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

Linear viscoelastic behavior was investigated for aqueous solutions of perf luorooctyl sulfonate (C8F17SO3-; abbreviated as FOS) micelles having a mixt ure 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, phi(Li) = 0-0.6. and the FOS micelles in these solutions formed threads which further organized into de ndritic networks, At T less than or equal to 15 degrees C, the terminal rel axation rime tau and the viscosity eta, governed by thermal scission of the networks. increased with increasing phi(Li) up to 0.55. A further increase of phi(Li) resulted in decreases of tau and eta and in broadening of the r elaxation mode distribution. These rheological changes are discussed in rel ation to the role of TEA ions in thermal scission: Previous NMR studies rev ealed that only a fraction of TEA ions were tightly bound to the FOS micell ar surfaces and these bound ions stabilized the thread,network structures. The concentration of non-bound TEA ions, C-TEA*, decreased and finally vani shed on increasing phi(Li) up to phi(Li)* congruent to 0.6. and the concent ration of the bound TEA ions significantly decreased on a further increase of phi(Li) The nonbound TEA ions appeared to catalyze the thermal scission of the FOS threads, and the observed increases of tau and eta for phi(Li) < 0.55 were attributed to the decrease of C-TEA*. On the other hand, the dec reases of tau and II as well as the broadening of the mode distribution, fo und for phi(Li) > 0.55 (where C-TEA* congruent to 0), were related to desta bilization of the FOS threads/networks due to a shortage of the bound TEA i ons and to the existence of concentrated Li+ ions, Viscoelastic data of pur e FOSTEA and FOSTEA/FOSLi/TEACl solutions lent support to these arguments f or the role of TEA ions in the relaxation of FOSTEA/FOSLi solutions.