Solubilities of solid mixtures in supercritical carbon dioxide: a review

A wealth of information on the solubility of materials in supercritical fluids (SCFs) has been published in the literature. For the most part, this information is concerned with the solubility of individual solutes in supercritical CO2 (binary systems). It has become clear from the relatively few studies on the solubility of solid mixtures that binary solubility data represent a limited picture of the complex intermolecular interactions that may occur in the SCF phase. In particular, the solubility of a solid may be enhanced significantly, compared to its binary system, in the presence of a second solid. In this review, published data on the solubilities of solid mixtures in supercritical CO2 are examined to highlight unique solubility phenomena associated with these systems. An extensive compilation of solubility enhancement data, the first of its kind, is presented for this purpose. The phase behaviour of solid mixtures under high pressure is considered as well as the effect of melting on solubility enhancement. The impact of solubility enhancement on selectivity is assessed and opportunities for improving the selectivity of extraction are highlighted. For most solid mixtures, solubility enhancement can be explained in terms of an entrainer effect similar to that observed in cosolvent systems. However, if the solid mixture partially melts, solubility enhancement becomes heavily dependent on which species is present as an excess solid phase. An examination of the phase behaviour of solid mixtures is therefore essential for the interpretation of solubility enhancement data.