*Article* **Molecular Dynamics Simulation for the Demulsification of O/W Emulsion under Pulsed Electric Field**

**Shasha Liu 1,2, Shiling Yuan <sup>1</sup> and Heng Zhang 1,\***


**\*** Correspondence: zhangheng@sdu.edu.cn

**Abstract:** A bidirectional pulsed electric field (BPEF) method is considered a simple and novel technique to demulsify O/W emulsions. In this paper, molecular dynamics simulation was used to investigate the transformation and aggregation behavior of oil droplets in O/W emulsion under BPEF. Then, the effect of surfactant (sodium dodecyl sulfate, SDS) on the demulsification of O/W emulsion was investigated. The simulation results showed that the oil droplets transformed and moved along the direction of the electric field. SDS molecules can shorten the aggregation time of oil droplets in O/W emulsion. The electrostatic potential distribution on the surface of the oil droplet, the elongation length of the oil droplets, and the mean square displacement (MSD) of SDS and asphaltene molecules under an electric field were calculated to explain the aggregation of oil droplets under the simulated pulsed electric field. The simulation also showed that the two oil droplets with opposite charges have no obvious effect on the aggregation of the oil droplets. However, van der Waals interactions between oil droplets was the main factor in the aggregation.

**Keywords:** bidirectional pulsed electric field; O/W emulsion; demulsification; molecular dynamics simulation

### **1. Introduction**

With the increase of oil production activities, oil pollution, particularly oily wastewater, has become an environmental concern nowadays. Enormous quantities of oily wastewater are generated during different industrial processes all around the world, including petroleum refining, industrial discharges, petroleum exploration, food production operations, etc. [1–5]. The oils in wastewater include fats, lubricants, cutting oils, heavy hydrocarbons and light hydrocarbons [6]. These oils can be further divided into free oils and emulsified oils. The free oils in wastewater are easier to separate by physical techniques such as gravity separation and skimming [7,8]. However, the emulsified oil droplets are more difficult to handle due to their high stability in water [9,10]. A widely used separation technique for emulsified oils involves the addition of chemicals, such as ferric or aluminum salts, to induce colloidal destabilization [1,5]. However, the cost is expensive, and the chemicals would dissolve in water or form-settling sludge after the treatment, which is not recommended from the perspective of green chemistry.

An alternative approach is the use of an electric field, especially for the dehydration of crude oil [11–17]. Electric field demulsification has practical advantages such as a lack of extra chemicals, simple equipment, short process flow, etc. It can achieve physical separation of oil and water mixtures and recover oily substances to a certain extent, without the pollution from added chemicals [18,19]. The demulsification mechanism of W/O emulsion by electric fields has also been widely researched. The demulsification was attributed to the droplets' polarization and elongation under the electric field, which then induces interactions between dipoles, leading to aggregation [20–22]. However, the utilization of an electric field to separate oil and water in O/W emulsion is rarely studied. It is generally believed that electric field demulsification does not work for O/W

**Citation:** Liu, S.; Yuan, S.; Zhang, H. Molecular Dynamics Simulation for the Demulsification of O/W Emulsion under Pulsed Electric Field. *Molecules* **2022**, *27*, 2559. https://doi.org/ 10.3390/molecules27082559

Academic Editor: Enrico Bodo

Received: 23 February 2022 Accepted: 12 April 2022 Published: 15 April 2022

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emulsions, because water is conductive, and the electrical energy could dissipate easily in aqueous solution [5]. Ichikawa et al. [23] investigated the demulsification process of dense O/W emulsion in a low-voltage DC electric field and found that a mass of gas bubbles occurred and surged in the emulsion during the demulsification process. Furthermore, Hosseini et al. [24] applied a non-uniform electric field to demulsify the benzene-in-water emulsion. Bubbles were also generated in the emulsion when the electric field was introduced. The occurrence of these phenomena is attributed to the overly large electric current in the emulsion, leading to water's electrolysis.

To resolve this problem, Bails et al. [25,26] applied a pulsed electric field (PEF) to W/O emulsion and found that the electric current generated by a pulsed electric field is small at a high voltage. After this, Ren et al. [27] applied bidirectional pulsed electric field (BPEF) to separate O/W emulsion; this was prepared by mixing 0 # diesel oil and SDS solution. They found that BPEF induced the aggregation of oil droplets, and BPEF had a distinct demulsification effect on O/W emulsion with surfactant. The demulsification effect under different BPEF voltages, frequencies and duty cycles were investigated by evaluating oil content and turbidity of the clear liquid after demulsification. Moreover, they put forward a hypothesis that charges on the oil drop surface would redistribute under BPEF to promote the mutual attraction and coalescence of oil drops. However, the mechanism of oil droplet movement and aggregation in O/W emulsion at the molecular level under BPEF has not been well studied; still less studied is the effect of surfactants on demulsification. Molecular dynamics (MD) simulation is considered a useful tool to carry out microscopic analysis of the dynamic behavior of nanodroplets based on the basic laws of classical mechanics [28]. Chen et al. [29] used MD simulation to study the influence of a direct current electric field on the viscosity of waxy crude oil and the microscopic properties of paraffin. They found that the electric field strength affects the distribution of oil molecules. He et al. [30] simulated the aggregation process and behavior of charged droplets under different pulsed electric field waveforms by MD simulation. They discovered that the deformation of droplets is greatly affected by the waveform. Moreover, the additive in the emulsion has an important influence on its emulsifying stability [31–33]. For example, an experimental study found that BPEF had a distinct demulsification effect on O/W emulsion with SDS surfactant [27] . However, to the best of our knowledge, there has been no report on the microscopic level of the demulsification of O/W systems with SDS surfactants under the action of BPEF electric field. In addition, the crude oil composition was relatively distinct when the behavior of crude oil in electric field was simulated previously [34]. Therefore, it is necessary to study the movement and coalescence behavior of oil droplets in O/W emulsion under BPEF by MD simulation. We believe that this will provide a theoretical basis for the application of BPEF in O/W emulsion demulsification.

In this paper, we investigated the movement and aggregation behavior of crude oil droplets in O/W emulsion with differing contents of SDS under BPEF. First, the structural changes of oil droplets in each system and their collision time were analyzed to determine the behavioral difference between the oil droplets with and without SDS. Second, the centroid distance between oil droplets, the average elongation length of oil droplets and the MSD of SDS and asphaltene molecules of oil droplets were calculated to explain why SDS can reduce demulsification time. Finally, we investigated the aggregation behavior of oil droplets after the shut-off of BPEF and discussed the aggregation mechanism of oil droplets under BPEF.
