Modifying Wax Crystal Morphology Using Hydrophobized Silica Nanoparticles

Nanoparticles are widely adopted to control crystallization due to their remarkable ability to modify crystal properties at the molecular level. Nanoparticles with suitable surface characteristics can selectively enhance or suppress nucleation and crystal growth. Silica nanoparticles, in particular, are extensively used as texture modifiers in the food industry and flow improvers in the petroleum industry. However, the precise details of their modus operandi remain poorly understood. We report the effect of silica nanoparticles, hydrophobized with octadecyl chains, on the kinetics of wax crystallization and the flow properties of a model waxy oil. Quantification of the density of hydrocarbon chains on nanoparticles showed that the grafting consists of multiple layers. The addition of these nanoparticles inhibited wax nucleation, estimated as a nanoparticle concentration-dependent effective activation energy of nucleation. There was also a change in the morphology of wax crystals from plate-like crystals to large, branched structures, resulting in a reduction of viscoelastic moduli and yield stresses by more than one order of magnitude. SEM images further revealed that nanoparticles are adsorbed along plate edges, reducing growth in that direction and thereby causing branching. These insights can be extended to other systems where nanoparticles can be used to tune the nucleation and growth mechanisms during crystallization.