Solvents supercritical/subcritical water

In liquid-solid extraction (LSE) the analyte is extracted from the solid by a liquid, which is separated by filtration. Numerous extraction processes, representing various types and levels of energy, have been described steam distillation, simultaneous steam distillation-solvent extraction (SDE), passive hot solvent extraction, forced-flow leaching, (automated) Soxh-let extraction, shake-flask method, mechanically agitated reflux extraction, ultrasound-assisted extraction, y -ray-assisted extraction, microwave-assisted extraction (MAE), microwave-enhanced extraction (Soxwave ), microwave-assisted process (MAP ), gas-phase MAE, enhanced fluidity extraction, hot (subcritical) water extraction, supercritical fluid extraction (SFE), supercritical assisted liquid extraction, pressurised hot water extraction, enhanced solvent extraction (ESE ), solu-tion/precipitation, etc. The most successful systems are described in Sections 3.3.3-3.4.6. Other, less frequently... 

Conversion of polymers and biomass to chemical intermediates and monomers by using subcritical and supercritical water as the reaction solvent is probable. Reactions of cellulose in supercritical water are rapid (< 50 ms) and proceed to 100% conversion with no char formation. This shows a remarkable increase in hydrolysis products and lower pyrolysis products when compared with reactions in subcritical water. There is a jump in the reaction rate of cellulose at the critical temperature of water. If the methods used for cellulose are applied to synthetic polymers, such as PET, nylon or others, high liquid yields can be achieved although the reactions require about 10 min for complete conversion. The reason is the heterogeneous nature of the reaction system (Arai, 1998). 

Solvent extraction Microwave extraction Solid phase microextraction Subcritical water extraction Subcritical water extraction Supercritical fluid extraction... 

Batch and continuous hydrolysis of vegetable oils in sub- and supercritical water have also been investigated (193, 194). Water, in this case, acts both as a solvent and a reactant in the hydrolysis reaction. Although the reaction could be carried out effectively in subcritical water, thermal degradation of products and reactants were observed under supercritical conditions (193). 

When water is used as the solvent, PLE is referred to as superheated water extraction, subcritical water extraction (SWE), or pressurized (hot) water extraction (PWE). Hot water is very effective as an extraction solvent for PAHs from soil and sediment. Superheated water is water above the boiling point but below the supercritical point, and under sufficient pressure to maintain... 

SEE is today not the only hyphenated extraction technique available. Both microwave-assisted solvent extraction (MAE) and pressurized fluid extraction (PEE) have evolved in parallel with SFE during the last decade. A thorough comparison of the different techniques can be found in the analytical literature, and will assist in choosing an appropriate technique for a specific application. These newer techniques offer both speed and simplicity, while some of the main benefits of SFE are still inherent to the usage of supercritical fluids as MAE and PFE utilize liquid solvents. More recently, subcritical water extraction has also been introduced as an environmental friendly alternative but the reader is referred to the analytical literature on this research topic. 

Coal liquefaction under supercritical (or subcritical) water condition has some advantages over organic solvents. Supercritical water is miscible with H2, CO, aromatics, and oils, which provides a unique, homogeneous reaction medium for coal liquefaction. CTL process with supercritical water is more environment-friendly than SRC-II process and has higher conversion for low-rank coal [31,32]. 

Many density-dependent properties of H2O, such as viscosity, polarity (dielectric constant s changes from 74 to 2), heat capacity at constant pressure (which is infinite at the critical point), ion product and solvent power can be tuned for specific requirements by setting the correct temperature and pressure, and they show significant changes near the critical point (Figure 25.2). Several studies have demonstrated that the transition from sub- to supercritical conditions also affects the elementary steps in reaction mechanisms, and radical intermediates are favoured over ionic species. Another consequence is that subcritical water shows potential for acid catalysis. Reactions can be run either under non-polar/aprotic or polar/pH controlled conditions (water can take part in these reactions). Consequently, non-polar compounds like aromatics become soluble whereas inorganic salts precipitate. Therefore, the properties of water as a solvent are tunable over much wider parameter ranges than for most other compounds. 

Given the concerns about the use of toxic organic solvents in food chemistry, many new techniques have been developed to overcome or minimize this problem. For instance, environmentally clean extraction techniques, such as those based on the use of compressed fluids (pressurized liquids, PLE supercritical fluids, SFE and subcritical water, SWE or PHWE), are widely used as alternatives to conventional procedures, such as solid—liquid extraction (SEE), liquid—liquid extraction (LLE), and the like. These alternative processes have in common the use of lower amount of solvents (from hundred milliliters to few milliliters), the lack of toxic residues, higher efficiency extraction (in terms of yields and energy used), and the improved selectivity of the process. SFE has been used in food analysis as a sample preparation technique, mainly for lipophilic compounds, while PLE has been extensively used for many compositional food applications, because the selectivity of this technique... 

Saponins from plant materials can be extracted using different techniques and solvents. The conventional techniques for saponin extraction used soxhlet, liquid-liquid or solid-liquid extraction (Berhow et al. 2002 Hassan et al. 2010a, b). These methods consume a lot of solvent, time and may lead to potentially deleterious degradation of labile compounds (Kerem et al. 2005). Therefore, in recent years, new extraction techniques include accelerated solvent extraction, supercritical fluid extraction, solid-phase microextraction, sonication, extraction with supercritical or subcritical water, and microwave-assisted extraction have been developed and are considered to be more efficient than the conventional methods (Wu et al. 2001 Kerem et al. 2005 Ligor et al. 2005 Gii lii-Ustundag and Mazza 2007). Ultrasonication-assisted extraction of ginseng saponins was about three times faster than the liquid-liquid extraction and can be carried out at lower temperature (Wu et al. 2001). Kerem et al. (2005) reported that methanol- microwave assisted method to extract saponin of chiclqtea proved to be faster and more efficient than soxhlet extraction. 

In the last years, several alternatives to conventional solvent-based methodologies have been developed involving the use of subcritical or supercritical extraction conditions, such as supercritical fluid extraction, subcritical water extraction, or superheated liquid extraction. 

Properties of substances highly depend on temperature. As an extreme example, water can be considered. We know very well the behaviour of normal water, but it is less familiar that water heated up near to its critical temperature behaves like a completely different solvent. Subcritical water and supercritical water are unpolar liquids and able to dissolve fats. Why should not we utilise the extraordinary properties of such solvents in electrochemistry The methods presented in this book further down will show that we can do experiments of this kind even with everyday instruments, without application of external pressure or spending a lot of heat energy. Examples for the novel experimental facihties offered by the scientific field named here modem thermoelectrochemistry or alternatively in sim thermoelectrochemistry will be presented in this book. 

Table 12.2 gives the properties of sub- and supercritical water. Subcritical water is the water that is in a state under a pressurized condition at temperatures above its boiling point under ambient pressure and below the critical point Tc = 374°C Pc = 22.1 MPa, pc = 320 kg/cm ). The dielectric constant of liquid water decreases with increasing temperature (Nanda et al., 2014b). At temperatures from 277 to 377°C, the dielectric constant becomes as low as those of polar organic solvents. The ionic product of water is maximized at temperatures between 227 and 372°C depending upon the pressure (Kruse and Dinjus, 2007). Thus, subcritical water acts as acid and/or base catalysts for reactions, such as hydrolysis of ether/ester bonds, and also as a solvent for the extraction of low molecular mass products (Brunner, 2009). 

Water reaches supercritical conditions at 373.9 °C (Table 6.13) but it becomes a suitable solvent at 200-350 °C and at pressures generated solely by the expansion of the liquid medium, about 20-100 bar (subcritical or superheated water). 

Soxhlet, sonication, supercritical fluid, subcritical or accelerated solvent, and purge-and-trap extraction have been introduced into a variety of methods for the extraction of contaminated soil. Headspace is recommended as a screening method. Shaking/vortexing is adequate for the extraction of petroleum hydrocarbons in most environmental samples. For these extraction methods, the ability to extract petroleum hydrocarbons from soil and water samples depends on the solvent and the sample matrix. Surrogates (compounds of known identity and quantity) are frequently added to monitor extraction efficiency. Environmental laboratories also generally perform matrix spikes (addition of target analytes) to determine if the soil or water matrix retains analytes. 

---