Molecular Dynamics Simulation of a Brine Droplet under an Electric Field: Distinct Behavior Shown by NaCl and CaCl2.

Abstract

Electrostatic demulsification is a promising technique to disrupt petroleum emulsions. However, the presence of salts in the emulsion can influence the effectiveness of the electric field. In this work, we target an understudied area, namely, the effect of salt ion type and concentration on the stability of brine droplets when exposed to an electric field. Molecular dynamics (MD) simulations are performed on a series of water-in-oil emulsion systems consisting of a water or brine droplet surrounded by an oil phase containing toluene and model asphaltene molecules (N-(1-hexylheptyl)-N'-(5-carboxylicpentyl) perylene-3,4,9,10-tetracarboxylic bisimide (C5Pe)). The brine droplet contains either NaCl or CaCl2, with concentrations varying from 0 to ∼11 wt %. An external electric field is applied, which has a strength ranging from 0 to 1 V/nm. Our results show that as the electric field increases, the bare water droplet exhibits progressive deformation from the original spherical shape to an ellipsoid, a spindle, and finally a cylinder. When the brine droplets are exposed to a low electric field (≤0.5 V/nm), they behave similar to the bare water droplet. However, at a high electric field (≥0.75 V/nm), both NaCl and CaCl2 brine droplets are stabilized in the bulk oil and maintain the spherical or ellipsoidal shape by ejecting salt ions toward the electrodes at high salt concentrations (≥7.8 wt %), which induces a counter electric field that weakens the destabilization of the droplet by the applied field. At low salt concentrations (≤4.5 wt %), brine droplets containing NaCl or CaCl2 display different behaviors: the former tends to shift toward an electrode, whereas the latter stays in the bulk oil phase. The contrasting phenomena are the result of combined effects of brine droplet net charge and C5Pe adsorption on the droplet surface: a large net charge and low C5Pe adsorption tend to drive the droplet toward an electrode. This study provides useful insights into the important role of salt ions in electrostatic demulsification of petroleum emulsions.