ABSTRACT
The determination of KRS from the surface tension measurements (γ) requires that at least one of the two species, generally the substrate, changes the γ of the solution, as is the case of surfactant molecules. Several binding isotherms have been proposed in the last decade from cyclodextrins/surfactant studies; most of them are empirical relations between γ and the substrate activity, taking into account several stoichiometries for the complex.[78-83] Funasaki et al.[78,79] established an empirical relation with various fitting parameters, while Czapkiewicz and Tutaj[80] proposed a logarithmic expression (γ=b0+b1 In ai). However, the most conceptual model is that developed by Dharmawardana et al.[82] and Tunçay and Christian[83] based on Langmuir, Gibbs, and Frumkin models, which gives the following equations:
γ=γ0+tDln(1−θ)+αθ2 (32)
θ/(1−θ)=BaS exp(2αθ) (33)
where θ is the fraction of the occupied surface and γ0, tD, and B are constants. These equations, together with mass balances and Eq. 4, allow for the determination of KRS as a fitting coefficient of the NLR analysis of the experimental γ values vs. [S] at constant [R], or vice versa.
