**3. Water under sub- and supercritical conditions**

Water under sub- and supercritical conditions behaves totally different to that at ambient conditions, and its properties have been extensively reviewed by G. Brunner in his recent publication [10], which is basically based on the work by M. Modell in the 70s. To summarize, water with a dielectric constant of about 80 at ambient condition changes its properties in near critical (or subcritical) conditions, which can be used as a solvent for ionic or polar species. But, as the temperature approaches the critical temperature (Tc=373°C, Pc=22.1 MPa) and beyond (supercritical), the dielectric constant dramatically decreases to less than 10 (similar to that of methylene chloride) [11], behaving like a non-polar solvent that can dissolve and degrade a variety of non-polar organic compounds (please refer to Fig. 1 for the typical phase diagram of a substance).

Other than the dielectric constant, which indicates polarity, the ion product also exhibits changes in the properties of water. Water dissociates to produce hydrated hydrogen (hydro‐ nium ion) and hydroxyl ions. Under ambient conditions, the product of their concentrations is about 10–14 (mol/L)2 . The ion product also changes dramatically with increasing tempera‐ ture, and the maximum product is obtained at a temperature of about 250°C, under saturated vapor pressure. Having low dielectric constant and high ion product, water under near critical (subcritical) conditions become a suitable solvent for hydrolysis of organic compounds, even without any additives. With these properties, water serves as a potential solvent and a catalyst for organic reactions.

The review of G. Brunner [10] also showed several physicochemical properties of water under the above-mentioned conditions including density, viscosity and solubility of some gases and hydrocarbons. Sophisticated techniques for the measurement of those properties under high temperature and high pressure conditions were also presented. A micro-cell for potentiometric pH measurements of aqueous solutions in supercritical state has been reported [12]. An apparatus utilizing an optical cell under flow conditions was also developed to measure solubility, salt deposition and Raman spectroscopic studies in aqueous solutions near the critical point [13]. Mixing of solutions under supercritical water conditions had been a

**Figure 1.** Typical phase diagram of any substance (for H2O: Tc=373°C, Pc=22.1 MPa).

comprehensive review of the technology summarized by Peterson et al. [3]. Reviews on its application to various biomass components such as protein, carbohydrates, lignin and fats have also been reported [4]. The most recent applications to the liquefaction of microalgae and marine biomass have also been summarized [5–8]. This hot-compressed water has also been compared with other hydrolysis methods [9] and shows it to be very attractive economically and environmentally because of higher sugar recovery even in the absence of acid and chemical

The increasing popularity of this method is due to the deemed tremendous opportunities that biomass of any sources can offer as energy carrier, combined with the promising properties of water at high temperature and high pressure, especially under sub- and supercritical condi‐

Water under sub- and supercritical conditions behaves totally different to that at ambient conditions, and its properties have been extensively reviewed by G. Brunner in his recent publication [10], which is basically based on the work by M. Modell in the 70s. To summarize, water with a dielectric constant of about 80 at ambient condition changes its properties in near critical (or subcritical) conditions, which can be used as a solvent for ionic or polar species. But, as the temperature approaches the critical temperature (Tc=373°C, Pc=22.1 MPa) and beyond (supercritical), the dielectric constant dramatically decreases to less than 10 (similar to that of methylene chloride) [11], behaving like a non-polar solvent that can dissolve and degrade a variety of non-polar organic compounds (please refer to Fig. 1 for the typical phase

Other than the dielectric constant, which indicates polarity, the ion product also exhibits changes in the properties of water. Water dissociates to produce hydrated hydrogen (hydro‐ nium ion) and hydroxyl ions. Under ambient conditions, the product of their concentrations

ture, and the maximum product is obtained at a temperature of about 250°C, under saturated vapor pressure. Having low dielectric constant and high ion product, water under near critical (subcritical) conditions become a suitable solvent for hydrolysis of organic compounds, even without any additives. With these properties, water serves as a potential solvent and a catalyst

The review of G. Brunner [10] also showed several physicochemical properties of water under the above-mentioned conditions including density, viscosity and solubility of some gases and hydrocarbons. Sophisticated techniques for the measurement of those properties under high temperature and high pressure conditions were also presented. A micro-cell for potentiometric pH measurements of aqueous solutions in supercritical state has been reported [12]. An apparatus utilizing an optical cell under flow conditions was also developed to measure solubility, salt deposition and Raman spectroscopic studies in aqueous solutions near the critical point [13]. Mixing of solutions under supercritical water conditions had been a

. The ion product also changes dramatically with increasing tempera‐

**3. Water under sub- and supercritical conditions**

catalysts.

460 Biofuels - Status and Perspective

tions.

diagram of a substance).

is about 10–14 (mol/L)2

for organic reactions.

**Figure 2.** Summary of tunable physical properties of sub- and supercritical water and comparison of critical tempera‐ tures and pressures of some commonly used supercritical solvents.

challenging and hot topic, a device for direct observation of channel-tee mixing has been developed for this purpose [14]. Further, a reactor for the studies of in-situ X-ray scattering studies of nanoparticle formation in supercritical water system was also developed [15]. The advantages of supercritical fluids with regard to its physicochemical properties are summar‐ ized in Fig. 2, including easily tunable physical properties which make the water suitable for highly selective extraction and reaction processes involving biomass samples.
