## 1 Introduction  
Membrane distillation (MD) is an emerging non-isothermal membrane separation process, being one of the promising techniques for the desalination of highly saline waters [1; 2; 3; 4; 5; 6; 7]. Contrary to pressure-driven techniques, the driving force in the membrane distillation process is related to the difference in chemical potential (generated by difference of temperatures) between the two sides of the hydrophobic membrane [8; 9; 10]. The non-wetted porous hydrophobic membranes are required for an efficient process realization. There are various MD modes utilized, for example, sweep gas membrane distillation (SGMD), direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), vacuum membrane distillation (VMD) [1; 5; 9; 11]. MD can be used in different applications such as waste water treatment, desalination, and food processing [1; 3; 5; 11; 12; 13; 14; 15; 16; 17; 18; 19]. In all configurations, the liquid-vapor equilibrium is the determining factor yielding to the selectivity of the MD processes. The mass transfer in MD follows three subsequent steps: liquid-vapor phase transition at the membrane pores entrance on feed side; transfer of vapors through the pores of the membrane; and condensation of vapors on the permeate side of the membrane [1; 2; 3; 4; 5; 6; 7]. Despite the fact that, in the MD process, high pressure is not required and the process offers a very promising performance for both the stand-alone and the desalination process, full-scale commercialization of MD still copes with various problems. These difficulties are related to the lack of suitable and effective module design, proper membranes, as well as intensive energy consumption (in cases when waste heat, solar, or different alternative energy sources are not utilized). In the presented article, the authors address the issues related to the lack of suitable membranes. The major requirements for the membrane features are related to hydrophobic character and porous structure [20]. During the MD process only vapors of solvent (water) are allowed to pass across the membrane. Concerning scientific literature focused on MD application, it is possible to find application of commercially-available hollow fiber or flat-sheet microfiltration membranes [16; 21; 22; 23; 24; 25; 26; 27; 28; 29]. The utilization of these membranes in MD is associated with their hydrophobic properties, decent porosity, as well as adequate pore sizes. Nevertheless, these membranes are not the perfectly-designed and dedicated to membrane distillation, therefore they suffer from wetting problems as well as low permeability and short life span [9; 21; 22; 23; 24; 25; 26; 30]. To design and form hydrophobic surfaces with certain properties, two approaches can be selected. One possibility is the creation of a rough surface with a structure responsible for its hydrophobicity. By generating heterogeneity on the surface, it is possible to form highly hydrophobic surfaces due to generation of pillars and the presence of air pockets on the surface. The other way is utilization of chemical modification applying grafting agent materials possessing the low surface free energy [31; 32]. Up till now, a number of methods have been developed to produce rough surfaces, like solidification of melted alkylketene dimer (AKD) [33], plasma polymerization/etching of polypropylene in the presence of polytetrafluoroethylene (PTFE) [34], microwave plasma-enhanced chemical vapor deposition (MWPE-CVD) of trimethylmethoxysilane (TMMOS) [35], anodic oxidization of aluminum [36], or an immersion of porous alumina gel films into boiling water [37]. Chemically attached coupling agents with low-surface-energy can be also efficiently used to turn the hydrophilic character materials to hydrophobic one [38; 39; 40; 41]. Within that group of compounds, perfluoroalkylsilane (PFAS) molecules [42; 43; 44; 45], hydrophobic polymers [46], as well as Grignard compounds [47; 48] have been found to generate hydrophobic surfaces.