PI & S: Subcritical Water as an Alternative Solvent for Particle Engineering Process
Water is used in many reaction and separation processes due to more chemical industries shifting towards sustainability [1-3]. The strong hydrogen-bond cohesive energy between the water molecules causes low solubility of hydrophobic compounds in water. However, the hydrogen-bond in water weakens when the temperature increases, which leads to the solvating power of water for hydrophobic compounds increasing . Hence, water at temperatures above the ambient boiling temperature can be used for materials extraction, such as ionic and polar species extraction at modest temperature and non-polar materials extraction at higher temperature (near critical condition) . Besides extraction, pressurised water at high temperature can also be used in hydrolysis reactions by hydrothermal transformations to obtain fuel from non-food plant material .
Subcritical water, or superheated water, is water heated below its critical temperature and held at a pressure that maintains in a liquid state. The solvating power of subcritical water is highly dependent on temperature and polarity than pressure as the effect of pressure has been described as insignificant over moderate pressure ranges . Hence, the solvating power of subcritical water for a wide range of non-polar compounds increases as the temperature is increased .
Subcritical water has recently been under extensive research as an alternative solvent to organic solvents for particle engineering, and is particularly used in micronizing hydrophobic organic compounds (HOC) and Active Pharmaceutical Ingredients (APIs). By regulating the size and shape of APIs, their dissolution rates in the body can be enhanced, which can assist APIs to reach the site of drug reception. In addition, minimising the size of drug particles can aid in increasing the effect of drugs, which could potentially reduce the dosage of drugs required. Consequently, the cost can also be reduced as well as the risk of adverse side effects .
The first few HOCs that were studied using subcritical water in our laboratory included budesonide, griseofulvin and naproxen . Some of the scanning electron microscopy (SEM) images of the abovementioned HOCs are shown below:
Subcritical or superheated water has been proven to be a useful alternative and tunable solvent for particle engineering. The use of subcritical water has not only overcome the limitations of conventional micronization processes, it is a promising tool to move industry processes towards sustainability.
 W.H. Teoh, Fundamental solubility study of polycyclic aromatic hydrocarbons in subcritical water and ethanol mixtures, in: N. Foster, R. Mammucari (Eds.) School of Chemical Engineering, School of Chemical Engineering, UNSW Australia, 2012.
 R.O. Canıaz, C. Erkey, Process intensification for heavy oil upgrading using supercritical water, Chemical Engineering Research and Design, 92 (2014) 1845-1863.
 A.G. Carr, Subcritical water as a tunable solvent for particle engineering, in: N. Foster, R. Mammucari (Eds.) School of Chemical Engineering, School of Chemical Engineering, UNSW Australia, 2010.
 G. Brunner, Near critical and supercritical water. Part I. Hydrolytic and hydrothermal processes, Journal of Supercritical Fluids, 47 (2009) 373-381.
 W.H. Teoh, R. Mammucari, S.A.B. Vieira De Melo, N.R. Foster, Solubility and solubility modeling of polycyclic aromatic hydrocarbons in subcritical water, Industrial and Engineering Chemistry Research, 52 (2013) 5806-5814.