What is a Supercritical Fluid?
A supercritical fluid (SCF) is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases no longer exist. In this state, it exhibits properties intermediate between those of a gas (high diffusivity, low viscosity) and a liquid (high density, good solvating power).
Key Properties and Behavior
Beyond the critical point, increasing pressure does not cause condensation into a separate liquid phase; instead, the fluid's density increases continuously, maintaining a single, homogeneous phase. SCFs can penetrate porous solids like a gas but dissolve substances like a liquid, making them powerful solvents.
A Practical Example: Supercritical CO2
Supercritical carbon dioxide (scCO2) is a widely used example, as its critical point is relatively accessible (31.1°C and 7.38 MPa). Above these conditions, scCO2 effectively dissolves organic compounds, allowing for applications such as decaffeinating coffee, extracting essential oils, and dry cleaning, often replacing toxic traditional solvents.
Importance and Applications
Supercritical fluids are highly valued for their role in green chemistry due to their tuneable solvent properties and ease of separation from dissolved solutes by simply reducing pressure. They are crucial in pharmaceuticals, food processing, chemical synthesis, and waste treatment, offering efficient and environmentally friendlier processes.