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Figure 6.3 Diagrammatic representation of an interface between two bulk phases in the presence of an adsorbed layer.

it is assumed that the properties of the two bulk phases remain unchanged up to the dividing surface. A thermodynamic derivation of the Gibbs equation is provided in Box 6.2

The form of the Gibbs equation applicable to dilute solutions is r=-

c dy RT dc

where activity in equation (6.11) has been replaced by concentration, c. Equation (6.12) is the form of the Gibbs equation applicable to or the adsorption of nonionic surfactants at the surface of the solution. For ionic surfactants the derivation becomes more complex since consideration must be taken of the adsorption of both surfactant ion and counterion. The general form of the Gibbs equation is then written r=-

0 0

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