Supercritical fluids, properties dielectric constant

The examples presented in this chapter concern oxides impregnated with aqueous solutions but the field of nonaqueous solvents, which exhibit different coordinative properties, dielectric constant, viscosity, and boiling temperature, and in which precursors like carboxylates and acetylacetonates can be dissolved, should probably be explored more thoroughly. The use of supercritical fluids can also be considered. The fact that more catalysts are being prepared on supports such as carbon fibers or polymers, whose interfacial chemistry is less known than that of oxides, adds interest to these directions of research. 

Reactions. Supercritical fluids are attractive as media for chemical reactions. Solvent properties such as solvent strength, viscosity, diffusivity, and dielectric constant may be adjusted over the continuum of gas-like to Hquid-like densities by varying pressure and temperature. Subsequently, these changes can be used to affect reaction conditions. A review encompassing the majority of studies and apphcations of reactions in supercritical fluids is available (96). 

Water in its supercritical state has fascinating properties as a reaction medium and behaves very differently from water under standard conditions [771]. The density of SC-H2O as well as its viscosity, dielectric constant and the solubility of various materials can be changed continuously between gas-like and liquid-like values by varying the pressure over a range of a few bars. At ordinary temperatures this is not possible. For instance, the dielectric constant of water at the critical temperature has a value similar to that of toluene. Under these conditions, apolar compounds such as alkanes may be completely miscible with sc-H2O which behaves almost like a non-aqueous fluid. 

In general, the properties of supercritical fluids make them interesting media in which to conduct chemical reactions. A supercritical fluid can be defined as a substance or mixture at conditions which exceed the critical temperature (Tc) and critical pressure (Pc). One of the primary advantages of employing a supercritical fluid as the continuous phase lies in the ability to manipulate the solvent strength (dielectric constant) simply by varying the temperature and pressure of the system. Additionally, supercritical fluids have properties which are intermediate between those of a liquid and those of a gas. As an illustration, a supercritical fluid can have liquid-like density and simultaneously possess gas-like viscosity. For more information, the reader is referred to several books which have been published on supercritical fluid science and technology [1-4],... 

With traditional solvents, the solvent power of a fluid phase is often related to its polarity. Compressed C02 has a fairly low dielectric constant under all conditions (e = 1.2-1.6), but this measure has increasingly been shown to be insufficiently accurate to define solvent effects in many cases [13], Based on this value however, there is a widespread (yet incorrect ) belief that scC02 behaves just like hexane . The Hildebrand solubility parameter (5) of C02 has been determined as a function of pressure, as demonstrated in Figure 8.3. It has been found that the solvent properties of a supercritical fluid depend most importantly on its bulk density, which depends in turn on the pressure and temperature. In general higher density of the SCF corresponds to stronger solvation power, whereas lower density results in a weaker solvent. 

Liquefied or Supercritical Cases as Solvents for Electrolytes For very special applications, where the increased efforts for low temperature and/or pressurized cells are acceptable, liquefied gases, for example, sulfur dioxide or ammonia, can be interesting solvents for electrolytes (see e.g. [3a]). Supercritical fluids show remarkable properties that are very different from other solvents. Detrimental to electrochemistry is that especially the dielectric constant in the supercritical state becomes low. For supercritical carbon dioxide, no supporting electrolyte with sufficient conductivity is known. 

Ultimately physical theories should be expressed in quantitative terms for testing and use, but because of the complexity of liquid systems this can only be accomplished by making severe approximations. For example, it is often necessary to treat the solvent as a continuous homogeneous medium characterized by bulk properties such as dielectric constant and density, whereas we know that the solvent is a molecular assemblage with short-range structure. For an example of this tool with supercritical fluids see Ting et al., 1993. 

Important solvent properties of SC-CO2 (e.g., dielectric constant, solubility parameter, viscosity, density) can be altered via manipulation of temperature and pressure. This unique property of a supercritical fluid could be exploited to control the behavior (e.g., kinetics and selectivity) of some chemical processes. 

Many physicochemical properties describe a chemical substance or mixture. For example, the boiling point, density, and dielectric constant can all be used to characterize a particular species or system as a solid, liquid, or gas. However, if a substance is heated and maintained above its critical temperature it becomes impossible to liquify it with pressure (1). When pressure is applied to this system a single phase forms that exhibits unique physicochemical properties (1-14). This single phase is termed a supercritical fluid and is characterized by a critical temperature and pressure (Tc and Pc). 

The physical properties of the supercritical fluid differ from those of the bulk liquid. One of the most notable changes is the lower dielectric constant of polar solvents such as water which allows the accumulation of low-polarity solutes at this interface. This explains the crucial role of the hydro-phobicity of solutes during reactions in the solution. Thermolysis as well as radical abstraction reactions occur in this region. A temperature of approximately 800 K was determined for the interfacial region surrounding the... 

Due to its compressibility in the liquid (near the critical point) and in the supercritical fluid state, the dielectric constant and density, and thus the solvent quality of C02, are tunable with pressure and temperature (Keyes and Kirkwood, 1930). As illustrated in Figure 1.2, this compressibility provides for control of the density and therefore solvent-dependent properties such as dielectric constant and overall solvent strength (Giddings et al., 1968). While supercritical C02 can have high liquidlike densities, it shares many of the... 

We now turn attention to a completely different kind of supercritical fluid supercritical water (SCW). Supercritical states of water provide environments with special properties where many reactive processes with important technological applications take place. Two key aspects combine to make chemical reactivity under these conditions so peculiar the solvent high compressibility, which allows for large density variations with relatively minor changes in the applied pressure and the drastic reduction of bulk polarity, clearly manifested in the drop of the macroscopic dielectric constant from e 80 at room temperature to approximately 6 at near-critical conditions. From a microscopic perspective, the unique features of supercritical fluids as reaction media are associated with density inhomogeneities present in these systems [1,4],... 

However, if the solvent is a supercritical fluid (SCF), it is possible to examine the effects of temperature and density on VER independently. The role of other solvent properties, such as viscosity, dielectric constant, and correlation length, can also be studied. A supercritical fluid is a substance that has been heated above its critical temperature (Tc) and, therefore, no longer undergoes the liquid/gas phase transformation. A typical phase diagram for an SCF is shown in Fig. 1. In an SCF, it is possible to fix the temperature and vary the density continuously (by varying the pressure) from gas-like densities to liquid-like densities. It is also possible to vary the temperature at fixed density. 

The physical properties of supercritical fluids tend to lie between those of gases and liquids. The increased density relative to a gas, and the decreased viscosity relative to a liquid, allow supercritical fluids to be used as excellent solvents in many laboratory and industrial applications (19-25). Also, some notable solvation peculiarities of supercritical fluids have been discovered. For example, supercritical water can dissolve nonpolar oils because the dielectric constant of supercritical water decreases drastically near the critical point (26). 

Water possesses vastly different properties as a reaction medium in its supercritical state than in its standard state. The diagram in Fig. 14.7 is that of a pure substance and shows the regions of temperature and pressure where the substance exists as a solid, liquid, gas, and supercritical fluid. The supercritical point for water is met at a temperature of 400°C and above and at high pressure (about 25 MPa). At the supercritical point, water behaves as a nonpolar dense gas, and hydrocarbons exhibit generally high solubility. However, the solubility of inorganic salts is very low in such liquid. Note that the dielectric constant of water is 80 at the standard state reaches approximately 0 at the supercritical point the Aw of 10 14 at the standard state reaches approximately 10-24 at the supercritical point. 

Other important physical chemical properties are polarity and dielectric constant. Water has a high dielectric constant (78.5 at STP), which would effectively mask ionic charges and lead to high solubility of ionic compounds. The dielectric constant of CO2 at 200 bar and 40°C is approximately 1.5, and CO2 is considered a very non polar solvent. As would be expected, polarity influences solubility for supercritical fluids. Carbon dioxide has a dipole moment of 0.0 Debye, while the value for NH3 is approximately 1.5. Therefore, C02 by itself is poorly suited for dissolving polar compounds. 

Some properties of supercritical fluids can be moni-fored (manipulafed) continuously by adjusting the temperature and pressure or density of the fluid. Dielecfric consfanf is such a property and the solvent s dielectric constant can influence the rate of the reaction. 

The critical point on a phase diagram designates the pressure (pc) and temperature (Tq) at which the vapor and liquid phases of a substance become indistinguishable. By definition, a supercritical fluid (SCF) is above pc and Tc- Generally, the physical properties of an SCF (density, viscosity, and dielectric constant) are intermediate between those of a liquid and a gas, and these properties vary dramatically as a function of temperature and pressure [1,2]. Because of these unique features, there is enormous interest in the use of SCFs as solvents for chemical reactions [3-6]. 

Consider a spherical point dipole solute molecule with a dipole moment, p, at infinite dilution in a spherical container of supercritical fluid. With a continuum assumption, the fluid s electrical properties may be represented by a homogeneous dielectric constant, e. The inhomogeneous field of the dipole polarizes the fluid which reacts and gives rise to a field, R, at the dipole. R will be proportional to p as long as no saturation effects occur. 

Chapter 8 briefly introduced the concept of supercritical fluids in the context of undersea thermal vents. The supercritical point for water occurs at a temperature of 705°F (374°C) and a pressure of 222.3 bar (atmosphere). Above this temperature, no pressure can condense water to its liquid state. For carbon dioxide (CO2), the critical temperature (88.0°F or 31.1°C) and critical pressure (73.8 bar) are much lower. Above the supercritical point, CO2 behaves as a liquidlike gas liquidlike densities, gaslike viscosities. The solubility properties of supercritical CO2 are mnable by varying temperature and/or pressure. Density and dielectric constant increase with increasing pressure and decreasing temperature. Water and ionic substances are insoluble in supercritical CO2. The ability of supercritical CO2 to dissolve and extract relatively non-polar substances has been known for decades. The range may be extended by adding polar solvents such as methanol or acetone. The addition of surfactants helps to disperse microscopic particles to form colloidal suspensions. Carbon dioxide is nonflammable, nontoxic, and inexpensive. 

The dielectric constant is an important property for chemical reactions and reaction theory. Although it is not technically a thermodynamic property, it will be discussed here. For supercritical fluids such as CHF3 and water, temperature and pressure can be adjusted to achieve large variations in the dielectric constant. For others, such as CO2, the dielectric constant changes little between ambient pressure to several times the reduced pressure. Figure 5 illustrates the large differences in the dielectric constant between CO2 and CHF3. Both have similar qualitative behavior... 

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