# UV-Polymerized Vinylimidazol Ionic Liquids for Permselective Membranes
## 3 Results and Discussion
### Separation Experiments
The PIL-containing layer detached from the polyethylene plate, which was used as spacer. After establishing the filtration again, the same results as before were achieved (185-200 h). A continuous procedure overcame this drawback of the setup and confirmed the previous observations (205 to 280 h). Due to the low starting concentration of about 10.65 mol-L-1, retention of 95% was obtained. Nevertheless, the precedent linear enrichment of the sugar to 18.45 mol-L-1 reduced the retention to 74%. Next, a steady concentration of approximately 11 mol-L-1 was used in a continuous experiment. Therefore, the volume of the filtrate was replenished to the cell in the form of pure water. Between 270 and 400 operating hours, the concentration of calcium gluconate in the vessel was constant at 11.3 \(\pm\) 0.3 mol-L-1. Simultaneously, the retention was above 83% and comparable to the retention obtained at the same concentrations in previous experiments. The permeance was unaffected by adding water at about 0.0535 L-m-2-h-1-bar-1. By lowering the concentration to 7.7 mmol-L-1, the retention increased to 87% and was constant. The retention is, as previously described, dependent  
Figure 3: Retentate (blue triangles), filtrate (orange circles) and retention (green squares) in dead-end (90–270 h) and diafiltration experiments (270–490 h) using PILs BD2. Conditions: \(d_{\text{membrane}}\) = 43.35 mm, \(p_{\text{air}}\) = 6 bar, Schleicher&Schuell stirred dead-end cell, \(n_{\text{TyBuIm}}\)Br = 0.00465 mol, \(n_{\text{TyDodeclm}}\)Br = 0.00155 mol, \(n_{\text{IL}}\):\(n_{\text{ACN}}\):\(n_{\text{Syr}}\) = 1:3.1, 5 wt% TPO, 2 wt% divinyl benzene (DVB) (wt% based on \(m_{\text{IL}}\)), 0.5 h ultrasonic bath, \(h_{\text{gap}}\) = 300 \(\upmu\)m, 0.5 h UV lamp, 24 h EtOH, 24 h H\({}_{2}\)O.  
on the retenate concentration. Lower retenate concentration results in higher rejections of calcium gluconate, while higher concentrations immediately impair the retention. Sodium gluconate provides a smaller molecular weight and the same structure as calcium gluconate. The experiment was started with a 0.025 mol\(\cdot\)L\({}^{-1}\) solution of sodium gluconate and the PIL BD2 membrane (Figure 4). The retention was about 50% higher for over 60 h with a permeance of 0.048 L\(\cdot\)m\({}^{-2}\cdot\)h\({}^{-1}\cdot\)bar\({}^{-1}\). Even with increased retenate concentration, no decreasing retention was observed when compared to that of the calcium gluconate (Figure 3). Equally, the retention was dependent on the initial feed concentration. By decreasing the initial feed concentration, the retention increased (90-120 h, Figure 4) and was only 20% lower than when using the calcium gluconate. The comparison with neutral sugars showed that the charge has a significant influence on the separation performance. d-Glucose, p-mannitol, sucrose and raffinose were chosen because of their different structure and molecular weights. The retentions are summarized in Table 2. Further experimental details are presented in the SI. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sugar** & \(M\)**/g\(\cdot\)mol\({}^{-1}\)** & **Charge** & **R** \\ \hline K-gluconate & 234.25 & yes & 25\% \\ Na-gluconate & 218.14 & yes & 53\% \\ Ca-gluconate & 448.39 & yes & 75\% \\ d-glucose & 180.16 & no & 3\% \\ d-mannitol & 182.17 & no & 6\% \\ sucrose & 342.30 & no & 14\% \\ Raffinose & 504.44 & no & 23\% \\ \hline \hline \end{tabular} Conditions: \(c_{\text{sugar}}=0.025\) mol\(\cdot\)L\({}^{-1}\), \(d_{\text{membrane}}=43.53\) mm, \(p_{\text{air}}=6\) bar, Schleicher&Schuell stirred dead-end cell, \(n_{\text{IL}}=0.0031\) mol, \(n_{\text{IL}}\cdot\)_ACN\(;n_{\text{SHF}}=1:3.1\), 5 wt\(\%\) PI, 2 wt\(\%\) CL (wt\(\%\) based on \(n_{\text{IL}}\)), 0.5 h ultrasonic bath, \(h_{\text{SPP}}=300\) μm, 0.5 h UV lamp, 24 h EtOH, 24 h \(H_{\text{D}}\).  
\end{table} 
Table 2: Retention \(R\) of the sugars tested with PILs BD2. The retention increased from mono- to tri-saccharide to about 23%. This was much lower for the charged carbohydrates.