Local Density Augmentation Research Articles

Substantial breakdown of the hydrogen-bonding network, local density inhomogeneities and fluid-liquid structural transitions in supercritical octanol-1: A molecular dynamics investigation.

Molecular dynamics simulations have been employed to explore the hydrogen-bonding structure and dynamics in supercritical octanol-1 at a near-critical temperature and up to high densities and pressures. A substantial breakdown of the hydrogen-bonding network when going from ambient-liquid to supercritical conditions is revealed. The fraction of the non-hydrogen bonded molecules significantly increases in supercritical octanol-1, and a substantial decrease in the intermittent hydrogen-bond lifetime is observed. This behavior is also reflected on the maximum local density augmentation, which is comparable to the values obtained for non-polar and non-hydrogen bonded fluids. The existence of a structural transition from an inhomogeneous fluid phase to a soft-liquid one at densities higher than 2.0 ρc is also revealed. At higher densities, a significant change in the reorientational relaxation process is observed, reflected on the significant increase in the ratio of the Legendre reorientational times τ1R/τ2R. The latter becomes much higher than the value predicted by the Debye model of diffusive reorientation and the corresponding ratio for ambient liquid octanol-1. The non-polar tail of octanol-1 under supercritical conditions reorients more slowly in comparison with the polar tail. Interestingly, the opposite behavior is observed for the ambient liquid, further verifying the strong effect of the breakdown of the hydrogen bonding network on the properties of supercritical octanol-1. In accordance with the above-mentioned findings, the static dielectric constant of supercritical octanol-1 is very low even at high densities and pressures, comparable to the values obtained for non-polar and non-hydrogen bonded fluids.

Solvation structure and dynamics of favipiravir in supercritical CO2. A molecular dynamics investigation

The solvation structure and related dynamics of favipiravir diluted in supercritical CO2 in a wide density range (0.2–––2.0 ρc) along a near-critical isotherm (1.03 Tc) have been extensively studied by performing classical molecular dynamics simulations. The potential model of favipiravir developed in our previous studies was used in combination with the well-established EPM2 model of CO2 in the simulations. The results obtained have revealed that the local density augmentation of CO2 inside the first solvation shell of favipiravir is maximized at 0.67 ρc and is about 5.6 times larger than the corresponding value for pure supercritical CO2 at the same conditions. The local solvation structure around the favipiravir solute has also been investigated in terms of the atom–atom radial distribution functions. The intermittent residence dynamics of CO2 molecules in the first solvation shell of favipiravir, as well as the single translational and reorientational dynamics of favipiravir have also been investigated, revealing the important role of the local density augmentation of CO2 around the favipiravir solute upon these specific dynamic properties.* Corresponding authors.

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

Sub-to-supercritical properties and inhomogeneity of JP-10 using molecular dynamics simulation

JP-10, as one single-component aviation fuel, usually undergoes a sub-to-supercritical phase transition through channels due to the extreme temperature and pressure when applied in the regeneration cooling process. Predicting JP-10 properties during this transition has always been one task which is far from trivial. In this study, equilibrium molecular dynamics simulations with three forcefields (OPLS-AA, COMPASS (II) and TraPPE-UAf) are employed to model JP-10 under the pressure from 1.48 MPa to 6.00 MPa and temperature from 250 K to 900 K. The OPLS forcefield achieves the most satisfactory result in predictions of density, viscosity, thermal conductivity and critical parameters, with few mispredictions in the transcritical area. To further explore the divergences, inhomogeneity, as an inherent property of supercritical fluids which disturbs EMD simulations near the critical point, is described by Voronoi tessellation and quantitatively analyzed by evaluating the local density augmentation and enhancement factor under various conditions. Simulation results show an increasing tendency in local density augmentation of supercritical JP-10 with the decreasing of the bulk density in the range of this work.

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.

Local density augmentation of supercritical water probed by 4,4′-bpyH[rad] radical: A pulse radiolysis study

Solvatochromic shift of 4,4′-bpyH in aqueous solutions at elevated temperatures up to supercritical conditions and in various organic solvents with different dielectric constants, is investigated by pulse-radiolysis technique. 4,4′-bpyH shows a stronger solvent–solute interaction in water than in organic solvents, perhaps due to the hydrogen bond between 4,4′-bpyH and water. At 380°C, local density augmentation, namely ρlocal–ρbulk, in supercritical water becomes 280kgm−3 (ρbulk=208kgm−3), and the density enhancement factor is 8.9. Density fluctuation maximizes when ρbulk is around 120kgm−3. Density inhomogeneity decreases as temperature rises, but is still remarkable at 400°C.

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 T(c)) 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 CO(2) around the isomers have been found significantly larger than the corresponding values for pure supercritical CO(2). 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 ρ(c), 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.

Significant substitution effects in dipolar and non-dipolar supercritical fluids

Vibrational Raman spectra of C=C stretching modes of ethylene derivates (cis-C(2)H(2)Cl(2), cis-stilbene, and trans-stilbene) were measured in supercritical fluids along an isotherm as functions of their densities. The substitution effect of the Raman shift is so significant that a difference among three solutes can be 20 times and is observed similarly in dipolar (CHF(3)) and non-dipolar (CO(2)) fluids. In particular, the shifts of trans-stilbene were enormously large among all systems for studies of vibrational spectroscopies of supercritical fluids and were equivalent to those of typical hydrogen-bonded fluids. Such large shifts arising from the significant attractive energy between solute and solvent molecules were attributed to a site-selective solvation around a phenyl group, which was driven by a dispersion force in the absence of steric hindrance. We found that the absence of steric hindrance causes the significant local density augmentation. To the best of our knowledge, Raman experiments and their theoretical analysis are the first ones quantifying how the difference of steric hindrance produces solvation structures in solution as well as supercritical solutions.

Site-Selective Solvation in Supercritical CO2Observed by Raman Spectroscopy: Phenyl Group Leads to Greater Attractive Energy than Chloro Group

Vibrational Raman spectra of the C=C stretching modes of cis-stilbene and cis-1,2-dichloroethylene (C(2)H(2)Cl(2)) were measured in supercritical CO(2) in a density range of 0.08 < ρ(r) = ρ/ρ(c) < 1.5 at an isotherm of T(r) = T/T(c) = 1.02. As the fluid density increased, the peak frequencies of cis-stilbene and cis-C(2)H(2)Cl(2) shifted toward the low-energy side. The shifted frequencies of cis-stilbene were consistently greater than those of cis-C(2)H(2)Cl(2) in all density regions, by a factor of 4. By analyzing these density dependencies using the perturbed hard-sphere theory, the shifted frequencies were decomposed into attractive and repulsive components. By quantifying these components as a function of fluid density, we investigated how each solute is solvated in supercritical CO(2). The results indicate that the attractive energy between cis-stilbene and CO(2) is twice that between cis-C(2)H(2)Cl(2) and CO(2). A local density augmentation around the solute molecule was not observed in the cis-C(2)H(2)Cl(2)/CO(2) system, but it was observed in the cis-stilbene/CO(2) system because of site-selective solvation around the phenyl group of cis-stilbene. To the best of our knowledge, this is the first time that the site-selective solvation of a solute molecule has been observed using Raman spectral measurements of a solution system. Based on theoretical calculations and Raman spectral measurements of cis-stilbene in the supercritical fluid of dipolar CHF(3), it is concluded that a driving force for site-selective solvation is the dispersion force.

ChemInform Abstract: Homogeneous Organic Reactions as Mechanistic Probes in Supercritical Fluids

An interesting and intriguing aspect of supercritical fluid solutions is their microscopic inhomogeneities. The most well-known of these are the long-range densities fluctuations that occur in a pure fluid at conditions close to the critical point. However, there are also short-range inhomogeneities around dissolved solutes in supercritical fluids, which are frequently referred to as local density augmentation and local composition enhancements. Many questions have arisen about the potential influences that short-range solvation and long-range density fluctuations might have on complex chemical reaction schemes. In attempts to answer these questions, researchers have sought to study the effect of supercritical fluid solvents on simple unimolecular and bimolecular reactions. In doing so, the reactions themselves have become probes of the molecular structure and the influence of solvation on reactivity in supercritical fluids (SCFs). In this paper, the authors attempt to review those studies that involve simple reactions as probes of molecular structure and its effect on reactivity in SCFs. The paper discusses (1) the potential ways for solvation in supercritical fluids to affect reactivity and (2) the reactions that probe solvation in supercritical fluids, including diffusion-controlled reactions, kinetically-controlled reactions, equilibrium reactions, SCF solvent cage effects, and other important reactions and directions.

A study of solvation of benzaldehyde and cinnamaldehyde in CO 2 by molecular dynamics simulation

The solvation of benzaldehyde and cinnamaldehyde in CO 2 was simulated at 323 K at a density range from 0.135 to 0.807 g/cm 3. It was observed that the spatial distribution of CO 2 molecules around solutes closely follows the negative electrostatic potential in the solute molecules. The solvent density distribution maps and radial distribution maps at different densities provided the details of the local solvent density augmentation around each solute. In order to understand the difference of the local density augmentation, the interaction energies between CO 2 molecules and solutes were discussed.

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

Molecular dynamics simulations have been performed in a wide range of densities along a near critical isotherm of supercritical water in order to reveal the interconnection between the local hydrogen bonding (HB) network and several related dynamic properties. The results obtained have revealed a significant slowing down of reorientational dynamics of the water molecules as the value of the number of hydrogen bond per molecule increases and this is reflected on the increase in the reorientational correlation times. The calculated reorientational times exhibit also an increasing trend by increasing the bulk density, and this effect is more pronounced in the case of the first-order Legendre reorientational correlation functions. A clear nonlinear dependence of the librational mode frequencies of the water molecules on the augmented local density around them has also been revealed. This result could be regarded as an additional support of experimental observations suggesting the use of a nonlinear relation when analyzing the density dependence of spectroscopic peak frequencies in order to extract information about local density augmentation in supercritical fluids. The HB dynamics have been also investigated, revealing a plateau in the calculated HB lifetimes at intermediate and higher liquidlike densities and a small increase at low, gaslike densities.

Molecular Dynamics Simulation of Benzene−Propene−Cumene Mixtures in Different Phases

Molecular dynamics simulations have been performed to study the microscopic configuration and dynamic behavior of mixtures of benzene, propene, and cumene for the cumene synthesis process. The comparisons have been made for the intermolecular radial distribution functions of the binary and ternary mixtures at the conditions that are near, below, and above their respective critical points. The results have shown that in both binary and ternary mixtures propene molecules have a small tendency to cluster in the liquid state, but at supercritical conditions they tend to be uniformly distributed. Contrary to propene, cumene molecules have a tendency to cluster in ternary mixtures. A moderate local density augmentation is also found in the benzene-propene binary supercritical fluid. The excess functions for benzene-propene binary mixtures have shown that there exists an enhancement of the potential energy when benzene mixes with propene. This enhancement provides a rational explanation for the experimental critical properties, which exhibit the behavior of the nonmonotonous dependence of critical pressure on compositions.

Raman OH stretching frequency shifts in supercritical water and in O 2- and acetone-aqueous solutions near the water critical point

Raman OH stretching frequency shifts in supercritical water and in O 2- or acetone- aqueous solutions near the water critical point were observed to discuss the microscopic solution structure of these fluids. In neat water, the peak frequencies shifted almost linearly to the density except near the critical density. As the temperature approached the critical temperature, the density dependence of the shift became non-linear. This behavior was considered to be due to some microscopic structural non-uniformity caused by the density fluctuation. In the solutions, such non-linear density dependences were decreased, suggesting the decrease in the local density augmentation around water molecules.

Local Structural Effects and Related Dynamics in Supercritical Ethanol. 2. Hydrogen-Bonding Network and Its Effect on Single Reorientational Dynamics

The local hydrogen-bonding (HB) network and its possible interconnection with the single reorientational dynamics in pure supercritical (sc) ethanol have been systematically investigated by employing molecular dynamics simulation techniques. The results obtained reveal a nonlinear density dependence of the calculated average number of hydrogen bonds n(HB), similar to that of the calculated coordination numbers N(c), signifying also the interrelation between the local HB network and the local density augmentation in sc ethanol. Additionally, the HB dynamics were investigated in terms of several appropriate time correlation functions. The results obtained reveal that the density dependence of HB dynamics has some similarities with local density and residence dynamics, corresponding to very short length scales. Moreover, the effect of mutual reorientation on HB dynamics seems to be more important than that of mutual diffusion. Finally, the reorientational dynamics of several intramolecular vectors of ethanol have been systematically studied. From the results obtained, we may observe that the dynamics of several reorientational modes in the molecule exhibit significant differences between them. Furthermore, the effect of the HB state of each molecule on these dynamics has also been studied, revealing significant differences, especially in the case of the dynamics of HB free molecules.

Local Structural Effects and Related Dynamics in Supercritical Ethanol. 1. Mechanisms of Local Density Reorganization and Residence Dynamics

The length scale effects on the relaxation processes describing the local density reorganization and residence dynamics of pure supercritical ethanol have been systematically investigated by employing molecular dynamics simulation techniques. The calculated static local density augmentation and enhancement values of ethanol have been found to be comparable to those of methanol at similar conditions but significantly lower than those of the strong associating fluid water, indicating thus the effect of hydrogen-bonding interactions on the creation of local density inhomogeneities in supercritical fluids. The bulk density dependence of local density reorganization dynamics has been studied as a function of the shell cutoff radius, revealing a significant change at length scales higher than the position of the first maximum of the radial pair distribution function. At length scales higher than this cutoff, the local density inhomogeneities affect more significantly the reorganization of the local environment and this becomes even more apparent at length scales corresponding to the second solvation shell radius. Additionally, the residence dynamics for the region extending up to the first solvation shell of ethanol was investigated, and the results obtained reveal that the density dependence of local density dynamics exhibit differences in correspondence to local density dynamics, something which becomes also apparent at length scales higher than the position of the first peak of the radial pair distribution function.

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.

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 (T(r) = T/T(c) = 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(rho) 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.

Advanced dielectric continuum model of preferential solvation

A continuum model for solvation effects in binary solvent mixtures is formulated in terms of the density functional theory. The presence of two variables, namely, the dimensionless solvent composition y and the dimensionless total solvent density z, is an essential feature of binary systems. Their coupling, hidden in the structure of the local dielectric permittivity function, is postulated at the phenomenological level. Local equilibrium conditions are derived by a variation in the free energy functional expressed in terms of the composition and density variables. They appear as a pair of coupled equations defining y and z as spatial distributions. We consider the simplest spherically symmetric case of the Born-type ion immersed in the benzene/dimethylsulfoxide (DMSO) solvent mixture. The profiles of y(R) and z(R) along the radius R, which measures the distance from the ion center, are found in molecular dynamics (MD) simulations. It is shown that for a given solute ion z(R) does not depend significantly on the composition variable y. A simplified solution is then obtained by inserting z(R), found in the MD simulation for the pure DMSO, in the single equation which defines y(R). In this way composition dependences of the main solvation effects are investigated. The local density augmentation appears as a peak of z(R) at the ion boundary. It is responsible for the fine solvation effects missing when the ordinary solvation theories, in which z=1, are applied. These phenomena, studied for negative ions, reproduce consistently the simulation results. For positive ions the simulation shows that z>>1 (z=5-6 at the maximum of the z peak), which means that an extremely dense solvation shell is formed. In such a situation the continuum description fails to be valid within a consistent parametrization.

Solvation of 5‐fluorouracil in supercritical CO2

AbstractThe solvation properties of 5‐fluorouracil in supercritical carbon dioxide (SC‐CO2) is investigated by means of molecular dynamics computer simulations using a force field that treats all atoms explicitly. Solvation shell distribution functions and solvent density maps around the solute were computed at different supercritical temperatures in the range 308–328 K and densities varying from 0.117 to 0.94 g/cm3. At the lowest density, we detect a strongly inhomogeneous local density augmentation effect manifested as the clustering of solvent molecules above and below the solute molecular plane and around its carbonyl and NH groups. The free‐energy benefit for a CO2 molecule to be in the first solvation shell relative to the bulk is roughly twice the thermal energy at such conditions. The distribution of the relative orientation of CO2 with respect to the solute suggests that local bond multipolar solute–solvent interactions are responsible for the solvent density enhancement in these systems. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008