Drying techniques solvent-exchange process

The PCA process uses supercritical fluid drying to help preserve fine microstructures in the material. Supercritical fluid drying is a technique that has been used for many years to dry biological materials and, more recently, aerogels (qv). The original solvent is replaced by exchange with a supercritical fluid, such as C02, and the system is depressurized above the critical temperature of the SCF. SCFs have no vapor—liquid interface. Thus fine microstructures are... 

The methods employed for isolation of the alkaloids depend on the nature of the compounds, and specific conditions have frequently been devised for the selective isolation of particular types of compounds. Usually, fresh or dried plant material is extracted with dilute acid solution or with alcohol, and the extract obtained is further fractionated by extraction into organic solvents with variation of pH. Extraction columns (288), membrane processes (425), and ion-exchange materials (288-290) may be particularly useful for subfractionation or isolation procedures. For further identification and isolation of separate compounds, preparative thin-layer chromatography, (288, 291, 292, 426), liquid chromatography (293, 294), or gas chromatography may be used (202, 296, 297). Because some of the products reviewed in this chapter occur naturally in very small amounts, they have not been isolated in crystalline form. Gas chromatography-mass spectrometry (87, 213, 299), mass fragmentography (192), and mass spectrometry-mass spectrometry (301, 359) have proved to be particularly useful techniques for identification of trace alkaloids in complex mixtures. 

Kawakami et al. prepared dense and asymmetric membranes from 2,2 -bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and bis[4-(4-aminophen-oxy)phenyl]sulfone (APPS) by solvent evaporation (dense) and by the dry-wet phase inversion technique [47]. The surface morphology was studied by AFM. They reported that the solvent evaporation method adopted for the preparation of the dense membrane influenced the formation of nodules, while the dry-wet process in which solvent/nonsolvent exchange was involved determined the roughness of the skin layer. 

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