Effects of solubilized additives on the rheological properties of CTAB solutions

Abstract

The effects of cosurfactant additives on the evolutions of microstructure in micellar solution are investigated. The microstructure is developed via two-step shape transitions in micellar solution when the surfactant concentration exceeds a critical micelle concentration. First, the initial spherical micelles undergo shape transition ro rodlike or disklike micelles because of the surface characteristics of micelle demand the compactly packing as the surfactant concenration increases. The packing density strongly depends on the rate of decrease in the repulsions between micelle headgroups as well as on temperature. Further increment in the surfactant concentration makes the anisotropic rodlike micelles overlapped each other. Solutions in these states exhibit polymeric solution behaviors such as shear thinning at the high shear rates. Addition of solubilized additives enhances the transitions which are affected by the additive concentration and its chemical structure. Presence of cosurfactants with long alkyl chains reduces the repulsion by forming surfactant-alcohol mixed micelles as the dissociated counterions of ionic salts do. In the present study, we examine the effects of chemical structure of alcoholuc additives which possess a very weakly hydrophilic hydroxyl group. The primary heptanol can be easily penetrated into the micelles and aligned parallel to the surfactant molecules, and thus it enhances the two-step shape evolutions. On the other hand, the secondary and tertiary heptanols retard the shape transition since compact packing of the surfactant molecules is suppressed.