Scaling of self and Fickian diffusion coefficients in the critical region

The behavior of diffusion coefficients in the critical region is important to understanding transport phenomena in supercritical fluids. For example, it is often claimed that mass transfer rates are high in the critical regio n. However, it is also generally accepted that diffusion processes slow dow n dramatically as the critical point is approached, although there appear t o be conflicting results on this point in the literature. From published ex perimental data for the self-diffusion constant of pure fluids in the criti cal region one could conclude that this property: (1) approaches zero at th e critical point (Cini-Castagnoli et al. Physica 48 (1970) 153), (2) remain s finite there, showing no anomalous behavior (Etesse et al. Physica B 183 (1993) 45) or (3) displays singular behavior, approaching infinity, at the critical point itself (Duffield and Harris, Ber.Bunsenges-Gesellschaft 80 ( 1976) 157). We discuss these and related issues from the vantage point of d ynamic scaling theory and the Onsager regression hypothesis. We conclude th at while self-diffusion coefficients are not predicted to show anomalous be havior at the critical point. consistent with much of the available data, F ickian (transport) diffusion coefficients are predicted to approach zero th ere. We are unaware of any measurements of this latter property in a pure f luid consistent with scaling predictions. (C) 2001 Elsevier Science Ltd. Al l rights reserved.