Measurements of Gas Adsorption Systems

An instrument to measure the dielectric permittivity of a gas adsorption system basically consists of an electric capacitor (plates, cylinders, spheres) placed within an adsorption vessel. The vessel should be placed within a thermostat (water, oil etc.) and provided with tubes for gas supply and evacuation. Also manometers and thermometers are needed to measure the gas pressure (p) and temperature (T) inside the chamber. The capacitor is filled with sorbent material (powder, pellets, continuous matter etc.) which can be considered to be homogenous as long as its characteristic length - for example the diameter of cylindrical pellets - is small compared to a characteristic length of the capacitor. 

Assuming a small constant electric voltage (U) to be applied to the plates of the capacitor producing a weak electric field (E 1 V/m), the static capacitance (Cs) of the system can be determined by the relation 

Measurements of the capacitance (C) of an electric capacitor allow one to calculate the relative dielectric permittivity or dielectric constant (e,) of the material included in the capacitor. For plate capacitors we have, cp. Figs 6.1, 6.8, [6.1, 6.2]  

The parameter e o in (6.11, 6.13) is the electric field constant or dielectric permittivity of the vacuum. Its numerical value depends on the system of units chosen [6.3, 6.21-6.24]. For the international system of units that we prefer here, its value is given in (6.12), [6.20, 6.21]. 

These equations represent different forms of the dielectric equation of state (DEOS) for the sorbent / sorbate system. It can be used to characterize the system and is of importance for electrostriction phenomena and / or the electro-adsorptive effect - especially for continuous non-rigid materials, [6.16, 6.17, 6.4]. The dielectric permittivity (8r) is a phenomenological measure of the interaction between the electric field and the material within the capacitor. Numerical values of e, for static electric fields range as follows [6.5-6.7, 6.20, 6.21]  

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Dielectric measurements of gas adsorption systems can be performed fairly quickly, typically within a few seconds [6.3]. Hence the kinetics of adsorption processes being slow on this time scale can be observed. Indeed these processes are sometimes invisible to purely manometric or even gravimetric measurements. As examples we mention internal diffusion, reorientation or catalytically induced chemical reaction processes of admolecules within a sorbent material. The mass of the adsorbed phase normally is constant during processes of this type, whereas the dipole moment of the admolecules and hence their polarization changes, cp. Sect. 3.2. 

Impedance/capacitance measurements of gas adsorption systems in thermodynamic equilibrium ... 

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