Vacuum thermal evaporation limitations

As was mentioned earlier, distillation and subsequent solvent extraction remains popular in the aroma research area Q). In this method for aroma analysis, the Likens-Nickerson apparatus has been a standard for over 20 years (17, 18). The primary limitation of the Likens-Nickerson distillation/ extraction procedure has been its operation at reduced pressure. It is desirable to operate the system under vacuum in order to reduce the sample boiling point to minimize the formation of thermally induced artifacts. The fact that the solvent side of the distillation-extraction apparatus is also under vacuum makes it difficult to retain the solvent in the apparatus. Even modifications of the apparatus to include a dry ice/acetone condenser followed by a liquid nitrogen trap do not permit easy operation under vacuum. Problems arise in that the solvent or aqueous vapors reach the cryogenic traps, thereby eventually blocking the exit of the condenser. The need to minimize exposure of the sample to heat has resulted in the more frequent use of two step procedures. Very often, the sample is simply placed in a flash evaporator, a certain volume of distillate collected and the distillate is solvent extracted via either separatory funnel or a continuous extractor. In this manner, the distillation process and solvent choice are not conflicting processes. 

The upper limit of the liquid range is usually bounded by the thermal decomposition temperature of the ionic liquid, since most ionic liquids exhibit extremely low vapor pressure to their respective decomposition temperatures. In contrast to molten salts, which form tight ion pairs in the vapor phase, the reduced Coulombic interactions between ions energetically restricts the ion pair formation required for volatilization of ILs, leading to low vapor pressures. This leads to high upper temperature limits, defined in many cases by decomposition of the IL rather than vaporization. However, it has also been demonstrated recently that certain, thermally very stable, ionic liquids do evaporate under harsh temperature and vacuum conditions [22]. It has even been demonstrated that mixtures of such ionic liquids can be separated by distillation. 

Atoms at a metal surface exhibit unsatuiated bonds that are available for fixing reactive species, atoms or molecules, present in the gas or liquid surrounding the surface. Such a reaction, when limited to one monolayer or a fraction of a mono-layer, is known as an adsorption phenomenon or chemisorption. Similarly, atoms present in the bulk metal may diffuse toward and enrich the surface by so-called thermal segregation. Segregation may also occur by selective evaporation of the metal in vacuum or in an inert gas or by selective dissolution of the metal in a liquid phase (anodic segregation). Whatever the mechanism of surface enrichment, there is strong experimental evidence that the same structural and chemical states can be achieved by adsorption or segregation. 

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