Stability of Pickering emulsions to control drug delivery
The stability of Pickering emulsions, i.e. nanoparticle-stabilized emulsions, has attracted enormous attention because of a variety of applications including drug delivery, cosmetics, food-stuffs, waste-water treatment, and even energy applications. Indeed, particles and nanoparticles (NPs) of different chemical and mechanical properties can be used to stabilize emulsions. Qualitative analysis is thus needed to improve the stability of Pickering emulsions and understand the fundamentals and applications of the phenomena that contribute to this stability.
Because NPs are used as emulsifiers, their characteristics have critical effects on the emulsions properties such as interfacial tension, droplet size, and emulsion stability. The density of NPs on the droplet surface is known to be the most important parameter for this stability. Indeed, the interfacial tension of the droplet depends strongly on the NPs coverage, which also depends on the NPs affinity to the interface, i.e. their desorption energy. Furthermore, when the density is large enough, the three-phase contact angle discriminates between NPs that are effective at stabilizing emulsion and those that are not. For instance, to be effective in preventing droplet coalescence, the solid particles should be able to prevent fluid molecules to transfer from one droplet to the other when the droplets are not at contact. Moreover, particles, once adsorbed, are rather difficult to displace from the interfaces. It is this property which makes that particles such a good colloidal stabilizers of emulsions and bubbles.
Sequence of simulation snapshots representing buckling processes of water in oil droplets armored with Janus (top) and homogeneous (bottom) NPs after successive removals of water (Sicard et al., 2017).
Besides the structure of NP monolayers, the NPs diffusion also affects the system stability. Janus nanoparticles play a specific role in the stability of Pickering Emulsion as they are believed to be more effective than other type of nanoparticles (like homogeneous NPs) when droplets are forced to buckle or coalesce. Indeed, Janus NPs typically have high adsorption energies, and therefore are expected to pack densely at an interface, achieving large interfacial tension reduction. NPs with small desorption energy like homogeneous NPs, yield lower surface coverage than those required to induce large interfacial tension reduction. We recently highlighted that the stability mechanism strongly depends on the collision speed achieved numerically and/or experimentaly during the coalescence process. Considering Adiabatic Biased Molecular Dynamics simulations and the Dissipative Particle Dynamics framework, it emerged that the type of NPs used as stabilizer no more plays a critical role when the collision speed is slower than a threshold velocity related to the self-diffusion constant of the stabilizing NPs. We underlined that, for collision velocity faster than the self-diffusion threshold velocity, the characteristics of NPs used to stabilize the Emulsion is a discriminate parameter, but remains evasive when the collision velocity is slower (Sicard et al., 2016).
We extended this work to the issue of the buckling mechanim in droplets stabilized by solid particles. We studied the interplay between the evolution of droplet shape, layering of the particles, and their distribution at the interface when the volume of the droplet is reduced. We showed that Janus particles affect strongly the shape of the droplet with the formation of a crater-like depression. This evolution is actively controlled by a close-packed particle monolayer at the curved interface. On the contrary, homogeneous particles follow passively the volume reduction of the droplet, whose shape does not deviate too much from spherical, even when a nanoparticle mono/bilayer transition is detected at the interface (Sicard et al., 2017). Current and future work are focusing on the relevance of armored droplets in various applications including potential drug delivery systems and biomimetic design of functional surfaces.
- F. Sicard and A. Striolo, Understanding the Assembly of Anisotropic Building Blocks via Computer Simulation. In N. Wu, D. Lee, and A. Striolo (Eds), Anisotropic Particle Assemblies: Synthesis, Assembly, Modeling, and Applications, Elsevier (1st Ed.), ISBN: 9780128040690 (2018)
- F. Sicard and A. Striolo, Buckling in Armored Droplets, Nanoscale 9, 8567 (2017)
- F. Sicard and A. Striolo, Numerical analysis of Pickering Emulsion stability: insights from ABMD simulations, Faraday Discuss.191, 287 (2016)
last update: June 2018