Because the ability of a surface to adsorb molecules onto its surface depends on energies of interaction, thermodynamics of adsorption can be used to understand the driving forces for adsorption. To measure the thermodynamics of polymer surfaces, contact angles are often used to easily obtain useful information. The thermodynamic description of contact angles of a drop of liquid on a solid surface are derived from the equilibrium formed between the chemical potentials of the solid–liquid, solid–vapor, and liquid–vapor interfaces.
At equilibrium, the contact angle of a liquid drop on a surface does not change. Therefore, the Gibbs free energy is equal to 0:
The chemical potentials of the three interfaces must cancel out, producing Young's equation for the relationship between surface energies and contact angles:
where:
:is the surface tension of the liquid
:  is the contact angle of the liquid
: is the surface tension of the solid–vapor interface
: is the surface tension of the solid–liquid interface
: is the vapor pressure of the liquid at equilibrium.
However, this equation cannot be used to determine the surface energy of a solid surface by itself. It can be used in conjunction with the following equation to determine the relationship between contact angle and surface energy of the solid, as surface tension ≈ surface energy for a solid:
where
: is the surface energy of the solid
: is the surface tension of the liquid.
: and  are the dispersive and polar components of the surface energy of the solid
Using these two equations, the surface energy of a solid can be determined simply by measuring the contact angle of two different liquids of known surface tension on that solid's surface.