Under-Oil Superhydrophilic TiO2/Poly(sodium vinylphosphonate) Nanocomposite for the Separation of Water from Oil

Abstract Under-water superoleophobic filters lose their water separation capability if they are contaminated with oil. In contrast, an oil contaminant in a filter that is both superoleophobic underwater and superhydrophilic underoil can be readily displaced with water. Even after contamination by oil, such a filter can self-clean in water and re-create and maintain an integral water film in the pores to enable sustained water separation. Unfortunately, there have been only a few reported examples of under-oil superhydrophilic materials. We report herein a novel under-oil superhydrophilic material consisting of TiO2 nanoparticles that are coated by an excess of a sodium salt of poly(vinylphosphonic acid) (PVPA.nNa). This nanocomposite can be applied as a coating onto various substrates such as glass plate, glass wool, cotton ball, and stainless-steel metal mesh for permanent self-cleaning and hexadecane collection. More interestingly, a coated cotton ball could be affixed to the bottom end of a standing tube and allows the selective permeation of water from a stirred water/hexadecane mixture. Unfortunately, a coated metal mesh with larger pores and thus much higher water flux could not achieve the separation most likely because the weakness of the water film in the mesh pores induces a negative value of Laplace pressure. We achieve this separation for the coated metal mesh by attaching a thin electrospun superhydrophobic mat onto its lower side to thus yield a Janus filter. Our assumption is that the unidirectional upward water-pulling force of the Janus filter in this case helps to stabilize the water film in the metal mesh. Due to the facile synthesis of TiO2-PVPA·nNa and the demonstrated applications of the TiO2-PVPA·nNa coating, this study should inspire further research in high-flux filters for water separation from oil.