Thermally regulated flocculation-coalescence process by temperature-responsive cationic polymeric surfactant for enhanced crude oil-water separation

Polymeric surfactants play a crucial role in the flocculation-assisted coalescence process due to their unique bridging effect. However, the steric hindrance induced by their large molecules severely impedes the coalescence of oil droplets. Herein, temperature-responsive polymeric surfactants (quaternary ammonium chitosan-g-PNIPAM, Q-g-PN) with thermally-modulated structure were designed by integrating thermal responsive moieties onto cationic chitosan. The as-prepared Q-g-PN exhibited enhanced oil-water separation efficiency through a thermally regulated flocculation-coalescence process. At low temperatures, the thermal-responsive Q-g-PN remains in a flexible hydrophilic extended state, facilitating the flocculation of dispersed oil droplets through bridging via electrostatic interaction. At high temperatures, the Q-g-PN structure collapses into a hydrophobic coil state, reducing steric hindrance and enhancing hydrogen bonding to asphaltenes, thus effectively promoting oil droplet coalescence. Bottle tests demonstrated that the demulsification performance of Q-g-PN could reach up to 94 % with the Q-g-PN concentration only of 0.001 wt% via the proposed temperature-regulated flocculation-coalescence process, which was further confirmed by investigating oil droplet behavior and phase transition of Q-g-PN during through molecular dynamic (MD) simulation of the reaction process. This work presents new insights into enhancing oil-water separation efficiency by regulating the flocculation-coalescence process through precise modulation of the molecular interactions between polymeric surfactants and emulsion droplets.