Experimental techniques for inverse gas

An important question regarding SN2 reactions in the gas phase concerns the stereochemistry and the extent to which a Walden inversion occurs at the reaction site. Since the experimental techniques monitor exclusively ion concentration, the actual nature of the neutrals produced in the reaction is subject to some doubt. An indirect method to ascertain the nature of the products is to assess the thermochemistry of other possible reaction channels. In the case of methyl derivatives, the alternatives are few and result in highly endothermic reactions, as exemplified in (22) and (23). For more complicated systems, this argument may not be satisfactory or may not yield an unequivocal answer. 

Probing polymer-polymer interactions in miscible blends is an experimentally difficult task. Most methods available for this purpose are elaborate and limited in their applicability. In recent years, research has shown that inverse gas chromatography (IGC) offers great promise for the study of polymer-polymer interactions. Conceptually, the technique involves the following the elution behavior of volatile organic compounds (probes) is measured for one or more blend columns and compared with the retention behavior of two homopolymers studied under identical conditions. An excess retention can then be characterized and treated as a measure of polymer-polymer interaction strength. This polymer-polymer interaction is the cause of the miscibility phenomenon and is of practical interest. 

The reversed-flow gas chromatography (RFGC) technique, a subtechnique of the inverse gas chromatography (IGC), can be used for the determination of physicochemical quantities pertaining to environment and pollutants. Experimental setup and appropriate mathematical analysis for the calculation of physicochemical parameters are reviewed, taking into account i) the interaction between air pollutant(s) and a solid surface in the absence, or in the presence, of a chemical reaction between two pollutants in the gas phase over the solid material (synergistic effects) and ii) exchange of gas pollutant(s) between atmospheric and water environment. 

During the past 40 to 50 years, inverse gas chromatography (IGC) has developed into a widespread, popular, and fruitful technique for the physico-chemical characterization of various materials, as well for providing descriptions of the interactions between components in various systems. Indeed, during the past 20 year several reviews detailing the theoretical background of IGC, as well as its parameters, the interpretation of experimental data and applications have been produced [1-8]. 

Figure 13.6 shows a schematic for IGC operation. Inverse, in this instance, refers to the observation that the powder is the unknown material, and the vapor that is injected into the column is known, which is inverse to the conditions that exist in traditional gas chromatography. After the initial injection of the known gas probe, the retention time and volume of the probe are measured as it passes through the packed powder bed. The gas probes range from a series of alkanes, which are nonpolar in nature, to polar probes such as chloroform and water. Using these different probes, the acid-base nature of the compound, specific surface energies of adsorption, and other thermodynamic properties are calculated. The governing equations for these calculations are based upon fundamental thermodynamic principles, and reveal a great deal of information about the surface of powder with a relatively simple experimental setup (Fig. 13.6). This technique has been applied to a number of different applications. IGC has been used to detect the following scenarios ...

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