Utilising CO2 as a solvent in reaction chemistry commonly results in increased reaction speeds, largely due to its low viscosity, which bolsters mass transfer. Additionally, CO2 presents distinctive selectivity traits that are not found in conventional solvents, potentially paving the way for novel reactive pathways, such as leading to a single diastereomer. Upon completion of the chemical reaction, the CO2 is depressurised, thus effectively rendering a process that is practically free of solvent remnants.

Although enzymes typically perform optimally in water-based environments at around room temperature and neutral pH, the conditions within supercritical carbon dioxide (scCO2) are markedly different and can potentially influence the structure and functionality of enzymes. Nevertheless, certain enzymes, particularly those that demonstrate resilience in organic solvents, are capable of reactions such as esterification and transesterification more effectively in environments that aren't water-based making CO2 an ideal solvent for such processes.

The CO2 Rapid Expansion of Supercritical Solutions (RESS) process is a method used to produce fine particles of substances. This method has been widely used in the pharmaceutical industry for drug formulation.

The process involves using supercritical carbon dioxide (scCO2) as a solvent. The substance to be formed into particles is dissolved in the scCO2 under high pressure and temperature, creating a supercritical solution. This solution is then allowed to expand rapidly through a small nozzle into a region of lower pressure.

The rapid expansion causes the supercritical solution to become unsaturated, leading to the precipitation of fine particles of the solute. The CO2 then evaporates, leaving behind the fine particles.

One of the major advantages of the RESS process is that it avoids the use of organic solvents, making it an environmentally friendly method for particle production. It's also useful for processing heat-sensitive materials, as the process can be conducted at relatively low temperatures. However, the method does have limitations, such as a relatively low solubility for many substances in scCO2 and the need for high pressures.

In the SAS process, a substance is first dissolved in an organic solvent. This solution is then mixed with a supercritical fluid, often carbon dioxide (CO2). The supercritical fluid acts as an anti-solvent, meaning it decreases the solubility of the substance in the organic solvent. This leads to the precipitation of fine particles of the substance.

The production of aerogels largely depends on the supercritical drying technique. By modulating the temperature and pressure of CO2, a supercritical fluid is generated that dissolves and takes the place of the gel's interior liquid. Following this, the supercritical CO2 undergoes a controlled depressurisation, facilitating the direct shift of CO2 from a supercritical condition to a gaseous one, thereby bypassing the liquid phase and circumventing the associated surface tension typically observed in traditional drying processes. This method safeguards the gel structure from damage during the gas evacuation. The end result is an aerogel distinguished by its high porosity and low density.

Both supercritical drying and freeze drying are designed to remove liquid from a material without causing it to shrink or lose its structure, which is common in conventional drying methods due to the surface tension of the liquid.

Freeze drying involves freezing the material, then reducing the surrounding pressure to allow the frozen water to sublime directly from a solid state to a gas. This process is generally slower than supercritical drying and requires lower temperatures, but it is effective for heat-sensitive substances, such as proteins, pharmaceuticals, and food.

Supercritical drying, on the other hand, is a high-temperature and high-pressure process, which makes it unsuitable for heat-sensitive materials. However, it is faster than freeze drying and can create materials with extremely high porosities, such as aerogels.