Unexpected excellent under-oil superhydrophilicity of poly(2-(dimethylamino)ethyl methacrylate) for water capture from oil and water-induced oil self-dewetting

Abstract

The anti-oil-fouling property of cross-linked poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) was investigated by testing not only the under-oil superhydrophilicity but also the dewetting ability of oil that adheres on PDMAEMA coating surface. Different to other common hydrophilic polymers (e.g., polyacrylamide (PAM) and poly(methacrylic acid) (PMAA)), PDMAEMA coating shows the unique under-oil superhydrophilicity (wetting time <3 s for a 6 μL water drop), and even more interestingly, the oil that adheres on dry PDMAEMA coating surface can spontaneously dewet and detach off from the surface when immersed in water. The PDMAEMA coating shows lower surface energy (γs) (53.3 mN m−1) than PAM and PMAA (70.5 and 61.1 mN m−1, respectively) and moderate zeta potential (69 mV; 29 and −148 mV for PAM and PMAA, respectively), while it has much higher moisture absorption rate (∼16%) than PAM (∼6%) and PMAA (∼3%) coatings. The overall test results suggest that PDMAEMA coating has stronger hydration ability than PAM and PMAA coatings, and meanwhile, the hydrated water molecules in the coating can weaken the interaction of the PDMAEMA with oil, thus inducing the under-oil superhydrophilicity. Moreover, it is demonstrated that dry PDMAEMA coating can capture water drops from bulk oil, and thus the PDMAEMA coating can be utilized to prepare filtration membrane for the separation of surfactant-stabilized water-in-oil emulsions with high separation efficiency (>99.0%) and long-lasting stability.