Water chain triggered droplet coalescence through hydrogen bonding rearrangement in water-in-oil emulsions

Abstract

The addition of demulsifiers is a widely used chemical method for demulsification, yet the microscopic mechanisms involved are not fully understood. This work employs molecular dynamics simulations to investigate the coalescence process of water droplets in water-in-oil emulsions. By analyzing the molecular spatial configurations and concentration distributions at various stages, we discovered that demulsifier molecules insert themselves into the oil-water interface, displacing surrounding asphaltene molecules and adsorbing onto the interface. This confirms the competitive adsorption between asphaltene and demulsifier molecules at the molecular level. The demulsifier molecules positioned between the water droplets induce a rearrangement of water molecules on the droplets' surfaces, pulling the water molecules to diffuse and ultimately forming a water chain between the two droplets. A water chain can expand into a water bridge, displacing surrounding molecules and creating a channel in the oil phase through which water molecules can move freely. The formation of water chains is essential for the coalescence of water droplets, as demonstrated by potential mean force calculations. Moreover, an estimation of hydrogen bond energy during coalescence indicated that hydrogen bonds primarily drive the interactions between demulsifiers and water molecules. That facilitates the rearrangement and directional diffusion of water molecules, thereby promoting the coalescence of the droplets. The analysis of hydrogen bond lifetimes indicated that demulsifier molecules can form shorter hydrogen bond networks with water compared to asphaltene, which accelerates the rearrangement of water molecules. This work suggests that the formation of water channels, resulting from the rearrangement of hydrogen bonds induced by demulsifiers, is essential for the coalescence of water droplets. It provides insights into the demulsification mechanism of water-in-oil emulsions at the atomic level when demulsifiers are present.