Spectroscopic Investigations of Viscoelastic Solutions Containing the Cetyltrimethylammonium Ion

It has previously been shown [1] that very dilute solutions of cetyltrimethylammonium salts can show strong viscoelastic behaviour. In order to elucidate the molecular organization specific for the viscoelastic behaviour we have, using several spectroscopic techniques, investigated aqueous solutions containing cetyltrimethylammonium salicylate. Using proton magnetic resonance it is found by simulation of the bandshapes [2] that in these dilute (<1 %) micellar solutions the viscoelastic behaviour correlates with the aggregate size, both with respect to changes in composition and in temperature. Thus a necessary condition for viscoelastic behaviour is aggregates larger than the minimal spherical micelle. The viscoelastic solutions were oriented by Couette flow. The orientation was monitored through the linear dichroism (LD) of the solution and it is found that the LD increases with increasing concentration. The temperature for obtaining zero LD is in agreement with the disappearance of the viscoelastic properties of the solution. An independent measure of aggregate sizes was obtained by studying the quasielastic light scattering (QLS) of the solutions which also indicates a transition from large to smaller aggregates at higher temperatures. It is suggested that electrostatic and anisotropic interactions between the large aggregates cause a specific periodic colloidal structure that is responsible for the viscoelastic behaviour.