Its about a hundred years since my chemistry O level... here goes! I expect it works on a couple of principles based around the different properties of the filtrants - 1. the imiscibility of a polar covalent solvent (water) and a non-polar hydrophobic covalent solvent (aliphatic hydrocarbons in this case) so buzzin is right up to a point - the first principle is to allow the water to drop out to the bottom as it has a higher SG. 2. Low ability of a pure covalent solvent to form a stable suspension with insoluble (principally ionic) matter - grit and crud drops to the bottom and is too massive to pass through the filter anyway 3. The weak inter-molecular bonds (hydrogen bonding) that give water its unique properties to hold it together more strongly than its molecular structure indicates - and gives rise to- 4. The very different surface tension of water and hydrocarbons - Water is much higher I would guess that a filter like this can only work if there is a great degree of insolubility between the two elements - so if you put in a load of a surfactant and form micelles, or an alcohol, which water and fuel are miscible with, the water properties are changed enough so it would go through the filter with the fuel.
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