) of a particle with bulk density. The local density dynamics of the first and second solvation shell of each fluid were further analyzed and related to our previously proposed [Skarmoutsos, I.; Samios, J. J. Chem. Phys. 2007, 126, 044503] different time-scale relaxation mechanisms. Finally, the effect of the different strength of the molecular interactions corresponding to these fluids upon the local density dynamics has also been revealed in the behavior of the predicted appropriate time correlation functions and their corresponding correlation times. © 2009 American Chemical Society