Silica-Based Janus Nanoparticles at the Water-Decane Interface: Evidence of Emergent Behavior From Equilibrium Molecular Dynamics Simulations and Experimental Observations

This work stems from a recent paradigm-changing proof-of-concept result reported by the group of Daniel Resasco at the University of Oklahoma [Science 327 (2010) 68]. The experimental results showed that it is possible to perform in-situ upgrade of bio-oil (the pyrolisis product of lignocellulosic biomass) when solid particles are used to both stabilize water-in-oil emulsions and support heterogeneous catalysts. The solid particles used were hybrid materials obtained by fusing silica particles on carbon nanotubes. To generalize this proof of concept to large-scale industrial applications it is necessary to design simpler and cheaper particles that stabilize oil-in-water emulsions and support the catalysts. It is necessary to understand how the molecular-level features characterizing the solid particles determine macroscopic properties such as drop size and shape, as well as the mechanism of droplets coalescence. It is also desirable that the particles can be recovered after the bio-oil upgrade is complete. It is well known that solid particles adsorb at water-oil interfaces to reduce the contact area between the two immiscible phases. Stable emulsions are obtained when the particles strongly adsorb at the interfaces. It is plausible that by adding appropriate surface-active compounds the particles can be easily released from the interfaces. Once in the continuous phase, the particles tend to agglomerate, facilitating their recovery. Quantification of these qualitative expectations will transform the bio-energy field. We report herein all atom molecular dynamics simulation results for silica-based nanoparticles functionalized with hydrophobic moieties at the decane-water interface. The simulation results are quantified in terms of contact angle at the water-nanoparticle-decane interface, mobility of the nanoparticles, association of multiple nanoparticles at the interface, and free-energy landscapes that dictate the nanoparticle adsorption at the interface. The results are quantified based on the chemical features of the nanoparticle surface. Comparison with experimental data, including but not limited to TEM and cryo-TEM images of water-oil emulsions, are provided.