The Surface Excess

When a surfactant accumulates at a surface it modifies the surface tension. We consider two species in a uniform solution.

The Gibbs energy is:

G = G(a) + G(b)

However, one component may accumulate that an interface. The Gibbs energy is decreased by the surface Gibbs energy G(s).

G = G(a) + G(b) - G(s).

Similarly, the amount of component J is:

nJ = nJ(a) + nJ(b)

and when J accumulates on the surface the amount is decreased by nJ(s):

nJ = nJ(a) + nJ(b) - nJ(s)

The excess can be expressed as an amount per unit area of the surface. This is known as the surface excess:

GJ = nJ(s)/s.

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