The Frenkel, Halsey, Hill FHH Isotherm

The Frenkel-Halsey-Hill (FHH) isotherm has found much utilization due to the range specified for its application. It seems especially handy for porosity determinations. It seems to work well between relative pressures in the range 0.4-0.9. The equation is 

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Since z oc V, this may be written In (p0/p) = const. V where V is the volume of gas adsorbed. A more general form of this isotherm, called the Frenkel-Halsey-Hill (FHH) isotherm, treats the power dependence of V as an unknown n and writes... 

The Frenkel-Halsey-Hill (FHH) isotherm was originally developed to describe the growth of thick films and wetting phenomena on a flat surface and was later extended to studying adsorption on fractal surfaces [3, 55]. In contrast to BET theory, FHH theory applies to long-range adsorbate-absorbent interactions and its approach is closely related to the so-called potential theory of adsorption of Eucken and Polanyi (see Ref. [35]). 

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