Rheology of wormlike micelles with varying hydrophobicity of the counterion

Dynamic viscoelastic experiments were performed on mixtures of cetyl trimethylammonium tosylate (CTAT) and cetyl trimethylammonium 3-hydroxy naphthalene 2-carboxylate (CTAHNC) to understand the effect of hydrophobicity of the counterions on the structure of aggregates formed. The total surfactant concentration is fixed at 100 mM, and the ratio of the two counterions HNC- to tosylate(-) (x(2)) is varied from 0 to 0.7. The zero shear viscosity (eta) and the terminal time of stress relaxation (tau(R)) initially increases slowly with x(2) and then decreases drastically for x(2) > 0.5. The initial increase of both eta and tau(R) can be explained by an increase of the micellar length owing to the more hydrophobicity of HNC-, which reduces the scission energy of the micelles due to the decrease of the electrostatic contribution. A decrease of the eta and tau(R) above x(2) = 0.5 is attributed to the formation of intermicellar connections, which is favored at very high salt content. The presence of intermicellar branching speeds up the reptation process and decreases the average micellar lengths thereby reducing eta and tau(R). Analysis of the data by the theory of Cates suggests that increasing the fraction of HNC- increases the rate of entanglement length to the contour length of the micelles. This is consistent with the temperature dependence of the rheological data. The present investigation supports the earlier observation that branched micelles constitute the intermediate structures between linear micelles and bilayers.