# Fooling in Membrane Distillation, Osmotic Distillation and Osmotic Membrane Distillation
## 2 Membrane Characteristics  
The membranes used in MD, OD and OMD must be hydrophobic and porous with pore sizes ranging from some nanometers to few micrometers [2; 19]. Their characteristics such as the thickness, tortuosity, pore size and porosity dictate the resistance to mass transfer in these three processes [20]. Their pore size distributions should be as narrow as possible and the maximum pore size should be small enough to prevent liquid penetration in such pores [2; 19; 21]. The liquid entry pressure (_LEP_), which is the minimum transmembrane pressure required for a liquid or a given feed solution to enter into the pore, is a significant membrane characteristic for MD, OD and OMD. _LEP_ is high for small maximum pore sizes and more hydrophobic membranes [1]. The membrane thickness is inversely proportional to the rate of mass and heat transfer through the membrane. In the case of multi-layered membranes, the hydrophobic layer should be as thin as possible [2; 19]. In order to achieve a high thermal efficiency in MD and OMD, the thermal conductivity of the membrane material should be as low as possible [19]. More details on the properties needed for a membrane to be used in MD are summarized elsewhere [15; 18; 19; 22; 23]. [PERSON], et al. [23] gave a comprehensive overview of the optimal membrane properties, specifically for MD process. The wetting resistance is the key factor considered in the optimization study. The recommended optimal membrane properties were summarized as a pore diameter of 0.3 \(\upmu\)m to balance between a high _LEP_ (preferably >2.5 bar) and a high permeate flux, an optimal membrane thickness between 10 and 700 \(\upmu\)m depending on the process conditions in order to take into account the compensation between mass transport and energy loss, a membrane porosity that should preferably be as high as possible (>75%) in order to improve both the mass transfer and energy efficiency, a pore tortuosity factor that should be as low as possible (1.1-1.2) and a membrane thermal conductivity that should be also as low as possible (>0.06 W\(\cdot\)m\({}^{-1}\cdot\)K\({}^{-1}\)) in order to reduce the heat loss due to the heat transfer by conduction through the membrane matrix. Additionally, it was stated that thinner membranes with a thickness below 60 \(\upmu\)m exhibited low mechanical properties. As it is well known, a way to improve the mechanical properties of a membrane without scarifying its other characteristics is the design of supported membranes using baking materials with a high porosity, a low thickness and a high thermal conductivity. In addition, the feed side of the membrane must be formed by a material of high fouling resistance properties. Different membrane surface modification techniques have been considered such as coating, interfacial polymerization, plasma treatment, etc. [2; 19; 24]. For instance, to avoid wetting and membrane fouling the porous hydrophobic membrane surface needs to be modified by coating a thin layer of a hydrophilic polymer [24; 25; 26]. This was necessary for concentration of oily feeds, because the uncoated membranes were promptly wetted even for low concentrations of oil in water solutions [25]. Recently, the effects of surface energy and its morphology on membrane surface omniphobicity (i.e., membranes resistant to wetting to both water and low surface tension liquids, e.g., oil and alcohols) have been systematically studied by means of wetting resistance evaluation using low surface tension liquids [27; 28]. It was found that the negatively charged nanofibrous membrane fabricated by a blend of poly(vinylidene fluoride-_co_-hexafluoropropylene), PVDF-HFP, and the cationic surfactant benzyltriethylammonium, and then grafted by a negatively charged silica nanoparticles, exhibited excellent wetting resistance against low surface tension aqueous solutions and organic solvents (i.e., mineral oil, decane and ethanol). [PERSON] et al.