Structure, Rheology, and Thermodynamics of Wormlike Micelle-nanoparticle Mixtures
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Published: 2009
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ISBN-13: 9781109248746
DOWNLOAD EBOOKWormlike micelles (WLMs) have become widely used in a number of industrial and consumer products and processes where they come in contact with colloidal species. Still, relatively little is understood regarding the interactions between WLMs and colloids and resulting changes in macroscopic properties. As such, the goals of this thesis are to uncover the mechanisms by which WLMs interact with colloidal particles, and to determine how these interactions affect changes in macroscopic properties for mixtures of model WLMs and colloidal nanoparticles. Aqueous mixtures of cetyltrimethylammonium bromide (CTAB) and sodium nitrate (NaNO 3) serve as a model system in which to study the self-assembly, phase behavior and rheology of WLMs. Characterization of CTAB/NaNO 3 solutions across a wide range of state variables enables full exploration of WLM phase behavior, including micellization, the sphere-to-rod transition, overlap, and entanglement. Combining rheological measurements and small angle neutron scattering (SANS) yields measurement of the relevant length scales of WLMs, and provides a microstructural basis for the resulting changes in WLM rheology. In the concentrated regime, CTAB micelles undergo shear banding, an important flow instability that occurs in a variety of WLMs. Combination of rheometry, velocimetry, and spatially-resolved flow-SANS shows that shear banding of CTAB results from a shear-induced isotropic-paranematic phase transition. The use of a constitutive model enables direct, quantitative coupling of shear banding to equilibrium phase behavior, and enables construction of non-equilibrium phase diagrams for shear banding fluids. Establishment of structure property relationships for model WLMs thus serves as a basis for examining the interactions and changes in the resulting properties of CTAB/NaNO 3 WLMs in the presence of model cationically modified silica nanoparticles. Structural and thermodynamic measurements of interactions at the colloid-surfactant interface show that adsorption of surfactant results in the formation of hemimicellar surface structures, which in turn interact with WLMs to form micelle-nanoparticle junctions. These junctions give rise to unique rheological modification of WLMs, such as significant increase in viscosity and viscoelasticity, and arise from the contribution of effective cross-linking of the micellar solution to form a viscoelastic network. Furthermore, micelle-nanoparticle junctions suppress shear banding due to hindered orientational mobility of the micelles. Conversely, the formation of micelle-nanoparticle junctions gives rise to new colloidal interactions mediated by the bridging of micelles between particles, leading to thermoreversible phase separation of the colloid. In order to better understand and describe these interactions, a statistical mechanical model has been developed to capture the phenomenology of wormlike micellar bridging. The model predicts strong-long range attractions arising from the distribution in length of the micelles. Because the model is based on experimentally measureable parameters, it yields a priori predictions of interaction potential, which are in good agreement measurements of the colloidal phase behavior and of the second osmotic virial coefficient. The results of this work demonstrate that rheological properties of WLMs can be uniquely tuned by the addition of particles, and conversely the interactions of colloids in the presence can be uniquely controlled through the self-assembly of WLMs and interactions at the surfactant-colloid interface.
