The goal is to thin a vegetable oil so that it spreads easily on top of water. It is not for consumption and the flash point of the mixture should be >=95° Celsius". Vegetable oils tend to have flash points greater than 225° Celsius so the flash point of any additives can be well below 95°, as long as special precautions would not be necessary in a manufacturing process.

Since my original post, I've discovered another reason for the low viscosity. It is required for the delivery system to work properly.

I am open to other means to achieve the goal.

  • $\begingroup$ If you're looking for food based products, you might be better posting this question on the 'seasoned advice' stack exchange. $\endgroup$
    – NotEvans.
    Jul 16, 2016 at 16:33
  • $\begingroup$ He said it was not for consumption, so this should be the right place. $\endgroup$
    – ChemBird
    Jul 16, 2016 at 18:32
  • $\begingroup$ Flash point >95C -are you planning to do this experiment at that temperature, or are you going to do it at room temperature, and this is just a general safety concern? Do you want to increase the speed at which oil spreads or the size of the oil spot when it reached equilibrium (stopped spreading)? $\endgroup$
    – sixtytrees
    Jul 17, 2016 at 20:25
  • $\begingroup$ Sorry sixtytrees, I did not see your comment sooner. It's a general safety concern to comply with restrictions for air shipments. I should have said "so that the flash point of the mixture >=95° Celsius". Vegetable oils tend to have flash points greater than 225° Celsius so the flash point of any additives can be well below 95°. I'll change the original question accordingly. I'm more interested in size of the spread then speed. The max size of the spread is about .3 sq m, and the time to 100% spread can be a minute or more. $\endgroup$
    – steveorg
    Aug 5, 2016 at 16:04

2 Answers 2


If the goal is to have the oil cover the water surface, then you might need to add a natural surfactant, such as lecithin, to cause the oil to spread, rather than form globules. You can try egg-yolk or soy beans as a source of lecithin, or you can buy refined lecithin.

As for the smoke point, or maximum cooking temperature for the oil, many common vegetable oils could do: safflower, sunflower or refined canola (rape seed) oil all have a good margin of safety.

One issue might be that if the water is at a "roiling" boil, the turbulence will mix the oil into the body of water. For that reason, you'd need to keep the temperature below 100° C and prevent local hot-spots in the heat source from causing strong convection currents.

  • $\begingroup$ Thank you so much for the lecithin idea! I've purchased some and will test it this week. The water surface will always be at room temperature. The flash point requirement is based on postal regulations for an item to be shippable by air. $\endgroup$
    – steveorg
    Jul 17, 2016 at 17:03
  • $\begingroup$ You may need only a very small amount of lecithin! Sorry I didn't mention that earlier. $\endgroup$ Jul 17, 2016 at 17:04
  • $\begingroup$ Thanks for the extra info. That helps reduce my testing. $\endgroup$
    – steveorg
    Jul 17, 2016 at 17:07
  • $\begingroup$ I just thought of a potential issue. I think of lecithin as an emulsifier, so I am concerned that the oil will mix with the water. Will the oil only spread on top, or will it also mix with water? $\endgroup$
    – steveorg
    Jul 17, 2016 at 17:59
  • $\begingroup$ I just reread the paragraph on roiling water. As I mentioned, the temperature will always be room temperature. The surface will mostly be calm, but there will be times that the surface is moderately disturbed. When the disturbance ends, will the oil resume it's original state spread across the surface? $\endgroup$
    – steveorg
    Jul 17, 2016 at 18:05

Surface tension is responsible for the spreading of one liquid on top of another at equilibrium. The viscosity dictates how fast oil spreads over water, but the surface tension is the one dictating the size of the spot at equilibrium.

If you need to increase the size of the final spot - use surfactants. Soap or dish washing liquid will work. If you want to decrease viscosity then you while keeping flashpoint low then you are in a tough spot. Ligher hydrocarbons are less viscose, but their flash point is low. Pentachloroethane has low viscosity and high flash point, but it isn't readily available and is toxic.

  • 1
    $\begingroup$ Related molecules, chloroform or tetrachloroethylene, which have lower viscosities and densities and are far more common than pentachloroethane will all sink in water (not float). The density PCE is 1.68 g/ml, the density of TCE is 1.62 g/ml, and the density of CHCl4 is 1.49. The viscosity of CHCl4 is 0.563 cP (20 °C). The viscosity of TCE is 0.89 cP at 25 °C, and the viscosity of PCE is 2.45 cP at 20 °C. $\endgroup$ Jul 22, 2016 at 19:23
  • $\begingroup$ Thanks Ben. Sorry that I didn't see your post sooner. If I understand correctly, all of the chemicals that you listed do not meet my criteria? If I'm mistaken, are any of these easy to use and safe in a manufacturing process? Can any be considered natural? $\endgroup$
    – steveorg
    Aug 5, 2016 at 15:29
  • $\begingroup$ I wish that I had seen the original post sooner. The goal is to increase the final spot. I haven't had much luck with a couple of soaps or dish washing liquid. Should I be looking for any special characteristic? I had assumed that low viscosity helped increase the spread, but that seems to not be true. In any case, I still need low viscosity because it is required by the delivery system. $\endgroup$
    – steveorg
    Aug 5, 2016 at 15:35

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