Impact of Interfacial Tension and Critical Micelle Concentration on Bilgewater Oil Separation

Oil-in-water emulsions created in shipboard bilgewater can be challenging to treat to hydrocarbon-limiting environmental discharge regulations. While emulsion behavior has been widely studied in other disciplines, research on bilgewater emulsions has been limited due to the variability of bilgewater composition. In this context, surfactant types and concentrations are generally unknown. Therefore, experimental properties of neat surfactants and commercial cleaners were evaluated for predicting bilgewater emulsion stability. Critical micelle concentration (CMC), CMC in the presence of oil (CMC_IFT), and equilibrium interfacial tension (IFT) were investigated for emulsion stability using different surfactant types, oil concentrations, and homogenization energies. It was found that CMC was significantly larger in the presence of mineral oil. Γ_∞ values of all three surfactants were similar for the oil-water and air-water cases; however, the κ values were larger for the oil-water interface suggesting that, for a given surfactant, the adsorption of the surfactant molecules to the oil-water interface was more active than to the air-water interface. It was also determined that emulsion stability was most closely related to CMC_IFT. Coalescence into a separated oil layer was only observed in emulsion samples with surfactant concentrations below CMC_IFT. This relationship was observed for different homogenization intensities and oil concentrations. Experiments also validated the relationship between CMC_IFT and oil separation in more complex formulations of commercial cleaners commonly found aboard ships. Four different cleaners with unique compositions all demonstrated separated oil at concentrations below CMC_IFT. Overall, CMC_IFT represents a practical way to evaluate a cleaner's likelihood of developing stable (> 72 h with no observable oil separation) emulsions in bilgewater.