Developing New Recyclable and pH-Sensitive Amphiphile for Heavy Oil Emulsion and Demulsification: A Molecular Dynamics Study

Abstract

We report a long-chain N-alkyl-N, N-dihydroxyethylammonium salt (ADHA) that can be reversibly transformed into charged surfactants by decreasing pH, thereby stabilizing(or destabilizing) water/oil emulsions. As the conventional oil reserves in the world continue to decline, the immense deposits of heavy crude oil attract much attention. A growing number of heavy oil reservoirs are being proven and developed. However, heavy oil’s recovery and pipeline transport pose new challenges due to its high viscosity. The emulsion of heavy oil by surfactant floods is able to lower the viscosity but consequently leads to oil–water separation problem. Therefore, the application of this technique will benefit from an efficient, rapid method of demulsification at specially desired stage, which raises stringent requirements to the surfactants. The increase of the pH reverses the reaction, deprotonates the surfactants into uncharged tert-ammonia (ADHA), and destabilizes the emulsion. In addition, the introduction of two hydroxyls in the head groups reduces the lipophilic of the ADHA. Hence, it is inclined to flocculate in water when the base is added and it is simple to realize the recycle of the surfactant. MD simulations are used to study the mechanisms of this novel surfactant. Demulsification was studied in a beaker; the emulsion separates into two layers within 3 min, revealing the ADHA’s function as a demulsifier. Aiming for deeper insights into the mechanisms of the transformation of ADHA, using MD simulation tools, we studied the behavior and properties of ADHA at the oil–water interface. Computational results suggest cohesive bindings with experimental outcomes and also give qualitative and quantitative explanations at molecular level. In summaries, these emulsion–demulsification processes suggest that the switchable surfactants are potentially useful for heavy oil production and pipeline transports.