Impact of mixed surfactant composition on emulsion stability in saline environment: anionic and nonionic surfactants

The influence of anionic and nonionic surfactant ratios on emulsion stability in saline environments was investigated in the context of bilge water. Sodium laureth sulfate (SLES) and Triton X-100 were selected as surfactants likely present in bilge water emulsions. Interfacial tension results revealed that with 0.42 M NaCl, SLES is a more surface-active surfactant with a smaller head group area than Triton X-100. Increasing the proportion of SLES resulted in a higher surface charge, thus creating a greater repulsive interaction energy barrier between oil droplets, preventing flocculation. Size distribution results showed emulsions with 500 ppm surfactants at different surfactant ratios had long-term (more than 20 days) stability against coalescence. However, SLES-rich emulsions had smaller oil droplets (<8 μm) generated with the same energy input and were more stable against flocculation when compared to Triton X-100-rich emulsions. Visual observation showed more small oil droplets (<10 μm) with a slow creaming rate in SLES-rich emulsions than in Triton X-100-rich emulsions. Turbidity measurements showed that SLES-rich emulsions had higher remnant oil content in the subnatant than Triton X-100-rich emulsions, which is undesirable for phase separation and bilge water treatment. This work provides experimental results on how anionic and nonionic surfactant ratio influences emulsion properties in saltwater environments, which is valuable in developing strategies to minimize and remove remnant oil in bilge water.