## 3 Results and Discussion
### Characterization of the Membranes
#### 3.2.4 Sem  
Figure 5 shows the effect of various concentrations of DES-lipin (0-1 wt %) on the cross-section of the resultant membranes. The cross-section SEM images are represented in three magnifications (100, 50, and 10 \(\upmu\)m) for better visualization. In general, all the fabricated membranes possess an asymmetric structure including a thin top skin layer, a porous middle sub-layer, and a bottom sub-layer. As can be seen in the SEM images, the sub-layers appear to possess cavities and morphologically different finger-like macrovoids in their structures. Kinetic hindrance and thermodynamic enhancement are of two most prominent factors which influence the ultimate morphology of membrane substantially, which in turn alters the membrane performance. In the case of the overall viscosity of the dope solution, no significant differences were found between the pure membrane and the modified ones, and it can thus be postulated that the kinetic effect of viscosity compared to the thermodynamic enhancement effect is minor. In general, the addition of non-solvent polymer additives in the initial casting solution reduces the thermodynamic miscibility of the casting solution with a non-solvent additive (i.e., increase of thermodynamic instability of the dope solution) and thus accelerates the diffusional exchange rate between the solvent (NMP:DMF) and the non-solvent (water). Furthermore, the presence of DES-lipin on the surface of the membrane increases the hydrophilicity of the membrane, which in turn enhances the diffusion of water as non-solvent into the cast film, and thereby a more open sub-layer can be formed in the membrane structure. As can be seen in the figures, the size and length of finger-like microvoids and cavities increases as the DES-lipin dosage increases in the dope solution. However, although the surface porosity and pore size distribution play determining roles in the membrane performance, the bulk porosity can also change the membrane permeability. Image] software (National  
Figure 4: The effect of different wt % of lignin as an additive on the surface charge of modified membranes.  
Institutes of Health, Bethesda, MD, USA) was also used as a tool to estimate the bulk porosity of the membrane structure (cross-section) [51]. As can be seen in Table 1, the results of image analysis corroborate the increase of bulk porosity and the flux of fabricated membranes as a result of successive increase of DES-lignin in the dope solution. An increase of the bulk porosity of the membrane sub-layer as a result of incorporated alkali lignin into a polysulfone membrane has also been reported [16]. The presence of some trapped particles (0.2-0.5 \(\upmu\)m) in the membrane matrix can be clearly seen in Figure 5 (zoom section of the top layer). This might be attributed to the precipitation of highly localized concentrated PES as a consequence of crystallization. The same crystallization-related precipitation conducts have been observed by [PERSON] et al. [52] for PVDF membranes. In fact, at a lower temperature, gelation prompted by crystallization arises first in the membrane formation processes and thus the crystals can grow as a result of delayed demixing [52].  
Figure 5: _Cont._