Unraveling The Interfacial Behavior of Double Functionalized Silica Nanoparticles at Oil-Water Interfaces: Insights from Atomic Force Microscopy, In Operando Ultrasmall and Small-Angle and X-Ray and Interfacial Rheology Measurements

Tuning interfacial interactions of emulsions is of utmost importance in addressing hydrocarbon contamination of water resources and facilitating sustainable synthesis and processing of organics across different industries, such as energy recovery, pharmaceuticals, and the food industry. In this comprehensive study, we investigated the assembly of colloidally stable silica nanoparticles (NPs) at the oil-water interface, employing a novel double functionalization strategy. The NPs were functionalized with a blend of two silanes, one based on a quaternary ammonium group and the other combining primary and secondary amine groups, thereby endowing them with a positive charge. By modulating the charges on both the NP's surface groups and the oil phase, either by pH adjustments or the addition of electrolytes, we controlled the assembly and behavior of the emulsion. The careful combination of tunable charge and specifically chosen silane coupling agents provided the NPs with outstanding colloidal stability, which is significant for practical applications.<br/>To evaluate the colloidal stability of the NPs, we conducted accelerated Space-Time Extinction Profiles (STEP^®) measurements in brine solutions up to 60,000 ppm containing monovalent and divalent electrolytes and temperatures up to 60 °C. Remarkably, even under these harsh conditions, the NPs displayed exceptional stability. Assembly of the NPs at the oil-water interface can be controlled by controlling the pH or in the presence of electrolytes. Specifically, at pH 9 or in high salt concentrations, the interfacial tension (IFT) was substantially reduced from 35 mN/m to 5 mN/m and 2 mN/m, respectively.<br/>To gain further insights into the interfacial properties of the emulsion, we used Ultrasmall/Small-angle X-ray scattering measurements, which unequivocally confirmed the presence of nanoparticles at the interface. Moreover, we found a correlation between the time-dependent scattering profiles and the dynamics of the growth of the interfacial film. To explore the nanomechanical characteristics of the interface, Atomic Force Microscopy (AFM) was employed. Notably, in the presence of the NPs at pH 9, the stiffness of the interface was substantially lower compared to acidic and neutral conditions, in brine or at pH 9 the assembly leads to jamming, reminiscent of an elastic membrane characterized by high dilatational and storage moduli.<br/>Leveraging the jamming phenomenon, we demonstrated the efficacy of our approach in herding oil droplets, thereby showcasing its potential for oil spill remediation. Additionally, our method facilitated the production of highly moldable and printable oil-in-water emulsions, presenting novel opportunities for various applications in the fields of environmental remediation, catalysis, drug delivery, food technology, and oil recovery.