Subcritical Water

Why Subcritical H2O

Subcritical Water Systems

There is a great demand for environmentally friendly extraction procedures that eliminate the need for toxic organic solvents. Supercritical carbon dioxide is the most common, but one can also use water in some innovative ways.

Water can be held in a liquid state above its normal boiling temperature (100°C) by increasing the pressure. This process is demonstrated using a simple pressure cooker. Water above its normal boing point but below its supercritical temperature (374°C), is “subcritical” water.

Water can be used as an alternative to toxic organic solvents because the dissolving power of water can be changed by changing its temperature. Water can be physically altered to become “subcritical” water

What is Subcritical water?

Subcritical water is a LIQUID. Simply described, it is that area on the phase diagram from the normal boiling point of water (100°C at 1 atm) to the supercritical point of water (374°C at 218 atm). As the temperature is increased the pressure is also increased to ensure liquidity and not drift into the vapor (gaseous) phase.

Liquid water exhibits many changes in its characteristics as it is heated. Viscosity and surface tension decrease, and the solubility of solutes and their diffusion increases.

Viscosity and surface tension decrease

  • Viscosity and surface tension decrease with increasing temperature
  • Solubility of solutes increases
  • Solute diffusion increases, decreasing time to solubilize

Replace alcohol with subcritical water

  • Since the dielectric constant of water decreases significantly as it is heated, it can behave like alcohol

Subcritical water may be used to extract many organic molecules

  • Eliminate the use of conventional organic solvents.

In addition, water has a very unusual property. Increase the temperature and it becomes more NON polar. Just as one can fractionate by changing the pressure in supercritical CO2. Similarly, one can fractionate by changing the TEMPERATURE of the liquid water. As you increase the temperature, the dielectric decreases making water increasingly NON polar, behaving like non-polar solvents - and salts are insoluble.

Subcritical water

  • Complimentary SC-CO2 for an all-natural processing platform
  • Substitute for ethanol & polar solvents
  • Lower energy is required than steam distillation
  • Possible control of microbial & enzymatic activity
  • Modifying or degradation agent for various materials

Not only does one have the advantages of the above, but also, from an analytical point of view, it’s:

  • Non-toxic
  • Inexpensive
  • Readily obtained and disposed of
  • And the sample does not need to be dried

A few of the many applications:

  • Extraction of anthocyanins from grape pomace
  • Extraction of antioxidant compounds from rosemary plants
  • Extraction of medicinal plant essential oil:
  • Continuous subcritical water extraction as a useful tool for isolation of edible essential oils - oregano
  • Selective extraction of oxygenates from savory and peppermint
  • Continuous subcritical water extraction of marjoram essential oil
  • Subcritical water extraction of essential oils from Thymbra spicata
  • Subcritical water extraction of nutraceuticals with antioxidant activity from oregano
  • Subcritical water extraction of kava lactones from kava root
  • Subcritical water extraction of essential oils from coriander seeds (Coriandrum sativum)
  • Subcritical water extraction of catechins and proanthocyanidins from winery by-products
  • Subcritical water extraction of anthocyanins and other phenolics from dried red grape skin
  • Static–dynamic subcritical water extraction of laurel essential oil
  • Extraction of clove using pressurized hot water
  • Subcritical water extraction and β-glucosidase-catalyzed hydrolysis of quercetin glycosides in onion waste
  • Antioxidant bioactives extracted from canola meal by subcritical water
  • Scale-up of Subcritical Water Extraction of Polyphenolic Compounds from Fruit Waste Material