Cosolvent-modified supercritical fluids

Current work with supercritical fluids can also illustrate the importance of cosolvents. Cosolvent effects in supercritical fluids can be considerable for systems where the cosolvent interacts strongly with the solute. A correlation suggests that both physical and chemical forces are important in the solvation process in polar cosolvent supercritical CO2 mixtures. The model coupled with the correlation represents a step toward predicting solubilities in cosolvent-modified supercritical fluids using nonthermody-namic data. This method of modeling cosolvent effects allows a more intuitive interpretation of the data than either a purely physical equation of state or ideal chemical theory can provide (Ting et al., 1993). 

DL Tomasko, BL Knutson, F PouUlot, CL Liotta, CA Eckert. Spectroscopic study of structure and interactions in cosolvent-modified supercritical fluids. J Phys Chem 97 11823, 1993. 

Cosolvent-modifled supercritical fluids are also used routinely in supercritical fluid chromatography (SFC) to modify solute retention times (11-20). In these reports, cosolvents are used to alter the mobile and stationary phase chemistries (16t17t20). However, distinguishing between such effects in a chromatography... 

Optimizing solvents and solvent mixtures can be done empirically or through modeling. An example of the latter involves a single Sanchez-Lacombe lattice fluid equation of state, used to model both phases for a polymer-supercritical fluid-cosolvent system. This method works well over a wide pressure range both volumetric and phase equilibrium properties for a cross-linked poly(dimethyl siloxane) phase in contact with CO2 modified by a number of cosolvents (West et al., 1998). 

The benefits from tuning the solvent system can be tremendous. Again, remarkable opportunities exist for the fruitful exploitation of the special properties of supercritical and near-critical fluids as solvents for chemical reactions. Solution properties may be tuned, with thermodynamic conditions or cosolvents, to modify rates, yields, and selectivities, and supercritical fluids offer greatly enhanced mass transfer for heterogeneous reactions. Also, both supercritical fluids and near-critical water can often replace environmentally undesirable solvents or catalysts, or avoid undesirable byproducts. Furthermore, rational design of solvent systems can also modify reactions to facilitate process separations (Eckert and Chandler, 1998). 

Solvation in supercritical fluids depends on the interactions between the solute molecules and die supercritical fluid medium. For example, in pure supercritical fluids, solute solubility depends upon density (1-3). Moreover, because the density of supercritical fluids may be increased significantly by small pressure increases, one may employ pressure to control solubility. Thus, this density-dependent solubility enhancement may be used to effect separations based on differences in solute volatilities (4,5). Enhancements in both solute solubility and separation selectivity have also been realized by addition of cosolvents (sometimes called entrainers or modifiers) (6-9). From these studies, it is thought that the solubility enhancements are due to the increased local density of the solvent mixtures, as well as specific interactions (e.g., hydrogen bonding) between the solute and the cosolvent (10). 

The technique may be viewed as an alternative to the addition of cosolvents or modifiers (sometimes termed entraimrs) that are commonly used in supercritical fluid technology to enhance the polarity of the fluid. For cleaning processes, however, these cosolvents may be toxic or detrimental in various ways to the substrate. In addition, these modifiers are usually more difficult to separate downstream from the process due to their high volatility. In contrast, surfactants typically have very low volatility and thus interact to a much lesser degree with the substrate. Furthermore, they often dramatically improve the solubility of polar species, well beyond that of simple modifiers. 

The solvent characteristics of a supercritical fluid can be altered by adding a modifier (also known as an entrainer or cosolvent ). The mechanism of action of the modifier depends on both the type of matrix concerned and the form in which the analytes occur in it. A modifier can have four different effects, namely (a) increase the analyte solubility by interacting with the solute in the fluid phase (b) facilitate solute desorption by interacting with bound solutes, the matrix active sites or both (c) favour diffusion of the solute within the matrix and (d) hinder diffusion of the solute within the matrix through contraction, which will result in decreased recovery. 

Thus, taken as a whole, the tunable densities and tunable solubilities, the availability of cosolvents as property modifiers, the ability to utilize fluids that are gases under ambient conditions, and the possibility of employing fluids that are environmentally friendly (e.g., CO2 or water) (the unique properties of supercritical fluids) make SFE a valuable alternative to normal liquid extraction processes. 

Supercritical or near-critical fluids can be used both for extraction and chromatography. Many chemicals, primarily organic species, can be separated and analyzed using this approach [6], which is particularly useful in the food industry. Substances that are useful as supercritical fluids include carbon dioxide, water, ethane, ethene, propane, xenon, ammonia, nitrous oxide, and a fluoroform. Carbon dioxide is most commonly used, typically at a pressure near 100 bar. The required operating pressure ranges from about 43 bar for propane to 221 bar for water. Sometimes a solvent modifier is added (also called an entrainer or cosolvent), particularly when carbon dioxide is used. 

The solvent power of the supercritical phase, and the selectivity of the column can be enhanced by using a cosolvent, usually called an entrainer or modifier in this application. A two-phase region may be inadvertently entered at P-T conditions for which the pure supercritical fluid is in one phase. Knowledge of the phase behavior of the binary system is therefore necessary. 

Supercritical fluid extraction (SFE) is gaining popularity as a prefractiona-tioii technique because it can reduce extraction time with concomitant good recoveries and ease of automation [II]. SFE frequently employs carbon dioxide, which is converted into a supercritical state by pressure and temperature. Adjustment of these parameters and addition of modifiers and cosolvents yield selective extractions of specific classes of molecules [11-13]. Like Soxhlet extraction, SFE is typically used for extraction of small molecules. 

Other carotenoids such as lycopene from tomato and its industrial waste [65-68] and lutein esters from marigold (Tagetes erecta) petals [69-71] had been extracted with supercritical fluids, achieving better extractiOT yields when modifiers and cosolvents were used as acetone, chloroform, ethanol, and vegetable oils. 

Sub- and supercritical fluid extraction with CO2 has several advantages. One is that CO2 is nontoxic, nonflammable, and noncorrosive. Also, the end product is obtained as a powder without the need for drying [197]. However, the relatively high equipment purchase and maintenance pose a major constraint. This technique has been successfully used to extract polyphenols from grape pomace, generally using CO2 modified with methanol [198-201] or ethanol as cosolvent [202]. 

Based on the overwhelming prevalence of CO, as SF extractant, all applications described in this section refer to this fluid (whether neat or modified by a cosolvent) unless otherwise stated. Because of its special features, the uses of supercritical water are dealt with in another section. 

SFE using CO2 usually yields good recoveries for nonpolar lipids, but polar lipids may remain partially unextracted because of their lower solubility in this fluid. For this reason, organic modifiers are added as cosolvents to the primary fluid to enhance the extraction efficiency the addition of 1 to 10% of methanol or ethanol to CO2 t) ically expands the extraction range to include more polar lipids. By performing the extraction with supercritical carbon dioxide and 20% of ethanol, more than 80% of the phospholipids could be recovered [27]. 

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