Influence of substitutional defects in ZIF-8 membranes on reverse osmosis desalination: A molecular dynamics study

In this study, molecular dynamics simulation is used to investigate the effects of water-based substitutional defects in Zeolitic imidazolate frameworks (ZIF)-8 membranes on their reverse osmosis (RO) desalination performance. ZIF-8 unit cells containing up to three defect sites are used to construct the ZIF-8 membranes. These substitutional defects can either be Zn defects or linker defects. The RO performance of the membranes is tested in terms of the water flux and ion rejection rate. The effects of defects on the interactions between the ZIF-8 membranes and NaCl are investigated and explained by the radial distribution function (RDF) and ion density distribution. The results show that ion adsorption on the membranes occurs at either the nitrogen atoms or the defect sites depending on the number of defect sites. Complete ion rejection can be achieved for NaCl in the starting period by introducing defects to change the size of the aperture and cavity diameters, rendering it harder for the ions to pass through the membranes by the size exclusion effect. It is also discovered that the presence of linker defects increases membrane hydrophilicity, which reduces water flux due to the increase in water interactions at the defect sites. Overall, molecular dynamics simulations were used in this study to show that water-based substitutional defects in the ZIF-8 structure reduce the water flux and influence its hydrophilicity and ion adsorption performance, which is useful in predicting the type and number of defect sites required for RO applications. Of the seven ZIF-8s tested, pristine ZIF-8 performs the best in the starting period.