(195g) Rheology and Behavior of Wormlike Micelles: Using Neutron Scattering and Specific Salt Interactions to Understand the Onset of Shear Alignment

Wormlike micelle (WLM) solutions are a widely used both industrially and as model systems in academic studies. One challenge has been understanding the precise effects of the persistence length (or micelle flexibility) on the micelle behavior. This research uses rheological and neutron scattering techniques to characterize the effect of persistence length on the behavior of WLM solutions. A collection of salts (NaCl, LiCl, MgCl₂, NaBr, NaI, and Na₂SO₄) were used along with the widely used anionic surfactant sodium lauryl ether sulfate (SLES). The different salts resulted in a wide range of rheological behavior and a variation in the persistence lengths measured using neutron scattering. The specific salts were chosen to also investigate the effect of both anions and cations on WLMs analogously to the Hofmeister series with proteins. WLM's solution have common factors controlling their behavior which are believed to be contour length and persistence length. Those factors are results of the effects of additives on the morphology of surfactant caused by shrinkage of their head group and therefore change on the packing parameter. Salt type and concentration study is like any other WLMs solution study where persistence length and contour length of WLMs are varied by solution additives. From salt ion to packing parameter relationship using the packing parameter model given by Pleines and his team showing the morphological change as a function of salt concentration between all the studied salt types, we will show for each salt type the relationship between stiffness factor fitted by packing parameter model and persistence length obtained from static small angle neutron scattering (SANS) experiment. Universal behavior between all salt additives on WLMs solutions is found to be controlled by persistence length and ion dissociation. Persistence length obtained using a small angle neutron scattering experiment is found to be directly related to the stiffness factor fitted by the packing parameter model. Ion dissociation, found from the packing parameter model, controls the molarity peak of zero shear viscosity. Then, the significance effects of anions and cations on the ordering of Hofmeister series that is found to be analogous to the Hofmeister effects seen in protein solutions is highlighted. Flow behavior is also believed to be affected by either persistence length, contour length, entanglement length, or the combination of these factors that all can be calculated and varied by salt concentration and salt types. Beside rheology, this research is targeting a RheoSANS study of the alignment of WLM’s along the shear flow for the different types of salt solutions soon (not included in this presentation).