(73g) Silica-Polyelectrolyte Nanocomposites As Stabilizers for Aqueous Microcapsules with Adjustable Properties

Particle-stabilized (Pickering) emulsions offer enhanced kinetic stability of dispersed liquid droplets, due to the adsorption of solids at the oil-water interface, preventing droplet coalescence. The simple preparation and diverse properties of these systems make them useful for applications in drug delivery, catalysis, separations, or as stiffness-enhancing agents. While numerous studies have investigated the formation of microcapsules from particle-stabilized emulsions by using inorganic particles or interfacial polymerization, the development of robust, but deformable, microcapsules for stimuli-responsive applications remains a challenging goal in soft matter research.

Here, we report the preparation and characterization of viscoelastic microcapsules formed from water-in-oil emulsions, which are stabilized by anionic silica nanoparticles, cationic polyelectrolytes, and non-ionic surfactant molecules that influence the mechanical properties of the microcapsule. The surface densities of negative charges on colloidal silica nanoparticles with different morphologies and with or without alumina surface groups were measured as functions of pH via streaming potential titrations. The respective nanoparticles were mixed with a water-soluble non-ionic surfactant and a cationic polyelectrolyte in the aqueous phase, before adding an oil (decane) phase containing an alkane-soluble non-ionic surfactant. Ultrasound-induced cavitation of the decane-water mixture resulted in the formation of submicron water-filled capsules in a continuous decane phase. Subsequent fractionation of the nanocomposite-stabilized emulsion allowed the extraction of microcapsules with a narrow distribution of sizes in the range of 600 ± 90 nm. Solid-state two-dimensional heteronuclear correlation NMR spectra of dried silica-polyelectrolyte nanocomposites revealed strong interactions between the quaternary ammonium moieties of the polyelectrolyte and the silica nanoparticle surfaces. The influences of the silica-polyelectrolyte nanocomposite on the mechanical properties of the water-decane microcapsule system was probed by interfacial rheology measurements, which revealed viscous and elastic responses that depended on the nanocomposite composition. Higher loadings of spherical silica nanoparticles, relative to elongated nanoparticles, resulted in microcapsules that exhibited greater elastic moduli across the frequency range investigated (0.1 to 200 rad/s), thereby demonstrating adjustable stiffness at the oil-water interface. Control of elasticity based on interface composition is expected to enable the design of pliable microcapsules for packaging stimuli-responsive guest species, such as pharmaceutical agents for drug delivery or protein molecules with triggerable functions.