I believe this is at least part of the explanation. Although there might have been some further developments in the 100 years since this was published Olei

I believe this is at least part of the explanation. Although there might have been some further developments in the 100 years since this was published Oleic acid on water forms a film one molecule deep, in which the hydrocarbon chains stand vertically on the water surface with the COOH groups in contact with the water. Acetic acid is readily soluble in water because the COOH group has a strong secondary valence by which it combines with water. Oleic acid is not soluble because the affinity of the hydrocarbon chains for water is less than their affinity for each other. When oleic acid is placed on water the acid spreads upon the water because by so doing the COOH can dissolve in the water without separating the hydrocarbon chains from each other. When the surface on which the acid spreads is sufficiently large the double bond in the hydrocarbon chain is also drawn down on to the water surface, so that the area occupied is much greater than in the case of the saturated fatty acids. Oils which do not contain active groups, as for example pure paraffin oil, do not spread upon the surface of water https://zenodo.org/records/1429064 https://zenodo.org/records/1429064