Probing the Effect of NaCl Concentrations on a Model Asphaltene Adsorption onto Water Droplets of Different Sizes

While a large amount of attention has been paid to investigating the effects of various factors (e.g., solvent and asphaltene molecular structure) on asphaltene adsorption, the effect of salinity, particularly brine water, has not been systematically studied at the atomistic level despite the ubiquitous presence of salt in crude oil processing. In this work, the adsorption behaviors of a model asphaltene at the oil/brine interface were investigated using molecular dynamics simulations. It was found that the addition of salt greatly enhances the adsorption kinetics in systems with a small droplet, while this enhancement was insignificant for systems with a large droplet. Furthermore, dependent on droplet size, salinity has different effects on the adsorbed structures. In systems with a small droplet, the addition of ions facilitates the direct contact of adsorbed model asphaltene molecules with water surfaces, while in systems with a large droplet, such direct contact was suppressed. Detailed analysis of hydrogen bonding, water/ion distributions, and shape transformation of the water droplet suggests that these observations result from the different degrees of hydration in systems with small and large droplets: small droplets have large surface area to volume ratio, and thus cannot facilitate strong ion hydrations, while large droplets manifest strong ion hydrations, leading to reduced model asphaltene and water attractions. The results reported here can enhance our understanding of emulsification mechanisms in high-salinity crude oil at an atomistic level.