Molecular Dynamics Study of the Coalescence of Water Droplets with Anionic Asphaltene Molecules under a DC Electric Field

During the extraction of crude oil, water-in-oil (W/O) emulsions are mostly formed at a high pH, where water droplets can be stabilized by anionic asphaltene molecules on the surface. The study of driving forces in the electro-coalescence of these emulsions is fundamental to the efficient design of the oil dehydration process. We studied by molecular dynamics the electro-coalescence of two asphaltene-laden droplets suspended in <i>n</i>-hexane as a model oil. The findings indicate that a low number of anionic asphaltenes per water droplet and a moderate electric field strength (<i>E</i>) allow optimal droplet-droplet coalescence conditions to be reached, which is favored by high electrical polarization of water droplets. Under these conditions, it has been found that the diffusion and polarity of water molecules are enhanced, favoring the formation of the liquid bridge between colliding droplets and reducing the droplet-droplet coalescence time. On the contrary, with a high number of asphaltenes per droplet and <i>E</i>, the droplet-droplet coalescence is hindered and/or retarded due to the steric effect of asphaltene aggregation at the interface between water droplets. Here, the high ionic conductivity (σ) of water droplets and low interfacial tension (γ) before the formation of the liquid bridge led to the formation of a water chain (WC) between electrodes, an undesirable phenomenon impairing the dehydration efficiency in the coalescer. This study demonstrates that W/O emulsions with anionic asphaltenes under conditions of relatively low σ and somewhat high γ at moderate <i>E</i> (around the critical <i>E</i>) promote complete droplet-droplet coalescence and prevent WC formation.