(198f) Nanostructure and Properties of Nanoparticle Dispersions with Tunable Interaction Potentials

Wagner, N. J. - Presenter, University of Delaware

Colloidal systems ranging from proteins to intermediates in the manufacturing of particulate suspensions such as paints, and inks can exist in distinct thermodynamic equilibrium states similar to molecular systems. For instance, isolated, condensed, and ordered particles are analogous to gas, liquid, and crystalline states. Additionally, aggregation, gelation, stable and metastable phases, and arrested states give rise to a rich phase diagram that is dependent on the delicate interplay between interaction potentials and physical barriers with respect to the thermal energy. Such complex systems are of fundamental scientific interest and pose challenges to industrial formulation and application because of the hierarchy of structures that connect particle properties to bulk material properties. Understanding the mechanisms leading to aggregation with the ultimate goal of designed control is of central importance to achieving a desired particulate microstructure and related bulk mechanical properties.

In this work we explore the relationship between colloidal interaction and structure relating to thermodynamic states and phases in both dynamic and static environments. Specifically, we explore the mechanisms of leading to aggregation, structure, and mechanical properties of silica nanoparticle gels and attractive driven nanoparticle glasses formed by thermally quenching a model dispersion. The model thermoreversible system is composed of nano-sized silica coated with a relatively thin oligomeric surface layer (end-grafted 1-octadecanol) that provides steric stability in a good solvent. In a poor solvent, short range attraction drives aggregation, flocculation, gelation, phase separation, and crystallization depending on the particle volume fraction and quench path.

Single particle properties are studied using fiber optic quasi-elastic light scattering (FOQELS), small-angle neutron scattering (SANS), and Zeta potential measurements. At the particle surface the molecular phase behavior of the brush is studied using neutron reflectivity and is directly connected to the change in interparticle potential with contrast match variation SANS experiments. The nanostructure is connected to the colloidal phase behavior as a function of particle volume fraction and temperature using static-SANS. Finally, rheological measurements allow for the connection between interparticle potential, structure and bulk mechanical properties at corresponding concentrations and temperatures. The goals of this study are to identify the hierarchical structures that quantitatively relate the state of the gel or glass, interparticle potential, and particle properties to the bulk rheology.