A capacitor is formed when two conducting plates are separated by a non-conducting medium, called the dielectric. The value of the capacitance depends on the size of the plates, the distance between the plates, and properties of the dielectric. The relationship is:
ε0 = electrical permittivity
εr = relative electrical permittivity
A = surface area of one plate
d = distance between two plates
Whereas the electrical permittivity is a physical constant, the relative electrical permittivity depends on the material. The following table gives you a few useful εr values.
Notice the large difference between the electrical permittivity of water and that of an organic coating. The capacitance of a coated substrate changes as it absorbs water. EIS can be used to measure that change in capacitance.