Pressurized hot (subcritical) liquids
Chemical and physical properties of pressurized hot liquids can be tuned by changing the temperature, since at higher temperature intermolecular interactions are weakened. This is especially true for water due to its extraordinary strong hydrogen bonding. At elevated temperatures this results in lower dielectric constant and polarity, enbaling water to be used as a solvent for more nonpolar compounds. Furthermore, elevated temperature also results in higher diffusion rates, lower viscosity and surface tension and higher thermal energy that result in more breaking of molecular bonds and weakening of intermolecular forces.
Water is one of the most sustainable solvents, as we do have an existing production and distribution system of almost negligible environmental impact, and water is naturally readily available. Depending on contaminations dissolved in the water during processing, the used water can be returned to natural waters through existing wastewater treatment plants. Hence, in our research, we optimize extraction methods using liquid water at elevated temperature and pressure as a “green and clean” solvent.
A molecule reaches the supercritical state by increasing its temperature and pressure above its critical point, as defined by its critical temperature and pressure. Supercritical fluids have viscosities similar to gases and densities similar to liquids, properties that result in fast diffusion rates and strong solvent strength, and make them extraordinarily useful as extraction, reaction and separation media. Supercritical carbon dioxide is by far the most commonly used supercritical fluid, mainly because it is a non-toxic fluid that leaves no organic solvent residues in the sample; it has rather mild critical parameters (31°C and 74 bar) which is useful when working with thermolabile compounds; and it is readily available at high purity and low cost. Carbon dioxide is a solvent similar to heptane and hexane, but has the advantage that it is gaseous at room temperature and ambient pressure, which enables separation of solids or liquids upon depressurization. In our research, we use supercritical carbon dioxide as a solvent for "lipophilic" compounds and as a precipitation media for more polar compounds.
Supercritical water is a more aggressive solvent, as its critical point is relatively high, at 374°C and 221 bar, and gases are completely miscible in water under such conditions. In our research supercritical water is used for degradation of biomass to monomers of sugars, phenols, and other smaller organic compounds.