AN OPTICAL-CELL FOR RAMAN-SPECTROSCOPIC STUDIES OF SUPERCRITICAL FLUIDS AND ITS APPLICATION TO THE STUDY OF WATER TO 500-DEGREES-C AND 2000-BAR

A high-temperature, high-pressure optical cell has been developed for the study of aqueous solutions by Raman spectroscopy. The disk-shaped cell has a sample volume of < 1 ml and utilizes diamond or sapphire wi ndows set at 90-degrees to one-another. Temperatures to 700-degrees-C and pressures to 4000 bar have been attained as measured using an inte rnal thermocouple and a strain gauge. The apparatus was employed in th e study of water to 500-degrees-C and 2000 bar with spectra of the O-H stretching mode being collected at intervals of 50-degrees-C and 250 bar. A low-frequency shoulder between 3250 and 3300 cm -1 was found to persist to the maximum temperatures to at least 450-degrees-C at pres sures above those of the liquid vapor curve; its intensity decreased w ith increasing temperature and decreasing pressure. The frequency of t he maximum intensity of the spectral envelope increased dramatically w ith temperature to above 250-degrees-C and was found to be linear with respect to density and independent of temperature at constant density above 250-degrees-C. Similar behavior is seen for the viscosity, diel ectric constant and the limiting equivalent conductances of pure water . The data indicate the presence of intermolecular hydrogen bonding to temperatures to well above 300-degrees-C at densities above the criti cal density.