Supercritical-fluid studies, transition

A subsequent picosecond electronic absorption spectroscopic study of TPE excited with 266- or 355-nm, 30-ps laser pulses in cyclohexane found what was reported previously. However, in addition to the nonpolar solvent cyclohexane, more polar solvents such as THF, methylene chloride, acetonitrile, and methanol were employed. Importantly, the lifetime of S lp becomes shorter as the polarity is increased this was taken to be evidence of the zwitterionic, polar nature of TPE S lp and the stabilization of S lp relative to what is considered to be a nonpolar Sop, namely, the transition state structure for the thermal cis-trans isomerization. Although perhaps counterinmitive to the role of a solvent in the stabilization of a polar species, the decrease in the S lp lifetime with an increase in solvent polarity is understood in terms of internal conversion from to So, which should increase in rate as the S -So energy gap decreases with increasing solvent polarity. Along with the solvent-dependent hfetime of S lp, it was noted that the TPE 5ip absorption band near 425 nm is located where the two subchromophores— the diphenylmethyl cation and the diphenylmethyl anion—of a zwitterionic 5ip should be expected to absorb hght. A picosecond transient absorption study on TPE in supercritical fluids with cosolvents provided additional evidence for charge separation in 5ip. 

The first section features new approaches to investigating physicochemical properties. Its final two chapters facilitate the transition to the second section, on chemical reactions, a new topic of fundamental importance. Phase equilibria are described in the final section of principles. Here initial chapters are devoted to modeling, and the final chapters report solubility studies. The final three sections are devoted to important applications of supercritical fluids chromatography, fractionation and separation, and fuel applications. The chapters in each of these sections are also arranged so that there is a transition to more applied topics in the later chapters. 

The Diels-Alder cycloaddition reaction of maleic anhydride with isoprene has been studied in supercritical-fluid CO2 under conditions near the critical point of CO2 [759]. The rate constants obtained for supercritical-fluid CO2 as solvent at 35 °C and high pressures (>200 bar) are similar to those obtained using normal liquid ethyl acetate as the solvent. However, at 35 °C and pressures approaching the critical pressure of CO2 (7.4 MPa), the effect of pressure on the rate constant becomes substantial. Obviously, AV takes on large negative values at temperatures and pressures near the critical point of CO2. Thus, pressure can be used to manipulate reaction rates in supercritical solvents under near-critical conditions. This effect of pressure on reacting systems in sc-fluids appears to be unique. A discussion of fundamental aspects of reaction kinetics under near-critical reaction conditions within the framework of transition-state theory can be found in reference [759],... 

Gas flow processes through microporous materials are important to many industrial applications involving membrane gas separations. Permeability measurements through mesoporous media have been published exhibiting a maximum at some relative pressure, a fact that has been attributed to the occurrence of capillary condensation and the menisci formed at the gas-liquid interface [1,2]. Although, similar results, implying a transition in the adsorbed phase, have been reported for microporous media [3] and several theoretical studies [4-6] have been carried out, a comprehensive explanation of the static and dynamic behavior of fluids in micropores is yet to be given, especially when supercritical conditions are considered. Supercritical fluids attract, nowadays, both industrial and scientific interest, due to their unique thermodynamic properties at the vicinity of the critical point. For example supercritical CO2 is widely used in industry as an extraction solvent as well as for chemical... 

Nagano et al. [22] measured the CO2 uptake in Cgg in a study of the effects of supercritical fluid treatment, the aim of which was to remove solvent molecules from Cgg. Carbon dioxide was found to interact strongly with Cgg, and to have a remarkable effect on the orientational phase transition of Cgg crystals at 250 K. The kinetic features of the process suggested that CO2 absorbs inside the C g... 

The manufacturing factors that may be affected by the choice of a particular polymorphic form include granulation, milling and compression, stability (particularly for semisolid forms), amount of dose delivered in metered inhalers, crystallization from different solvents at different speeds and temperature, precipitation, concentration or evaporation, crystallization from the melt, grinding and compression, lyophilization, and spray drying. In the manufacturing processing, crystallization is a major problem and it can be avoided by a careful study of polymorphic transition, particularly in supercritical fluids. 

It should be noted that very little phase behavior data is available for ligands with CO2. Since p-diketones have been shown to be viable for the supercritical fluid extraction of a variety of metals, ranging from transition metals to lanthanides and actinides, we will focus on this set of compounds. In particular, the goal of this research is to determine the binary phase behavior of several P-diketones with CO2 because the phase behavior of the ligand/C02 systems is necessary for the design of in-situ chelation processes. Table I lists the lUPAC and abbreviated names for the p-diketones investigated here. These particular p-diketone ligands were chosen because they could be obtained commercially and have been used in a number of supercritical fluid extraction studies (1-4). 

As with the Tenax system, there is evidence here that the sorbent may be perturbed by the supercritical fluid. Wang and coworkers (T5) have shown that pressurized CO2 can severely plasticize polymers. Creep tests ( ) indicate that appreciable changes in bulk polymer moduli as well as differential changes in the glass transition temperature (up to 50 to 70 C) occur at pressures under 100 atmospheres in polystyrene. Additional dilation data for carbon dioxide absorbing into polystyrene (37) support the above study and indicate that the superimposition of a polymer phase change may also influence the retentive capacity of the polymeric sorbent. 

This brief survey begins in Sec. II with studies of the aggregation behavior of the anionic surfactant AOT (sodium bis-2-ethylhexyI sulfosuccinate) and of nonionic pol-y(ethylene oxide) alkyl ethers in supercritical fluid ethane and compressed liquid propane. One- and two-phase reverse micelle systems are formed in which the volume of the oil component greatly exceeds the volume of water. In Sec. Ill we continue with investigations into three-component systems of AOT, compressed liquid propane, and water. These microemulsion systems are of the classical Winsor type that contain water and oil in relatively equal amounts. We next examine the effect of the alkane carbon number of the oil on surfactant phase behavior in Sec. IV. Unusual reversals of phase behavior occur in alkanes lighter than hexane in both reverse micelle and Winsor systems. Unusual phase behavior, together with pressure-driven phase transitions, can be explained and modeled by a modest extension of existing theories of surfactant phase behavior. Finally, Sec. V describes efforts to create surfactants suitable for use in supercritical CO2, and applications of surfactants in supercritical fluids are covered in Sec. VI. 

Watkins, J. J. (1998)Phase transitions in polymer blends and block copolymers induced by selective dilation with supercritical CO2, Presented at the NATO Advanced Study Institute on Supercritical Fluids, Kemer, Turkey, July 12-24, 1998 [Manuscript is included in this volume]. 

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]. 

The discussion of chemical reactions here concentrates on fundamental studies being carried out principally in academic chemistry and chemical engineering departments in the USA, Europe, Japan and other countries. A review of some of the literature reports on near-critical fluids as reaction media is given first to illustrate the three perceived advantages of using this medium. Finally, there is an evaluation of the application of transition state theory to reactions in near-critical and supercritical fluids. 

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