An Introduction to Supercritical Fluids

Introduction

Supercritical fluids are highly compressed gases which combine properties of gases and liquids in an intriguing manner. Fluids such as supercritical xenon, ethane and carbon dioxide offer a range of unusual chemical possibilities in both synthetic and analytical chemistry.

We are exploring the use of supercritical CO₂ (scCO₂) as an environmentally acceptable alternative to conventional solvents for reaction chemistry, so called "Clean Technology". In addition, supercritical fluids can lead to reactions which are difficult or even impossible to achieve in conventional solvents.

The book by McHugh and Krukonis gives an excellent introduction to the properties of supercritical fluids. The definition of a supercritical fluid usually begins with a phase diagram, which defines the critical temperature and pressure of a substance. (CO₂ ; T_c = 31.1 °C, P_c = 73.8 bar). A recent review by Poliakoff & Darr gives a comprehensive overview of 'New Directions in Inorganic and Metal-Organic Coordination Chemistry in Supercritical Fluids' Chem. Rev. 1999, 99, 495-541.

Supercritical fluids have solvent power similar to a light hydrocarbon for most solutes. However, fluorinated compounds are often more soluble in scCO₂ than in hydrocarbons; this increased solubility is important for polymerisation.

Solublity increases with increasing density (i.e. with increasing pressure). Rapid expansion of supercritical solutions leads to precipitation of a finely divided solid. This is a key feature of our flow reactors.

The fluids are completely miscible with permanent gases (eg N₂ or H₂) and this leads to much higher concentrations of dissolved gases than can be achieved in conventional solvents. This effect has been exploited in both organometallic reactions and hydrogenation.