My question exactly.. I hope someone can chime in :)

EDIT: Thinking a bit more... I suppose it's a reasonable assumption that the molecules (mostly) wouldn't stack on top of each other. They all want to get lower and perhaps the resistance to the oil spreading out is much lower proportionately that the gravitational force encouraging the oil to flatten

But if I spill some oil on my plate it doesn't look a molecule thin to me. Why is it different with water?

I guess oil is repelled by water, so when it's poured on top of water it's more like floating on top. So the water pushes up, gravity pulls down and the oil molecules pull on each-other, there is no horizontal friction, allowing the oil to spread out this way. Whereas the oil does slightly stick to your plate, as can be observed when moving the plate around, so it won't spread as thinly?

There’s a couple of possible reasons. First, you probably spilled more oil onto the plate. In the experiment, 0.81 milliliters of oil spread out until it covered a circle with diameter 84 cm. Most spills would be more than a mL of oil, and most plates are much smaller.

Second, most plates aren’t flat. If you have an area of the plate that is at a lower elevation than the rest, the oil would pool up in that area.

Third, even if you fill the plate with water, you could have elevation changes due to surface tension of the water. If the water is concave up, the oil would float upward and form a ring around the edge. If the water is above the surface of the plate, held in just by surface tension, the oil would float upwards and form a bubble at the center of the plate.

Fourth, you could have something else on the plate that acts as an emulsifier. Whether a bit of egg, some pasta water, leftover detergent, these would break up the oil and prevent a film from forming.

The easiest way to have a flat surface is to do the experiment in the center of a much larger body of water, since any effects from the surface tension would be at the edge.

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

there has to be some missing information on how he found the area of water that fully consumed exactly that amount of oil as it simply doesn’t make sense without that. for instance one can spread a teaspoon of oil over 1, 2, n square meters and at some point the oil goes from m later thick to one to less than one.

Such an experiment was described in a science book I read as a kid. They dissolved oil in alcohol so as to measure a very small quantity of oil, and then a a knitting needle was used to stretch the oil film across a plastic container filled to the edge with water (until the film breaks).

Perhaps he was observing the layer making sure it had integrity? Oil layers famously have an optical effect (iridescence from interference of reflections). This effect would transition as the layer goes from >= 1 molecule thick to <= 1 molecule thick (on average). It's likely possible to pinpoint this transition experimentally and then using the oil volume obtain the molecular layer thickness.

To me, the much more questionable assumption is that volume is preserved when a liquid spreads out to a layer one molecule thick.

There's no reason why the volume you get when each oil molecule is surrounded on all sides by other oil molecules should be the same as when each oil molecule has air above and water below. Can anyone explain why this is so?