Surface Chemistry and Particle Morphology Govern the Multiscale Interactions and Properties of...

Particle-stabilized emulsions offer a strategy for forming mechanically robust microcapsules based on coassembly of silica nanoparticles, polyelectrolytes, and surfactants at oil-water interfaces. Such systems have complicated distributions of inorganic colloidal solids, surfactants, solvents, and ions that influence their compositions and structures over multiple length scales, which have been challenging to characterize and establish. To do so, silica-polyelectrolyte microcapsules were prepared with nearly monodisperse dimensions in the submicron range from water-in-oil (W/O) emulsions that were stabilized by a combination of nonionic surfactants, anionic silica nanoparticles, and cationic polyelectrolyte chains. Nonionic surfactants were used to establish oil as the continuous phase, while silica-polyelectrolyte complexes, self-assembled at the oil-water interfaces, prevented coalescence between droplets and provided mechanical elasticity. Particle charge measurements show that the surface charge density of the silica nanoparticles can be controlled by adjusting the pH conditions or by substituting aluminate ions at their surfaces. These promoted strong electrostatic and hydrogen-bonding interactions with the cationic polyelectrolyte and nonionic surfactant species, direct atomic-scale evidence of which is provided by solid-state two-dimensional (2D) ²⁹Si{¹H} NMR. For nanoparticles with higher surface charge densities, strong electrostatic interactions are the basis for particle coassembly with cationic polyelectrolyte species, and the resulting silica-polyelectrolyte complexes adsorb at the oil-water interface, as revealed by cryogenic electron microscopy. Relative to larger spherical nanoparticles, the elongated nanoparticles exhibit more extensive hydrogen bonding with polar organic moieties, which contributes to polyelectrolyte bridging between particles with lower densities of surface negative charges, consistent with interfacial rheology analyses. In addition to interfacial compositions and conditions, noncovalent polyelectrolyte-silica interactions, governed by nanoparticle surface compositions, charge density, and surface area, can be adjusted to control the macroscopic mechanical properties of the microcapsule interfaces.