Surface Wetting-Driven Separation of Surfactant-Stabilized Water–Oil Emulsions

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Abstract
Four fluorocarbon polymers including PTFE and PVDF were coated respectively on a stainless steel felt to separate emulsified water droplets from ultralow sulfur diesel fuels (ULSD). The original fuel treated with clay to remove additives was additized again with four known surfactants including Pentaerythrityoleate, (octadecadienoic acid) dipolymer, (octadecadienoic acid) tripolymer and monoolein individually. The different surfactants adsorbed on the fuel-water interface reduce the interfacial intension with different intensity. The separation efficiency at various surfactant concentrations was used to evaluate the coalescence effect exerted by these coatings. It was found the separation was both surfactant and coating dependent. A fluoro-polyurethane coating (FC1) stood out to counteract the adverse effect of all the surfactants. Solid free energy was then measured using Acid-Base and Kaelble-Uy adhesion theories for all the coatings, but its correlation with coalescence was not found at all. Coating aging in surfactant additized fuel on the coating’s water wettability was also examined to better understand how historical wetting affects separation. A tumbled model for fluorocarbons was identified that well explained the continuous decline of water contact angle on the FC1 coating in fuel. Subject to the challenge of foreign environment, the fluoroalkyl chains of the polymer tilt to expose the carbonyl groups underneath, resulting in favored coalescence separation in the presence of surfactants.
Funding Information
  • Ministry of Science and Technology of the People's Republic of China (2017YFB0308002)
  • National Natural Science Foundation of China (21476237)

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