Supercritical solvents, properties

Carbon dioxide has a conveniently low critical point (31 °C, 7.39 MPa), and supercritical CO2 has become the most widely used fluid where supercritical solvent properties are required, as it is also inexpensive and nontoxic. The solvent powers of supercritical fluids generally increase with increasing density, which can be regulated at will by varying the pressure. The absence of a gas-liquid interface and associated surface tension in a supercritical fluid enables the fluid to penetrate porous solids freely, and also to... 

The explosion of interest in supercritical fluids during the past decade has seen a bridging of these extremes, a movement toward balance. Investigators have sought to understand and develop applications on the basis of underlying physicochemical principles, and recent reports of semi-empirical treatment of supercritical solvent properties have shifted these fluids squarely into the mainstream of chemical research. 

Numerous experimental studies have been conducted on solute-solvent interactions in supercritical fluid solutions. In particular, issues such as the role of characteristic supercritical solvent properties in solvation and the dependence of solute-solvent interactions on the bulk supercritical solvent density have been extensively investigated. Results from earlier experiments showed that the partial molar volumes 02 became very large and negative near the critical point of the solvent (4-12). The results were interpreted in terms of a collapse of the solvent about the solute under near-critical solvent conditions, which served as a precursor for the solute-solvent clustering concept. Molecular spectroscopic techniques, especially ultraviolet-visible (UV-vis) absorption and fluorescence emission, have since been applied to the investigation of solute-solvent interactions in supercritical fluid solutions. Widely used solvent environment-sensitive molecular probes include Kamlet-Taft jt scale probes for polarity/polarizability... 

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). 

Above the critical temperature and pressure, a substance is referred to as a supercritical fluid. Such fluids have unusual solvent properties that have led to many practical applications. Supercritical carbon dioxide is used most commonly because it is cheap, nontoxic, and relatively easy to liquefy (critical T = 31°C, P = 73 atm). It was first used more than 20 years ago to extract caffeine from coffee dichloromethane, CH2C12, long used for this purpose, is both a narcotic and a potential carcinogen. Today more than 10s metric tons of decaf coffee are made annually using supercritical C02. It is also used on a large scale to extract nicotine from tobacco and various objectionable impurities from the hops used to make beer. 

Above 30.9 °C, CO2 cannot be liquefied by compression it exists in a supercritical fluid phase (SC-CO2) that behaves like a gas that is denser than liquid CO2. Below 30.9 °C, CO2 can be maintained as a liquid under relatively modest pressure generally SC-CO2 has better solvent properties than CO2 in the sub-critical liquid phase. 

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. 

Solvation properties, of supercritical solvents, 14 80-81 Solvatochromic materials, 22 708t Solvatochromic probes, 26 853—855 Solvatochromic spectral shifts, 23 96 Solvatochromy, 20 517 Solvay, 7 641 Solvay process, 15 63... 

Supercritical regime, 11 756 Supercritical solvents, solvation properties of, 14 80-81... 

Because their solvent properties are very good and their viscosities are very low, supercritical fluids can be used for very efficient extraction of analytes from solid phase samples. The solid phase sample is held in a tube or cartridge and the supercritical fluid made to flow through (minimal pressure required). The fluid with the analyte is then made to flow through a trap solvent. The analyte dissolves in this solvent and the fluid reverts back to the gas phase. 

A supercritical fluid is a state of matter achieved by high temperature and extremely high pressure, exceeding the so-called critical temperature and pressure for that substance. The solvent properties of a supercritical fluid are much improved over the normal solvent properties of that fluid. 

The polymerization of other fluoroolefins such as TFE with hexafluoropro-pylene (HFP), TFE with ethylene, and vinylidine difluoride - " further demonstrates the broad applicability of liquid and supercritical CO in the production and processing of fluorinated polymers. Many of the aforementioned advantages associated with CO2, including tunable solvent properties, integrated synthesis, separation and purification processes, negligible chain transfer in the presence of highly electrophilic species, and relative ease of recycling, make it an ideal solvent for fluoroolefm polymerization. 

Supercritical solvents have a number of advantages which make them excellent reaction media, such as the low cost, non-toxicity, and low viscosity. These advantages have meant that they are increasingly utilized in reactions. Supercritical solvents can be described as fluids with attributes of both liquids and gases. Solubility of the solute in the fluid depends on the vapour pressure of the solute however, addition of different polar/nonpolar compounds can change the solubility. What makes the supercritical solvent to be so imique is that properties, such as solubility, can be tuned by varying the pressure, so the solvent becomes more gas-like or liquid-like [46]. 

Examples are the benefits in the area of extraction of vegetable cuticular waxes being separated from the more valuable essential oils, using supercritical CO2 (Stassi and Schiraldi, 1994). A molecular understanding of how a phenomenon like supercritical behavior affects solvent properties is important (Kazarian and Poliakoff, 1995). 

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. 

The addition of a dispersed droplet phase (forming a microemulsion) provides a convenient means of solubilizing highly polar or ionic species into the low polarity environment of the SCF phase. Hence, the combination of supercritical solvents with microemulsion stractures provides a new type of solvent with some unusual and important properties of potential interest to a range of technologies. These droplets have high diffusion rates in SCF and the properties of the continuous phase can be readily controlled by manipulation of system pressure (Beckman et al., 1995). 

The process involves the use of supercritical fluids rather than liquids as solvents. A fluid is in the supercritical state when its pressure and temperature exceed the pl ical properties which defines its critical point. Carbon dioxide is by far the most widely used supercritical solvent. Many other selected fluids have potential use for SFE technologies. 

A way around this issue may have been found with the use of supercritical fluids. These materials, such as liquid carbon dioxide, have many interesting properties from the point of view of pharmacutical processing since they combine liquid-like solvent properties with gas-like transportation properties. Small changes in the applied pressure or temperature can result in large changes of the fluid density and, correspondingly, the solvent capacity and properties of the resultant particles. 

This makes it possible to tune solvent properties to optimize chromatographic separations. Because of the lower viscosity and higher diffusivity of supercritical fluids compared to common solvents, a higher mobile phase velocity can be used in the column, leading to a higher process throughput than that of liquid chromatography. 

Apart from Section 12.7, which deals with supercritical fluids and room-temperature ionic liquids, only molecular liquid solvents are considered in this book. Thus, the term solvents means molecular liquid solvents. Water is abundant in nature and has many excellent solvent properties. If water is appropriate for a given purpose, it should be used without hesitation. If water is not appropriate, however, some other solvent must be employed. Solvents other than water are generally called non-aqueous solvents. Non-aqueous solvents are often mixed with water or some other non-aqueous solvents, in order to obtain desirable solvent properties. These mixtures of solvents are called mixed solvents. 

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