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 windows set at 90-degrees to one-another. Temperatures to 700-degrees-C and pressures to 4000 bar have been attained as measured using an internal thermocouple and a strain gauge. The apparatus was employed in the 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 pressures above those of the liquid vapor curve; its intensity decreased with increasing temperature and decreasing pressure. The frequency of the maximum intensity of the spectral envelope increased dramatically with 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, dielectric 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 critical density.