ABSTRACT
This chapter considers the behavior of gemini (gemini) surfactants in aqueous solution in the submicellar range of concentration that is at concentrations below the critical micellization concentration (cmc). The understanding of this behavior is extremely important for the interpretation of different types of results. Consider, for instance, the determination of the surface area a occupied by one surfactant at the air-water interface from the variation of the surface tension γ with the surfactant concentration C. On the basis of Gibbs law, the relationship between a and the surface excess Γ is:
(1)
In Eq. (1), R is the gas constant, T is the absolute temperature, NA is Avogadro’s number, and n is a constant that takes the values 1 for nonionic surfactants, 2 for univalentunivalent ionic surfactants, and 3 for divalent-univalent ionic surfactants. The ionic gemini surfactants investigated are all of the divalent-monovalent type or conversely. The value n=3 must therefore be used if at C<cmc, the gemini surfactant is completely ionized in the solution from which adsorption occurs, whereas the value n=2 should be used if the gemini surfactant ion binds one counterion (ion pairing). This uncertainty concerning the value of n has been a real problem in the interpretation of surface tension data for gemini surfactants (see Sec. II).
