A study of near- and super-critical fluids using diamond anvil cell and in-situ FT-IR spectroscopy

The phase relation and solution structure of water and NaCl aqueous solutio n have been observed and examined by using the hydrothermal diamond anvil c ell (WDAC) at elevated temperatures and pressures and the in situ FT-IR spe ctroscopy. The temperature of observations ranges from 25 to 850 degrees C and the pressure up to 10 or 30 kb. At first, we observed the phase transit ion process from halite+liquid+vapour (H+L+V) to L+H, then to L (or super-c ritical fluid, SCF), and another path: H+L+V-->L+V-->L (or SCF) in heating process. By means of the visual microscope, the authors found that in the L +V immiscibility field L+V exhibits an ordered structure, i.e. a large visu al cluster of solvent around ions. The liquid phase is manifested by vapour bubbles. When phase transitions are observed, the authors examined their i nfrared spectra by using the FT-IR microscopy simultaneously. In the case o f the phase transition from liquid (L) to liquid + vapor (L+V) immisciblity field of NaCl solutions, a sudden change (strong frequency shift) of infra red spectra of the aqueous solution is observed near the critical temperatu re of water as the temperature is raised from 25 to 650 degrees C. The freq uency of the maximum intensity of OH symmetric and asymmetric vibration var ies with respect to temperature. The sharp peak of the OH stretching vibrat ion of the maximum intensity appears in an interval from 300 to 400 degrees C. It is indicated that the hydrogen bonding network is weakened and broke n at last near the critical point of water, which causes the aqueous soluti on to become more associated. Besides, a pressure indicator (a mineral or c ompound) was introduced to the HDAC.