(651b) The Roles of Surface Chemistry and Particle Morphology in the Formation and Properties of Silica-Polyelectrolyte Microcapsules

Particle-stabilized (Pickering) emulsions provide a means to prepare robust, hybrid microcapsules through the adsorption of solids at liquid-liquid interfaces. The co-assembly of silica nanoparticles and polymer species at oil-water interfaces is particularly promising for the development of monodisperse, submicron capsules with adjustable mechanical properties as hosts for water-soluble materials. However, understanding the complicated interactions between multiple components at these interfaces remains challenging, especially at an atomic level.

Here, we establish the atomic-scale interactions between silica surface moieties and polyelectrolyte groups and correlate them with interfacial mechanical properties in a decane-water system. The behavior of anionic silica nanoparticles at oil-water interfaces is fundamentally linked to the surface charge density, which can be modified through surface functionalization with aluminum moieties and further tuned with the pH of the aqueous phase. Two-dimensional ²⁹Si{¹H} NMR analyses enable the unambiguous identification of specific polyelectrolyte and silica surface moieties that bind via strong electrostatic interactions and weaker hydrogen bonding to form nanocomposites that adsorb at oil-water interfaces. Furthermore, these analyses reveal key differences in the populations of silica surface moieties between spherical and elongated silica nanoparticles, resulting in distinct charge densities that are influenced by particle morphology. Cryogenic electron microscopy images show differences in the distributions of spherical and elongated particles on the surfaces of water-filled microcapsules. As shown in rheological analyses of a decane-water system, the interfacial viscosity and stiffness can be controlled by adjusting the relative fraction of these particles in the aqueous phase. By unraveling the coupled relationships between surface chemistry, particle morphology, and macroscopic mechanical properties, these results provide design criteria for the preparation of silica-polyelectrolyte microcapsules with optimized composition and properties for a variety of applications.