Frumkin-Fowler-Guggenheim isotherm

This equation is sometimes called the Frumkin-Fowler-Guggenheim (FFG) isotherm [374— 376], For j3 = nEP/RT < 4 lateral interactions cause a steeper increase of the adsorption isotherm in the intermediate pressure range. Characteristic of all Langmuir isotherms is a saturation at high partial pressures P/Po —> 1. 

Figure 9.7 Left Frumkin-Fowler-Guggenheim (FFG) adsorption isotherms (coverage 0 versus the pressure in units of Kj 1). The curves were calculated using Eq. (9.35) with ft 0,2,4,6. For ft 6 the physically correct adsorption curve is plotted as a continuous curve while the one calculated with Eq. (9.35) is plotted as a dotted curve. Right Adsorption isotherms for krypton adsorbing to the (0001) plane of graphite at two different temperatures. The dotted curves were fitted using Eq. (9.35) with ft = 4.5. Experimental results were taken from Ref. [377],...

In practice, most models are based on the assumption of localization, the reasoning being that many substances adhere rather strongly to surface sites. As a consequence, frequently used Individual isotherms Include those of Langmuir or Frumkin-Fowler-Guggenheim (FFG) but not those of Volmer or Hlll-De Boer (see app. 1). 

FIGURE 14.5 Frumkin-Fowler-Guggenheim adsorption isotherms for different interactions between solvent and adsorbate molecnles. 

Equation (34) was deduced by Frumkin [32] as a general case of Langmuir isotherm, which corresponds to a = 0. A statistical derivation of Eq. (34) was carried out by Fowler and Guggenheim [42] however, the applicability of Frumkin isotherm to the experimental data appears to be substantially wider than it follows immediately from the derivation conditions. 

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