Phase transitions and supercritical fluids

Thomas Russell is Silvio O. Conte Distinguished Professor, Polymer Science and Engineering Department Director, Energy Frontier Research Center (EFRC), Polymer-Based Materials for Harvesting Solar Energy. His research interests are polymer-based nanoscopic structures, polymer-based nanoparticle assemblies, electrohydrodynamic instabilities in thin polymer films, surface and interfacial properties of polymers, polymer morphology kinetics of phase transitions, and supercritical fluid/polymer interactions. 

As it has appeared in recent years that many hmdamental aspects of elementary chemical reactions in solution can be understood on the basis of the dependence of reaction rate coefficients on solvent density [2, 3, 4 and 5], increasing attention is paid to reaction kinetics in the gas-to-liquid transition range and supercritical fluids under varying pressure. In this way, the essential differences between the regime of binary collisions in the low-pressure gas phase and tliat of a dense enviromnent with typical many-body interactions become apparent. An extremely useful approach in this respect is the investigation of rate coefficients, reaction yields and concentration-time profiles of some typical model reactions over as wide a pressure range as possible, which pemiits the continuous and well controlled variation of the physical properties of the solvent. Among these the most important are density, polarity and viscosity in a contimiiim description or collision frequency. 

Matrix isolation spectroscopy has proved an invaluable technique for the isolation and characterization of transition metal—noble gas complexes (see Table III). However, this technique has obvious limitations. Although photoproducts in low-temperature matrices can be made to react with added dopants, it is impossible to accurately predict their reactivity and mechanisms in solution at room temperature. Therefore, in the years following the original discovery of transition metal-noble gas interactions in matrices, new techniques have been used to probe these species in solution, gas phase, and supercritical fluids. 

The effect of dissolved CO2 on the miscibility of polymer blends and on phase transitions of block copolymers has been measured with spectroscopy and scattering (40). The shifts in phase diagrams with CO2 pressure can be pronounced. Polymer blends may be trapped kinetically in metastable states before they have time to phase separate. Metastable polymer blends of polycarbonate (PC) and poly(styrene-cn-acrylonitiile) were formed with liquid and supercritical fluid CO2 in the PCA process, without the need for a surfactant. Because of the rapid mass transfer between the CO2 phase and the solution phase, the blends were trapped in a metastable state before they... 

Solubility behavior of compounds in supercritical fluids is interpreted using a simplified solution model. Solid-fluid and liquid-fluid phase equilibria data are presented and discussions of the phase behavior are provided. Phase transitions and transport properties are briefly discussed. 

This chapter provides an introduction to supercritical fluid behavior. As a tutorial, qualitative fluid behavior is stressed rather than quantitative description. Solubilities in solid-fluid systems are interpreted with a simple fluid model. Enhancement factors are introduced to demonstrate the importance of repulsive forces. Intermolecular interactions in cosolvent systems are discussed. Liquid-fluid phase behavior and phase transitions in liquid-fluid and solid-fluid systems are briefly presented. Transport properties are briefly presented to stress their density dependence. 

The roots of this method can be traced back to the pioneoing work of the Rosenbluths in the 1950s [64]. However, the CCB method in reality is a direct descendant of the Scanning method of Meirovich [65-68], in partkular of the version for attractive random walks [68]. A related idea was introduced by Harris and Rice [69]. The method has recently attracted much intnest, and has been fully developed as a simulation tool through the work of Siepmann [42], Frenkel et al. [43], and Siepmann and Frenkel [44]. de Pablo et al. [45] implemented the CCB method for the off-lattice treatment of realistic polymer systems. The initial off-lattice applications have demonstrated that the method can be used in a wide variety of important problems in polymer systems, most notably the determination of equilibrium thermodynamic properties, chemical potentials of polymers, soluUlitks d gi t mol ades in polymer melts, studies of phase transitions, and polymer-sdivent interactions in supercritical fluids [70-72]. 

A key feature of SFC is the supercritical nature of the CO2 and the advantageous physical properties that result. Supercritical fluids have intermediate physical properties of both gas and liquid phases and exhibit the positive chromatographic qualities of both. Low viscosity and high solute diffusivity, relative to HPLC, result in faster analysis times, shorter reequilibration times, and higher throughput without loss in efficiency. A supercritical fluid is a defined state, not an additional state of matter. A fluid is supercritical when it remains above its critical temperature (Tc) and critical pressure (Pc). For CO2, Tc is 31 °C and Pc is 74 bar. Transition from a gas to a supercritical state occurs without a phase transition and without significant changes... 

The physical-chemical properties of a supercritical fluid are between those of liquids and gases supercritical fluids (SCFs) indicate the fluid state of a compound in pure substance or as the main component above its critical pressure (pc) and its critical temperature (Tc), but below the pressure for phase transition to the solid state, and in terms of SCF processing, a density close to or higher than its critical density. 

Microemulsions. Systems comprising microwater droplets suspended in an scCO T oil phase can be achieved with the use of appropriate surfactants, of which the best appear to be fluorinated. Microemulsions in supercritical hydrofluoro carbons are also possible. Potential may also exist for speciality coatings via low concentration solutions of fluorinated products in supercritical fluid for, e.g., thin-fitm deposition, conformal coatings, and release coatings. Supercritical CO2 will dissolve in formulated systems to improve flow and plasticize melt-processable materials to improve melt-flow characteristics and lower the glass transition temperature. 

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