Magnitude of the diffusion coefficient anomaly in the critical region and its effect on supercritical fluid chromatography

Abstract

Literature measurements of diffusion in a supercritical fluid, especially those of naphthalene in supercritical fluid carbon dioxide, are discussed. These show that at very high dilution there is no measurable anomaly, but that large reductions in the apparent diffusion coefficient are observed at higher, though still low, concentrations. Three effects contributing to the anomaly are described: those of chemical potential, augmented density and fluid motion. Measurements were made of the effect of on-column naphthalene solute concentration on column efficiency in supercritical fluid chromatography using carbon dioxide as the mobile phase. The experiments were perfumed at 35°C (308 K) at a variety of mobile phase densities and a constant linear flow-rate. The results are interpreted in terms of the above-mentioned phenomena and their implications for the Van Deemter and Golay equations. Calculations of quantities related to chemical potential, augmented density and fluid motion were performed for the same system (naphthalene in carbon dioxide) in order to estimate their importance. The significance of the diffusion coefficient anomaly for analytical and preparative supercritical fluid chromatography is discussed.