VISCOELASTIC MICELLAR SOLUTIONS - MICROSCOPY AND RHEOLOGY

Artifact-free electron micrographs and rheological properties of aqueous solutions of cetyltrimethylammonium chloride (CTAC), with salicylate (from NaSal) as the binding counterion, are presented as a function of counterion concentration. At low [Sal-], CTAC solutions form globular (spherical) micelles (5 nm in diameter) and behave as Newtonian fluids. With increasing [Sal-], these solutions form threadlike or wormlike micelles (5 nm in diameter with lengths as long as several micrometers) that are viscoelastic and entangled. At moderate [Sal-], these solutions have a spectrum of relaxation times similar to semidilute polymer solutions. At high [Sal-], a single relaxation time dominates. The viscosity of the viscoelastic solutions shows shear thinning and power law behavior. As a function of [Sal-], the zero-shear viscosity (eta(0)) reaches a maximum 5 orders of magnitude greater than the initial viscosity of the solution with no salicylate. With further increase in [Sal-], the zero-shear viscosity then decreases 3 orders of magnitude. After the formation of threadlike micelles, the rheological properties change as a function of [Sal-], whereas the electron micrographs all show similar images of entangled threadlike micelles. Some of the images have a small number of closed circular threadlike structures, indicating the existence of ringlike micelles. Samples for electron microscopy were thermally fixed before and after relaxation. In some cases, the samples imaged before relaxation revealed the threadlike micelles in an ordered state due to the shear induced during sample preparation.