Solute-fluid interactions in supercritical

Elucidation of Solute—Fluid Interactions in Supercritical CF H by Steady-State and Time-Resolved Fluorescence Spectroscopy... 

BETTS ET AL. Solute—Fluid Interactions in Supercritical CFJH... 

Betts, T. A., J. Zagrobelny, and F. V. Bright 1992, Investigation of solute-fluid interactions in supercritical CF3H A multifrequency phase and modulation fluoresence study . J. Supercritical Fluids 5, 48. 

Investigations of Solute—Cosolvent Interactions in Supercritical Fluid Media... 

Studying vibrational dynamics in supercritical fluids can aid in understanding the vibrational relaxation process as well as providing insights into the nature of solute-solvent interactions in supercritical fluids. One important result that emerged from the studies is the success of the theory to describe the data over a range of solvents, densities, and temperatures. The theory contains details of the solvent through the... 

Numerous experimental studies have been conducted on solute-solvent interactions in supercritical fluid solutions. In particular, issues such as the role of characteristic supercritical solvent properties in solvation and the dependence of solute-solvent interactions on the bulk supercritical solvent density have been extensively investigated. Results from earlier experiments showed that the partial molar volumes 02 became very large and negative near the critical point of the solvent (4-12). The results were interpreted in terms of a collapse of the solvent about the solute under near-critical solvent conditions, which served as a precursor for the solute-solvent clustering concept. Molecular spectroscopic techniques, especially ultraviolet-visible (UV-vis) absorption and fluorescence emission, have since been applied to the investigation of solute-solvent interactions in supercritical fluid solutions. Widely used solvent environment-sensitive molecular probes include Kamlet-Taft jt scale probes for polarity/polarizability... 

Sun et al. carried out a more systematic investigation of the TICT molecules DMABN and ethyl p-(A,A-dimethylamino)benzoate (DMAEB) in supercritical fiuoroform, CO2, and ethane as a function of fluid density (1). They found that the absorption and TICT emission spectral maxima shifted to the red with increasing fluid density. The results were comparable to those reported by Kajimoto et al. (51-55). More importantly, the spectral shifts and the fractional contribution of the TICT state emission changed with fluid density following the characteristic three-density-region pattern (Figures 11 and 12) (1). In fact, these results furnished the impetus for the development of the three-density-region solvation model for solute-solvent interactions in supercritical fluid solutions (2,3). 

The K, Py, and TICT solvation scales discussed above have been the basic techniques used in the investigation of solute-solvent interactions in supercritical fluid solutions. In addition, other methods have been applied for the same purpose, including the use of unimolecular reactions and vibrational spectroscopy and the probing of rotational diffusion the results obtained have been important to the understanding of the fundamental properties of supercritical fluids. 

Substantial evidence suggests that in highly asymmetric supercritical mixtures the local and bulk environment of a solute molecule differ appreciably. The concept of a local density enhancement around a solute molecule is supported by spectroscopic, theoretical, and computational investigations of intermolecular interactions in supercritical solutions. Here we make for the first time direct comparison between local density enhancements determined for the system pyrene in CO2 by two very different methods-fluorescence spectroscopy and molecular dynamics simulation. The qualitative agreement is quite satisfactory, and the results show great promise for an improved understanding at a molecular level of supercritical fluid solutions. 

Fluorescence Investigation of Cosolvent—Solute Interactions in Supercritical Fluid Solutions... 

Angus, S. Armstrong, B. de Reuck, K. M. eds., International Thermodynamic Tables of the Fluid State Carbon Dioxide. Pergamon Press, Oxford, 1976. Brennecke, J. F. Intermolecular Interactions in Supercritical Fluid Solutions from Fluorescence Spectroscopy. PhD Thesis, University of Illinois, Urbana, 1989. Brennecke, J. F. Eckert, C. A. AIChE J.. 1989, 35(9), 1409-1427. 

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. 

The effect of temperature, pressure and density on solute retention (k1) in supercritical fluid chromatography (SFC) has been well studied.(1-6) Retention in SFC depends upon both solute solubility in the fluid and solute interaction with the stationary phase. The functional relationship between retention and pressure at constant temperature has been described by Van Wasen and Schneider. ( 1 ) The trend in retention is seen to depend on the partial molar volume of... 

In this paper we present new results of fluorescence spectroscopy studies of dilute organics in pure supercritical fluids. We compare those results to make observations about the strength of solute/solvent interactions in solution, especially near the critical point, as well as the in ortance of solute/solute interactions even at extremely low concentrations. 

In this paper we have presented speotroscopio evidence of unusual Intermolecular Interactions in supercritical fluids. The strength of the solute/solvent interactions is much stronger near the critical point of the solvent, indicating an aggregation or clustering of the solvent around the solute. This is deduced from relative intensity ratios in the spectra and overall fluorescence intensities. In addition, the formation of excimers in dilute solutions indicates the importance of solute/solute interactions, even at concentrations as low as 5 x 10 mol fraction. 

The numerator quantifies the effect of hydrostatic pressure on the fugacity of the solid phase. The exponential term is known as the Poynting correction (17). The denominator quantifies the fluid phase intermolecular interactions and density effects. Note that the enhancement factor is dependent on the solid volume as well as the interactions in the supercritical fluid. A solute with a large solid molar volume will have a larger enhancement factor than a solute with a smaller solid molar volume at the same temperature and pressure when the interactions in the supercritical phase are identical. To further understand the molecular interactions in supercritical fluids, it is interesting to decompose the enhancement factor into these two effects. We may define a fluid enhancement factor, Ep, and a Poynting enhancement factor, Ep,... 

Morita, A. and O. Kajimoto 1990, Solute-solvent interaction in nonpolar supercritical fluid A clustering model and size distribution . J. Phys. Chem. 94, 6420. 

On the topics of solute-solvent and solute-solute clustering, there is a significant number of publications by research groups from around the world, demonstrating the tremendous interest of the international research community. This chapter is a review of representative literature results, especially those based on molecular spectroscopy and related experimental techniques. Discussion of the fundamental properties of supercritical fluids will be within the context of enhanced solute-solvent and solute-solute interactions in supercritical fluid solutions, and the current understanding of the reasonably well-established solute-solvent clustering model and the somewhat controversial solute-solute clustering concept will be presented. 

The investigation of high-critical-temperature supercritical fluids is a more challenging task. One of the significant difficulties associated with these studies is probe-molecule thermal stability many molecular probes commonly used with ambient supercritical fluids decompose at the temperatures required by these high-critical-temperature fluids. Fortunately, pyrene can be employed for such tasks. Several reports have been made of the use of pyrene as a molecular probe to investigate solute-solvent interactions in high-critical-temperamre supercritical fluids (e.g., pentane, hexane, heptane, octane, cyclohexane, meth-cyclohexane, benzene, toluene, and water) (44,48,49). In supercritical hexane... 

---