Conditions for spontaneous oil–water separation with oil–water separators
Separating oil and water from oil-in-water or water-in-oil systems is immensely important. Such separation is typically achieved through specially fabricated surfaces that demonstrate distinct differences in wettability between water and the specified oil; in other words, these surfaces are either hydrophobic–oleophilic or hydrophilic–oleophobic. While this difference is sufficient to trigger the separation – and this has been the fundamental basis of preparing all surfaces meant for oil–water separation – the spontaneity of the separation process is not a single-valued function of this difference. Considering the free energy change associated with the separation process as an indicator of the extent of the spontaneity of the separation, we show that this spontaneity is additionally dictated by the oil–water interfacial tension. More intriguingly our analysis establishes that for a hydrophobic–oleophilic surface, the separation of oil from oil-in-water systems is more spontaneous than the separation of water from water-in-oil systems, whereas for a hydrophilic–oleophobic surface the separation of oil from oil-in-water systems is less spontaneous than the separation of water from water-in-oil systems.