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

Abstract

The microstructure of wormlike micelles (WLMs) directly affects the rheological properties of their solutions. Investigating the structure-property relationships of WLMs has been a popular topic for researchers for several decades with a variety of theoretical and empirical models based on combinations of thermodynamics and polymer physics having been developed to describe their viscoelastic behavior. All these models rely on a collection of characteristic ‘length’ parameters that are often difficult to estimate. In particular, the micelle stiffness, as described by the persistence length, can be difficult to measure experimentally, while being very impactful on the bulk rheology of WLM solutions. Here, an array of inorganic salts has been used to induce wormlike micelle formation in an aqueous solution of an anionic surfactant. Specifically, NaCl, LiCl, MgCl2, NaBr, NaI, and Na2SO4 salts are used with a sodium lauryl ether sulfate surfactant solution. Ion dissociation/association with the surfactant head groups and the hydrogen bond network of the water alters the effective stiffness of the micelles, allowing for an estimation of the stiffness using three different methods: 1) small angle neutron scattering (SANS) measurements, 2) oscillatory rheological measurements, and 3) steady rheological measurements with a thermodynamic packing parameter model. Each of these methods are then compared and shown to be consistent with each other for the micelle solutions tested. The results are then validated with a rheological small angle neutron scattering (RheoSANS) by comparing the segmental alignment of a collection of samples with different dimensionless numbers. The consistency of the results across all the measurements suggests that the approach used in this study, which estimates micelle parameters using steady shear rheology and a thermodynamic model, could provide a simpler and more accessible method for estimating micelle parameters in a wide range of surfactant systems. Surfactant applications are numerous, but the basis to manage the self-assembly is still lacking. This experimental data also indicates that estimating the rotational diffusivity of WLMs can be best accomplished over the widest range of shear rates by using the 'local' viscosity rather than the solvent viscosity.