Molecular dynamics study of droplet electrocoalescence in the oil phase and the gas phase

Abstract

• Dispersive attraction and steric effect of oil molecules affect droplet coalescence. • Solvent accessible surface area could determine when droplets contact and coalesce. • The migration of molecules causes uneven distribution of charges in water droplets. • The electro-coalescence behavior of droplets in oil and gas are highly similar. Under a uniform electric field, water droplets in a water-in-oil (W/O) emulsion approach and coalesce by dipole–dipole forces, which fundamentally affect the efficiency of electric dehydration. In order to unravel the molecular mechanism of droplet electrocoalescence, we explored the coalescence behavior of droplets in the oil phase and the simplified oil phase (nitrogen) under an external electric field using molecular dynamics (MD) simulations. The results indicated that the coalescence process of droplets can be divided into three stages based on the solvent accessible surface area (SASA), before coalescence, coalescing and after coalescence. The two droplets completely coalesced when the center-of-mass distance between droplets was 2.73 nm. Weak interaction and interaction energy analysis revealed that the electrostatic attraction between water molecules dominated the coalescence process of water droplets due to the significant characteristics of electrostatic potential (ESP). The dispersion attraction of C 6 H 14 H 2 O and C 6 H 14 C 6 H 14 was very pronounced while the interactions related to nitrogen molecules were negligible, which was the underlying reason for the differences between droplet coalescence behaviors in the two media. Nevertheless, the coalescence behaviors of a droplet pair in oil and gas had a high level of similarity, which implied that the MD simulation results of the electric coalescence based on nitrogen is acceptable to some extent. The field-induced hydrogen and oxygen atoms migrated toward both ends of the droplet, causing the redistribution of charges and changing the overall dipole moment of the droplets. This study made it possible to use oil-phase molecules for MD simulation study on the electrostatic coalescence behavior of droplets in the W/O emulsion, and confirmed the feasibility of nitrogen replacing with the oil phase.