# Will a film of oil on water retard evaporation (of water) and prevent air and atmospheric impurities from diffusing into it?

I vaguely recall hearing that a layer of oil on water will slow down the evaporation of water, as well as serving as a barrier that would prevent (or retard) the diffusion of air and atmospheric impurities into it. How accurate is this claim?

Some Background:

I want to make zinc/air batteries. $\ce{KOH}$ is commonly used in the electrolyte, but $\ce{CO2}$ in the air can react with the $\ce{KOH}$ and form carbonates which hinders the battery's performance. It seems like oil on top will shield the electrolyte from atmospheric $\ce{CO2}$. The $\ce{H2O}$ also must be kept from evaporating, which would otherwise result in changes to concentration. $\ce{KOH}$ is also very hygroscopic and tends to attract water from air which would also cause variations in concentration.

When the batteries get discharged, $\ce{ZnO}$ is a final product. This should almost certainly precipitate and collect on the bottom of the tank since its considered insoluble in water.

Solar panels harness a vast amount of energy from the sun during the Summer. Storing surplus enrgy for use in Winter is a daunting task. A method that's been doing rounds these days: One could collect and reduce the $\ce{ZnO}$ back to $\ce{Zn}$, deposit it on plates/electrodes, and store these plates for Winter use. The plates could be put back in the batteries and the electrolyte (a fresh solution of $\ce{KOH}$) would have to be replenished.

• Water "dislikes" oil and oil "dislikes" water". So if a molecule of water would like to go through the oil it will be hard for it. Then water doens't try to escape and stay under the oil. It might have a very tiny amount of it which find a way to escape but this is very very slow. For the diffusion of gas in oil I don't know how much is it possible for the air to go through it. I need to find some publications. May 26 '17 at 2:40
• Long story short, it is "yes" to both questions. Oil would hinder the exchange between water and air, both ways. May 26 '17 at 10:38
• Who edited my question? Now it doesn't mean the same as I typed it. I'll have to do some balancing of the equations. The O in the ZnO may come entirely from the air and it may be the electrolyte solution returns back to a solution of KOH once the ZnO is removed-no replenishing necessary. What happens is the Zn turns into Zn(OH)2 which disassociates into ZnO and OH ̅ . There are rechargeable zinc/air batteries on the market which are not suitable and apparently not available to me anyway. Their configurations won't work well in this solar application-self discharge too easily. May 27 '17 at 0:21
• Regarding the edits: Click on the "edited X hours ago" link to see the edit history and who edited what. Keep in mind they have nothing to gain except trying to help you, but they may well have made a mistake. All you can really do is to edit it back to how you like it. May 27 '17 at 4:00
• It wasn't clear what was meant when the editor said "making the rounds". Now I assume this same idea has been discussed before. It would be silly of me to think only I had thought of this. The cathode side of the battery may be the difficult one; methods have been developed to keep the zinc from depositing there. Could someone point me to previous discussions on this? May 27 '17 at 14:25

I don't know about oil (fun fact, putting oil on water was a traditional approach to calming waves, thus the phrase "pour oil on troubled waters"). But cetyl alcohol has been used to decrease evaporation from reservoirs, which might be what you heard about. For instance, here is a report on this, with lots of references. They used a mix of steryl and cetyl alcohols (long-chain alcohols that will form a monolayer on the water).

• Interestingly, I know of another chemical called aspartic acid which is made by Flexible Solutions Inc which is biodegradable for the same purpose u mentioned. I own some stock in this company. They are now claiming it slows evaporation by 45% to 47%. I think it lasts only for a few days though. They might get some contracts from San Diego for their reservoirs and are expecting more from Brazil. Anyway, I'm going in a different direction. I'm amazed at how many amp hours u can get out of 1 mol of oxygen in this reaction. I will show my figures. Jun 4 '17 at 15:42

A oil film definitely slows down evaporation. For example, when the meibomian glands don't work properly, there isn't enough "oil" in tears, and people who have a problem with these glands are subject to dry eyes.