Linear viscoelastic behavior of perfluorooctyl sulfonate micelles: effects of counter-ions

Abstract

Linear viscoelastic behavior was investigated for aqueous solutions of perfluorooctyl sulfonate (C8F17SO− 3; abbreviated as FOS) micelles having a mixture of tetraethylammonium (N+(C2H5)4; TEA) and lithium (Li+) ions as the counter-ions. The solutions had the same FOS concentration (0.1 mol l−1) and various Li+ fractions in the counter-ions, φLi = 0−0.6, and the FOS micelles in these solutions formed threads which further organized into dendritic networks. At T ≤ 15 °C, the terminal relaxation time τ and the viscosity η, governed by thermal scission of the networks, increased with increasing φLi up to 0.55. A further increase of φLi resulted in decreases of τ and η and in broadening of the relaxation mode distribution. These rheological changes are discussed in relation to the role of TEA ions in thermal scission: Previous NMR studies revealed that only a fraction of TEA ions were tightly bound to the FOS micellar surfaces and these bound ions stabilized the thread/network structures. The concentration of non-bound TEA ions, CTEA *, decreased and finally vanished on increasing φLi up to φLi * ≅ 0.6, and the concentration of the bound TEA ions significantly decreased on a further increase of φLi. The non-bound TEA ions appeared to catalyze the thermal scission of the FOS threads, and the observed increases of τ and η for φLi < 0.55 were attributed to the decrease of CTEA *. On the other hand, the decreases of τ and η as well as the broadening of the mode distribution, found for φLi > 0.55 (where CTEA * ≅ 0), were related to destabilization of the FOS threads/networks due to a shortage of the bound TEA ions and to the existence of concentrated Li+ ions. Viscoelastic data of pure FOSTEA and FOSTEA/FOSLi/TEACl solutions lent support to these arguments for the role of TEA ions in the relaxation of FOSTEA/FOSLi solutions.