Self-assembled surfactant micelles are ubiquitous to a wide variety of products and industrial processes, ranging from consumer products to enhanced oil recovery, where phase behavior and rheology are critically linked to material performance. These applications require a particularly detailed understanding of the hierarchical nature of micellar self-assembly, whereby molecular interactions, mesoscale structure, and macroscopic properties are intimately connected. Recent interest in novel surfactant fluids involves the incorporation of colloidal particles, specifically on the nanoscale, into self-assembled surfactant solutions in order to impart additional structure and functionality to materials of interest. Despite the demand for such formulations, the affect of colloid addition on the structure and properties of micellar solutions (and vice versa) is poorly understood. The goal of this work is to understand the structure and rheology of model mixtures of nanoparticles and wormlike micelles (WLMs), in order to better engineer fluids for targeted applications. The work shown here focuses on two aspects of this research that combine methods that probe length scales ranging from the particle surface to macroscopic micellar networks to develop a detailed knowledge of novel colloid-surfactant mixtures.

Modification of viscoelastic wormlike micellar solutions via nanoparticle incorporation

This work demonstrates that the addition of nanoparticles to like-charged WLM solutions allows for unique tunability of the structure and rheology of such fluids. Specifically, nanoparticle addition enables the formation of entangled micellar solutions under normally dilute conditions, and enhances the existing viscoelasticity of semi-dilute solutions. Studies of the structure by scattering methods and energetics using isothermal titration calorimetry (ITC) at the surfactant-nanoparticle interface show that this rheological modification results from association of micelles with an adsorbed layer at the particle surface. This results in the formation of micelle-nanoparticle junctions which impart additional network formation to the micellar fluid. Additionally, these network junctions restrict the local orientational mobility of WLMs, and can be used as a method to suppress flow instabilities such as shear banding in concentrated solutions. Studying these phenomenon for a variety of particle sizes and surface chemistries leads to an engineering understanding of the structuring of WLMs by nanoparticle addition.

Interactions and phase behavior of colloids dispersed in wormlike micellar media

The unique structuring of surfactants at the particle interface leads to modified colloidal interactions between nanoparticles suspended in WLM solutions. Specifically, the presence of WLMs leads to aggregation and thermoreversible phase separation of nanoparticles at sufficient concentrations. We have successfully modeled these WLM-mediated colloidal interactions using a simple statistical mechanical description of reversibly breaking, end-adsorbing micellar chains. The resulting interaction potential contains model parameters that can be experimentally determined, and can be used to simultaneously describe the observed phase behavior as well as structural measurements using small angle neutron scattering. The results suggest that the model is a more appropriate description than previously used potentials, and describes the complex colloidal interactions observed in WLM-colloid mixtures.