## 3 Results and Discussion  
### SEM and Zeta Potential  
The surface morphology of the prepared membranes with different ZIF-8 contents are displayed in Figure 1. The surface of the primary Fujifilm membrane was flat, whereas that of a MPD/TMC membrane (M-1) exhibited the typical ridge-and-valley feature. This phenomenon resulted from the violent reaction between small molecular amine and acyl chloride. With increasing ZIF-8 loading to the surface layer, the membrane showed a more net-like structure with denser and smoother zones. At higher ZIF-8 loadings, especially at 0.08%, more \"cubic\" structures are visible, which represent ZIF-8 nanoparticles that are covered by polyamide. No direct relation between the morphology and the ZIF-8 concentration was observed, indicating that nanoparticles were covered by a continuous PA film. It is believed that interfacial polymerization is a diffusion control process. MPD adheres on the membrane surface and diffuses and reacts with TMC in the organic phase. When no MPD permeates across the barrier layer, the reaction stops. For the monovalent ion exchange membrane that was prepared with ZIF-8 nanoparticles, ZIF-8 deposits on the membrane first and it is embedded under the PA layer, leading to similar SEM images [15]. The elemental weight distribution on the membrane surface was determined by EDAX to confirm the presence of ZIF-8. In Table 1, a low Zn content can be observed. With increasing concentration of ZIF-8 nanoparticles, a higher Zn content can be achieved. The EDAX mapping (Figure 2) demonstrated a uniform distribution of ZIF-8 on the membrane surface. However, the variation of the Zn concentration is not obvious. Thus, the results of SEM and EDAX suggest a successful encapsulation of ZIF-8 nanoparticles. The PA surface structures with various crosslinking degree can be explored by analyzing the element ratios between O and N. For the modified membrane without ZIF-8 nanoparticles, the O/N ratio is about 1.75. It was obvious that no distinct difference can be seen in the O/N ratio with the increase of ZIF-8 concentration. This indicated that the crosslinking degree of MPD/TMC copolymer was maintained as stable, while the ZIF-8 concentration is below 0.08%. The zeta potential of the as-prepared membranes was listed in Table 2. Both the primary ion exchange membrane and the MPD/TMC modified membrane are negatively charged, with zeta potentials of \(-16.4\) and \(-20.3\) mV, respectively. After introducing ZIF-8 into the selective layer, the zeta potential of the modified membrane shifted to \(-22.3\) mV. With a further increasing ZIF-8 content, the zeta potential values continued to decrease. It should be noticed that the slight reduction of zeta potential is not consistent with the O/N ratio results that were obtained from EDAX, which can be explained by the detection depth of EDAX. A detection depth around 1 \(\upmu\)m for EDAX with complex background can reduce the impact of carboxylic acid groups on O/N ratio. Thus, the ZIF-8 loading inside the MPD/TMC surface layer could generate more carboxylic acid groups. Figure 1: Scanning electron microscope (SEM) images of the unmodified and modified ion exchange membrane. Nanoparticle loadings are **(M-1)** 0.00%, **(M-2)** 0.02%, **(M-3)** 0.04%, **(M-4)** 0.06%, and **(M-5)** 0.08% (w/v). The water contact angle measurements also confirmed the presence of ZIF-8 under the PA film (Figure 3). After modification, contact angles increased from 27.6\({}^{\circ}\) to 43.7\({}^{\circ}\). This indicates that the primary commercial cation exchange membranes are highly hydrophilic; by introducing the MPD/TMC layer, the surface becomes more hydrophobic. The addition of a certain content of ZIF-8 nanoparticles to the TFN membranes greatly reduce the surface hydrophilicity, which is in accordance with previous research [28]. Assuming that some ZIF-8 nanoparticles would be bared on the modified layer, water contact angles would increase with the increase of the ZIF-8 nanoparticles content.