Diffusion behaviors of ethanol and water through g-C3N4-based membranes: Insights from molecular dynamics simulation

Graphitic carbon nitride (g-C3N4) with a layered lamellar structure was successively used to fabricate highly water-selective membranes via hybrid or free-standing methods. Molecular dynamics (MD) simulations were carried out to study the diffusion behaviors of water and ethanol molecules through pristine g-C3N4 (Pri-g-C3N4) and benzene ring-doped g-C3N4 (Ben-g-C3N4) membranes. MD results indicated that hydrogen bonds formed between water molecules and Pri-g-C3N4 or Ben-g-C3N4 nanosheets affected the permeation of water across these nanosheets. Interestingly, the simulation analysis showed that the lamellar structures of Pri-g-C3N4 nanosheets played a significant role in the separation of ethanol and water. The diffusion coefficients of water and ethanol through Pri-g-C3N4 lamellae were 2.24 x 10(-5) cm(2) s(-1) and 1.04 x 10(-5) cm(2) s(-1), respectively. Besides, the movement behaviors of water and ethanol molecules between lamellae of the sheet-shaped g-C3N4 (pristine g-C3N4, Pri-g-C3N4) and distorted-type carbon nitride g-C3N4, i.e., benzene ring-doped into g-C3N4 (Ben-g-C3N4) were studied in detail. For the different configurations of g-C3N4, pure water flow was slower in Pri-g-C3N4 confinement compared to Ben-g-C3N4, as the lifetime of hydrogen bonds between water molecules in Ben-g-C3N4 was shorter than that in Pri-g-C3N4. Importantly, for ethanol/water binary mixtures, both Pri-g-C3N4 and Ben-g-C3N4 surfaces preferentially adsorb ethanol, where the hydroxyl group of ethanol is closer to the membrane surface. The observed interesting feature suggested that the lamellar structures of Pri-g-C3N4 (or Ben-g-C3N4) nanosheets were efficient in the separation of ethanol and water.