Phase layer distribution and influencing factors for the static sedimentation process of W/O emulsions

The variation of phase‑layer distribution during the settling process is fundamental to understanding the gravitational sedimentation‑separation mechanism of W/O emulsions. Since the existence of the dense-packed zone was verified, few studies have systematically investigated the effects of various factors on phase distribution. Meanwhile, current characterization techniques suffer from insufficient detection precision or high equipment costs. In this work, a Turbiscan LAB Expert kinetic stability analyzer combined with static multiple light scattering (SMLS) was innovatively adopted. By tracking variations in backscattered light intensity during emulsion sedimentation, distinct profiles of all phase interfaces, including the dense‑phase boundary, can be precisely captured. With this method, we systematically analyzed the influences of initial dispersed droplet size and surfactant dosage on phase‑layer distribution. The results demonstrate that reducing the initial droplet size or increasing the dosage of Span 80 surfactant both hinder static oil-water separation. Smaller droplets slow the thickening rate of the dense-packed zone, while extra Span 80 accelerates its formation and reduces the average water content inside the dense phase. Further experiments were conducted to explore the effects of temperature and demulsifier. Higher temperature and demulsifier dosage both accelerate static sedimentation. The demulsifier severely disrupts the internal structure of the dense-packed zone and triggers rapid separation of the aqueous phase. Instead of promoting free water generation, moderate heating only speeds up the formation of the dense-packed zone. These observations and conclusions not only provide guidance for oil-water gravitational sedimentation separation in engineering practice, but also lay a theoretical foundation for the structural design of gravitational oil-water separators.