Supercritical fluids solvent-solute interactions

Because the strength of such interactions depends on intermolecular distances, the relative importance of each contribution is density dependent. A large body of work exists that studies the various solvent-solute interactions using liquid solvents however, very few results have been reported for supercritical fluid solvents. 

This study was aimed at investigating solvent-solute interactions in the supercritical fluid (dense gas) regime, particularly... 

The wide variety of possible solvent-solute interactions requires that any scale used to quantify solvent properties will be complex. Unfortunately, no universally accepted scale of solvating power has been devised. It does not seem reasonable to develop an entirely new scale for supercritical fluid solvents, especially since it is desirable to compare the solvent behavior of supercritical fluids with that of liquid solvents. 

Local solvent compression. The next application of the solvato-chromic data will be to determine the magnitude of the local compression of a supercritical fluid solvent in the immediate environment of the solute. The of a dye such as phenol blue can be predicted in liquids where no specific interactions are present by treating the solvent as a homogeneous polarizable dielectric (22,29). The intrinsic "solvent strength", E, °, describes dispersion, Induction, and dipole-dipole forces and is given by (22). 

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. 

They were the calculation of the Hildebrand solubility parameter as a function of density using tabulated thermodynamic data for carbon dioxide and Raman spectroscopy of test solutes dissolved in supercritical carbon dioxide compared to liquid solvents to evaluate solvent-solute interactions. The results of these recent approaches indicated that while the maximum solvent power of carbon dioxide is similar to that of hexane, probably somewhat higher, there is some solvent-solute interaction not found with hexane as the solvent. The limiting solvent power of carbon dioxide is resolved by choosing the alternative of a supercritical fluid mixture as the mobile phase. The component added to the supercritical fluid to increase its solvent power and/or to alter the chromatograph column is referred to as the "modifier."... 

The determination of whether a given solute will dissolve in a particular supercritical fluid solvent depends on the free volume differences between the solute and the solvent, and the intermolecular forces in operation between solvent-solvent, solvent-solute, and solute-solute pairs in solution. We first address the issue of how intermolecular interactions affect solubility. To gain a better understanding of why certain solutes readily dissolve in certain SCF solvents, we use simplified expressions to interpret qualitatively the properties of the components that fix the strength of their intermolecular forces (Prausnitz, 1969). 

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. 

Removing an analyte from a matrix using supercritical fluid extraction (SEE) requires knowledge about the solubiUty of the solute, the rate of transfer of the solute from the soHd to the solvent phase, and interaction of the solvent phase with the matrix (36). These factors collectively control the effectiveness of the SEE process, if not of the extraction process in general. The range of samples for which SEE has been appHed continues to broaden. Apphcations have been in the environment, food, and polymers (37). 

A variety of equations-of-state have been applied to supercritical fluids, ranging from simple cubic equations like the Peng-Robinson equation-of-state to the Statistical Associating Fluid Theoiy. All are able to model nonpolar systems fairly successfully, but most are increasingly chaUenged as the polarity of the components increases. The key is to calculate the solute-fluid molecular interaction parameter from the pure-component properties. Often the standard approach (i.e. corresponding states based on critical properties) is of limited accuracy due to the vastly different critical temperatures of the solutes (if known) and the solvents other properties of the solute... 

Adding an organic solvent (such as methanol or acetone) to the supercritical fluid can modify its solvating properties. Since the polarity of C02 in its supercritical state (at 100 atm and 35 °C) is comparable to that of hexane, it can be altered by introducing a modifier. Nonetheless, isolating analyte from the matrix requires knowledge about the solubility and the transfer rate of solute in the solvent as well as chemical and physical interactions between matrix and solvent (Fig. 20.6). 

Solvation in supercritical fluids depends on the interactions between the solute molecules and die supercritical fluid medium. For example, in pure supercritical fluids, solute solubility depends upon density (1-3). Moreover, because the density of supercritical fluids may be increased significantly by small pressure increases, one may employ pressure to control solubility. Thus, this density-dependent solubility enhancement may be used to effect separations based on differences in solute volatilities (4,5). Enhancements in both solute solubility and separation selectivity have also been realized by addition of cosolvents (sometimes called entrainers or modifiers) (6-9). From these studies, it is thought that the solubility enhancements are due to the increased local density of the solvent mixtures, as well as specific interactions (e.g., hydrogen bonding) between the solute and the cosolvent (10). 

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. 

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. 

The main purpose of our work is the improvement of molecular level understanding of solute-solvent interactions under supercritical conditions. Unique nuclear magnetic resonance (5) techniques are employed to obtain new information about dynamics of molecules in supercritical fluids at high pressures. 

S. C. Tucker, Solvent density inhomogeneities in supercritical fluids, Chem. Rev., 99 (1999) 391—418 O. Kajimoto, Solvation in supercritical fluids Its effects on energy transfer and chemical reactions, Chem. Rev., 99 (1999) 355-89 S. Nugent and B. M. Ladanyi, The effects of solute-solvent electrostatic interactions on solvatochromic shifts in supercritical C02, J. Chem. Phys., 120 (2004) 874-84 F. Ingrosso and B. M. Ladanyi, Solvation dynamics of C153 in supercritical fluoroform a simulation study based on two-site and five-site models of the solvent, J. Phys. Chem. B, 110 (2006) 10120-29 F. Ingrosso, B. M. Ladanyi, B. Mennucci and G. Scalmani, Solvation of coumarin 153 in supercritical fluoroform, J. Phys. Chem. B, 110 (2006) 4953-62 Y. Kimura and N. Hirota, Effect of solvent density and species on static and dynamic fluorescence Stokes shifts of coumarin 153, J. Chem. Phys., Ill (1999) 5474 ... 

---