Near-critical region

Chemical reactions at supercritical conditions are good examples of solvation effects on rate constants. While the most compelling reason to carry out reactions at (near) supercritical conditions is the abihty to tune the solvation conditions of the medium (chemical potentials) and attenuate transport limitations by adjustment of the system pressure and/or temperature, there has been considerable speculation on explanations for the unusual behavior (occasionally referred to as anomalies) in reaction kinetics at near and supercritical conditions. True near-critical anomalies in reaction equilibrium, if any, will only appear within an extremely small neighborhood of the system s critical point, which is unattainable for all practical purposes. This is because the near-critical anomaly in the equilibrium extent of the reaction has the same near-critical behavior as the internal energy. However, it is not as clear that the kinetics of reactions should be free of anomalies in the near-critical region. Therefore, a more accurate description of solvent effect on the kinetic rate constant of reactions conducted in or near supercritical media is desirable (Chialvo et al., 1998). 

Promotion of Lipase-Catalyzed Esterification of N-Valeric Acid and Citro-neUol in Supercritical Carbon Dioxide in the Near-Critical Region (Ikushima et al 1996). 

Figure 2.10a displays details of some Van der Waals PV isotherms in the near-critical region. As shown in the figure, the isotherms below Tc exhibit pronounced oscillatory loops, quite unlike the experimental plateau-like behavior shown in Fig. 2.8. Indeed, no simple cubic equation could exhibit true plateau-like behavior, and the near-horizontal looping pattern in Fig. 2.10a is apparently the best that a cubic polynomial can do to represent such a flattened region. [The fact that the Van der Waals P = P(V) isotherm is a cubic polynomial will be apparent from expanding (2.13).]...

It seems unlikely that deactivation of the HT transition state would be much different than that of the HH isomer. Clearly, the sharp change in regioselectivity must be due to factors other than changes in solvent deactivation. Because the sizes of the regioisomers are similar, this effect can not be attributed to the repulsive contribution to the equation of state in transition state theory. However, in this near-critical region, large solute-solute fluctuations (i.e. solute-solute clusters) must... 

Local density enhancements, being by definition short-ranged, are not peculiar to the highly compressible near-critical region. Very close to the solute molecule, the local environment differs markedly from the bulk (for example, the local density in the first solvation shell at bulk near-critical conditions is p (R) = 1.43 pc when p = 0.31 and T/Tc = 1.02). However, even this region does not appear to have a liquid-like character, as suggested by other spectroscopic experiments (35-36),... 

Solvatochromic shift data have been obtained for phenol blue in supercritical fluid carbon dioxide both with and without a co-solvent over a wide range in temperature and pressure. At 45°C, SF CO2 must be compressed to a pressure of over 2 kbar in order to obtain a transition energy, E, and likewise a polarizability per unit volume which is comparable to that of liquid n-hexane. The E,j, data can be used to predict that the solvent effect on rate constants of certain reactions is extremely pronounced in the near critical region where the magnitude of the activation volume approaches several liters/mole. 

In addition to the organic compounds, waste water contains dissolved salts. This causes severe corrosion problems under conditions of operation in the near-critical region of water. The corrodibility of different materials in contact with waste water and oxidizing agents was investigated at temperatures up to 573 K and pressures up to 15 MPa. Furthermore, the applicability of appropriate inhibitors for corrosion protection was studied under mentioned conditions of wet oxidation. 

Supercritical fluids (SCFs) have long fascinated chemists and over the last 30 years this interest has accelerated. There is even a journal dedicated to the subject— the Journal of Supercritical Fluids. These fluids have many fascinating and unusual properties that make them useful media for separations and spectroscopic studies as well as for reactions and synthesis. So what is an SCF Substances enter the SCF phase above their critical pressures P and temperatures (Tc) (Figure 4.1). Some substances have readily accessible critical points, for example for carbon dioxide is 304 K (31 °C) and is 72.8 atm, whereas other substances need more extreme conditions. For example for water is 647 K (374 °C) and P is 218 atm. The most useful SCFs to green chemists are water and carbon dioxide, which are renewable and non-flammable. However, critical data for some other substances are provided for comparison in Table 4.1. In addition to reactions in the supercritical phase, water has interesting properties in the near critical region and carbon dioxide can also be a useful solvent in the liquid phase. Collectively, carbon dioxide under pressurized conditions (liquid or supercritical) is sometimes referred to as dense phase carbon dioxide. 

Bai Sh.-P. Studies on the adsorption behavior of CO2 for the near-critical region. Dissertation for PhD degree (2002) School of Chemical Engineering, Tianjin University, Tianjin, China. 

Fig.3 Modeling the adsorption of CO2 on activated carbon for the near-critical region. Points experimental Curves model predicted.

Selectivities to certain products may likewise be adjusted using pressure or temperature. In SCF water, reaction chemistry is governed by fiee-radical (homolytic) mechanisms for an ion product of water, Kw, less than 10 and by ionic (heterolytic) mechanisms for larger values of Ku/(16). This describes the observation that decomposition reactions shift from pyrolysis to hydrolysis as the density of water and thus Kw is increasedQl). In the near-critical region, Kw can be manipulated by small changes in temperature and pressure to control the reaction selectivity. 

Alkylation of phenols using primary, secondary and tertiary alcohols was achieved using supercritical water (at the near-critical region, 250-350 °C). This process eliminates the need for environmentally hazardous organic solvents and acid catalysts. Both ortho-and para-alkylphenols were formed in these reactions, their ratio being dependent on the... 

At the critical point the mole fraction of CO2 Xi is 0.888 (Figure 9). In Figure 9 the part of the curve with Xi < 0.888 is the bubble point curve, and a homogenous mixture above the bubble point can be regarded as a subcritical fluid. The part of the curve with X] > 0.888 is the dew point curve, and a homogeneous mixture above the dew point is a vapor or a supercritical mixture. The mixed solvent near critical region at fixed temperature is defined as the solvent of which the composition and pressure are close to the critical composition and critical pressure ofthe mixture. 

Enzymatic catalysis in SCFs exploits the ability to alter solvent strength with small changes in temperature and pressure in the near-critical region. The versatile solvation characteristics of SCFs, the extreme catalytic specificity of enzymes, and the ability to achieve complete removal of the SCF solvent by depressurization make enzymatic catalysis in SCFs attractive to the pharmaceutical and food industries. 

Because the correlation length increases as the critical solution point is approached, enhanced adsorption occurs in the pore space specifically in the near-critical region of the phase diagram. In the two-phase region the pore walls are wet, either partially or completely, by that phase in which the... 

The observed system-size dependence clearly indicates that the correlation length associated with density fluctuations in the confined fluid exceeds the dimensions of the simulation cell [178]. This is indicative of a near-critical thennodynamic state of the confined fluid. Because of the density of the participating phases in this near-critical region we conclude that the critical point is the one at which fluid bridge and liquid-like phases become in-... 

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