Mixing behaviour of charged and non-charged surfactants described by BET isotherms

We analysed various sets of published results concerning the mixing behaviour of charged and non-charged surfactants, beyond their aggregation threshold, by means of the BET model modified by Stokes and Robinson. This approach, which is mainly used to account for properties of concentrated aqueous salt solutions, provides a good description of the properties of mixed micelles across the complete scale of compositions between the two pure surfactants. We use analyses of the behaviour of several systems reported in the literature for illustration. We consider the significance of the parameters C and r introduced in the BET model. For mixed micelle solutions in the presence of a support electrolyte, the value of the solvation enthalpy of the charged amphiphile by the non-ionic surfactant is small (C close to one). This model is compared to the regular solution theory (RST).