Particle synthesis by rapid expansion of supercritical solutions (RESS): Current state, further perspectives and needs

The application of supercritical fluids in particle formation processes has attracted great attention due to numerous advantages over classical size reduction technologies. These classical approaches can be subdivided into top-down and bottom-up technologies. Typical top-down methods are milling, grinding and crushing, thereby one starts with a (large) bulk material and obtains the sub-micron particles due to size reduction. On the other hand, by bottom-up techniques, one starts with atoms or molecules which growth is caused by, e.g. co-precipitation or crystallization. In some cases, the disadvantages of the classical top-down and bottom-up approaches which are, among others, the control of size, size distribution, composition, and morphology, can be overcome by the utilization of supercritical fluids, which are characterized by liquid-like densities and gas-like viscosities and diffusion coefficients. The present article gives a survey (that never can pretend to be exhaustive!) of knowledge published in the period from end of the eighties until today about the micronization of organic compounds based on supercritical fluid induced phase transition processes. Here, the focus is on rapid expansion of supercritical solution (RESS). In this process organic solids are dissolved in the supercritical fluid, typically CO 2 , and are precipitated caused by a pressure drop. • The basic principles of the RESS process are presented and discussed. • A critical overview about experimental und modeling results is given. • Strategies for overcoming the knowledge gaps are discussed. • Essential future research directions are pointed out.