Interfacial properties of unsymmetrical bolaform amphiphiles with one ionic and one nonionic head group

We report the results of a combined experimental and theoretical investigation of the equilibrium interfacial properties of unsymmetrical bolaform. surfactants that possess one ionic and one nonionic head group. By synthesizing the surfactant (11-hydroxyundecyl)trimethylammonium bromide (HTAB) and by performing measurements of the equilibrium surface tension of aqueous solutions of HTAB as a function of surfactant concentration, we conclude that the hydroxyl group of HTAB is associated with the aqueous subphase, thus forcing this surfactant to adopt a looped configuration at the surface of aqueous solutions. Surface tension measurements of HTAB as a function of added salt reveal the surface pressure of this class of surfactant to be less sensitive to the ionic strength of the aqueous subphase than classical surfactants such as dodecyltrimethylammoniurn bromide (DTAB) or symmetrical ionic bolaform surfactants. Through the development of a molecular thermodynamic model for Gibbs monolayers of HTAB, we reveal that the effects of added salt on the electrostatic and configurational contributions to the surface pressure tend to compensate each other (thus giving rise to the experimentally measured insensitivity to added salt). In contrast, the effects of salt on the electrostatic and configurational contributions to the surface pressure of a Gibbs monolayer of DTAB are found to be additive. (C) 2001 Academic Press.