Thermodynamics and the Langmuir Isotherm

Note that besides the entropy associated with the internal degrees of freedom, we have the so-called configuration entropy. 

It is easy to see (see section 2.8.4) that 0 is the probability of finding a specific site occupied and (1 - 0) the probability of finding it empty, i.e.. 

Pr(empty) = (1 - 0), Pr(occupied) = 0. Hence the configurational entropy has the general form 

This has the general form of a chemical potential in an ideal dilute solution, pi A when 0- 0. The other extreme case is when 0- 1 then - + oo. The form 

Next we assume that the adsorbed molecules are at equilibrium with an ideal gas consisting of A molecules. The chemical potential of the molecules in the gaseous phase is 

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