Viscosity sulfur hexafluoride

The first use of supercritical fluid extraction (SFE) as an extraction technique was reported by Zosel [379]. Since then there have been many reports on the use of SFE to extract PCBs, phenols, PAHs, and other organic compounds from particulate matter, soils and sediments [362, 363, 380-389]. The attraction of SFE as an extraction technique is directly related to the unique properties of the supercritical fluid [390]. Supercritical fluids, which have been used, have low viscosities, high diffusion coefficients, and low flammabilities, which are all clearly superior to the organic solvents normally used. Carbon dioxide (C02, [362,363]) is the most common supercritical fluid used for SFE, since it is inexpensive and has a low critical temperature (31.3 °C) and pressure (72.2 bar). Other less commonly used fluids include nitrous oxide (N20), ammonia, fluoro-form, methane, pentane, methanol, ethanol, sulfur hexafluoride (SF6), and dichlorofluoromethane [362, 363, 391]. Most of these fluids are clearly less attractive as solvents in terms of toxicity or as environmentally benign chemicals. Commercial SFE systems are available, but some workers have also made inexpensive modular systems [390]. 

Sono Vue (BRl) is an ultrasound contrast medium that consists of microbubbles containing sulfur hexafluoride gas stabilized by phospholipids. The diameter of each microbubble is 2.5 pm and there are about 0.5 x 10 microbubbles/ml. It is isotonic with human plasma, has the same viscosity as blood, and has minimal antigenic potential, since it contains no protein material. 

The chemically inert character of sulfur hexafluoride is responsible for the almost complete lack of exchange of fluorine atoms between SFe and HF (249). It does react with hot alkali metals, however, and a study has been made of the rate of reaction of Na atoms with SF6 gas using the sodium diffusion flame technique. The rate constant at 247° is 2.23 X 10-1 cm mole-1 sec-1 and the energy of activation for the reaction SF6 + Na — SF6 + NaF, is about 37 keal. A film of sodium on a glass wall does not react with SF at room temperature. The reaction sets in at about 200° (57). The fluorides, SF , SF4, and S2F2, have no effect upon the viscosity of liquid sulfur in the range 180-195° (93). Sulfur hexafluoride forms a solid hydrate which has a crystal constant of 17.21 A. It decomposes just above 0° (285). 

R. D. Trengove and W. A. Wakeham, The viscosity of carbon dioxide, methane, and sulfur hexafluoride in the limit of zero density, J. Phys. Chem. Ref Data, 16,175, 1987. 

Strehlow, T., and Vogel, E., Temperature Dependence and Initial Density Dependence of the Viscosity of Sulfur Hexafluoride, Physica A 161,101,1989,... 

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