# UV-Polymerized Vinylimidazol Ionic Liquids for Permselective Membranes
## 3 Results and Discussion
### Separation Experiments  
After the characterization of the different PIL-containing layers, a separation experiment was conducted to determine the efficiency of the membranes. Calcium gluconate, a charged sugar derivate, was used as a test substrate in an aqueous solution. A 0.025 mol\(\cdot\)L\({}^{-1}\) solution was used initially to avoid the precipitation of the sugar when overcoming the solubility limit (\(>\)0.069 mol\(\cdot\)L\({}^{-1}\)). As the polymeric layers containing D monomer provide high mechanical stability, a mixture of 95 mol\(\%\) B and 5 mol\(\%\) D (PILs BD1) was used. Overall, PILs BD1 was not suitable for the filtration of calcium gluconate, as the retention collapsed after 16 h (additional data and graphics in SI). The concentration of the sugar in the filtrate was increasing from the beginning. The permeance showed that after five hours, which could be ascribed to conditioning, a constant flow through the membrane was obtained. Furthermore, the concentrations and retention were constant after this time. More than 70% of the calcium gluconate was retained. Predictably, the concentration in the retentate increased over time. The permeance was 60% lower than in previous experiments. As the concentration in the filtrate increased significantly after twelve hours, a rupture of the membrane was assumed, which was confirmed by the inspection of the membrane after completing the experiment. When increasing the amount of D (PILs BD2) in the membrane formation, higher mechanical stability should affect the long-term stability positively. Figure 3 presents some of the results of the filtration experiments. The complete diagram, including all experimental data, is shown in the Supplementary Materials. \begin{table}
\begin{tabular}{c c} \hline \hline
**Membrane** & \(\mathbf{L_{\text{H2O}}/L\ m^{-2}\ h^{-1}\ bar^{-1}}\) \\ \hline Ultracell® 30 kDa & 193.4 \(\pm\) 10.92 \\ PILs B & 74.09 \(\pm\) 8.63 \\ Ultracell® 5 kDa & 12.24 \(\pm\) 0.57 \\ PILs C & 5.160 \(\pm\) 0.53 \\ DuraMem® 200 & 1.880 \(\pm\) 0.06 \\ PILs D & 0.027 \(\pm\) 0.0021 \\ PILs BD1 & 0.291 \(\pm\) 0.0002 \\ PILs BD2 & 0.259 \(\pm\) 0.0041 \\ PILs BD3 & 0.051 \(\pm\) 0.0025 a  \\ \hline \hline \end{tabular} Conditions: \(d_{\text{membrane}}\) = 43.53 mm, Schleicher&Schuell stirred dead-end cell, \(n_{\text{IL}}\) = 0.0031 mol, \(n_{\text{IL}}\):\(n_{\text{ACCN}}\):\(n_{\text{SQT}}\) = 1:3:1, 5 wt% PI, 2 wt% CL (wt% based on \(n_{\text{IL}}\)), 0.5 h ultrasonic bath, \(h_{\text{gap}}\) = 300 \(\upmu\)m, 0.5 h UV lamp, 24 h EtOH, 24 h H\({}_{2}\)O.  
\end{table} 
Table 1: Overview of permeance \(L_{\text{H2O}}\) of different polymerized ionic liquid (PIL)-containing and commercial membranes. After a short time, in the beginning, the permeance was constant at approximately 0.05 L-m-2-h-1-bar-1. The retention \(R\) was about 75%. Compared to the pure water flux (Table 1), the permeance was reduced by more than 85% and displayed a stronger decrease than in other experiments. Within the filtration, the concentration \(c_{\text{sample}}\) increased slightly, while other parameters remained constant. This increase corresponds to the growing concentration \(c_{\text{cell}}\) in the filtration cell. It is assumed that a high concentration of calcium gluconate in the cell reduces the efficiency of the filtration process. Originating from a lower concentration \(c_{\text{cell}}\) with 14.45 mol-L-1 (after 90 h), this observation was investigated more in detail. In over 100 h of filtration, both, the \(c_{\text{cell}}\) and \(c_{\text{sample}}\) increased equally. Higher retentate concentrations decreased the efficiency from about 84% to less than 40%. Here, the increase in calcium gluconate in the permeate seems to be linear and directly dependent on the concentration of the retentate. In addition, at high concentrations, the permeance is slightly decreased by about 10%. The fluctuation in retention, concentration and permeance trace back to the experimental setup and an interruption overnight.