Head Group-Induced Interfacial Water Rearrangement during Demulsification: A Study at the Molecular Level

Abstract Tight oil emulsions produced in tertiary oil recovery display ultrahigh stability, whereas conventional block polyether demulsifiers show inadequate demulsification performance under harsh high-temperature and high-salinity conditions. However, the relationship between the microstructure of demulsifiers and their demulsification performance remains inadequately understood at the molecular level. In this work, 12 anionic-nonionic and nonionic triblock polyether demulsifiers with distinct block sequences (PO–EO–PO, EO–PO–EO) and diversified head groups were rationally designed, and their demulsification performance and microscopic interaction mechanisms were systematically investigated by integrating molecular dynamics simulations and quantum chemical calculations. The results demonstrate that the type of head group imposes a more decisive effect on demulsification efficiency than the block sequence. Potential of mean force calculations reveal that demulsifiers functionalized with anionic head groups universally exhibit a low free energy barrier during the demulsification process. Analysis of the number and binding energy of hydrogen bonds between demulsifier molecules and water molecules validates that hydrogen bonding interactions dominate the entire demulsification process. Moreover, electrostatic potential analysis of demulsifier molecules verifies that the strength of such hydrogen bonding is not directly correlated with the electronegativity extremum of the head group, but with the area of the electrostatic potential region of the whole demulsifier molecule. Specifically, a larger regional area corresponds to more exposed hydrogen-bond acceptor sites and stronger hydrogen-bonding interactions. Calculations of the radius of gyration and solvent-accessible surface area confirm that demulsifier molecules adopt a more extended conformation in aqueous solution, resulting in higher interfacial coverage at the oil–water interface and more sufficient displacement of asphaltenes. More importantly, two-dimensional water density distribution profiles clearly visualize that anionic-nonionic block polyethers can induce the formation of multilayer ordered hydration layers around their head groups, which is distinctly different from the featureless uniform water distribution observed for nonionic demulsifiers. Finally, we define the concepts of interfacial water and bulk water in the demulsification process, and find that a higher proportion and greater thickness of interfacial water accelerate the rearrangement of the hydrogen bond network and the construction of water channels between droplets, thereby facilitating water droplet coalescence. These theoretical findings are highly consistent with reported experimental results showing that strongly electronegative functional groups in demulsifiers reconstruct the interfacial hydrogen bond network and break the rigid interfacial film, achieving a dehydration rate of up to 98.3% for tight oil emulsions. This study reveals the regulatory mechanism of head group type on demulsification performance at the molecular level and enriches the microscopic understanding of demulsification behaviors.