## 3 Results
### Structure of the Coating  
Apart from the contact angle measurements, another important issue is the possibility that the coating will block the pores. This reduces the porosity and pore size, and in the ultimate case, a dense membrane is obtained, obstructing the mass transport. Pore blockage can easily be seen by porometry because it reduces both the gas flow through the membrane and the pore size compared to the untreated membrane. For membranes 1-3 and 5-7, the pore sizes and the gas flows of the uncoated and coated membranes are equal. As an example, the pore size distribution obtained using porometry is presented for an untreated membrane and membrane 3 and 4 are given in Figure 7. For membrane 4, coated with a 30 wt. % solution, no pores are detected using porometry, indicating that the pores are completely blocked by the coating in this case. These measurements reveal that a 30% solution is not suitable for membrane modification since it significantly affects the porosity.  
The position of the coating is investigated using energy-dispersive X-ray spectroscopy (EDX). As an example, Figure 8 shows the EDX spectrum of membrane 3. The peaks of the oxygen and sulfur atoms in the spectrum correlate with the presence of the PES membrane material or pores at the measured position. In the first 20 um at surface 1, a first increase of the silicon and fluorine atoms is observed, which correlates with the position of the coating. A second increase is observed between 30 and 50 um, which is also the densest zone of the membrane (Table 3) and therefore, more surface is  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Membrane**} & \multirow{2}{*}{**Coated Side**} & \multicolumn{2}{c}{**\(\theta\) (\({}^{\circ}\))**} & \multirow{2}{*}{_LEP (bar)_} \\ \cline{3-3}  & & **Surface 1** & & **Surface 2** \\ \hline
6 & Surface side 1 only & 97 \(\pm\) 1 & 41 \(\pm\) 3 \({}^{\ast}\) & 1.8 \(\pm\) 0.2 \\
7 & Both sides & 102 \(\pm\) 1 & 107 \(\pm\) 1 & 2.2 \(\pm\) 0.1 \\ \hline \hline \end{tabular} \({}^{\ast}\) Droplet wets the membrane within 2 min.  
\end{table}
Table 6: Contact angle and LEP of the coatings applied by the spray system.  
Figure 7: Pore size distribution untreated PES-membrane, membrane 3, and membrane 4 using porometry.  
available to deposit the coating. In the part between 50 and 120 \(\upmu\)m, the silicon and fluorine content is lower. The permeation of the coating might be inhibited by the denser structure of the membrane. Further in the membrane cross-section, the silicon and fluorine content decreases, but are not equal to zero. The increase of the oxygen and silicon at 120 to 140 \(\upmu\)m is caused by the silicon glue used to fix the sample. Based on the EDX on the chemical composition, the structure is hydrophobic until a depth of at least 50 \(\upmu\)m. Unfortunately, it is impossible to indicate the exact hydrophobicity or hydrophilicity in terms of a water contact angle at a certain point of the membrane cross-section based on the elemental composition obtained with EDX. Therefore, the exact hydrophobic thickness cannot be derived from these EDX figures.  
In summary, these results indicate that no difference in membrane structure in terms of thickness, pore size, and porosity was found for the coatings applied using less than 10 wt. % silanes in the coating solution. Only coating 4, with very high load of silanes, the MD-flux and N2 flux during porometry decreased to zero, showed a strong difference, which is a strong indication for pore blocking.