A systematic study of the impact of hydrophobicity on the wetting of MD membranes

In membrane distillation desalination (MD), a hydrophobic membrane acts as a barrier between feed and distillate solutions. MD membranes are often fabricated using a composite method: a hydrophilic or mildly hydrophobic base membrane is coated with a highly hydrophobic material to produce a membrane with adequate water liquid entry pressure (LEP_w) for MD. This composite method increases the range of materials that can be used for MD membranes. However, environmental safety concerns about the hydrophobic coating materials persist. In this study we systematically quantify the relationship between the hydrophobicity of the coating material (its contact angle with water) and the LEP_w of the coated MD membrane. This relationship determines the conditions when lower-hydrophobicity, more environmentally-friendly coating chemistries can be used to prepare successful MD membranes. For the membranes in this study, we found that for coating materials with intrinsic advancing contact angles (ACA) not much greater than 90°, water liquid entry pressure (LEP_w) was in a range suitable for MD. We explained this result with a new model for liquid entry pressure (LEP). This work predicted that the hydrocarbon, poly(divinyl benzene) (pDVB), an environmentally-friendly but low-hydrophobicity polymer, could be a successful coating chemistry for MD membranes. We thus fabricated a nylon-pDVB composite membrane and demonstrated it in an MD desalination system.