Mechanistic insights into heat-induced demulsification of oil/water emulsions driven by surfactant type
Shideng Yuan, Haibo Wang, Qing Du, Qing You
Abstract
Understanding the thermal demulsification behavior of oil droplets stabilized by different surfactants is crucial for efficient emulsion separation and oil recovery processes. The working hypothesis posits that differences in surfactant architecture regulate interfacial stability and control the coalescence pathways of droplets under thermal conditions. Here, molecular dynamics (MD) simulations were employed to investigate the thermal coalescence mechanisms of oil droplets emulsified by anionic (sodium dodecyl sulfate, SDS) and nonionic (polyoxy ethylene nonyl phenyl ether, NP-4) surfactants. The demulsification process can generally be divided into three stages: the free oil droplet stage, the oil droplet contact stage, and the oil droplet stabilization stage. Oil droplets emulsified by the non-ionic surfactant NP-4 undergo thermal demulsification more readily than those emulsified by SDS. NP-4 accelerates droplet coalescence through bridging interactions, whereas SDS, due to its strong interfacial anchoring capability, maintains interfacial stability and hinders demulsification. Post-demulsification, droplets stabilized by SDS were more compact (with radius of 0–3 nm) compared to those stabilized by NP-4 (with radius of 0–4 nm), with NP-4 systems displaying higher molecular mobility, making them more likely to collide and undergo demulsification. These findings offer molecular-level insights into how surfactant structure influences interfacial dynamics and thermal demulsification efficiency.