Hydrophobic sponge with superhydrophilic sites for the controlled capture of surfactant-stabilized emulsified oil droplets

Hydrophobic sponges have been identified as a promising material for the recovery of floating or dispersed oil, but they still face challenges in recovering oil from surfactant-stabilized emulsions. In this study, the melamine sponge (MS) was modified with Fe 3 O 4 magnetic particles (MPs), forming a hydrophobic sponge skeleton interspersed with superhydrophilic particles, denoted as MP@MS. The hydrophilic sites of the sponge can attract and capture the emulsified oil droplets with the hydrophilic ends of the surfactants stretching into the aqueous phase; then oil droplets spread along the hydrophobic skeleton; thereby recovering surfactant-stabilized emulsified oil droplets. To prepare the dual-wettability sponge, dopamine/polyethyleneimine (DA/PEI) was used to achieve hydrophilic modification of MPs, hydrophobic modification of MS, and bond them together synchronously by a facile one-pot synthesis. Compared with traditional hydrophobic sponges, the separation efficiency for surfactant-stabilized emulsions treated by MP@MS can increase from 5 % to approximately 90 %. MP@MS can effectively capture both anionic and nonionic-stabilized emulsified oil droplets, among which, the controlled continuous capture can be realized for oil droplets stabilized by nonionic surfactant. Furthermore, nonionic surfactant-coated oil droplets were verified through molecular dynamics (MD) simulations that they were more likely to detach from hydrophilic sites, and in this process remove surfactant molecules. This was conducive to exposing the hydrophilic sites promptly and thus demulsifying continuously. The new design principles and processing strategies will support the development and application of materials with dual-wettability structures in many fields, such as oil recovery, smart separation, and liquid manipulation.