Fast formation of superhydrophobic octadecylphosphonic acid (ODPA) coating for self-cleaning and oil/water separation

A simple and fast method to prepare robust superhydrophobic octadecylphosphonic acid (ODPA) coating on oxidized copper mesh for self-cleaning and oil/water separation is reported here. The substrate of the copper mesh was first oxidized by simple immersion in an aqueous solution of 1.0 M NaOH and 0.05 M K2S2O8 at room temperature for 30 min, which was then covered with micro- and nanoscale Cu(OH)2 on the surface. Subsequently, the oxidized copper mesh was immersed in 2 × 10(-4) M octadecylphosphonic acid/tetrahydrofuran (ODPA/THF) solution, an ODPA coating formed on the oxidised copper mesh. The ODPA coating formation process takes place rapidly, almost in 1 second, which makes the as-prepared mesh exhibit superhydrophobicity with the water contact angle of approximately 158.9° and superoleophilicity with the oil contact angle of 0°. Moreover, the as-prepared mesh has self-cleaning effect and can be repeatedly used to efficiently separate a series of oil/water mixtures like gasoline/water and diesel/water. Interestingly, straightforward oxidation of a copper substrate produces a "water-removing" type oil/water separation mesh with underwater superoleophobicity, whereas ODPA coating on the oxidized copper mesh produces an "oil-removing" type oil/water separation mesh with superhydrophobicity and superoleophilicity. This interesting conversion results from a small amount of ODPA and takes place very rapidly.