Relative adsorption and surface segregation

We can now use the relative adsorption to describe a two-component system at constant temperature. The relative adsorption of B with respect to A for rA = 0 is given by eq. (6.81) as 

Thus the adsorption of B at the interface is given by the variation of the surface tension with chemical potential. 

Equation (6.87) is the basis for most adsorption measurements associated with liquid solutions. When(d(j/dfi )T is negative, Tg is positive and there is an excess of the solute at the interface. For (dct/dfiB)r 0, Tg is negative and there is a deficiency of the solute at the interface. In other words, solutes that reduce the surface tension are enriched at the surface. 

The relative adsorption of component B is clearly affected by its partial pressure. Let us consider a binary system A-B where rA = 0 and B is an ideal gas with partial pressure pB. The chemical potential of B can be expressed in terms of the partial pressure of B and this is then also the case for the relative adsorption of B 

The effect of an impurity on the surface energy is often discussed in terms of the surface activity of the impurity BJb, defined as the slope of the surface tension or energy versus composition at infinite dilution  

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