Solubilities of solid mixtures in supercritical carbon dioxide: a review

A wealth of information on the solubility of materials in supercritical flu ids (SCFs) has been published in the literature. For the most part, this in formation is concerned with the solubility of individual solutes in supercr itical CO2 (binary systems). It has become clear from the relatively few st udies on the solubility of solid mixtures that binary solubility data repre sent a limited picture of the complex intermolecular interactions that may occur in the SCF phase. In particular, the solubility of a solid may be enh anced significantly, compared to its binary system, in the presence of a se cond solid. In this review, published data on the solubilities of solid mix tures in supercritical CO2 are examined to highlight unique solubility phen omena associated with these systems. An extensive compilation of solubility enhancement data, the first of its kind, is presented for this purpose. Th e phase behaviour of solid mixtures under high pressure is considered as we ll as the effect of melting on solubility enhancement. The impact of solubi lity enhancement on selectivity is assessed and opportunities for improving the selectivity of extraction are highlighted. For most solid mixtures, so lubility enhancement can be explained in terms of an entrainer effect simil ar to that observed in cosolvent systems. However, if the solid mixture par tially melts, solubility enhancement becomes heavily dependent on which spe cies is present as an excess solid phase. An examination of the phase behav iour of solid mixtures is therefore essential for the interpretation of sol ubility enhancement data. (C) 2000 Elsevier Science B.V. All rights reserve d.