Particles located along the interface between two immiscible fluids or two different polymers can stabi-lize the interface by reducing the free energy of the system, thus acting as surfactants. In comparison to conven-tional organic surfactants, particle surfactants exhibit highly stable, quasi-irreversible adsorption to the oil？water interfaces, thus producing emulsions that are stable against coalescence. Furthermore, particle surfactants have the interesting photonic, magnetic, electrical, and catalytic properties that can be combined to produce synergis-tic effects with the intrinsic properties of the polymer or fluid matrix. Particularly, the efficiency of particle sur-factants is strongly affected by their geometry (e.g., size and shape) and surface chemistry, which influences their adsorption energy to the interface, and the entropic contribution to the system. In this regards, many efforts have been devoted to developing particle surfactants with a careful design of geometry and surface properties to localize particles at the interface between the two different phases.
In this thesis, we developed graphene-based surfactants. Unlike particle surfactants with spherical shape, the efficiency of graphene as surfactants can be greatly amplified due to their two-dimensional geometry and high surface-to-volume ratio. Particularly, graphene oxide (GO) has been used as a particle surfactant because of their amphiphilic character. Recently, GO surfactants have been demonstrated to help create emulsions by preventing coalescence and enhance the morphology of polymer blends. However, a system including GO with controlled surface properties and geometry is needed to determine the size effects of the GO on the interfacial and morphological properties of immiscible fluids or polymer blends. To generate the GO surfactants, the surface properties of GO were modified by the thermal reduction of oxygen-containing groups. In addition, the size of GO were precisely controlled by a simple, and reproducible approach based on ultrasonication method. To determine the size effects of the GO, three different GO with sizes of 1200, 700 and 85 nm were prepared. All GO surfactants can produce highly stable Pickering emulsions of oil-in-water systems. Particularly, the average emulsion droplet size decreased as the size of GO decreases.
While GO exhibit their effectiveness as particle surfactants, GO sheets are not typically suitable for pro-ducing submicron-sized emulsion droplets due to the curvature effect caused by their large size. Therefore, we developed graphene-based nanoparticles, graphene quantum dots (GQDs) surfactants with tailored amphiphilicity, and we used them to produce Pickering emulsions and novel polymer particles. To generate the GQD surfactants, the surface properties of 10 nm sized, non-reduced GQDs (nGQDs), which have strong hydrophilicity, were synthesized and modified in a systematic manner by the thermal reduction of oxygen-containing groups at different treatment times. In stark contrast to the behavior of the nGQDs, thermally reduced GQDs (rGQDs) can produce highly stable Pickering emulsions of oil-in-water systems. To demonstrate the versatility of the rGQD surfactants, they were applied in a mini-emulsion polymerization system that requires nanosized surfactants to synthesize submicron-sized polystyrene particles. In addition, the use of rGQD surfactants can be extended to generating block copolymer particles with controlled nanostructures. Particularly, the polymer particles were highly luminescent, a characteristic produced by the highly fluorescent GQD surfactants, which has great potential for various applications, including bioimaging, drug delivery, and optoelectronic devices.
To exploit the full potential of the surface-tailored GQDs as surfactants, 10-nm sized GQDs were used as efficient surfactants to produce poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) particles with tunable external shape and internal morphology. The surface properties of GQDs were modified by grafting different alkyl ligands such as hexylamine and oleylamine to generate the surfactant behavior of GQDs. In stark contrast to the behavior of the unmodified GQDs and hexylamine-grafted GQDs, oleylamine-grafted GQD surfactants were selectively positioned on the P4VP domain at the particle surface, which effectively tuned the interfacial interaction between two different PS/P4VP domains of particles and the surrounding water and induced a dramatic morphological transition to the unconventional convex lens-shaped particles. Precise and systematic control of interfacial activity of GQD surfactants was also demonstrated by varying the density of alkyl ligands on the GQDs. Our design of surface-engineered GQDs combined with their significant advantages of 2D plate geometry with nanoscale size and tunable optical and electrical properties highlight the importance of GQD surfactants in producing novel functional colloidal particles.