High-pressure investigations on fluid systems—a challenge to experiment, theory, and application

Abstract

In the present lecture some activities of the author's group on the thermodynamics and dynamics of pure and mixed fluids under high pres- sure will be reviewed. First the complex dependences of liquid-liquid, liquid-gas and gas-gas equilibria and the related critical phenomena on pressure will be conside- red, and it will be shown on recent data up to about 300 MPa that continu- ous transitions exist between these different types of phase equilibrium in fluid mixtures. For some alkanol mixtures the concentrations of the monomeric and associated alkanol species in the coexisting phases have been determined separately by near-infrared spectroscopy (NIR). Some applications will be briefly described e.g. in supercritical fluid extraction (SFE) of low-volatile and/or thermolabile substances and recent high-pressure results will be presented for some ternary and quaternary systems being of interest for the use of low-volatile moderators in SFE. Analytical separations using supercritical fluid chromatography (SFC) and the determination of some physicochemical properties (e.g. capacity rati- os, binary diffusion coefficients) from constant-density or density-pro- grammed SFC runs will also be considered. The kinetics of phase separation in fluid mixtures has been studied using pressure-jump and other transient techniques. Here the accent is on the investigation of (nearly) isopycnic fluid systems (e.g. methanol+cyclohe- xane) in order to simulate microgravity conditions. Finally the effect of pressures up to about 2 GPa on the mesomorphic phase stability of some selected thermotropic liquid crystals will be demonstrated using differential thermal analysis (DTA) and diamond anvil cell (DAC) techniques.