Supercritical fluid Term applied

Chromatography. GC is the most common anal)d ical method used but liquid and supercritical fluid chromatographic methods are being increasingly developed. Like titration the sample is destroyed in the analysis process. The ideal situation depicted in Figure 8.8 cannot normally be applied for titration or chromatographic analysis since the analysis equipment needs to be close to the sampling device. This is often termed at-line analysis. 

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). 

Supercritical fluid extraction (SFE) has become more commonplace in analytical laboratories as an alternative technique to more traditional, manual techniques. While SFE is a relatively new technology to the analytical laboratory, feasibility in terms of affording acceptable levels of recovery (accuracy) has been demonstrated for a real diversity of samples by many researchers. Other workers have evaluated many of the usual technical and practical considerations applied in developing a new technique and the resulting conclusions have not indicated any insurmountable issues associated with those considerations which would preclude the continued development of SFE as a powerful tool for the analytical chemist. One major consideration which has received somewhat minimal attention thus far is that of the robustness of the technique. 

The objective of this paper is to propose a predictive method for the estimation of the change in the solubility of a solid in a supercritical solvent when another solute (entrainer) or a cosolvent is added to the system. To achieve this goal, the solubility equations were coupled with the Kirkwood-Buff (KB) theory of dilute ternary solutions. In this manner, the solubility of a solid in a supercritical fluid (SCF) in the presence of an entrainer or a cosolvent could be expressed in terms of only binary data. The obtained predictive method was applied to six ternary SCF-solute-cosolute and two SCF-solute-cosolvent systems. In the former case, the agreement with experiment was very good, whereas in the latter, the agreement was only satisfactory, because the data were not for the very dilute systems for which the present approach is valid. 2001 Elsevier Science B.V. All rights reserved. 

The term solvent extraction can apply to the removal of pollutants from soil with a solvent. However, this is properly called leaching and will be covered separately toward the end of this chapter along with supercritical fluid extraction (SEE). Initially,... 

Compounds not soluble in a supercritical fluid can be recrystallized in a process termed gas antisolvent (GAS) recrystailization. The process was first applied to the recrystailization of an explosive, RDX (cyclotrimethylenetrinitramine) into... 

Adidharma and Radosz provides an engineering form for such a copolymer SAFT approach. SAFT has successfully applied to correlate thermodynamic properties and phase behavior of pure liquid polymers and polymer solutions, including gas solubility and supercritical solutions by Radosz and coworkers Sadowski et al. applied SAFT to calculate solvent activities of polycarbonate solutions in various solvents and found that it may be necessary to refit the pure-component characteristic data of the polymer to some VLE-data of one binary polymer solution to calculate correct solvent activities, because otherwise demixing was calculated. GroB and Sadowski developed a Perturbed-Chain SAFT equation of state to improve for the chain behavior within the reference term to get better calculation results for the PVT - and VLE-behavior of polymer systems. McHugh and coworkers applied SAFT extensively to calculate the phase behavior of polymers in supercritical fluids, a comprehensive summary is given in the review by Kirby and McHugh. They also state that characteristic SAFT parameters for polymers from PVT-data lead to... 

In order to make calculations from this equation, two of the terms are combined to give a Gibbs function change for the reaction, which is still standard in terms of mole fractions, but which applies to the particular supercritical-fluid conditions used, AG where... 

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... 

---