Gas sorption measurements

Vapour and gas sorption measurements can be performed with static or dynamic methods, either of which can provide information on equilibrium behaviour. Furthermore, the measurements can be performed using gravimetric or volumetric based instrumentation. The most common flow methods are inverse gas chromatography (IGC) [1] for volumetric studies and dynamic gravimetric instrumentation [2]. 

Permeability should be demonstrated (e.g. by gas sorption measurements, spectroscopic evidence of guest exchange or crystallography). 

Table 2. Manufacturer of gravimetric gas sorption measuring instruments ...

As for the pore volume, the pore sizes of meso- and microporous adsorbents are characterised using gas sorption measurements, whereas the pore sizes of macroporous adsorbents are best estimated using mercury penetration measurements or by electron microscopy. The function dvpjddp = f dp) can be calculated using various models. In gas sorption on sorbents with mesopores the function is obtained using the Kelvin equation describing capillary condensation. 

The site-distribution model has recently been extended to mixed-gas sorption, and is able to account for competitive sorption effects (88). As with the dualmode model, the site-distribution model allows for the prediction of mixed-gas sorption by using parameters obtained from experimentally less complicated pure-gas sorption measurements. Like the dual-mode model, the site-distribution model is phenomenological, while the dual-mode model uses the bulk specific volume Vg and extrapolated equilibrium volume Ve as key parameters the site-distribution model relies upon the bulk shear modulus /u.g as a key parameter. The use of bulk parameters to characterize sorption which occurs at the atomistic scale is questionable from a fundamental point of view nevertheless, such models provide useful tools to describe and even predict the effect of conditioning and physical aging... 

Table 2.1 Manufacturer of volumetric and carrier gas sorption measuring instruments and of gas pycnometers.

The method most widely used to determine gas solubilities in ILs is gravimetric. As we first showed in 2001 [11] gravimetric techniques are unique, suited to measure the solubility of gases and vapors in ILs because the liquid does not evaporate into the vapor, complicating the measurements. Thus, apparatuses designed to measure gas sorption on solids can be used readily for ILs. 

Both the matrix-model and the dual-model represent the experimental data satisfactory (Fig. 1). After modeling sorption measurements in several gas-polymer systems we have observed no systematic differences between the mathematical descriptions of the two models. 

Another way of showing the same effect is to plot gas sorption against some convenient measure of saturation vapor pressure, such as the gas boiling point... 

When the gas or vapor feed stream contains a component that is highly soluble in the polymer membrane and causes plasticization, then the selectivity as defined by Equation 4.6 will depend on the partial pressure or the amount of the plasticizing component sorbed into the membrane. Furthermore, pure-gas permeation measurements are generally not a good indicator of the separation performance, and mixed-gas permeation measurements will be needed [21-23]. Often, the mixed-gas selectivity is less than predicted from pure-gas measurements [8] however, the opposite has been observed [24], Competitive sorption effects can also compromise the prediction of mixed-gas behavior from pure-gas measurements [25], For gas pairs where each component is less condensable than C02, like 02/N2, it is generally safe to conclude that the selectivity characteristics can be accurately judged from pure-gas permeabilities at all reasonable pressures. When the gas pair involves a component more condensable than C02, plasticization is likely to be a factor and pure-gas data may not adequately reflect mixed-gas selectivity. When C02 is a component, the situation depends on the partial pressures and the nature of the polymer. 

The micropore volume is defined as the pore volume of the pores < 2 nm. Microporous volumes calculated from the application of the Dubinin-Radushkevich equation to the N2 adsorption isotherms at 77 K. The mean pore size of each sample obtained from N2 adsorption was determined by applying Dubinin-Radushkevich equation. The hydrogen sorption isotherms were measured with the High Speed Gas Sorption Analyser NOVA 1200 at 77 K in the pressure range 0-0.1 MPa. 

Figure 6. Isotherms of hydrogen adsorption and desorption on carbon materials at temperature 77K, measured by the High Speed Gas Sorption Analyser NOVA 1200.

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