One of the defining features of polymer surfaces and coatings is the chemical regularity of the surface. While many materials can be irregular mixtures of different components, polymer surfaces tend to be chemically uniform, with the same distribution of different functional groups across all areas of the surface. Because of this, adsorption of molecules onto polymer surfaces can be easily modeled by the Langmuir or Frumkin Isotherms. The Langmuir equation states that for the adsorption of a molecule of adsorbate A onto a surface binding site S, a single binding site is used, and each free binding site is equally likely to accept a molecule of adsorbate:
where:
: is the adsorbate
: is the surface binding site
: is the bound adsorbate/binding site pair
The equilibrium constant for this reaction is then defined as:
The equilibrium constant is related to the equilibrium surface coverage , which is given by:
where:
:' is the surface coverage (fraction, 0 is empty, 1 is fully covered)
: is the adsorption equilibrium constant