Local Density Enhancement Research Articles

Local density enhancement in neat supercritical fluid due to attractive intermolecular interactions

Short-range structure in the neat supercritical fluid consisting of polar molecules is investigated by measuring Raman spectrum of fluoroform (CHF 3) at reduced temperature T r= T/ T c=1.02. The spectra are analyzed by the Schweizer–Chandler model, and the spectral shift is decomposed into attractive and repulsive components as a function of density varied by a factor of 50. The attractive component shows larger shift than the one anticipated from the uniform molecular distribution. A local density enhancement due to the attractive intermolecular interactions is definitely observed in the neat supercritical fluid and evaluated against bulk density.

Rotational Relaxation in Supercritical CO2

Though there are numerous studies of solute molecule rotational dynamics in ordinary liquids and supercritical fluids, few such studies have focused on the molecules of a neat supercritical fluid. In the present study, weinvestigate the structure and rotational dynamics of supercritical carbon dioxide at a temperature 1% above its critical temperature and over a 20-fold range of densities about the critical density. Our simulations reveal the presence of density inhomogeneities and thus regions of modest local density enhancement in this fluid over a significant range of bulk densities. Although these inhomogeneities account for the weak bulk density dependence of the CO 2 orientational correlation times in the near-critical regime, they are not reflected in a concomitant variation in the observed total rotational friction. Caution is warranted, however, in interpreting the usual diffusive dynamics parameters in this system because only at densities in excess of the critical density may the dynamics be characterized as diffusive; only at lower densities are orientational correlations found to decay on a time scale equal to or shorter than that characterizing the angular velocity correlations.

Response to “Comment on ‘Self-diffusion near the liquid–vapor critical point’ ” [J. Chem. Phys. 116, 6379 (2002)

It was predicted by us [J. Chem. Phys. 114, 4912 (2001)] that the presence of local density enhancements in a neat supercritical fluid near its liquid–vapor critical point should generate a weak anomaly in the fluid's self-diffusion constant. Herein, we examine possible reasons why the limited number of experimental studies conducted to date on diffusion constants in near-critical supercritical fluids do not provide clear evidence of this predicted weak anomaly.

Local density enhancement in neat supercritical fluids: dependence on the interaction potential

We study the local structural properties of neat supercritical fluids. Three forms of intermolecular potentials are considered – Lennard-Jones, Morse, and Stillinger-Weber. The inhomogeneous integral equation theory is employed to compute the local coordination number of a tagged solvent particle. The theory is shown to be in excellent agreement with the simulation for all intermolecular potentials throughout the entire density range. The degree of local density augmentation in neat supercritical fluids is shown to be sensitive to the details of intermolecular potential, with a particular important parameter being the ratio of the potential well-depth to the critical temperature of the fluid.

Study of Local Density Enhancement in Near-Critical Solutions of Attractive Solutes Using Hydrostatic Hypernetted Chain Theory

The properties of infinitely dilute solutes in near-critical solvents often show a strong change in curvature when plotted against the bulk fluid density, in some cases even a plateau region is observed. In this article, it is shown that this phenomenon, due to a local solvent density enhancement, can be construed as a consequence of the linear response of fluids having large compressibilities when the solutes interact strongly with the solvent molecules. The hydrostatic hypernetted chain (HHNC) solution of the Ornstein−Zernike (OZ) equation for inhomogeneous fluids was used to calculate the solvent−solute radial distribution function and the number of solvent molecules that surround the solute particle for different solute−solvent interaction energies and reduced temperatures. The use of HHNC eliminates physically unsound features of the calculations, which employ theories for homogeneous fluids, and it also limits the extent and range of the local density enhancement. It is shown that the performance of...

Local density augmentation in attractive supercritical solutions. III. How important is the solute–solvent interaction range?

We study the local solute–solvent structure in dilute supercritical solutions, using as a model system a dilute Yukawa solute in a supercritical Lennard-Jones fluid. Our primary interest is in the effect of the solute–solvent interaction range on the local solvent density around the solute. We employ the integral equation theory for inhomogeneous fluids to calculate the solute–solvent structural properties. The theory is shown to be in excellent agreement with Monte Carlo simulations and to provide a substantial improvement over the integral equation theory formulated for homogeneous fluids. In particular, it is demonstrated that the homogeneous theory greatly overestimates the local density enhancement for long-ranged solute–solvent interactions in the highly compressible supercritical regime.

Terahertz absorption spectra of supercritical CHF 3 to investigate local structure through rotational and hindered rotational motions

Far infrared absorption spectra of neat supercritical fluoroform (CHF 3) are measured with terahertz (THz) radiation. The spectra covering from 10 to 100 cm −1 are obtained at reduced temperature T/ T c=1.02 on densities varied by a factor of 200. As density increases, dominant component of spectra changes from rotational to hindered-rotational motions. However, the change is nonlinear to the density. Such specificity arises from difference between bulk and local densities, and the most enhanced local density is observed near the thermodynamic state of maximum density fluctuation. In the pure fluid system, the relationship between density fluctuation and local density enhancement is experimentally presented.

Pressure Tuning of Reaction Equilibrium of Esterification of Acetic Acid with Ethanol in Compressed CO2

The esterification of acetic acid and ethanol in compressed CO2 was studied at 333.2 K and at pressures up to 16 MPa, and the original molar ratio of the components was CO2:CH3COOH:CH3CH2OH:p-toluenesulfonic acid (catalyst) = 90:5:5:0.05. The phase behavior and the isothermal compressibility (kT) of reaction system was also determined under the reaction conditions. The conversion increases with increasing pressure in the two-phase region and reaches a maximum in the critical region of the reaction system where the system just becomes one phase. Then the conversion or apparent equilibrium constant Kx decreases with pressure after the pressure is higher than the critical value. In the supercritical region where the system is one phase, Kx is closely related to the isothermal compressibility and degree of inhomogeneity of the fluid, which is often referred to as the local density and/or local composition enhancement. At the higher pressures the “clustering” in the reaction system is not significant and the Kx is close to that of the reaction in the absence of CO2.

Temperature dependence of the structure of a 0.06 mole fraction tertiary butanol-water solution

Hydrogen/deuterium isotopic substitution neutron diffraction techniques have been used to measure the solute–solute, solute–solvent and solvent–solvent structural correlation functions in a 0.06 mole fraction solution of tertiary butanol in water. The measurements were made at two temperatures, 25 °C and 65 °C. Data analysis to the level of intermolecular orientational correlation functions was performed by the Empirical Potential Structure Refinement (EPSR) technique in combination with a spherical harmonic coefficient analysis. The results reveal in particular a subtle temperature dependence of the intermolecular structure of the solvent water. This structural perturbation is suggestive of a hydrophobic hydration induced, local density enhancement in the solvent water that could relate to the entropic driving force underlying the hydrophobic interaction mechanism.

Self-diffusion near the liquid–vapor critical point

While detailed descriptions of critical anomalies are well known for thermodynamic variables, the effect of critical fluctuations on microscopic properties is much less well understood. Herein we use the results of molecular dynamics simulation, for the distribution of local densities around a tagged particle in a neat Lennard-Jones fluid, to evaluate the behavior of the self-diffusion coefficient in the critical region (Tr≈1.05; 0.4<ρr<1.6). We find a weakly anomalous behavior at near critical densities which we attribute to both the broadening of this local-density distribution and the enhancement of mean local density.

Preferential solvation in supercritical fluids: An integral equation study

A statistical mechanical study of preferential solvation in supercritical fluids is presented. The study is focused on attractive mixtures, which are characterized by local density and composition enhancement in the vicinity of the solute molecule. The solute–solvent radial distribution functions are obtained from two versions of the integral equation formalism, both of which are designed for treating inhomogeneous systems. Model calculations are performed for a ternary Lennard-Jones mixture composed of a dilute solute and two solvent species, which are identical except for their interaction strength with the solute. It is shown that theoretical results agree well with Monte Carlo simulations in reproducing local solvent density and composition augmentation induced by the solute.

Global three‐dimensional MHD simulation of a space weather event: CME formation, interplanetary propagation, and interaction with the magnetosphere

A parallel adaptive mesh refinement (AMR) finite‐volume scheme for predicting ideal MHD flows is used to simulate the initiation, structure, and evolution of a coronal mass ejection (CME) and its interaction with the magnetosphere‐ionosphere system. The simulated CME is driven by a local plasma density enhancement on the solar surface with the background initial state of the corona and solar wind represented by a newly devised “steady state” solution. The initial solution has been constructed to provide a reasonable description of the time‐averaged solar wind for conditions near solar minimum: (1) the computed magnetic field near the Sun possesses high‐latitude polar coronal holes, closed magnetic field flux tubes at low latitudes, and a helmet streamer structure with a neutral line and current sheet; (2) the Archimedean spiral topology of the interplanetary magnetic field is reproduced; (3) the observed two‐state nature of the solar wind is also reproduced with the simulation yielding fast and slow solar wind streams at high and low latitudes, respectively; and (4) the predicted solar wind plasma properties at 1 AU are consistent with observations. Starting with the generation of a CME at the Sun, the simulation follows the evolution of the solar wind disturbance as it evolves into a magnetic cloud and travels through interplanetary space and subsequently interacts with the terrestrial magnetosphere‐ionosphere system. The density‐driven CME exhibits a two‐step release process, with the front of the CME rapidly accelerating following the disruption of the near‐Sun closed magnetic field line structure and then moving at a nearly constant speed of ∼560 km/s through interplanetary space. The CME also produces a large magnetic cloud (> 100 RS across) characterized by a magnetic field that smoothly rotates northward and then back again over a period of ∼2 days at 1 AU. The cloud does not contain a sustained period with a strong southward component of the magnetic field, and, as a consequence, the simulated CME is somewhat ineffective in generating strong geo‐magnetic activity at Earth. Nevertheless, the simulation results illustrate the potential, as well as current limitations, of the MHD‐based space weather model for enhancing the understanding of coronal physics, solar wind plasma processes, magnetospheric physics, and space weather phenomena. Such models will provide the foundation for future, more comprehensive space weather prediction tools.

Origins of Atom-Centered Local Density Enhancements in Compressible Supercritical Fluids

We used molecular dynamics simulation of a neat two-dimensional Lennard-Jones fluid to examine how the distribution of local densities found around an atom varies as the critical point is approached. Through this analysis, we discovered that mean local density enhancements arise as a necessary and direct consequence of the long-range density inhomogeneities present in such fluids, thereby establishing a relationship between the local and long-length-scale phenomena. Additionally, we uncovered a second, “potential-induced” mechanism which generates mean local density enhancements at low bulk densities. The competition between these two mechanisms of local density enhancement formation enables us to explain why these enhancements are not maximized at the critical density, as well as to explain how the location of this maximum will depend on intrinsic experimental parameters such as the size of the local region and the strength of the solute−solvent potential interaction.

Local density augmentation in attractive supercritical solutions: Inhomogeneous fluid approach

We study the local solute–solvent structure in supercritical solutions, using as a model system a dilute Lennard-Jones solute in a supercritical Lennard-Jones fluid. The study is focused on attractive mixtures, which are characterized by enhanced local solvent density around the solute molecule. We employ the integral equation theory for inhomogeneous fluids to calculate the solute–solvent radial distribution function and the local coordination number of the solute. We show that this theory is in excellent agreement with Monte Carlo simulations and provides a substantial improvement over the integral equation theory formulated for homogeneous fluids. Using the inhomogeneous fluid theory, we perform a detailed study of the local density enhancement in attractive supercritical mixtures, and analyze its dependence on the model potential parameters and on the solvent thermodynamic conditions.

HST ultraviolet observations of rapid variability in the accretion-disc wind of BZ Cam

We present results from time-resolved Hubble Space Telescope(HST) GHRS ultraviolet spectroscopy of the nova-like cataclysmic variable BZ Cam. Extensive changes are recorded in the extended absorption troughs of the UV resonance line profiles, which betray a highly unsteady and continuously variable supersonic outflow. The large-scale line-profile changes are almost exclusively confined to blueshifted velocities, although the Si iv and C iv emission components reveal orbital phase-linked velocity shifts. The character of the accretion-disc wind of BZ Cam can alter radically on time-scales down to 100 s or less. Correlated behaviour is noted in lines of low and high ionization stages, which may reflect localized density changes. Aside from the incidence of enormous line-profile changes on remarkably short time-scales, BZ Cam provides an important benchmark for the simultaneous presence of very strong absorption and well-developed emission components in Si iv, C iii and C iv, for an estimated low-inclination (i<40°) or possibly even face-on system. We illustrate the problems posed for pure-scattering line-synthesis models in matching the UV line profiles of BZ Cam, and set constraints for alternative thermal emission sources. A phenomenological interpretation is that the high-speed wind of BZ Cam is highly irregular in nature, and the outflow is riddled with structure arising from localized density enhancements. It is tempting to link our empirical results to recent hydrodynamic simulations of unsteady radiatively driven accretion-disc winds, such as those of Proga, Drew & Stone. Ultimately, line profiles synthesized from the hydrodynamic codes would have to match the substantial absorption equivalent width changes in the UV lines of BZ Cam, over the very short flow times across the line formation regions, i.e. the overall contribution of the, structured, disc-projected wind has to be substantially variable.

Local density enhancement in dilute supercritical solutions

We study the structure around a dilute Lennard-Jones (LJ) solute in a supercritical LJ fluid using integral equation theories and Monte Carlo simulation. We show that the Percus–Yevick approximation can be seriously in error in the near-critical region. The solute's coordination number as a function of the solvent density can be nearly linear, or quite non-linear, depending on the solute–solvent interaction. These results are rationalized as a crossover from a low-density energy-dominated regime to a high-density entropy-(packing-) dominated regime, and are discussed within the context of the local density enhancement concept.

Three-dimensional MHD simulation of coronal mass ejections

Abstract A parallel adaptive mesh refinement (AMR) scheme for solving the governing equations of ideal magnetohydrodynamics (MHD) in three space dimensions is used herein to investigate the structure and evolution of a coronal mass ejection (CME) propagating from the solar surface at 1 R s out into the inner heliosphere to distances approaching 1 2 AU . The highly parallelized solution algorithm makes use of modern finite-volume numerical methodology to provide a combination of high solution accuracy and computational robustness. Numerical results are discussed for a CME driven by local plasma density enhancement with a simplified representation of the background solar wind. The latter approximates the interplanetary magnetic field and two-stream nature of the solar wind for conditions near solar minimum. The predicted evolution and structure of the CME are described. The results demonstrate the potential of the parallel adaptive MHD model for enhancing the understanding of coronal physics and solar wind plasma processes.

What causes the vibrational lifetime plateau in supercritical fluids?

Using molecular dynamics simulation, we show that the vibrational relaxation lifetime of a diatomic in a two-dimensional supercritical rare-gas fluid exhibits the same unexpected plateau behavior as was observed experimentally for W(CO)6 in supercritical CO2 by Fayer and co-workers, and we link this behavior to average local solvent density enhancements arising from the solvent’s near-critical density fluctuations.

Solvent–solute interactions and the Raman CH stretching spectrum of cyclohexane-d11. II. Density dependence in supercritical carbon dioxide

We have measured the isotropic Raman CH stretching spectrum of cyclohexane-d11 in supercritical CO2 at 49.7 °C and in liquid CO2 at room temperature over a range of densities from 0.2ρc to 2ρc, where the critical number density ρc for CO2 is 6.4 nm−3. The axial and equatorial CH stretching bands in the spectrum shift to lower frequencies and broaden with increasing density. As was the case in an earlier study of cyclohexane-d11 in liquid solvents [G. J. Remar and R. A. MacPhail, J. Chem. Phys. 103, 4381 (1995)], the “perturbed hard-fluid model” of Ben-Amotz and Herschbach provides a satisfyingly consistent description of the observed shifts in terms of competing contributions from repulsive and attractive solute–solvent forces along the CH bond. In particular, when the repulsive contribution to the shift is calculated according to the prescription developed in the liquid solution study, the attractive contribution is found to scale linearly with the density and with the polarizability derivative of the CH bond, as predicted by the model. The ratio of the equatorial to axial linewidths has a density-independent value of 1.2, nearly the same value found for the liquid solutions and numerically equivalent to the ratio of polarizability derivatives for the CH bonds. This equivalence is consistent with Schweizer and Chandler’s theoretical result for the width of a band that is inhomogeneously broadened by attractive force fluctuations, but the density dependence is not; their result would predict a nonlinear density dependence with a maximum near ρc, whereas the observed linewidths show a nearly linear dependence on density. Neither the frequency shifts nor the linewidths show any clear evidence for a “local solvent density enhancement” that would be predicted for this mixture near the critical point. In the accompanying paper, Frankland and Maroncelli describe molecular-dynamics simulations of cyclohexane in supercritical CO2 that reproduce the observed linewidths nearly quantitatively. They show convincing evidence that the linewidths are dominated by binary, collisional interactions between the hydrogen and the solvent, and they discuss the apparent absence of a density enhancement.

Strong Gravitational Lensing on the Hubble Deep Field

The Hubble Deep Field, henceforth HDF, (Williams et al 1996) is a unique data set for studies of faint galaxies. A small area was chosen well away from known local density enhancements and sources of obscuration. It was imaged for 30 complete orbits in each of four filters centered on wavelengths λ300, 450, 606, 814 nm. There are three, contiguous WFPC2 frames and a solid angle 15,500 arcsec2 is usable. This tiny region is teeming with galaxies. At least 2500 significant features can be identified down to R = 30, though it is not clear that these are all entire galaxies.