Use of Drop Rest Technique To Evaluate the Stability of Waxy Crude Oil Emulsion

Waxy crude oil emulsions are difficult to demulsify because wax crystals at the oil-water interface form a rigid layer around dispersed water droplets. In this work, the coalescence of a single brine droplet at a stratified waxy crude oil-brine interface is investigated using the drop rest method. The time a droplet remains at the interface before coalescence, termed the drop rest time, is used to probe how surfactants modify wax-laden interfaces relevant to the chemical demulsification of waxy crude oil emulsions. The interaction of a brine droplet with the interfacial wax network is examined as a function of temperature, droplet size, and surfactant presence. The stochastic nature of drop rest time is described using a statistical framework based on a wax-wax connectivity model. Experiments are conducted using a modified crude oil system to enable controlled room-temperature studies. The effects of two anionic surfactants, AOT and SDS, are investigated. AOT increases the drop rest time due to enhanced wetting of wax crystals and the formation of a steric barrier at the interface. In contrast, SDS significantly decreases the drop rest time by generating Marangoni flows that remove SDS-wetted wax from the interface. In the presence of SDS, droplets exhibit pronounced center-of-mass oscillations prior to coalescence. These oscillations are analyzed using a modified force-balance model to estimate effective interfacial rigidity and viscous resistance, which correlate with measured drop rest times. When AOT and SDS are used together, progressive addition of SDS removes wax from the interface, leading to a dramatic reduction in drop rest time. Overall, this study provides mechanistic insight into wax-stabilized crude oil interfaces and demonstrates how surfactant chemistry can be used to design effective demulsification strategies.