(180b) Effect of Surfactants and Nanoparticles on Emulsions Stability

Understanding the processes involved in the instability of emulsions is of great interest for many practical applications. Although many have studied, for example, the ability of surfactant or/and nanoparticles to increase the lifetime of emulsions droplets, several open fundamental questions remain. Molecular dynamics simulations could be used to identify some of the molecular mechanisms responsible for emulsions instability. In this work, we present recent advances obtained via the implementation of the coarse-grained Dissipative Particle Dynamic simulation framework, DPD.

As any other simulation approach, DPD requires determining a few adjustable parameters to describe the systems of interest. We derived our interaction parameters based on a systematic investigation determined from known thermodynamic quantities. The results show that building DPD parameters starting from the Hansen theory of solutions, it is possible to reproduce simultaneously the interfacial tension between water and benzene, and the micellar properties of aqueous non-ionic surfactants representative of the octyl polyethylene oxide family. Using the DPD parameters so derived, we investigate the effect of surfactants on the stability of oil-in-water emulsions. Among the different processes that can lead to emulsion break down, we focused on the Ostwald ripening phenomena. The simulation results showed qualitative agreement with both experiments and the Lifshitz-Slyozov-Wagner theory in presence and absence of surfactants. Finally, we studied the effect of the attraction/repulsion interaction between surfactant and nanoparticles on the stability of a Pickering emulsion. In some cases, we can observe surfactant molecules displacing nanoparticles adsorbed on droplet surfaces, while other cases showed synergism between surfactant molecules and nanoparticles.