Simple model for mixed surfactant solutions

an exact expression should be derived which gives a relationship between the surface pressure of a surfactant mixture and the surface pressure of the individual solutions. For an ideal (ai = a2 = a = 0) mixture of homologues (co = (0 = C02), Eq. (3.27) can be rewritten in the form of a generalised von Szyszkowski equation 

Using the corresponding equations for the individual surfactants 1 and 2 (in the following the subscript 0 refers to the individual solution). 

The use of Eq. (3.41) would require only the knowledge of one pair of experimental surface pressure (tension) of individual solutions. The only parameter of the isotherm in Eq. (3.41), the molar area co, can be either calculated from the molecular geometry of the surfactant, or 

In the subsequent sections more complicated mixtures of surfactants, characterised by intermolecular interactions, different molar areas of the components, mixtures of ionic surfactants and other systems are considered. 

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