# A liquid that forms a surface membrane that prevents evaporation but allows oxygen to pass through

I'm a biologist. I have a water solution with living cells in a tiny well (10 um). I need to keep the solution from evaporating for several days in a warm incubator. I'm looking for a liquid to place on the surface of the solution that will create a surface membrane/film that prevents or slows evaporation but continues to allow oxygen exchange. I welcome any other suggestions for achieving the same result, prevent/slow evaporation while allowing oxygen exchange.

Here are some things that we've tried and some constraints:

• Increasing the humidity in the incubator is not sufficient (we've tried)
• Lowering the temperature in the incubator is not an option (we want the cells to keep doing what they do)
• Sealing the well is not an option (we need the solution to remain oxygenated)
• What can be added to the solution is limited to substance we are sure won't react with the cells
• Maybe a fluorosurfactant will work. Fluorocarbons provide high oxygen solubility but are very dense, whereas the surfactant should coat the surface. When you say you tried to increase of the humidity in the chamber what exactly did you attempt? If the atmosphere is saturated then any loss from your well means there is a sink somewhere else where water is condensing. Question is what is that sink. Also, there are humidity controlled incubators on the market. Jan 18, 2019 at 19:26
• An alternative is to increase the oxygen content in the chamber (a fire risk though) while narrowing the exposure of the water surface (for instance with a lid with a hole). But it becomes a balancing act because carbon dioxide may accumulate in the well, depending on how dense and active your cells are. Jan 18, 2019 at 19:41
• How about keep the solution in a closed container at $\pu{1 atm}$ $\ce{O2}$ pressure? Jan 18, 2019 at 20:15
• A ten µm droplet that is supposed to survive for days? Look for a poymer membrane with sufficient oxygen permeability and place it on top.The absolute amount of oxygen needed can't be too high in such a small system, I guess.
– Karl
Jan 18, 2019 at 20:51
• You should consider an non-ionic surfactant such as diethanolamine cocoamide or an ethoxylated alcohol.
– A.K.
Jan 19, 2019 at 0:02

## 4 Answers

I don't think you or your cells want any strange low molecular weight substance in your experiment.

You can buy polymethylpentene (PMP) foil, this stuff has a rather low density compared to PE and other polyolefines, and therefore a high permeability for gases (10x higher than PE for oxygen, and PE is already not exactly tight against gases, x20 higher permeability than PVC for example). 50µm films are readily available, you find them online in a minute.

1) Octadecanol, a solid, waxy material, not soluble in water, has been scattered on reservoirs (in Australia, and probably other places) to inhibit evaporation of water. It's biodegradable, so it only lasts a few days.

Octadecanol has been emulsified with equal amounts of soap and diluted for application onto fresh concrete to reduce evaporation, but the extra water in the diluted spray may be part of its success.

2) Melting point capillary tubes are small in diameter (~1 mm), but long enough to reduce the rate of evaporation of a microdrop in the bottom. Water would diffuse out (not really evaporate as if in the open) while O2 will diffuse in. Immersing the melting point tube in a large beaker of water so its water level is almost at the top of the melting point tube could keep the humidity high at the mouth and seriously impede evaporation. The melting point tube would tend to float, so it will have to be weighted down. And maybe put a watch glass over the beaker to form a somewhat enclosed pocket in the incubator. Oxygen will still get in easily, but the humidity will have an actual high level rather than one which is reduced by leaks, insufficient evaporation from the water source, etc.

If this works somewhat, you can draw hot glass (or warm PE tubes) to very small diameters and very long lengths. And I can guess that there will be a lot of breakage!

Thank you for the responses and suggestions. It turns out that there exists a product for exactly this purpose. "Light Mineral Oil is a sterile light mineral oil intended for use as an overlay when culturing in reduced volumes of media to prevent evaporation, and to protect the media from changes in osmolality and pH."

https://en.wikipedia.org/wiki/Mineral_oil#Cell_culture

Seal the well, add some phytoplankton and store with plenty of light. Or seal in a larger volume (1 ml) and hope the cells don't run out of oxygen.