Viscosity, of supercritical fluids

The viscosities of supercritical fluids are intermediate between those of a gas and a liquid, but this tine, are much closer to those of a gas than a liquid. For a fixed column pressure drop much longer columns or higher flow rates are possible in SFC compared to liquid chromatography. Liquids, however, are virtually incompressible while gases and... 

The popularity of this extraction method ebbs and flows as the years go by. SFE is typically used to extract nonpolar to moderately polar analytes from solid samples, especially in the environmental, food safety, and polymer sciences. The sample is placed in a special vessel and a supercritical gas such as CO2 is passed through the sample. The extracted analyte is then collected in solvent or on a sorbent. The advantages of this technique include better diffusivity and low viscosity of supercritical fluids, which allow more selective extractions. One recent application of SFE is the extraction of pesticide residues from honey [27]. In this research, liquid-liquid extraction with hexane/acetone was termed the conventional method. Honey was lyophilized and then mixed with acetone and acetonitrile in the SFE cell. Parameters such as temperature, pressure, and extraction time were optimized. The researchers found that SFE resulted in better precision (less than 6% RSD), less solvent consumption, less sample handling, and a faster extraction than the liquid-liquid method [27]. 

The second approach was taken by practicing liquid chromatographers. They routinely dealt with thermally labile, highly polar molecules and frequently sacrificed resolution, and speed in their separations because of the aqueous mobile phases that were required. With the enhanced diffusion and decreased viscosity of supercritical fluids over liquids, chromatographic run-time and resolution could be improved when supercritical fluids were used. But solubility in pure carbon dioxide mobile phases, which has the solvating powers from hexane to methylene chloride under normal density ranges, was a problem for these polar molecules. To compensate for this, experimentalists started working with mixed mobile phases. These mixed phases were based on... 

The low viscosity of supercritical fluids ensures effective penetration of the entire sample. 

Since commercial supercritical fluid extraction apparatus has become available, use of these materials as extractants has become attractive. Solvent evaporation and disposal are eliminated, and the extractions may be very efficient because of the low viscosity of supercritical fluids, which allows them to penetrate readily into the solid sample particles. Carbon dioxide, with or without modifiers such as methanol, is the most commonly used solvent. 

Supercritical fluid extraction (SFE) has been used for the recovery of analytes from hair this is a technique that offers several advantages due to the characteristics of low viscosity of supercritical fluid the speed of extraction, high extraction efficiency, and ultimately the ability to easily remove the extraction solvent. Also there is the possibility of working with automated systems that also allow the recycling of the solvent. The major limitation of this technique is the cost of the instrumentation [42],... 

There are several reasons why SFC may gain its place as a separation technique alongside GC and LC in the years ahead. From a fundamental point of view, the diffusion coefficients under typical SFC conditions are lower than those typically encountered in gases, but higher than those found in liquids. The viscosity of supercritical fluids is usually higher than that of typical gases, but much lower than that of common liquids. At the same time, supercritical fluids are good solvents for many low-volatile solutes, which are not compatible with GC. Therefore, SFC may offer the possibility to separate non-volatile... 

The viscosity of supercritical fluids is greater than that of gases. This is important because a pronounced viscosity is responsible for a noticeable drop of pressme in an extraction cell. 

The viscosity of supercrital fluids increases with pressure, but approaches that of a liquid less rapidly than the density. Even if very high pressures are used, the viscosity of a fluid in a supercritical state is inferior to that of a liquid. This is followed by improved penetration in porous materials coupled with a rapid kinetic. 

Very long columns can be used in SFC because the viscosity of supercritical fluids is so low. 

In addition to density, diffusivity of the supercritical fluids is higher than that of liquid solvents, and can be easily varied. For typical conditions, diffusivity in supercritical fluids is of the order of lO cm /sec as compared to 10 for gases and 10 for liquids. Typical viscosity of supercritical fluids is of the order of 10 g/cm/sec, similar to that of gases, and about 100-fold lower than that of liquids. High diffusivity and low viscosity provide rapid equilibration of the fluid to the mixture to be extracted, hence extraction can be achieved close to the thermodynamic limits. However, the main extraction benefit of supercritical fluids is their adjustable density that provides adjustable solvent strength. The compounds of choice can be dissolved/extracted in the supercritical fluid at high pressure and then this fluid mixture is carried to another vessel where simple lowering of the pressure... 

The slight variation in r(371°) can be explained as follows. The rate constants for 1°, 2°, or 3° hydrogen abstractions by Cl from alkanes are nearly diffusion-controlled in conventional solvents. Consequently, the intrinsic selectivity of Cl is diminished in conventional solvents because of the onset of diffusion control. In the gas phase, selectivity is slightly higher because the barrier imposed by diffusion is eliminated. The viscosity of a supercritical fluid (a) lies between that of conventional fluid solvent and the gas phase and (b) varies with pressure. Because of the low viscosity of supercritical fluids, bimolecular rate constants greater than the 10 ° M" s diffusion-controlled limit can be realized in SCF and, as a consequence, enhanced selectivity is achieved. Consistent with this interpretation is the observation that the plot of r(371°) versus inverse viscosity is approximately linear (Figure 4.4-7) [51]. 

Supercritical fluids exhibit gas-like mass transfer rates and yet have liquid-like solvating capability. The high diffusivity and low viscosity of supercritical fluids enable them to penetrate and transport solutes from porous solid matrices. From this point of view, SFE is an ideal method to extract uranium and lanthanides from solid wastes. Carbon dioxide (CO2) is most frequently used in SFE because of its moderate critical pressure (Pc) nd temperature (Jc), inertness, low cost, and availability in pure form. Figure 1 illustrates moderate values of Pc and Tc compared with those of water. 

As shown in Table 13.4, several physical properties of SCFs are intermediate between gases and liquids. Hence, SFC combines some of the characteristics of both GC and LC. For example, like GC, SFC is inherently faster than LC because of the lower viscosity and higher diffusion rates in supercritical fluids. High diffusivity, however, leads to band spreading, a signiflcant factor with GC but not with LC. The intermediate diffusivities and viscosities of supercritical fluids result in faster separations than are achieved with LC together with lower band spreading than encountered in GC. 

Several techniques have been proposed using supercritical fluids for precipitations. The main advantage is the lower viscosity of supercritical fluids compared to classical fluids, which facilitates mixing and thus higher supersaturations can be achieved. Due to its ready availability, negligible toxicity, and convenient critical point of 32 °C and 100 bar, carbon dioxide is used in most cases as a supercritical fluid. 

Some physical properties of supercritical fluids are in between those of typical gases than liquids. For example, the viscosity of supercritical fluids is about an order of magnitude lower (10 vs. 10 N s/m ) and the solute diffusivity of supercritical fluids is an order of magnitude higher (10 vs. 10 cm ) than for liquid solvents (64). These properties of viscosity and solute diffusivity contribute to improved mass transfer for solutes in the supercritical state, and, therefore, speed extraction rates. The density of carbon dioxide can be increased to densities... 

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