Facile Surface Hydrophobic Modification of Rice Husk for Efficient Crude Oil Emulsion Separation

Commercial demulsifiers are mainly polyethers derived from ethylene oxide and propylene oxide, with hazardous and complex production routes. Rice husk (RH), a renewable agricultural by-product with abundant cellulose, lignin and surface oxygen-containing groups, is a promising hydrophilic substrate for biomass demulsifiers. However, the reported RH-based demulsifiers generally need harsh synthesis conditions, and investigations into their interfacial mechanisms are still superficial. In this work, an RH-based demulsifier (RHPB) was prepared via a relatively mild one-step surface hydrophobic modification of RH with 1-bromooctadecane (C18Br). Its structure and surface morphology were characterized by FT-IR, XRD, ¹³C NMR, SEM, and N₂ adsorption-desorption analysis. Bottle test results revealed that RHPB attained a demulsification efficiency (DE) of 89.7% within 40 min and 95.2 % after 180 min at a dosage of 600 mg/L. In addition, RHPB exhibited good salt and pH tolerance, achieving 99.7% of DE, particularly under strongly alkaline conditions. Combined with density functional theory (DFT) and molecular dynamics (MD) simulations, the critical role of RHPB in regulating intermolecular non-covalent forces (dominated by hydrogen bonding) at the interface was revealed. RHPB forms HBs with water molecules through its oxygen-containing functional groups, competitively displacing water molecules originally bound to asphaltenes. Meanwhile, the hydrophobic long chains create steric hindrance effects that assist interfacial reconstruction, thereby effectively weakening the strength of the asphaltene-constructed oil-water interface (OWI) film. Owing to the low cost, naturally abundant feedstock, RHPB represents a sustainable alternative to traditional polyether demulsifiers for W/O emulsion remediation. This study provides a valuable strategy for designing demulsifiers by tuning the amphiphilicity of modified rice husk.