A Notes on the Use of Equations

In order to obtain valid (3 parameter values, i.e., values that do not change significantly with change in the ratio of the surfactant in the mixture, the following conditions must be met  

The two surfactants must be molecularly homogeneous and free from surface-active impurities. 

Since equations 11.1-11.4 neglect countenon effects, all solutions containing ionic surfactants should have the same total ionic strength, with a swamping excess of any counterions. 

The interaction parameters in the presence of a second liquid (hydrocarbon) phase, (Sjj and 3, for mixed monolayer formation at the aqueous solution-hydrocarbon interface and for mixed micelle formation in the aqueous phase, respectively, can be evaluated (Rosen, 1986) by equations analogous to 11.1, 11.2 and 11.3, and 11.4, respectively. The necessary data are obtained from interfacial tension-concentration curves. 

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