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

Electrostatic demulsification is a promising techniqueto disruptpetroleum emulsions. However, the presence of salts in the emulsioncan influence the effectiveness of the electric field. In this work,we target an understudied area, namely, the effect of salt ion typeand concentration on the stability of brine droplets when exposedto an electric field. Molecular dynamics (MD) simulations are performedon a series of water-in-oil emulsion systems consisting of a wateror brine droplet surrounded by an oil phase containing toluene andmodel asphaltene molecules (N-(1-hexylheptyl)-N & PRIME;-(5-carboxylicpentyl) perylene-3,4,9,10-tetracarboxylicbisimide (C5Pe)). The brine droplet contains either NaCl or CaCl2, with concentrations varying from 0 to & SIM;11 wt %. Anexternal electric field is applied, which has a strength ranging from0 to 1 V/nm. Our results show that as the electric field increases,the bare water droplet exhibits progressive deformation from the originalspherical shape to an ellipsoid, a spindle, and finally a cylinder.When the brine droplets are exposed to a low electric field (& LE;0.5V/nm), they behave similar to the bare water droplet. However, ata high electric field (& GE;0.75 V/nm), both NaCl and CaCl2 brine droplets are stabilized in the bulk oil and maintainthe spherical or ellipsoidal shape by ejecting salt ions toward theelectrodes at high salt concentrations (& GE;7.8 wt %), which inducesa counter electric field that weakens the destabilization of the dropletby the applied field. At low salt concentrations (& LE;4.5 wt %),brine droplets containing NaCl or CaCl2 display differentbehaviors: the former tends to shift toward an electrode, whereasthe latter stays in the bulk oil phase. The contrasting phenomenaare the result of combined effects of brine droplet net charge andC5Pe adsorption on the droplet surface: a large net charge and lowC5Pe adsorption tend to drive the droplet toward an electrode. Thisstudy provides useful insights into the important role of salt ionsin electrostatic demulsification of petroleum emulsions.