Supercritical solubility, determination

Water affects the reaction rate through its effect on reaction kinetics and protein hydration, which is required for optimal enzyme conformation and activity. Enzymes need a small amount of water to maintain their activity however, increasing the water content can decrease the reaction rate as a result of hydrophilic hin-drance/barrier to the hydrophobic substrate, or because of denaturation of the enzyme (189). These opposite effects result in an optimum water content for each enzyme. In SCFs, both the water content of the enzyme support and water solubilized in the supercritical phase determine the enzyme activity. Water content of the enzyme support is, in turn, determined by the distribution/partition of water between the enzyme and solvent, which can be estimated from water adsorption isotherms (141, 152). The solubility of water in the supercritical phase, operating conditions, and composition of the system (i.e., ethanol content) can affect the water distribution and, hence, determine the total amount of water that needs to be introduced into the system to attain the optimum water content of the support. The optimum water content of the enzyme is not affected by the reaction media, as demonstrated by Marty et al. (152), for esterification reaction using immobilized lipase in n-hexane and SCC02- Enzyme activity in different solvents should, thus, be compared at similar water content of the enzyme support. 

We propose the study of Lennard-Jones (LJ) mixtures that simulate the carbon dioxide-naphthalene system. The LJ fluid is used only as a model, as real CO2 and CioHg are far from LJ particles. The rationale is that supercritical solubility enhancement is common to all fluids exhibiting critical behavior, irrespective of their specific intermolecular forces. Study of simpler models will bring out the salient features without the complications of details. The accurate HMSA integral equation (Ifl) is employed to calculate the pair correlation functions at various conditions characteristic of supercritical solutions. In closely related work reported elsewhere (Pfund, D. M. Lee, L. L. Cochran, H. D. Int. J. Thermophvs. in press and Fluid Phase Equilib. in preparation) we have explored methods of determining chemical potentials in solutions from molecular distribution functions. 

The first summation is over the surface points with positive potential and the second over the points with negative potential. Vg and Vg are the positive and negative surface averages in V(r), respectively, Since the terms in eq. 10, are squared, Otot is, in contrast to fl, particularly sensitive to the extremes in V(r). The two quantities have also been found to be quite different and even been found to vary in opposite directions for some groups of molecules [106]. atot i considered to be indicative of a molecule s electrostatic interaction tendencies. For example, has been used in conjunction with measures of molecular size, i.e. surface area or volume, for correlating solubilities in supercritical fluids [101, 105]. It has been suggested that Ojot in these relationships reflect solute-solute interactions, since the supercritical solubility mainly is determined by the solute vapor pressure [105]. 

Among the cosolvents studied, methanol and acetone have received the greatest interest (36-38). Methanol may act as either a Lewis acid or a Lewis base while acetone is a weaker Lewis base and very slightly acidic ( ). The dipole moment of acetone is 2.88 Debeye compared to 1.7 Debeye for methanol. Based on these properties, Walsh, et al., (40), interpret the data of Van Alsten (37) and Schmitt (38) and present liquid phase IR measurements which show Lewis acid-base interactions in the systems methanol/acridine and acetone/benzoic acid. Supercritical solubility data of Dobbs, et al., ( ), exhibit trends which indicate the importance of acid-base interactions. Van Alsten and Schmitt present data which show that acid-base interactions are a secondary cosolvent effect superimposed on a primary effect determined by cosolvent concentration. 

Figure 1. (a). Zimm plot ofSANS data for 10% (w/v)high molecular weight POLYFOA in supercritical CO2 (b). Solubility determination using SANS for 3%(wAr) PFOA in CO2 at 65 OC... 

Regression correlation coefficient Regression coefficient of determination Rolling circle amplification Water solubility Sodium dodecyl sulfate Supercritical fluid extraction Standard operating procedure Solid-phase extraction Surface plasmon resonance Thymine... 

Various models of SFE have been published, which aim at understanding the kinetics of the processes. For many dynamic extractions of compounds from solid matrices, e.g. for additives in polymers, the analytes are present in small amounts in the matrix and during extraction their concentration in the SCF is well below the solubility limit. The rate of extraction is then not determined principally by solubility, but by the rate of mass transfer out of the matrix. Supercritical gas extraction usually falls very clearly into the class of purely diffusional operations. Gere et al. [285] have reported the physico-chemical principles that are the foundation of theory and practice of SCF analytical techniques. The authors stress in particular the use of intrinsic solubility parameters (such as the Hildebrand solubility parameter 5), in relation to the solubility of analytes in SCFs and optimisation of SFE conditions. 

Applications The majority of SFE applications involves the extraction of dry solid matrices. Supercritical fluid extraction has demonstrated great utility for the extraction of organic analytes from a wide variety of solid matrices. The combination of fast extractions and easy solvent evaporation has resulted in numerous applications for SFE. Important areas of analytical SFE are environmental analysis (41 %), food analysis (38 %) and polymer characterisation (11%) [292], Determination of additives in polymers is considered attractive by SFE because (i) the SCF can more quickly permeate throughout the polymer matrix compared to conventional solvents, resulting in a rapid extraction (ii) the polymer matrix is (generally) not soluble in SCFs, so that polymer dissolution and subsequent precipitation are not necessary and (iii) organic solvents are not required, or are used only in very small quantities, reducing preparation time and disposal costs [359]. 

Some typical applications in SFE of polymer/additive analysis are illustrated below. Hunt et al. [333] found that supercritical extraction of DIOP and Topanol CA from ground PVC increased with temperature up to 90 °C at 45 MPa, then levelled off, presumably as solubility became the limiting factor. The extraction of DOP and DBP plasticisers from PVC by scC02 at 52 MPa increased from 50 to 80 °C, when extraction was almost complete in 25 min [336]. At 70 °C the amount extracted increased from 79 to 95 % for pressures from 22 to 60 MPa. SFE has the potential to shorten extraction times for traces (<20ppm) of additives (DBP and DOP) in flexible PVC formulations with similar or even better extraction efficiencies compared with traditional LSE techniques [384]. Marin et al. [336] have used off-line SFE-GC to determine the detection limits for DBP and DOP in flexible PVC. The method developed was compared with Soxhlet liquid extraction. At such low additive concentrations a maximum efficiency in the extractive process and an adequate separative system are needed to avoid interferences with other components that are present at high concentrations in the PVC formulations, such as DINP. Results obtained... 

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. 

Saldana MDA, Tomberli B, Guigard SE, Goldman S, Gray CG and Temelli F. 2007. Determination of vapor pressure and solubility correlation of phenolic compounds in supercritical CO2. J Supercrit Fluids 40(1) 7—19. 

Supercritical solutions are characterized by very low solvent densities. As a result, they possess the interesting feature that solubility is determined more by solute-solute than solute-solvent interactions. Thus we were able to express the solubilities of naphthalene and a series of indole derivatives in four different supercritical solvents (C2H4, C2H6, C02 and the highly polar CHF3) in the same functional format, only the numerical coefficients varying from one to another.57 Solute-solvent interactions do occur,58 but solubility can be represented quite... 

Coimbra P, Blanco MR, Silva HSRC, Gil MH, de Sousa HC. (2006) Experimental determination and correlation of artemisinin s solubility in supercritical carbon dioxide. J Chem Eng Data 51 1097-1104. 

For the Diels-Alder reaction between isoprene and methyl acrylate (see fig. 6.4) in supercritical carbon dioxide at 323 K, the solubility parameter of the activated complex was determined in order to study the nature of the complex and the effect of the solvent on the reaction (Ikushima et al., 1994). 

Suitable gases in the form of supercritical fluids represent clean solvents/ carriers, which neither leave residues nor impose an environmental load. A number of factors determine the solubility of polymers in supercritical carbon dioxides (SCCO2) and these are given in Table 3.1. 

In the supercritical phase, both temperature and pressure play a significant role in determining the extraction efficiency. After the short-lived retrograde solubility effect subsides at about 55-60°C, a transition of the system back to the mass transfer controlled situation will take place where increasing temperature will, once again, bring about a surge in the extraction efficiency. In fact, for the supercritical phase,... 

The evaluation of the sublimation pressure is a problem since most of the compounds to be extracted with the supercritical fluids exhibit sublimation pressures of the order of 10 14 bar, and as a consequence these data cannot be determined experimentally. The sublimation pressure is thus usually estimated by empirical correlations, which are often developed only for hydrocarbon compounds. In the correlation of solubility data this problem can be solved empirically by considering the pure component parameters as fitting-parameters. Better results are obviously obtained [61], but the physical significance of the numerical values of the parameters obtained is doubtful. For example, different pure component properties can be obtained for the same solute using solubility data for different binary mixtures. 

Direct sampling and analysis of the effluent stream may be used to determine the solubility of the heavy phase in the volatile component (often a supercritical fluid). Alternatively, the composition can be determined from the total volume of gas (i.e., of the supercritical fluid after expansion) passed through the saturator, and from the known mass of solute extracted during die sample-collecting period. The efflluent stream is expanded to atmospheric pressure via an expansion valve. Then it passes through a cold trap, where the extract is quantitatively precipitated or condensed, and finally proceeds to a dry-test or a wet-test gas meter or other device, where the total amount of the passed gas is measured. The amount of extracted solute... 

In this paper, we present a preliminary analysis of the steady-state and time-resolved fluorescence of pyrene in supercritical C02. In addition, we employ steady-state absorbance spectroscopy to determine pyrene solubility and determine the ground-state interactions. Similarly, the steady-state excitation and emission spectra gives us qualitative insights into the excimer formation process. Finally, time-resolved fluorescence experiments yield the entire ensemble of rate coefficients associated with the observed pyrene emission (Figure 1). From these rates we can then determine if the excimer formation process is diffusion controlled in supercritical C02. 

In a dynamic extraction system, the supercritical fluid is pumped only once through the container with the sample to the receiver. In the receiver, the liquid is vaporized, leaving concentrated analytes that are then dissolved in a small volume of the solvent. Such extracts are analyzed to determine selected analytes. This manner of extraction is effective if the analytes are well soluble in the solvent and the sample matrix is penetrable. Apart from the aforementioned possibility of fractionated extraction, SFE has many other advantages accruing from the special properties of supercritical fluids ... 

To understand any extraction technique it is first necessary to discuss some underlying principles that govern all extraction procedures. The chemical properties of the analyte are important to an extraction, as are the properties of the liquid medium in which it is dissolved and the gaseous, liquid, supercritical fluid, or solid extractant used to effect a separation. Of all the relevant solute properties, five chemical properties are fundamental to understanding extraction theory vapor pressure, solubility, molecular weight, hydrophobicity, and acid dissociation. These essential properties determine the transport of chemicals in the human body, the transport of chemicals in the air water-soil environmental compartments, and the transport between immiscible phases during analytical extraction. 

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