# UV-Polymerized Vinylimidazol Ionic Liquids for Permselective Membranes
## 3 Results and Discussion
### AFM Measurements  
Atomic force microscopy (AFM) was used to determine the membrane surface morphology on the micro- and nano-scale. Figure 5 summarizes the key observations for three samples, of which two were polymer layers with only one ionic liquid used in the formation process (PILs B and D) and one containing two ionic liquid monomers in the polymer (PILs BD2) after the filtration of pure water. All surfaces are rather flat on a larger scale (\(\geq\)10 \(\upmu\)m) with height corrugations of a very few 100 nm or below, but they exhibit different morphologic features on a smaller scale. On PILs B (Figure 5a,b), large depressions with a typical lateral size of about 1 \(\upmu\)m are visible. Their apparent depth is about 150-200 nm, but this may be limited by the tip geometry of the AFM cantilever. Thus, these numbers represent a lower limit. Additionally, a grainy fine structure is visible with typical heights of 30-50 nm (Figure 5b). On PILs BD2 (Figure 5c), no pronounced holes are visible. Apart from a few grooves, no characteristic features are obvious on the microscale. On closer inspection (Figure 5d), again, a grainy structure is observed, but here the height corrugation is of the order of 10 nm or smaller. No pronounced holes are visible in the investigated area. At first glance, sample PILs D (Figure 5e) looks similar to PILs BD2, apart from the lower corrugation and a lower density of grooves. Even the appearance of the fine structure (Figure 5f) is similar to PILs BD2, but there are important differences: firstly, the corrugation height is considerably smaller, which is obvious from the comparison of line profiles (Figure 5d,f, inset); secondly, on PILs BD2, rather deep holes are visible (dark spots in Figure 5f and minima in the line profile, green curve) which are absent or at least much smaller on PILs D. Their depth cannot be directly quantified due to the tip geometry, but a lower limit is about 30 nm. Nevertheless, different morphologies of PIL layers were pictured, and AFM measurements of the glass support excluded the possibility that the surface was shaped by the plate itself during the casting process. Comparing AFM and flux data, an interesting correlation is found between the presence and size of surface holes and the measured permeance (compare Table 1): larger and more holes correspond to a higher flux. Thus, we tentatively assign the dominant liquid flux channels to the presence of hollow channels between the grains which are visible as holes in AFM topographies. This explains the observed flux data based on the membranes' nanomorphology and provides a way to inspect and prescreen membranes without the need for elaborating flux measurements. However, the high density of the observed holes also results in less mechanically stable membranes with low retentions of charged sugars; thus, a mixture of IL monomers proved to be the optimal approach. Figure 6 shows the surface morphology of a membrane (PILs BD2) used for a filtration experiment of calcium gluconate similar to that shown in Figure 3. Large lengthy objects are visible which bear resemblance to fibers or fiber bundles. Their height is about a hundred times larger than the corresponding surface corrugation of the unused membrane (Figure 5c,d). Hence, this surface is presumably dominated by filtration residues which may have been formed by aggregation of molecules from the sugar solution. Figure 5: Morphology of membrane surfaces (at top sides) obtained by atomic force microscopy (AFM). Bright (dark) colors correspond to large (small) heights. The left column (**a**,**c**,**e**) contains overview images, while the right column (**b**,**d**,**f**) shows higher resolved images of the fine structure of three samples, i.e., (**a**,**b**) PILs B, (**c**,**d**) PILs BD2, (**e**,**f**) PILs D. Line profiles are shown as insets, taken at locations indicated by straight lines in the images (**b**,**d**,**f**).