Phase equilibrium investigations of fluid systems at high pressures

Abstract

In the present review some selected results on the thermodynamics of pure and mixed fluids under high pressure are presented that have been obtained in the author's group at the University of Bochum since about 1990. Thus the paper is a continuation and extension of a more comprehensive earlier review in this Journal (Pure & Appl. Chem. 63, 1313 (1991)) where the fundamentals as well as the thermodynamical and theoretical background are discussed in more detail. The accent of the present review is on some applications e.g., in Supercritical Fluid Extraction (SFE) of low-volatile and/or thermolabile substances and recent high-pressure results are presented for some binary, ternary and quaternary systems (as solutions of fatty alcohols, fatty acids, alkanes etc in supercritical carbon dioxide) being of interest for the use of moderators in SFE. The phenomena are discussed with respect to cosolvency effects and the appearance of miscibility windows in the isobaric T(w1, w2) and isothermal p(w1, w2) phase prisms (w = mass fraction) as well as of closed miscibility gaps (so-called islands) in the triangular Gibbs phase diagrams at constant temperature and pressure. Some new applications of supercritical fluid solvents are shortly reviewed e.g., the dyeing of fibers. Analytical separations using Supercritical Fluid Chromatography (SFC) and the determination of some physico-chemical properties (such as capacity ratios, binary diffusion coefficients) from constant-density or density-programmed SFC runs (e.g., on liquid crystalline compounds) are also considered. Finally the effect of high pressures on the mesomorphic phase stability of pure as well as gas-saturated thermotropic liquid crystals is demonstrated using differential thermal analysis (dta). © 1993 IUPAC