Supercritical xenon

The first system we consider is the solute iodine in liquid and supercritical xenon (1). In this case there is clearly no IVR, and presumably the predominant pathway involves transfer of energy from the excited iodine vibration to translations of both the solute and solvent. We introduce a breathing sphere model of the solute, and with this model calculate the required classical time-correlation function analytically (2). Information about solute-solvent structure is obtained from integral equation theories. In this case the issue of the quantum correction factor is not really important because the iodine vibrational frequency is comparable to thermal energies and so the system is nearly classical. 

Holroyd RA, Wishart JF, Nishikawa M, Itoh K. (2004) Reactions of charged species in supercritical xenon as studied by pulse radiolysis. J Phys Chem S 107 7281-7287. 

The observed fast formation of excimers in supercritical xenon, occurring in the first 50 ps, corresponds to a second order rate constant for reaction (la) of 7.5 x 10 M s at a pressure of 67 bar. The density in this case was 1.33 g/cc and the ion concentration 1.5 the pulse width was 7 ps. ° This measured rate can be compared to the theoretical rate for electron-ion recombination as given by the reduced Debye equation ... 

The reaction of the electron with pyrazine is reversible and the rate of electron detachment, from the pyrazine anion has been measured in supercritical xenon ... 

To explain the small volume changes observed for electron attachment to pyrazine in supercritical xenon, it was proposed that clustering around the neutral pyrazine is comparable to that around the ion. The volume change in this reaction is given by ... 

Some electron transfer reactions have been studied in supercritical xenon. Two of them have been shown to be diffusion controlled and two are energy controlled. These reactions have been followed by changes in the optical absorption after the pulse. To carry out these studies requires that the rate of electron attachment to the solute be suffidendy fast to compete with ion recombination, which occurs on the picosecond time scale in pulse radiolysis. The solute hexafluo-robenzene satisfies this criterion the rate constant is sufficiently large (see Fig. 6) that millimolar concentrations will allow formation of anions. The rate constant for attachment to 4,4 -bipyridine (bipy) is also sufficiently large to satisfy this need. ° Another requirement for making these studies is to quench the excimers whose optical absorptions are strong and can interfere with detection of ions. As mentioned under Sec. 2, a small concentration of ethane (0.4%) is sufficient for this purpose. 

Holroyd RA, Itoh K, Nishikawa M. (2003) Density inhomogeneities and electron mobility in supercritical xenon. / Chem Phys 118 706-710. 

The terms polar, apolar and dipolar are often used to describe solvents and other molecules, but there is a certain amount of confusion and inconsistency in their application. Dipolar is used to describe molecules with a permanent dipole moment, e.g. ethanol and chloroform. Apolar should be used rarely and only to describe solvents with a spherical charge distribution such as supercritical xenon. All other solvents should, strictly speaking, be considered polar Therefore, hexane is polar because it is not spherical and may be polarized in an electric field. This polarizability is important when explaining the properties of such solvents, which do not have a permanent dipole and give low values on most polarity scales. Therefore, they are widely termed non-polar and, although... 

Figure 4 Temperature behaviour of H T values of Ti(C(H3 Bu3)2 in supercritical xenon...

In this paper we use dynamic light scattering (DLS) methods to examine micelle size and clustering in (1) supercritical xenon, (2) near-critical and supercritical ethane, (3) near-critical propane as well as (4) the larger liquid alkanes. Reverse micelle or microemulsion phases formed in a continuous phase of nonatomic molecules (xenon) are particularly significant from a fundamental viewpoint since both theoretical and certain spectroscopic studies of such systems should be more readily tractable. Diffusion coefficients obtained by DLS for AOT microemulsions for alkanes from ethane up to decane are presented and discussed. It is shown that micelle phases exist in equilibrium with an aqueous-rich liquid phase, and that the apparent hydrodynamic size, in such systems is highly pressure dependent. 

To calculate micelle size and diffusion coefficient, the viscosity and refractive index of the continuous phase must be known (equations 2 to 4). It was assumed that the fluid viscosity and refractive index were equal to those of the pure fluid (xenon or alkane) at the same temperature and pressure. We believe this approximation is valid since most of the dissolved AOT is associated with the micelles, thus the monomeric AOT concentration in the continuous phase is very small. The density of supercritical ethane at various pressures was obtained from interpolated values (2B.). Refractive indices were calculated from density values for ethane, propane and pentane using a semi-empirical Lorentz-Lorenz type relationship (25.) Viscosities of propane and ethane were calculated from the fluid density via an empirical relationship (30). Supercritical xenon densities were interpolated from tabulated values (21.) The Lorentz-Lorenz function (22) was used to calculate the xenon refractive indices. Viscosities of supercritical xenon (22)r liquid pentane, heptane, decane (21) r hexane and octane (22.) were obtained from previously determined values. 

The general observation from DLS studies is that the apparent hydrodynamic diameter increases as the pressure is decreased towards a phase boundary (where surfactant and water will precipitate to form a second phase). Figures 2 and 3 show DLS results for AOT/water micelles in supercritical xenon (at 25 C) and ethane (at 37 C), respectively. Results are presented for [H20]/[A0T] molar ratios (W) of 1 (a) and 5 (b). All measurements were obtained In single-phase systems at constant W. The apparent hydrodynamic micelle diameter decreases with increasing pressure or density of the continuous phase in both fluids. The second cumulant in Equation 1, which is a qualitative measure of the polydispersity of the system, is very close to zero for all conditions of this study. There is no statistically... 

Figure 2. Apparent hydrodynamic diameters of AOT reverse micelles In supercritical xenon as a function of pressure and density (of the pure fluid) at 25 C, with (a) W - 1 and (b) W - 5. (AOT) - 150 mM.

A supercritical fluid is defined as a substance above its critical temperature (Tc) and critical pressure (Pc). The critical point represents the highest temperature and pressure at which the substance can exist as a vapor and liquid in equilibrium (Tables 3 and 4). Supercritical fluids are highly compressed fluids that combine properties of gases and liquids in a synergistic manner. Fluids such as supercritical xenon, ethane, and carbon dioxide (CO2) offer a range of unusual chemical possibilities in both synthetic and analytical chemistry. 

The use of supercritical fluids as reaction media for organometallic species is also investigated. Reactions include photochemical replacement of carbon monoxide with N2 and H2 in metal carbonyls, where the reaction medium is supercritical xenon. Also, photochemical activation of C-H bonds by organometallic complexes in supercritical carbon dioxide is investigated. More recent studies on photochemical reactions also include laser flash photolysis of metal carbonyls in supercritical carbon dioxide and ethane and laser flash photolysis of hydrogen abstraction reaction of triplet benzophenone in supercritical ethane and... 

Holroyd R.A., Wishart J.F., Nishikawa M., Itoh K., Reactions of Charged Species in Supercritical Xenon as Studied by Pulse Radiolysis, J. Phys. Chem. B, 2003,107,7281-7287. 

McHugh, M. A., A. J. Seckner, and V. J. Krukonis. 1984. Supercritical xenon. Paper presented at the Annual AIChE Meeting, San Francisco, CA, November. 

Fig. 14. IR spectra showing the sequential formation of (r)5-C5H5)Re(CO)2(N2) (dark peaks), (7/ -C5H5)Re( CO)(N2)2 (medium peaks), and (rj5-C6H5)Re(N2)3 (light peaks ) from (r,5-C5H6)Re(CO)3 and N2 in supercritical xenon solution [reproduced with permission from Poliakoff, M. Howdle, S. Chem. Br. 1995, 31, 120].

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