Compressible Supercritical Fluids Research Articles

Atomistic simulation study of short pulse laser interactions with a metal target under conditions of spatial confinement by a transparent overlayer

The distinct characteristics of short pulse laser interactions with a metal target under conditions of spatial confinement by a solid transparent overlayer are investigated in a series of atomistic simulations. The simulations are performed with a computational model combining classical molecular dynamics (MD) technique with a continuum description of the laser excitation, electron-phonon equilibration, and electronic heat transfer based on two-temperature model (TTM). Two methods for incorporation of the description of a transparent overlayer into the TTM-MD model are designed and parameterized for Ag-silica system. The material response to the laser energy deposition is studied for a range of laser fluences that, in the absence of the transparent overlayer, covers the regimes of melting and resolidification, photomechanical spallation, and phase explosion of the overheated surface region. In contrast to the irradiation in vacuum, the spatial confinement by the overlayer facilitates generation of sustained high-temperature and high-pressure conditions near the metal-overlayer interface, suppresses the generation of unloading tensile wave, decreases the maximum depth of melting, and prevents the spallation and explosive disintegration of the surface region of the metal target. At high laser fluences, when the laser excitation brings the surface region of the metal target to supercritical conditions, the confinement prevents the expansion and phase decomposition characteristic for the vacuum conditions leading to a gradual cooling of the hot compressed supercritical fluid down to the liquid phase and eventual solidification. The target modification in this case is limited to the generation of crystal defects and the detachment of the metal target from the overlayer.

A high-pressure atomic force microscope for imaging in supercritical carbon dioxide

A high-pressure atomic force microscope (AFM) that enables in situ, atomic scale measurements of topography of solid surfaces in contact with supercritical CO(2) (scCO(2)) fluids has been developed. This apparatus overcomes the pressure limitations of the hydrothermal AFM and is designed to handle pressures up to 100 atm at temperatures up to ∼350 K. A standard optically-based cantilever deflection detection system was chosen. When imaging in compressible supercritical fluids such as scCO(2), precise control of pressure and temperature in the fluid cell is the primary technical challenge. Noise levels and imaging resolution depend on minimization of fluid density fluctuations that change the fluid refractive index and hence the laser path. We demonstrate with our apparatus in situ atomic scale imaging of a calcite (CaCO(3)) mineral surface in scCO(2); both single, monatomic steps and dynamic processes occurring on the (1014) surface are presented. This new AFM provides unprecedented in situ access to interfacial phenomena at solid-fluid interfaces under pressure.

The correlation between local and long-range structure in compressible supercritical fluids

We derive a family of local-density-specific radial distribution functions which enable us to analyze the fluid structure around a tagged solvent particle as a function of the local environment about that particle. With this new structural measure, we uncover a very strong correlation between the local density around a tagged atom and the surrounding long-range fluid structure in a neat, two-dimensional, Lennard-Jones fluid at a compressible supercritical state point. Indeed, we show that such an environmentally specific measure of structure provides an informative probe of compressible supercritical fluids, within which the distribution of accessible local solvent environments is typically very broad. Additionally, we find that these new functions provide a more sensitive measure of the mean size of the density inhomogeneities than does the correlation length, and thus that they provide an effective indicator of simulation convergence for molecular dynamics simulations performed in the supercritical regime.

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.

Effect of Critical Slowing Down on Local-Density Dynamics

Through molecular dynamics simulation we demonstrate that the time scale for local solvent reorganizations, which may control solute dynamics, can become very long for low- and near-critical-density supercritical fluids. We show that this can be ascribed to two processes, the first being due to direct, interparticle potential interactions, and the second, which dominates on all but the shortest length scales, being due to a coupling between the dynamics of the local solvent environment and those of the long-length-scale density fluctuations which are present in compressible supercritical fluids. Specifically, as the critical point is approached and the correlation length of these density inhomogeneities increases, we find that the associated slowing of these extended collective fluctuations, known as “critical slowing down”, generates a concomitant slowing of the local reorganization time.

Glass-like behavior in supercritical fluids: The effect of critical slowing down on solute dynamics

Compressible supercritical fluids exhibit extended density inhomogeneities which, due to their collective many-particle nature, fluctuate only very slowly, a phenomenon known as “critical slowing down.” Our computations demonstrate that these slowly fluctuating inhomogeneities may remain effectively frozen on the time scale of fast dynamic processes such as solute vibrational relaxation. Furthermore, we find that a given solute molecule may experience any of a broad distribution of effectively static local solvent environments and that each solute’s dynamical behavior will be controlled by its own local solvent environment. Consequently, an ensemble of solute molecules will relax with a correspondingly broad distribution of lifetimes. Although such inhomogeneous broadening effects are expected in amorphous solids and glasses, this is the first confirmation of their presence in supercritical fluids.

Design and Characterization of the Molecular Environment in Supercritical Fluids

Spectroscopic studies may be combined synergistically with phase equilibria data and molecular thermodynamic models to provide a better understanding of supercritical solutions to aid process design. Highly compressible supercritical fluids condense about solutes to form large physical clusters. This clustering phenomena has been characterized in terms of solubilities, partial molar volumes, solvatochromic shifts in UV-visible and fluorescence spectroscopy, Δv rxn for reversible reactions, Kirkwood-Buff solution theory, and computer simulation. In most cases, the clustering or the effect of clustering scales as the isothermal compressibility. The addition of a cosolvent, complexing agent, or surfactant to a fluid such as carbon dioxide can have large beneficial effects on solubilities and selectivities. Preferential solvation by co-solvents has been measured spectroscopically to develop and test advanced mixing rules. The complexing agent tributylphosphate increases the solubility of hydroquinone by a factor of 300. An anionic surfactant, aerosol-OT forms aggregates and reverse micelles in supercritical ethane, which can solubilize ionic substances such as tryptophan at levels of 0.4 wt %. The use of complexing agents and surfactants provides an opportunity to extend supercritical technology to hydrophilic substances, such as biomolecules.