Entropic Considerations
The formation of Langmuir monolayers by adsorption onto a surface dramatically reduces the entropy of the molecular system. This conflicts with the second law of thermodynamics, which states that entropy will increase in an isolated system. This implies that either another locally active force is stronger than the thermodynamic potential, or that our expression of the entropy of the system is incomplete.
To find the entropy decrease, we find the entropy of the molecule when in the adsorbed condition.
Using Stirling's approximation, we have,
On the other hand, the entropy of a molecule of an ideal gas is
where is the Thermal de Broglie wavelength of the gas molecule.
Read more about this topic: Langmuir Adsorption Model