gemini surfactants available in the literature pertains to their equilibrium behavior at the air–water interface [14] and [15]. In this study, the conformational entropy of the spacer chain was shown to play an important role in determining the equilibrium area per molecule. The model was able to describe qualitatively the non-monotonic behavior of the area per molecule at the air–water interface as a function of the spacer length for the 12-s-12,2 Br− surfactants