Supercritical Argon Research Articles

Comparison between integral equation method and molecular dynamics simulation for three-body forces: Application to supercritical argon

The prediction of the structural and thermodynamic properties of supercritical argon has been carried out by two independent routes: semianalytical calculations and numerical simulations. The first one is based on the hybridized mean spherical approximation (HMSA) conjugated with an effective pair potential that incorporates multipole dispersion interactions. The second one uses a very recent numerical simulation technique, inspired by the Car–Parrinello method [van der Hoef et al., J. Chem. Phys. 111, 1520 (1999)], which contains an effective quantum-mechanical representation of the underlying electronic structure. The latter approach allows us to treat the contribution of the three-body effects as well, and to validate the use of an effective pair potential for them in the framework of the self-consistent integral equation method. For all the supercritical argon states studied, the results obtained with the semianalytical approach are in good agreement with the predictions of the numerical simulation. Here it is shown that HMSA remains competitive with molecular dynamics simulation when the triple-dipole and the dipole–dipole–quadrupole three-body terms are taken into account.

Hysteresis-Associated Pressure-Shift-Induced Water Adsorption in Carbon Micropores

The remarkable adsorption increase of water-preadsorbed microporous carbons of different micropore widths w under a water vapor atmosphere upon introduction of supercritical methane or argon was measured at 303 K. The adsorption increment reached a maximum 1−2 h after the introduction of supercritical gas, decreasing gradually to the equilibrium after about 80 h. The precise quantitative analysis of gas-phase components after and before the introduction of supercritical gas using gas chromatography showed that this uptake was ascribed to additional adsorption of water induced by the introduction of supercritical gas. A variety of experimental data lead to the following hysteresis-associated adsorption mechanism of the water vapor. The transient compression of water vapor upon the introduction of supercritical gas gives rise to an increase of the partial pressure of water near the microporous carbon, inducing a predominant adsorption along the adsorption isotherm of water. Upon relaxation of the transient compression, additionally adsorbed water is desorbed along the desorption scanning curve.

Structural and thermodynamic description of supercritical argon with ab initio potentials

The self-consistent integral equation method is applied to calculate the structure and the thermodynamic properties of supercritical argon considered in a recent neutron-scattering experiment by Pfleiderer et al. [J. Chem. Phys. 111, 2641 (1999)]. Two kinds of potentials, different in nature, are used for the calculations. One is an empirical standard potential and the other is founded upon ab initio quantum chemical calculations. The small discrepancies between the two approaches are discussed, and the results are compared to the recent measurements for structure and experimental thermodynamic properties as well.

Contactless Weighing Up to High Pressures For Analysing Adsorption And Density of Fluids

Adsorption of supercritical fluids methane, nitrogen and argon by active carbons was studied up to a pressure of 500 bar. A three-parameter isothermal equation was used to represent the adsorption equilibrium. This isothermal equation is based on a physical model conception which had already been used for the modelling of adsorption processes with a pressure up to 150 bar. Beside the exact knowledge of the measurable parameters pressure, temperature and fluid composition, the density of the adsorbate are essential for the evaluation of the adsorption analysis. The fluid density can be determined either via equations of state, which is normally the most practicable and fastest way, or via lift measurements of a lowering body in the fluid based on the principle of Archimedes. This work represents and discusses the question of to what extent the fluid density determined under real conditions via equations of state, using, for example, equation of Bender, corresponds to the fluid density measured under high-pressure.

Viscosity dependence and solvent effects in the photoisomerization of cis-stilbene: Insight from a molecular dynamics study with an ab initio potential-energy function

Molecular-dynamics simulations of the photoisomerization of cis-stilbene in supercritical argon were performed. The stilbene molecule is represented by ab initio quantum chemistry, while the solvent, the interaction with solvent, and the time evolution were described by classical mechanics. Reaction rate constants are estimated and their dependence on temperature, pressure, and viscosity are investigated. Agreement with available experimental data was obtained. Our simulations strongly suggest a minimum on the excited-state potential-energy surface at a gauche conformation which is very rapidly reached after excitation, which leads to nonequilibrium barrier transitions. Specific solvent effects were identified. Implications on the current opinion on stilbene photoisomerization are discussed.

The structure of fluid argon from high-pressure neutron diffraction and ab initio molecular dynamics simulations

The structure of supercritical argon at 350 K over a broad range of density (pressures between 5 to 90 MPa) is determined experimentally by high-pressure neutron diffraction, and theoretically from quantum chemically calculated ab initio pair potentials applied in molecular dynamics simulations. The origins of small discrepancies between theory and experiment are discussed. Together with previous results at 85 K in the liquid state, the overall agreement between theory and experiment, over a wide range of state points, is found to be quite good. This is remarkable, as no empirical parameters are used for the calculations.

On-Line Nonmetal Detection for Argon Supercritical Fluid Extraction Using Inductively Coupled Plasma Optical Emission Spectroscopy

This paper describes the development of a solventless instrumental method for determining organic contaminants in soil by coupling argon supercritical fluid extraction (SFE) to inductively coupled plasma atomic emission spectroscopy (ICP-AES). In this method, organic compounds are first extracted by Ar SFE, transferred to the ICP directly in the supercritical (SC) argon, fragmented in the plasma, and then determined via their nonmetal atomic emissions. Supercritical Ar is superior to SC CO(2) for use in ICP-AES because it overcomes the disadvantages of plasma blowout, noisy baselines, and CO(2) interference in the determination of carbon. All instruments employed were commercially available, and the interface between the SFE and the torch of the ICP consisted of a simple glass capillary tube. Four nonmetals, C, S, P, and Si, were selected for this preliminary study. The selectivities obtained for these nonmetals, referenced to carbon, were found to be 345, 38 000, and 1400 for sulfur, phosphorus, and silicon, respectively. With the exception of carbon, the mass detection limits are in the range of 0.06-1.8 μg. For this work, the mass detection limit of carbon is 66 μg and is limited by the smallest aliquot of material that can be injected by syringe. Finally, total petroleum hydrocarbons (TPHs) were determined in two "real world" contaminated soil samples via the carbon emission at 247.9 nm. Dodecane was used as the standard reference compound because its response was found to be similar to other petroleum hydrocarbons (e.g., kerosene, gasoline, no. 1 fuel oil, and no. 2 fuel oil). Additionally, it was found necessary to add a small amount (i.e., 250 mg) of drying agent to the SFE vessel in order to prevent the plasma from extinguishing from the extracted water. The results of the on-line Ar SFE/ICP-AES determinations of TPH in the two samples agreed well with those obtained from conventional off-line CO(2) SFE and off-line Ar SFE.

Molecular dynamics simulation with an ab initio potential energy function and finite element interpolation: The photoisomerization of cis-stilbene in solution

An interpolation scheme for potential energy surfaces is presented. It employs a regular grid and finite element interpolation. The aim is the reduction of the computational expense for molecular dynamics simulation with a quantum chemical potential energy function. The methods used are described in detail. The feasibility is demonstrated and the efficiency and accuracy are evaluated for the photoisomerization of cis-stilbene in supercritical argon, using an ab initio configuration- interaction treatment for the first electronically excited state of the stilbene molecule and classical force fields for the solvent–solute interactions (quantum mechanical/molecular mechanical molecular dynamics). The number of required quantum chemical calculations of energy and gradients was substantially reduced compared to a simulation not using the interpolation scheme. On the other hand, the impact on the accuracy is insignificant.

Extraction of Petroleum Hydrocarbons from Soil Using Supercritical Argon

The use of supercritical argon is described for the extraction of petroleum hydrocarbons from soil samples. Argon is an attractive solvent because it is inexpensive and inert. Additionally, it has a clear spectral window in the infrared region which makes it useful for on-line (i.e., directly coupled) experiments. Spiking studies conducted with gasoline, no. 1 fuel oil, and no. 5 fuel oil on sand, loam, and clay show that component recovery rates for argon supercritical fluid extraction (SFE) generally increase with increasing pressure and/or temperature. The highest recovery rates (and recoveries) were obtained for Ar SFE at 500 atm and 150 °C. Under these conditions, the components of the gasoline and no. 1 fuel oil spikes could be recovered in as little as 12 min. However, the no. 5 fuel oil components could not be quantitatively removed from the loam and clay matrixes even for extraction times as long as 100 min. We also show in this work that Ar SFE performs similarly to CO(2) SFE for petroleum hydrocarbon contamination in real-world soil samples under moderate pressure and temperature conditions. Specifically, Ar SFE and CO(2) SFE have similar recoveries and reproducibilities, but Ar SFE requires a slightly longer extraction time.

Heterogeneous Atomistic-Continuum Representations for Dense Fluid Systems

We present a formulation and numerical solution procedure for heterogeneous atomistic-continuum representations of fluid flows. The ingredients from atomistic and continuum perspectives are non-equilibrium molecular dynamics and spectral element, respectively; the matching is provided by a classical procedure, the Schwarz alternating method with overlapping subdomains. The technique is applied to microscale flow of a dense fluid (supercritical argon) in a complex two-dimensional channel.

A Linear Gradient Theory Model for Calculating Interfacial Tensions of Mixtures

In this research work, we assumed that the densities of each component in a mixture are linearly distributed across the interface between the coexisting vapor and liquid phases, and we developed a linear gradient theory model for computing interfacial tensions of mixtures, especially mixtures containing supercritical methane, argon, nitrogen, and carbon dioxide gases at high pressure. With this model it is unnecessary to solve the time-consuming density profile equations of the gradient theory model. The model has been tested on a number of mixtures at low and high pressures. The results show excellent agreement between the predicted and experimental IFTs at high and moderate levels of IFTs, while the agreement is reasonably accurate in the near-critical region as the used equations of state reveal classical scaling behavior. To predict accurately low IFTs (σ < 0.1 mN/m), an equation of state with proper scaling behavior at the critical point is at least required.

Solvation Ultrafast Dynamics of Reactions. 12. Probing along the Reaction Coordinate and Dynamics in Supercritical Argon

In this paper, our focus is on the influence of the solvent density on the caging, recombination dynamics, and the nature of the reaction coordinate of iodine in supercritical argon at pressures of 0−2500 bar. Femtosecond probing with widely tunable pulses allows us to directly resolve the geminate recombination of iodine atoms and the subsequent relaxation processes. A nonzero recombination yield is found at argon pressures as low as 200 bar, and this yield increases strongly with increasing solvent density. The mechanism involves recombination onto the A/A‘ states. At high pressures, a large fraction of the iodine atoms undergo an ultrafast “in-cage” recombination which is measured on the subpicosecond time scale at 2500 bar of argon. In addition, a fraction of the iodine atoms break through the solvent cage and begin a diffusive motion through the rare-gas solvent. Experimental evidence is presented and indicates that this diffusive motion leads to reencounters and subsequent recombination of the geminate iodine pair. This diffusive recombination occurs on a significantly longer time scale than the rapid “in-cage” recombination. The newly-formed iodine molecules undergo vibrational relaxation within the A/A‘ state, and the dynamics of this process and its dependence on the solvent density are revealed. A key concept here is the solvent density-induced control of the rigidity of the first solvent shell surrounding the dissociating iodine atoms. As shown before [Liu et al. Nature 1993, 364, 427], such studies of solvation present a unique opportunity of examining the microscopic influence of the solvent structure on reaction dynamics in clusters and solutions.

Spectra and temperatures of diffusion flames at high pressures to 1000 bar

AbstractDiffusion flames have been produced by injection of oxygen into homogeneous dense mixtures of methane or hydrogen with supercritical water or argon. The mixture temperatures were around 500°C, pressures were varied between 50 and 1000 bar. The fuel concentrations were 30 mole percent or lower. Flames appeared spontaneously. Apparatus and procedure are described. Diffusion flame photographs to 1000 bar are shown. Emission spectra from 275 to 600 nm were recorded with a grating spectrometer and 1024 channel diode array. Emission in the visible is not of black body character and does not permit pyrometric temperature measurement. Flame temperature determination was based on the intense rotational emission of OH‐radicals in the near UV, produced by vibrational 0–0 and 1–0 transitions of the electronic A2Σ–X2∏ transition. The 0–0 band is near 306 nm. A fit program to determine rotational temperatures is developed which includes the influence of pressure broadening. The temperatures obtained for CH4 and H2 in supercritical water increase with pressure and reach 4000 to 4500 K at 1000 bar. Rotational temperatures obtained for dense argon environment are lower: 3000–3200 K. Adiabatic temperatures and calculated values for one‐dimensional diffusion flames are given for comparison.

Femtochemistry at high pressures. The dynamics of an elemantary reaction in the gas—liquid transition region

An extension of femtochemistry to the gas—liquid transition region is reported for an elemantary reaction, the dissociation and recombination of iodine in supercritical argon reaching a density of 30.5 mol ℓ −1. Three phenomena are systematically studied in the range 0 to 2000 bar of argon: the change of the coherent nuclear motion with solvent density, the rate of predissociation, and the caging by the solvent. The approach offers a unique way for examining the dynamics in real time from the gas to the liquid density limit.

Prewetting transition in supercritical argon

From the low frequency measurements of the dielectric constant of ultra low impurity liquid argon, we obtain the density of the liquid in the vicinity of the critical point. Along the 50 atm isobar slightly above the critical temperature, we find apparent discontinuity in the density, suggesting proximity to a prewetting transition. A simple mean field theoretical analysis provides a qualitative description of some aspects of this behavior Nous avons obtenu la densite de l'argon au voisinage du point critique a partir de mesures de la constante dielectrique a basse frequence. Nous avons observe une discontinuite de la densite le long de l'isobare correspondante a 50 atm a une temperature un peu au-dessus de la temperature critique. Cette discontinuite suggere une transition de pre-mouillage. Un calcul simple, selon le modele du champ moyen, reussit a rendre compte de quelques aspects des resultats de mesures

Spectroscopic studies of cryogenic fluids: Benzene in argon and helium

Energy shifts and bandwidths of the 610 vibronic feature of the 1B2u←1A1g optical absorption spectrum of benzene dissolved in supercritical argon and helium, and in liquid argon are reported as a function of pressure, temperature, and density. Benzene/Ar solutions display red shifts of the 610 transition with increasing density but the dependence is found to be nonlinear at high densities. Benzene/He solutions evidence blue shifts of the 610 transition as a function of increasing density which also becomes nonlinear at high densities. Only small spectral shifts are recorded if the density is kept constant and pressure and temperature are varied simultaneously. In addition, a small density independent temperature effect on the transition energy shift is identified. Experimental results are compared to dielectric (Onsager–Böttcher and Wertheim) and quantum statistical mechanical (Schweizer–Chandler) theories of solvent effects on solute absorption energy. Reasonably good agreement between experiment and theory is found only for the benzene/Ar system at relatively low densities. The theory fails to predict energy shifts for both the benzene/He and high density benzene/Ar systems. This result is different from the findings for the benzene/N2 and benzene/C3H8 solutions and can be interpreted qualitatively in terms of competition between dispersive attractive and repulsive interactions as a function of density. The failure of the theory to describe these transition energy shifts is attributed to the omission of explicit repulsive interactions terms in the theoretical models employed.

Comment on "Molecular-dynamics simulation of excess-electron transport in simple fluids"

A recent treatment of the saturation drift velocity ${v}_{\mathrm{sat}}$ of electrons in dense, supercritical argon [Leycuras and Levesque, Phys. Rev. A 32, 1180 (1985)] gives positive values of ${\mathrm{dv}}_{\mathrm{sat}/\mathrm{dn}}$ at n/${n}_{c}$=1.2\char21{}1.6, whereas the experimental values are negative in both the supercritical gas and the liquid at these densities [Jahnke, Meyer, and Rice, Phys. Rev. A 3, 734 (1971); Huang and Freeman, Phys. Rev. A 24, 714 (1981)]. Other difficulties are mentioned.

Three-body contributions to the thermodynamic properties of supercritical argon

Numerical calculations of the thermodynamic properties of argon were performed using Zwanzig perturbation theory in which the reference radial distribution functions were derived from the Percus–Yevick equation in conjunction with an accurate pair potential (HFD–C). Three-body contributions were approximated by an Axilrod–Teller induced triple dipole potential and the Kirkwood superposition approximation to the triple radial distribution function. Agreement with experimental data is very good over a range of densities up to twice the critical density and to a temperature approaching the critical. Deviation of the model from experiment is evident near the critical point, due to onset of macroscopic fluctuations.

Solvent perturbations of extravalence excitations of atomic Xe by rare gases at high pressures

In this paper we present the results of an experimental study of the lowest doublet excitations (1S0→3P1 and 1S0→1P1) of atomic xenon in dense supercritical fluid neon and helium and the 1S0→1P1 xenon excitation in dense supercritical and subcritical fluid argon. The spectral shift, the linewidth, the first moment, and the second moment of the absorption bands exhibit a strong density and a weak temperature dependence. The semiclassical statistical theory of line shapes was applied for the calculations of the first moment, the second moment, and the entire line shape. By fitting to the experimental moments over a broad density range, the excited state guest–host interaction potential parameters were derived. An analysis of the density dependence of the splitting and of the intensity ratio of the Xe doublet perturbed by high pressure rare gases was provided.

Medium perturbations of atomic extravalence excitations

In this paper we present the results of an experimental study of the lowest 1S0→3P1 extravalence electronic excitation of atomic xenon in dense supercritical and subcritical fluid argon over the density range 0.1–1.4 g⋅cm−3, spanning the temperature region 80–300°K. Solvent perturbations were characterized in terms of the spectral shift, the linewidth and the first and the second moments of the absorption band. These energetic parameters exhibit a weak temperature sensitivity and a strong density dependence. The experimental data were analyzed in terms of the semiclassical theory of line broadening extended to account for guest–host and host–host correlations and incorporating realistic pair potentials in the ground and in the excited electronic states. The semiclassical theory results in manageable expressions for the first and for the second moments of the absorption band which are expressed in terms of a difference guest–host interaction potential together with the solute–solvent and the solvent–solvent radial distribution functions. The theory provides a semiquantitative account for the weak temperature dependence and for the marked density dependence of the energetic parameters specifying solvent perturbations and was utilized to extract quite reliable information concerning the excited state Xe(3P1)+Ar(1S0) potential.