Design of near-superhydrophobic/superoleophilic PVDF and PP membranes for the gravity-driven breaking of water-in-oil emulsions

•Preparation of a set of 6 membranes with various surface properties.•Investigation of the oil/water wetting behavior in the three-phase interfaces.•Discussion on the parameters influencing the hydrophobicity/oleophilicity balance.•Separation of various water-in-oil emulsions under the action of gravity.•Selection of the VIPS PVDF membrane as the best (rate/efficiency) membrane. This study discusses the application of (i) polypropylene (PP) membranes, (ii) polyvinylidene fluoride (PVDF) membranes prepared by different phase separation processes, and (iii) CF4/CH4 plasma treated PP/PVDF membranes in gravity-driven filtration of water-in-oil (W/O) emulsions. The concept of hydrophobicity/superhydrophobicity depends on the dispersing medium (air vs. oil) and so does that of oleophilicity/superoleophilicity (air vs. water). The chemical contribution to the hydrophobicity/oleophilicity balance, obtained from an XPS analysis of CF and CH spectral area ratio, unveils that PP membrane plasma treated by CF4 (PP-CF4) would be the worst matrix for efficient filtration. Yet, it does not present the highest water contact angle in air/oil and often showed excellent wetting behavior by oil (in air/water) such that filtration results contradict this analysis. Thus, the physical contribution to the hydrophobicity/oleophilicity balance plays a major role. Gravity driven breaking-up of W/O was analyzed for all six membranes and four W/O emulsions. Based on this analysis, PVDF membranes prepared by vapor-induced phase separation showed an ideal combination of hydrophobicity in oil and oleophilicity in water arising from adequate chemical composition and physical structure, leading to fast separation (initial flux ranging between 220 and 3000kgm2h−1), still maintaining very low water content in permeate (ranging between 0.003 and 0.4wt%) and high separation efficiency (99.54–99.98%). [Display omitted]