isosteric enthalpy of adsorption

When the addition of the differential amount of component $i$ $d n i σ$ or $d n i s$ is effected at constant pressure $p$ , the differential molar enthalpy of adsorption, $Δ a H i σ$ or $Δ a H i s$ also called the isosteric enthalpy of adsorption ( $q st$ ) is defined as:

$Δ a H i σ = − q st , σ = U i σ − H i g$

$Δ a H i s = − q st , σ = H i σ − H i g$

where $H i s = ( ∂ H s ∂ n i s ) T , p , m , n j s$ and $H i g$ is the partial molar enthalpy of component $i$ in the gas phase, i.e. $( ∂ H g ∂ n i g ) T , p , n i g$

Source:

PAC, 1972, 31, 577 (Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry) on page 603