Evidence of Sticky Contacts between Wormlike Micelles in Viscoelastic Surfactant Solutions

Many cationic surfactants form highly viscoelastic solutions at concentrations of only a few percent. These solutions contain wormlike micelles (WLM) which can be several micrometers long. The structural relaxation time, τ_s in these solutions can be as long as many seconds, and the zero-shear viscosity can be in the range of 10⁶ Pas. Electric birefringence measurements on such solutions showed four different relaxation times with increasing concentration. The two shortest ones, τ₁ and τ₂ in the μs region were due to the alignment of small rodlike or ringlike micelles. The third one, τ₃, was observed in the viscoelastic region in the ms region and finally, a fourth one, τ₄, which was the same as the structural relaxation time, τ_s. In this article, it is shown that the τ₃ process is due to the formation and opening of contacts between the WLM. The reason for the contacts is the attraction between the WLM which is due to the hydrophobic surfaces of the micelles. The contacts crosslink the WLM and thus form a three-dimensional network. This network is the reason for the high viscosity and viscoelastic properties of the samples. If the attraction between the WLM is reduced by adding glycerol to the solution, the viscosity of the solution breaks down. The same happens if the surface of the WLM is made hydrophilic by the addition of small amphiphilic molecules. The WLM are not destroyed by these procedures.