3 ms·

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

zokier 2y ago
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
OvbiousError 2y ago
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?
MereInterest 2y ago
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 https://zenodo.org/records/1429064

OvbiousError 2y ago

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?

MereInterest 2y ago

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.