With the view that an insoluble monolayer is regarded as a limiting state of the adsorbed film at the air/water interface, the thermodynamic treatment of penetrated monolayer has been developed by extending the rigorous thermodynamics of mixed monolayer presented in Part I of this series and that of the adsorbed film at the air/water interface. The equations of the thermodynamic quantities of penetration have been derived on the basis of the surface excess quantities defined by Hansen; their evaluation has been found to require the measurement of surface tension as a function of the composition of the penetrating substance and the temperature over the whole range of area per mole of film-forming substance. It has been claimed that there is considerable doubt as to the validity of Pethica's equation.
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