## 3 Results
### Membrane Distillation Performance  
The flux and energy efficiency of the membranes produced in this study are compared to the PVDF GVHP membrane from Millipore commonly used in the literature and to a PE membrane currently used in pilot scale membrane distillation modules (Table 2). All coated membranes are coated on the same base membrane structure, except for membrane 4, where pore blocking occurs. The other membranes have equal porosity, pore size, and total thickness. However, the position of the coating and hence the thickness of the hydrophobic layer can be different. Since this thickness can affect the flux and energy efficiency of the membrane for desalination applications [20], a difference in MD performance is expected. Flux and energy efficiencies of the different membranes are summarized in Table 7.  
Membranes 1 and 2 are single component systems without organic network formation and show immediate breakthrough of the salts. The membrane wetting is also visually observed after demounting the module. This indicates that the inorganic network formed by these coatings is not sufficient to provide stable coatings for the process conditions used in membrane distillation. Figure 8: EDX (energy-dispersive X-ray spectroscopy) spectra of the cross section for membrane 3. Membranes 3 and 4 use the same components, but with a difference in mass fraction of 5 wt. % and 30 wt. % respectively (Table 1). Membrane 3 shows a much higher flux of 16.2 kg\(\cdot\)m\({}^{-2}\cdot\)h\({}^{-1}\) compared to membrane 4 with a flux of only 0.4 kg\(\cdot\)m\({}^{-2}\cdot\)h\({}^{-1}\). This low flux of membrane 4 is caused by the pore blocking shown in Figure 7, which hinders the transport of water vapor through the membrane. This shows that a mass fraction of 5 wt. % silanes in the coating solutions is balancing between a sufficient hydrophobicity and avoiding obstruction of the pores, which occurred at concentrations of 30 wt. %. The hydrophobic/hydrophilic membranes 3 and 6 show higher fluxes of about 16 kg\(\cdot\)m\({}^{-2}\cdot\)h\({}^{-1}\) compared to membranes 5 and 7 with a flux of about 14 kg\(\cdot\)m\({}^{-2}\cdot\)h\({}^{-1}\), which have two hydrophobic sides (Table 7). The effect of partial pore wetting on flux and heat transfer is comprehensively described in reference [47], where it was shown that an increase of the depth of pore wetting results in an increase of the flux. This difference in flux is explained by a different in hydrophobic non-wetted membrane thickness, which imposes the mass transport resistance for vapor transport. As discussed in the literature, the optimal hydrophobic thickness ranges from 10 to 60 \(\upmu\)m using 35 g\(\cdot\)L\({}^{-1}\) NaCl [20; 48; 49; 50]. While the membranes with two hydrophobic surfaces (5 and 7) are probably fully hydrophobic or at least they do not contain a wetted part, the cross-section of the hydrophobic/hydrophilic membrane structures (3 and 6) is partially wetted by the permeate liquid on surface side 2. Therefore, the hydrophobic layer thickness is much closer to the optimal values of 10-60 \(\upmu\)m for the hydrophobic/hydrophilic membranes. The fluxes achieved are higher compared to the commercial PVDF membrane, whereas the commercial PE membrane shows a flux slightly higher compared to the membranes with two hydrophobic sides, but lower compared to the membranes with a hydrophobic/hydrophilic structure. The energy efficiency varies from 43% to 55% and is lower compared to the commercial membranes and appears mainly to depend on the membrane base structure and porosity (Section 3.1). PE has the highest porosity and surface porosity and shows the highest energy efficiency as well. The energy efficiency of the PVDF membrane in negatively affected by the lower bulk porosity [14].