Binary supercritical fluid solution

Kim and Johnston (27), and Yonker and Smith (22) have used solute solvatochroism to determine the composition of the local solvent environment in binary supercritical fluids. In our laboratory we investigate solute-cosolvent interactions by using a fluorescent solute molecule (a probe) whose emission characteristics are sensitive to its local solvent environment. In this way, it is possible to monitor changes in the local solvent composition using the probe fluorescence. Moreover, by using picosecond time-resolved techniques, one can determine the kinetics of fluid compositional fluctuation in the cybotactic region. 

Additional evidence for PRODAN emitting from two environments is given by the temperature dependence of emission spectra in the CO2/CH3OH binary supercritical fluid (Figure 3). As the temperature is increased (at constant density) the contribution from the red-shifted peak decreases while the relative intensity of the peak at 400 nm increases. This indicates that the less polar environment surrounding PRODAN is preferred at higher temperatures. In other words, the specific cosolvent-solute interactions responsible for the red shift in emission are disrupted at elevated temperatures. 

We have utilized the static and dynamic fluorescence characteristics of an environmentally-sensitive solute molecule, PRODAN, to investigate the local solvent composition in binary supercritical fluids. In the two solvent systems studied (C02/1.57 mol% CH3OH and C02/1.44 mol% CH3CN), specific cosolvent-solute interactions are clearly evident. Time-resolved fluorescence emission spectra indicate that the cosolvent-solute interactions become more pronounced with time after excitation. Hence, the local composition of cosolvent around the excited-state solute becomes greater than that surrounding the ground-state solute. That is, the photon-induced increase in excited-state dipole moment drives picosecond cosolvent augmentation about PRODAN. 

UV-visible, fluorescence, and IR spectroscopy have been used to characterize the solvent strength of pure and mixed supercritical fluid solvents, and to study solute-solvent interactions. The use of spectroscopic probes for the determination of clustering of pure and binary supercritical fluids about solutes is discussed. Spectroscopic studies of solvent strength and solute-solvent interactions are valuable for the development of molecular thermodynamic theory, engineering models, and for the molecular design of separation and reaction processes. 

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. 

Bulk elastic modulus, of binary compound semiconductors, 22 145, 146-147t Bulk enzymes, from genetically engineered microbes, 22 480 Bulk erosion, 9 78 Bulk fluid velocity method, 16 688 Bulk gallium nitride, supercritical ammonia solution growth of, 14 96-97 Bulk gases... 

The evaluation of the sublimation pressure is a problem since most of the compounds to be extracted with the supercritical fluids exhibit sublimation pressures of the order of 10 14 bar, and as a consequence these data cannot be determined experimentally. The sublimation pressure is thus usually estimated by empirical correlations, which are often developed only for hydrocarbon compounds. In the correlation of solubility data this problem can be solved empirically by considering the pure component parameters as fitting-parameters. Better results are obviously obtained [61], but the physical significance of the numerical values of the parameters obtained is doubtful. For example, different pure component properties can be obtained for the same solute using solubility data for different binary mixtures. 

Solubilities of meso-tetraphenylporphyrin (normal melting temperature 444°C) in pentane and in toluene have been measured at elevated temperatures and pressures. Three-phase, solid-liquid-gas equilibrium temperatures and pressures were also measured for these two binary mixtures at conditions near the critical point of the supercritical-fluid solvent. The solubility of the porphyrin in supercritical toluene is three orders of magnitude greater than that in supercritical pentane or in conventional liquid solvents at ambient temperatures and pressures. An analysis of the phase diagram for toluene-porphyrin mixtures shows that supercritical toluene is the preferred solvent for this porphyrin because (1) high solubilities are obtained at moderate pressures, and (2) the porphyrin can be easily recovered from solution by small reductions in pressure. 

C02-solute systems in terms of a number of phase diagrams. E. L. Quinn and C. L. Jonesf °b(42] produced a number of papers in the 1920 s and 1930 s expanding this CO2 data, but an extensive study of CO2 was not made until much later in 1954, when A. W. Francisf produced a comprehensive report citing the solubility of 261 compounds and 464 ternary systems in liquid CO2 at 25 C. Since the 1970 s, the study of CO2 as a solvent has increased dramatically. In 1983, M. E. Paulaitis, V. J. Krukonis, R. T. Kumik and R. C. Reid produced a review called Supercritical Fluid Extractiod which detailed references on the solubility of a number of compounds in liquid and supercritical CO2. More recently Bartle et al. have compiled, from various references, binary phase diagrams for over 120 compounds mainly in supercritical CO2. 

So far the KBIs have been calculated for numerous binary systems, and the results were used to examine the solution behavior with regard to (1) local composition, (2) various models for phase equilibrium, (3) preferential solvation, and others One should also mention the use of the KB theory for supercritical fluids and mixtures containing supercritical components and for biochemical issues such as the behavior of a protein in aqueous mixed solvents. ... 

The objective of this paper is to propose a predictive method for the estimation of the change in the solubility of a solid in a supercritical solvent when another solute (entrainer) or a cosolvent is added to the system. To achieve this goal, the solubility equations were coupled with the Kirkwood-Buff (KB) theory of dilute ternary solutions. In this manner, the solubility of a solid in a supercritical fluid (SCF) in the presence of an entrainer or a cosolvent could be expressed in terms of only binary data. The obtained predictive method was applied to six ternary SCF-solute-cosolute and two SCF-solute-cosolvent systems. In the former case, the agreement with experiment was very good, whereas in the latter, the agreement was only satisfactory, because the data were not for the very dilute systems for which the present approach is valid. 2001 Elsevier Science B.V. All rights reserved. 

The addition of a small amount (usually less than 5 mol%) of a volatile cosolvent to a supercritical fluid (SCF) can lead to a very large enhancement in the solubility of a solute (up to several hundred percent) [1]. Similarly, the use of multiple solutes (usually binary) can enhance their solubilities [1]. These phenomena, often called the cosolvent and entrainer effects, have attracted attention both among experimentalists [2-7] and theoreticians [8-12]. Two types of ternary supercritical mixtures are of interest. 

Thermodynamic constraints to the SAS process can be summarized in the required miscibility between the liquid solvent and the supercritical antisolvent and the insolubility of the solute in the antisolvent and in the solvent-antisolvent mixture. Data are available for various binary mixtures liquid-supercritical fluid and can be described as type I using the classification of van-Konynenburg and Scott. If jet break-up is obtained and mass transfer is very fast, high-pressure VLEs of the ternary system liquid-I-solute-I-supercritical fluid can control the precipitation process. 

When determining the phase behavior of a ternary mixture, one of the pumps delivers the supercritical fluid of interest and the other delivers a binary mixture of fixed concentration. The overall loading of the solution delivered to the system is fixed by the flow rates of the pumps. The streams from each... 

In this chapter we describe the methods used to calculate solubility isotherms as well as the entire phase diagram for binary and ternary solute-SCF mixtures. The objective of the first part of the chapter is to discuss the relevant physical properties of the solute and solvent pair that are needed to describe the intermolecular forces in operation between molecules in a mixture that ultimately fix solubility levels. A brief description is provided on the application of solubility parameters to supercritical fluids. 

Fig. 11.6. A binary system (solution) that obeys the Lewis Fugacity Rule. The solution can be liquid, solid, gas, or supercritical fluid, although generally this rule is used for the latter two.

---