When the addition of the differential amount of component


or

is effected at constant pressure

, the differential molar
enthalpy of adsorption,

or

also called the isosteric
enthalpy of adsorption
(
) is defined as:
where

and

is the partial molar
enthalpy of component

in the gas phase, i.e.

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