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Homemade insect repellant

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Mar 27, 2018
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This is for those who like making their own essential oil based insect repellents. I am here to tell you that you are likely missing an important step the industry doesn't want you to know!!!

Lemon Eucalyptus essential oil is NOT the same oil being used in things like OFF! brand deet-free lemon eucalyptus spray. There is a difference between the natural product and what is being put in the bottle (not to mention the countless inferior oils being sold that aren't pure, but that's not what this discussion is about). Don't worry, hidden below is a solution that will make your half hour spray work for several hours!

Lemon Eucalyptus oil (not to be confused with the target substance "oil of lemon eucalyptus" aka PMD) contains 70-90% citronellal. The active ingredient, p-methane 3,8-diol (PMD) is only present in lemon eucalyptus essential oil at about 2% by weight. Compared to what the bottle of OFF contains, which is 65% PMD, you can see there is a big difference between what they sell and what you can buy as essential oil. Time for more chemistry! We need to perform a cyclization reaction, but don't worry, you do not need a degree to do it.

What you need to do is convert the citronellal that is in your oil to PMD by heating it with an acid catalyst. More specifically, you need to heat roughly 50% by volume oil with 50% by volume of a 7% citric acid solution at 50°C for 15 hours with constant stirring (you're gonna need a heated stir plate and magnetic stir bar for that). This process converts about 82% of the citronellal to PMD with a small portion becoming various terpenes (which have proven to aid in the repellent properties) and when the math from start to finish is considered, you end up with the same percentage PMD as the commercial product. Now you need to let it settle and remove the water layer, be patient, it will take a while. At this point, you could dry the oil with anhydrous sodium or magnesium sulfate, but something tells me the next step you do has water anyway, so don't bother.

This is then diluted to a 20-30% concentration with the strongest vodka you can get (or oils meant for skin). You can then add a few drops of other essential oils to make it truly yours! For that, I would recommend the other oils that actually work as insect repellents like lavender and rose geranium. And although there is debate on its effectiveness, I like to add a little cedarwood oil as well. The low volatility of PMD compared to other oils that evaporate off within an hour is the reason for it's long-term effectiveness. Now you know the secret!

EDIT: and don't toss that water/citric acid layer, it still contains oil and it can be reused as a catalyst in the next run!!!