## 1 Introduction  
Membrane distillation (MD) is a thermal separation technique using a hydrophobic microporous membrane as a contactor between two liquid phases. The membrane allows vapors (e.g., water vapor) to permeate, whereas the liquid phase including the dissolved components (e.g., salts) is retained by the membrane. A temperature difference induces the driving force and allows vapors to permeate from the hot feed side to the cold permeate side. The technique was initially proposed as an alternative technology for reverse osmosis in seawater desalination. However, due to the benefits of very high retentions and less dependence on salinity, it is recently also proposed for applications beyond the scope of reverse osmosis. The applications can include but are not limited to desalination and brine treatment [1], waste water treatment [2; 3], and resource recovery [4], where the dissolved components can be salts, proteins [5], acids [2; 6], and minerals [4]. Currently, hydrophobic microfiltration membranes are used in membrane distillation, although these membranes are not optimized for the MD process [7; 8]. The specific requirements for membrane distillation membranes are described in literature [9; 10; 11]. Most importantly, the membrane must consist of at least one layer that is not wetted by the liquid stream under the operational pressures used in the module. The minimum pressure required to wet a hydrophobic membrane is the liquid entry pressure (\(LEP\)), which depends on the membrane characteristics as well as on the feed composition and is defined by the following equation:  
\[LEP = \frac{-2B\gamma_{l}\text{cos}(\theta)}{r_{max}} \tag{1}\]  
where \(\gamma_{l}\) is the surface tension (N\(\cdot\)m\({}^{-1}\)) of the liquid, \(\theta\) the contact angle (\({}^{\circ}\)), \(r_{max}\) the maximum pore size (um), and \(B\) is a geometric factor. To ensure proper operation under fluctuating pressures and temperatures, an \(LEP\) of 2.5 bar is required [12]. To achieve a sufficient \(LEP\), membranes with maximum pore diameter between 0.1 and 1 um with a water contact angle above 90\({}^{\circ}\) are recommended for membrane distillation [9; 13; 14]. Moreover, it is generally agreed that a high membrane porosity is one of the most important membrane parameters in membrane distillation for both flux and energy efficiency, regardless of the MD configuration [15; 16; 17; 18; 19]. Additionally, membranes with a thickness between 30 up to 60 um are recommended; however, it was shown recently that this optimal value depends on salinity, and at high salinity, thicker membranes are preferred [20]. Currently, most commercial systems use membranes not specifically developed for membrane distillation (i.e., hydrophobic polyethylene (PE), polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) microfiltration membranes). However, in the literature, many efforts are described to improve the membrane performance. These efforts include the optimization of the phase inversion process, mainly using the hydrophobic polyvinylidene fluoride (PVDF) or poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) as polymer [21; 22; 23; 24] and the use of surface modifying macromolecules [25; 26; 27] and electrospinning [28; 29]. In addition to the optimization of the membrane structure, research is also oriented toward the enhancement of the surface properties for membrane distillation, including but not limited to plasma treatment [30; 31], fluorination of a TiO\({}_{2}\) coating [32], or the use of fluoroalkylsilane coatings on Tunisian Clay membranes [33]. Currently, these coatings are only applied in lab scale experiments and are not yet commercially available. In this publication, the use of hydrophobic sol-gel coatings forming an organic-inorganic network on hydrophilic polyethersulfone (PES) membranes is presented.