The ability of colloidal particles to penetrate fluid interfaces is a crucial factor in the preparation of particle stabilized disperse systems such as foams and emulsions. For hard micron-sized particles the insertion into fluid interfaces requires substantial energy input, but soft particles are known to adsorb spontaneously. Particle hardness, however, may also affect foam and emulsion stability. The high compliance of soft particles may compromise their ability to withstand the lateral compression associated with disproportionation. Hence, particles which can spontaneously adsorb onto fluid interfaces, and yet depict low compliance may be ideal as interfacial stabilizers. In the present work, we prepared core-shell particles comprising a hard, polystyrene core and a soft poly(N-isopropylacrylamide) based shell. We found that such core-shell particles adsorb spontaneously onto various fluid interfaces. The absence of a pronounced energy barrier for interfacial adsorption allowed the facile preparation of particle-stabilized bubbles as well as emulsion droplets. For bubbles, the stability was better than that of bubbles stabilized by entirely soft particles, but disproportionation was not stopped completely. Emulsion droplets, in contrast, showed excellent stability against both coalescence and disproportionation. Lateral compression of core-shell particles due to disproportionation was clearly limited by the presence of the polystyrene core, leading to long-lasting stability. For emulsions, we even observed non-spherical droplets, indicating a negligible Laplace pressure. Our results indicate that core-shell particles comprising a hard core and a soft shell combine the advantageous properties of hard and soft particles, namely spontaneous adsorption and limited compliance, and can therefore be superior materials for the preparation of particle-stabilized dispersions.
Stabilization of gas bubbles in water by applying solid particles is a promising technique to ensure long-term stability of the dispersion against coarsening. However, the production of large quantities of particle stabilized bubbles is challenging. The delivery of particles to the interface must occur rapidly compared to the typical time scale of coarsening during production. Furthermore, the production route must be able to overcome the energy barriers for interfacial adsorption of particles. Here we demonstrate that ultrasound can be applied to agitate a colloidal dispersion and supply sufficient energy to ensure particle adsorption onto the air-water interface. With this technique we are able to produce micron-sized bubbles, solely stabilized by particles. The interface of these bubbles is characterized by a colloidal shell, a monolayer of particles which adopt a hexagonal packing. The particles are anchored to the interface owing to partial wetting and experience lateral compression due to bubble shrinkage. The combination of both effects stops coarsening once the interface is jammed with particles. As a result, stable bubbles are formed. Individual particles can desorb from the interface upon surfactant addition, though. The latter fact confirms that the particle shell is not covalently linked due to thermal sintering, but is solely held together by capillary interaction. In summary, we show that our ultrasound approach allows for the straightforward creation of micron-sized particle stabilized bubbles with high stability towards coarsening.
There is a growing interest in the use of particles as stabilizers for foams and emulsions. Applying hard particles for stabilization of fluid interface is referred to as Pickering stabilization. By using hard particles instead of surfactants and polymers, fluid interfaces can be effectively stabilized against Ostwald ripening and coalescence. A drawback of the use of hard particles as interfacial stabilizers is that they often experience a pronounced energy barrier for interfacial adsorption and that hard particles are very specific with regard to the type of fluid interface they can adsorb to. Soft particles, on the other hand, are known as good stabilizers against coalescence and they spontaneously adsorb to a variety of different fluid interfaces. The aim of this thesis was to investigate core-shell particles comprising a hard core and soft shell with regard to their interfacial behaviour and their ability to act as sole stabilizers for foams and emulsions. We hypothesised that the presence of the soft shell allows for easier interfacial adsorption of core-shell particles compared to the hard core particles only. To test this hypothesis, we prepared core-shell particles comprising a solid polystyrene (PS) core and a soft poly-N-isopropylacrylamide (PNIPAM) shell. To ascertain the effect of shell thickness, we prepared a range of core-shell particles with different shell thicknesses, containing identical core particles. We found that core-shell particles are intrinsically surface active and can generate high surface pressures at the air-water interface and oil-water interfaces, whereas core particles seemed to experience a large energy barrier for interfacial adsorption and did not lower the surface tension. We also confirmed by microscopy that coreshell particles are actually adsorbing to the fluid interface and form densely packed interfacial layers. Further, we found that a certain critical thickness of the soft shell is necessary in order to ensure facile interfacial adsorption. If the PNIPAM shell on top of the core particles is well above 100nm thick, particle adsorption at the air-water interface was found to be diffusion limited.By gentle hand-shaking we were able to produce dispersion of air bubbles and emulsion droplets solely stabilized by core-shell particles. The resulting bubbles still underwent Ostwald ripening, albeit slowly. For oil-in-water emulsions of hexane and toluene, both of which have a relatively high solubility in the continuous phase, we found that core-shell particles can stop Ostwald ripening. The resulting emulsion droplets adopted pronounced non-spherical shapes, indicating a high elasticity of the interface. The high stability and the remarkable non-spherical shape of the emulsion droplets stabilized by core-shell particles were features we also observed for fluid dispersion stabilized by hard particles. This shows that in terms of emulsion stability core-shell particles behave similar to hard particles as interfacial stabilizer.As to why the differences between the st...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.