Fine-Tuning the Surface and Interfacial Chemistry of Silica Nanoparticles to Control Stability in High Ionic Strength Electrolytes and Modulate Assembly at Oil-Water Interfaces

Controlling the interfacial chemistry is critical to the behavior of several material systems. In this report, we demonstrate how subtle changes in the surface chemistry of silica nanoparticles have a profound effect on their colloidal stability, as well as their ability to assemble at oil-water interfaces. The ability to direct the assembly by fine-tuning the surface chemistry of nanoparticles, NPs, allows for controlling the interfacial properties and the overall behavior of the resulting Pickering emulsions. We start by showing that the colloidal stability of silica nanoparticles functionalized by a mixture of silanes is dramatically improved compared to that of silica functionalized with either one. For example, NP suspensions synthesized using a 50:50 mixture of N1-(3-trimethoxysilylpropyl)diethylenetriamine and N-trimethoxysilylpropyl-N,N,N-trimethylammonium in a complex brine containing a mixture of salts remain suspended when subjected to acceleration of 500 x g(->) and temperatures as high as 60 degrees C. In contrast, particles functionalized with either one of the silanes are far less stable under the same conditions. The colloidal stability of the particles is correlated to the silane-grafted layers and surface roughness obtained by atomic force microscopy (AFM). We next studied the mechanism of the particles' assembly at oil-water interfaces and characterized the interfacial properties under various conditions. In all cases, the assembly is driven by electrostatic interactions between the positively charged particles and the negatively charged oil surface. Ultrasmall-/small-angle X-ray scattering measurements confirm the assembly of nanoparticles at the interface, and time-dependent scattering measurements reveal the presence of two steps in the assembly in brine, consistent with the interfacial tension dynamics. The assembled particles at the interface lead to a solid-like behavior or jamming, with the interface behaving like an elastic membrane with high dilatational and storage moduli. This study provides fundamental insights into the surface and interfacial properties of silane-grafted NPs and ways to fine-tune their assembly at oil-water interfaces while improving their colloidal stability under harsh environmental conditions.