The interaction energy when they (1´ and 2´ in Fig. 2.3) are at the surface can be written as

(2.4)

where, is the pair-wise interaction between molecule of the type with that of type of . It can be shown that

(2.5)

is the energy required to form an interface/area between a pair of molecules.

If we assume that there are 1-2 pairs of molecules at the surface then total energy of the surface can be written as

(2.6)

If we assume that

then,

is a property of the material.