Fowler-Frumkin isotherm

Fowler-Frumkin isotherm - Frumkin adsorption isotherm... 

As an example of a 1/1 adsorption in Me UPD systems, the Fowler-Frumkin isotherm, which is derived using the so-called mean field approximation (Section 8.3), is given by [3.257, 3.258, 3.268] ... 

Explicit expressions for i(f) and i s/Me" r Quire the knowledge of the adsorption isotherm (cf. Section 3.3). For example, 2D Meads phase formation according to the Frumkin-Fowler adsorption isotherm gives by combining eqs. (3.18) and (3.20) ... 

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. 

The adsorption isotherm was modeled in order to deduce a numerical value for A. Following the model of Frumkin and Fowler reported in [21], is given by the following set of equations ... 

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],...

Fig. 6. A Frumkin-Fowler s adsorption isotherm for uniform surfaces. B, C Adsorption isotherm for heterogeneous surfaces with condensation [after Ref. 89]...

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. 

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