Admolecule diffusion

The activation energies for adatom or admolecule diffusion are significantly less than the heats of adsorption. In order to diffuse a molecule or atom has to change co-ordination (e.g. one-fold to two-fold, to one-fold etc.) These energy differences are typically 10% of the heats of adsorption. 

Admolecules diffuse within the pore system of a solid sorbent. This process can last many hours, days, and even weeks, as has been observed for adsorption of helium in activated carbon (NORIT Rl), [1.23]. As a consequence it can take the same time till thermodynamic equilibrium between the sorptive gas phase and the adsorbate is realized. In view of practical and industrial needs it is therefore necessary to introduce the concept of technical equilibrium defined as a state in which the relative uptake (Am/m) of mass at total mass (m) due to adsorption is less than a given value (e) typically e = 10 , within a certain time interval (At), typically At = 30 . These data will allow, together with cycle periods (tc) of an industrial process, one to define characteristic Deborah numbers... 

The admolecules in the first layer displayed in Figure 5.1.8 will diffuse around on the surface of the substrate until they desorb, join an existing cluster, or create a new nucleus. The creation of a new nucleus occurs when several molecules attach while diffusing on the substrate. Of course there is a complication in the fact that the free energy of a cluster with j molecules will be lowered for each particle taken from the vapor phase if it is thermodynamically favorable, but the free energy will... 

The areas of the peaks, 5peak. with the superimposed diffusive parts, are inversely proportional to the amount of admolecules, a ... 

Additionally, in highly porous solids like zeolites and activated carbons there may be internal diffusion processes of the adsorbed molecules (admolecules). These can occur without external exchange of mass, i. e. at constant mass adsorbed, cp. Sects. 4, 5. An example for such a phenomenon is presented in Chap. 6, Fig. 6.29, [1.4, 1.7-1.9]. 

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. 

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