Development of Underwater Oleophobic and Underoil Hydrophobic Strontium(II)–Cyclotriphosphazene Hexacarboxylate Framework with Prewetting-Induced Switchable Wettability and Self-Cleanability for Continuous Oil–Water Mixture and Emulsion Separations

Oil spill management presents significant challenges, particularly when addressing spills that occur beneath the water's surface. In this context, Sr-HCPCP (SRMIST-2) is an innovative MOF with underwater oleophobic and underoil hydrophobic properties, incorporating enhanced coordination, a strong affinity for water and hydrophilic strontium, and a nontoxic, eco-friendly, biocompatible, and hydrophobic cyclotriphosphazene. It is designed with switchable wetting properties and exceptional chemical and thermal stability. SRMIST-2 is synthesized via a hydrothermal reaction between strontium nitrate and hexakis(4-carboxylatophenoxy)-cyclotriphosphazene. Its structure consists of edge-sharing {Sr₃(COO)₆(H₂O)₃} polyhedra that form 1-D chains, which pair to create 2-D networks that further interact with HCPCP ligands to construct a three-dimensional framework. When coated onto cotton fiber using polydopamine, the resulting CF-PDA-SRMIST-2 demonstrates excellent oil-water separation. Depending on whether it is prewetted with water or oil, it achieves separation efficiencies of 88-99%, with high flux rates (3409 Lm^2-h^-1 for water and 2840 Lm^2-h^-1 for oil) and remains effective over 15 cycles. It effectively separates oil-in-water and water-in-oil emulsions with 98% and 95% efficiency, respectively. CF-PDA-SRMIST-2 remains stable under acidic, alkaline, saline, and extreme temperature conditions. Its self-cleaning, amphiphobic properties ensure durability and reusability. With its low-cost, scalability, and eco-friendly nature, CF-PDA-SRMIST-2 is a promising material for sustainable oil spill remediation and environmental protection.